Fungal Endophytes as Efficient Sources of Plant-Derived Bioactive Compounds and Their Prospective Applications in Natural Product Drug Discovery: Insights, Avenues, and Challenges

Abstract

Fungal endophytes are well-established sources of biologically active natural compounds with many producing pharmacologically valuable specific plant-derived products. This review details typical plant-derived medicinal compounds of several classes, including alkaloids, coumarins, flavonoids, glycosides, lignans, phenylpropanoids, quinones, saponins, terpenoids, and xanthones that are produced by endophytic fungi. This review covers the studies carried out since the first report of taxol biosynthesis by endophytic Taxomyces andreanae in 1993 up to mid-2020. The article also highlights the prospects of endophyte-dependent biosynthesis of such plant-derived pharmacologically active compounds and the bottlenecks in the commercialization of this novel approach in the area of drug discovery. After recent updates in the field of ‘omics’ and ‘one strain many compounds’ (OSMAC) approach, fungal endophytes have emerged as strong unconventional source of such prized products.

1. Introduction

Several recent reports suggest that natural products may play a substantial role in the drug discovery and development process as a source of diverse and novel templates for future drugs [1,2,3,4]. With the rapidly evolving recognition that significant numbers of natural products are either produced by microbes or a result of microbial interactions with their hosts, the area of endophyte research for natural products is positioned to take the drug discovery and development process to the next level [5,6]. In the backdrop of the past 25 years of studies, endophytes may be defined as a polyphyletic group of unique microorganisms residing in healthy living internal tissues of the plants with covert and/or overt positive effects on their hosts. They establish a variety of intricate biological intra- and inter-relationships among them and with their hosts, respectively. Endophytes are able to produce a multitude of secondary metabolites with diverse biological activities [7,8,9]. However, merely 0.75–1.50% of known plant species has been explored for their endophytes yet. So, the opportunity to find new potential bioactive metabolites from cryptic endophytic microorganisms of nearly 374,000–400,000 plant species congruently occupying millions of biological niches is considered high [5,10]. This opportunity has increased further with the innovative discovery of biosynthesis of Taxus derived anticancer compound ‘taxol’ from its endophytic fungus T. andreanae in 1993 by Stierle et al. [11]. This discovery leads to renewed attention in endophytic fungi for isolating plant-derived medicinal compounds [12,13,14]. Later, a series of works revealed that a reasonable number of plant-derived compounds are synthesized by endophytes rather than hosts [9,15]. However, there are unsettled and contradictory reports regarding the phylogenetic origin of genes related to the biosynthesis pathway of such plant-derived compounds in host plants and their microbial endophytes [16]. The above facts prompted us to use the word “plant/host-derived” rather than “plant/host-origin” for such compounds. Nevertheless, it is now an established fact that endophytes can co-/produce, induce, and/or modify a plethora of “specific plant-derived” metabolites in-/outside of host plants [12,14,17]. Such discoveries opened the new horizons for the up-scaled production of plant-derived medicinal compounds from endophytes. The recent increase in demand for natural products and difficulties in accessing them from plants make endophytes interesting targets for the assessment and isolation of typical host-derived compounds [18,19]. Since medicinal plants are an inherent source of many therapeutic compounds, it is vital to explore their endophytes to isolate such compounds. The current review aims to provide an up-to-date overview on the globally isolated specific plant-derived bioactive compounds synthesized by fungal endophytes from the period 1993 to mid-2020. It will also focus on applications and modes of actions of such compounds. This review will also provide insights about different challenges in employing endophytes as an alternative source for the synthesis of plant-derived bioactive compounds and their application in drug discovery. Its outcome would certainly lead to strategize the use of endophytes as an efficient novel source for plant-derived metabolites.

2. Plant-Derived Bioactive Natural Products from Fungal Endophytes

A wide array of secondary metabolites in fungi is biosynthesized from very few key precursor compounds by slight variations in basic biosynthetic pathways and can be classified into nonribosomal peptides, polyketides, terpenes, and alkaloids. Nonribosomal peptides are biosynthesized by multimodular nonribosomal peptide synthetases (NRPS) enzymes using both proteinogenic and nonproteinogenic amino acids. Polyketides are biosynthesized by polyketide synthase (PKS) enzymes from acetyl-CoA and malonyl-CoA units. Terpenes consisting of isoprene subunits are biosynthesized from the mevalonate pathway catalyzed by terpene cyclase enzymes. Alkaloids are nitrogen-containing organic compounds biosynthesized as complex mixtures through the shikimic acid and the mevalonate pathways, as they are usually derived from aromatic amino acids and dimethlyallyl pyrophosphate [20]. Other classes of fungal secondary metabolites are linked with the above four groups of compounds. For ease and better understanding, we have classified different fungal secondary metabolites as alkaloids, coumarins, flavonoids, lignans, saponins, terpenes, quinones, and xanthones, and miscellaneous compounds. Coumarins are a class of lactones consisting of a benzene ring fused to a α-pyrone ring and are mainly biosynthesized by the shikimic acid pathway from cinnamic acid. Flavonoids are synthesized by the phenylpropanoid pathway from phenylalanine using enzymes phenylalanine ammonia lyase (PAL), chalcone synthase, chalcone isomerase, and flavonol reductase [21]. Lignans are low molecular weight polyphenols biosynthesized by enzymes pinoresinol-lariciresinol reductase (PLR), PAL, cinnamoyl-CoA reductase (CCR), and cinnamyl-alcohol dehydrogenase (CAD) [22]. Saponins are glycosides containing a non-sugar triterpene or steroid aglycone (sapogenin) attached to the sugar moiety. Saponins are derived from intermediates of the phytosterol pathway using enzymes oxidosqualene cyclases (OSCs), cytochromes P450 (P450s), and UDP-glycosyltransferases (UGTs) [23]. Quinones are biosynthesized through several pathways; for example, isoprenoid quinones are synthesized by the shikimate pathway using chorismite-derived compounds as precursors, terrequinone by NRPS from L-tryptophan, dopaquinone by tyrosinase from tyrosine, and benzoquinone by catechol oxidase/PKS from catechol [24]. Xanthones comprise an important class of oxygenated heterocyclics biosynthesized through the polyacetate/polymalonate pathway by the internal cyclization of a single folded polyketide chain [25].

2.1. Plant-Derived Alkaloids from Fungal Endophytes

After a systematic literature survey, we enlisted 19 plant-derived medicinal alkaloids that have been produced by different endophytic fungi (Table 1), and some important alkaloids are described below.

Table 1.

Plant-derived alkaloids produced by endophytic fungi.

Plant-Derived Alkaloids	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Aconitine	Anticancer, anti-inflammatory, anti-neuralgic, cardiotoxic	Aconitum spp.	Cladosporium cladosporioides	Aconitum leucostomum	[26]
Berberine	Antibiotic, antidiabetic, antihypertensive, antiproliferative hepatoprotective, hypolipidemic, vasodilator	Berberis spp., Coscinium fenestratum, Hydrastis canadensis, Phellodendron amurense	Alternaria sp.	Phellodendron amurense	[27]
			Fusarium solani	Coscinium fenestratum	[28,29]
Caffeine	CNS stimulant	Coffea spp., Theobroma cacao	Anonymous endophytes	Osbeckia chinensis, Osbeckia stellata, Potentilla fulgens	[30]
Camptothecin	Antitumor	Camptotheca acuminata, Miquelia dentata, Nothapodytes nimmoniana, Ophiorrhiza spp.	Entrophospora infrequens	Nothapodytes foetida	[31]
			Entrophospora infrequens	Nothapodytes foetida	[32]
			Neurospora sp.	Nothapodytes foetida	[33]
			Valsa mali	Camptotheca acuminata	[34]
			Nodulisporium sp.	Nothapodytes foetida	[35]
			Fusarium solani	Camptotheca acuminata	[36]
			Botryosphaeria parva, Diaporthe conorum, Fusarium oxysporum, Fusarium sacchari, Fusarium solani, Fusarium subglutinans, Fusarium verticillioides, Galactomyces sp., Irpex lacteus, Phomopsis sp., Fusarium sp.	Nothapodytes nimmoniana	[37]
			Xylaria sp.	Camptotheca acuminata	[38]
			Fusarium solani	Apodytes dimidiata	[39]
			Botryosphaeria dothidea	Camptotheca acuminata	[40]
			Alternaria alternata, Fomitopsis sp., Phomopsis sp.	Miquelia dentata	[41]
			Trichoderma atroviride	Camptotheca acuminata	[42]
			Aspergillus sp.	Camptotheca acuminata	[36]
			Fusarium oxysporum	Nothapodytes foetida	[43]
Capsaicin	Anti-inflammatory, gastro-stimulatory	Capsicum annuum	Alternaria alternata	Capsicum annuum	[44]
Homoharringtonine	Anticancer	Cephalotaxus spp.	Alternaria tenuissima	Cephalotaxus sp.	[45]
Huperzine A	Acetylcholinesterase inhibitor, Alzheimer’s treatment	Huperzia serrata	Acremonium sp.	Huperzia serrata (syn. Lycopodium serratum)	[46]
			Blastomyces sp., Botrytis sp.	Phlegmariurus cryptomerianus	[47]
			Penicillium chrysogenum	Huperzia serrata	[48]
			Shiraia sp.	Huperzia serrata	[49]
			Cladosporium cladosporioides	Huperzia serrata	[50]
			Colletotrichum sp., Trichoderma sp.	Huperzia serrata	[51]
			Paecilomyces tenuis	Huperzia serrata	[52]
			Aspergillus flavus, Mycoleptodiscus terrestris, Penicillium griseofulvum	Huperzia serrata	[53]
			Penicillium sp.	Huperzia serrata	[54]
			Fusarium sp.	Phlegmariurus taxifolius	[55]
			Fusarium sp.	Huperzia serrata	[56]
Peimisine, Imperialine-3b-D-glucoside	Antiasthmatic, antitumor, expectorant	Fritillaria spp.	Fusarium sp.	Fritillaria unibracteata var. wabuensis	[57,58]
			Fusarium redolens	Fritillaria unibracteata var. wabuensis	[59]
Piperine	Anti-inflammatory, anticancer, antimicrobial, antidepressant, hepatoprotective	Piper longum, Piper nigrum	Periconia sp.	Piper longum	[60]
			Colletotrichum gloeosporioides	Piper nigrum	[61]
			Mycosphaerella sp.	Piper nigrum	[62]
			Phomopsis sp.	Oryza sativa	[63]
Cinchona alkaloids: Quinine, Quinidine, Cinchonidine, Cinchonine	Antimalarial, antiarrhythmic, analgesic	Cinchona spp.	Arthrinium, Fomitopsis, Diaporthe, Penicillium, Phomopsis, Schizophyllum	Cinchona ledgeriana	[64]
			Fusarium incarnatum, Fusarium oxysporum (only quinine and cinchonidine) Fusarium solani (only quinine)	Cinchona calisaya	[65]
Rohitukine	Anticancer, CDK inhibitor, cytotoxic	Amoora rohituka Dysoxylum binectariferum,	Fusarium proliferatum	Dysoxylum binectariferum	[66]
			Fusarium oxysporum, Fusarium solani	Dysoxylum binectariferum	[67]
			Gibberella fujikuroi	Amoora rohituka	[67]
Sanguinarine	Anticancer, antimicrobial, anti-inflammatory antioxidant, antihelmintic, neuroprotective	Macleaya cordata, Sanguinaria canadensis	Fusarium proliferatum	Macleaya cordata	[28,68]
Sipeimine	Antibechic, anti-ulcer	Fritillaria spp.	Cephalosporium corda	Fritillaria ussuriensis	[69]
Solamargine	Anticancer, cytotoxic	Solanum nigrum	Aspergillus flavus	Solanum nigrum	[70]
Swainsonine	Toxicosis in livestock	Astragalus, Oxytropis spp., Swainsona canescens	Embellisia sp.	Astragalus, Oxytropis spp.	[15,71]
			Undifilum cinereum, U. fulvum	Astragalus lentiginosus, Astragalus mollissimus	[72]
			Fusarium tricinctum	Oxytropis deflexa, Oxytropis kansuensis	[73]
			Undifilum sp.	Swainsona canescens	[74]
Vinblastine, Vincristine	Antitumor	Catharanthus roseus (syn. Vinca rosea)	Alternaria sp.	Catharanthus roseus	[75]
			Fusarium oxysporum	Catharanthus roseus	[76]
			Fusarium oxysporum	Catharanthus roseus	[77]
			Talaromyces radicus	Catharanthus roseus	[78]
			Eutypella spp.	Catharanthus roseus	[79]
			Geomyces sp.	Nerium indicum	[80]
Vincamine	Antihypertensive, vasodilator	Vinca minor	Anonymous	Vinca minor	[81]

2.1.1. Aconitine

Aconitine, a diterpenoid alkaloid found in Aconitum spp., is a voltage-gated sodium channel activator that effectively opens the Na⁺ channels causing the prolonged presynaptic depolarization of muscles and neurons. In Chinese folk medicine, aconitine is used for pain relief caused by trigeminal and intercostal neuralgia, rheumatism, migraine, and general debilitation. Aconitine is a strong cardiotoxic and neurotoxic agent, and its side effects may cause bradycardia, hypotension, ventricular dysrhythmia, and inhibition of the release of neurotransmitters [82]. Aconitine is also synthesized by endophytic fungus Cladosporium cladosporioides from Aconitum leucostomum [26].

2.1.2. Berberine

Berberine, an isoquinoline alkaloid found in Berberis spp. and some other plants (Table 1), is widely used in the treatment of hyperglycemia, hyperlipidemia, gastrointestinal, cardiovascular, renal, and neural disorders. The antidiabetic efficacy of berberine is comparable to that of the popular drug metformin. Its hypoglycemic effect is exerted via inhibition of mitochondrial function, stimulation of glycolysis, activation of AMP-activated protein kinase (AMPK)/AMPK pathway, and increasing insulin sensitivity. Moreover, berberine has additional advantageous effects on diabetic cardiovascular complications due to its antihypercholesterolemic, anti-arrhythmias, and nitric oxide (NO)-inducing properties. The antioxidant and aldose reductase inhibitory activities of berberine is useful in alleviating diabetic nephropathy [83]. Berberine specifically binds with DNA to inhibit replication, which confers its cytotoxicity and anticancer properties [28]. Moreover, the low toxicity of berberine makes it a potent future antidiabetic and antiproliferative agent. Berberine production has also been reported from endophytic fungi Alternaria sp. and Fusarium solani isolated from Phellodendron amurense and Coscinium fenestratum, respectively [27,29].

2.1.3. Camptothecin

Camptothecin (CPT), a potent anticancer quinoline indole alkaloid, was first isolated from the bark of the Camptotheca acuminata in 1966, but it was also produced by some other plant species, including Miquelia dentata, Nothapodytes nimmoniana, and Ophiorrhiza [84]. Inadequate water solubility and high toxicity are two limiting factors for the application of CPT as an anticancer agent. However, its two derivatives 10-hydroxycamptothecin (HCPT), and 9-methoxycamptothecin (MCPT) retain the same medicinal efficacy without above limitations [36]. CPT and HCPT reversibly stabilize the Top1–dsDNA complex by selectively inhibiting eukaryotic topoisomerase I (TopI) activity. In virtue of this, CPT derivatives are currently being used extensively as precursor compounds for efficient broad-spectrum anticancer drugs irinotecan, topotecan, and belotecan [36,85]. Recently, a chemically bespoke camptothecin–antibody drug conjugate named traztuzumabderuxtecan (Enhertu^®) has also been approved by the US Food and Drug Administration (FDA) [9]. Puri et al. in 2005 and Rehman et al. in 2008 isolated the camptothecin-producing potential endophytic fungi Entrophospora infrequens and Neurospora sp. respectively from the inner bark of Nothapodytes foetida [31,33]. Again, endophytic fungus F. solani isolated from C. acuminata was found to produce CPT, HCPT, and MCPT [36]. Endophytic fungus F. solani isolated from Apodytes dimidiata in the Western Ghats, India also yielded camptothecin [39]. Three fungal species, Alternaria alternata, Fomitopsis sp., and Phomopsis sp., isolated from fruits of M. dentata, were found as prominent CPT producers [41]. In another bioprospection study, 161 fungal endophytes from C. acuminata were screened for CPT production in which Botryosphaeria dothidea was found as a prominent producer of MCPT [40]. Two camptothecin-producing fungi, Trichoderma atroviride and Aspergillus sp., were also isolated from C. acuminata [42,85]. CPT-producing endophytic fungi have also been isolated from N. nimmoniana [37,86]. We found a total of 22 CPT-producing endophytic fungal species from five different host plant species, as listed in Table 1. These findings suggested that the endophytic fungi could be a future alternative source of not only CPT but also of its safer and more efficient analogues.

2.1.4. Capsaicin

Capsaicin, a spicy alkaloid of red pepper Capsicum annuum first crystallized in 1878, has antilithogenic, anti-inflammatory, thermogenic, gastro-stimulatory, antidiabetic, cardioprotective, and anticancer attributes [87]. Capsaicin selectively binds to calcium channel protein targeting transient receptor potential vanilloid 1 (TRPV1) expressed by nociceptors and lowers its opening threshold, resulting in nociceptor depolarization. That is why capsaicin is linked to the sensation of heat and pain as well as obesity regulation via increased thermogenesis. Capsaicin decreases glucose tolerance by inhibiting adipose tissue inflammatory responses via decreasing adipose tissue macrophages and levels of inflammatory adipocytokines like tumor necrosis factor alpha (TNF-α), monocyte chemoattractant protein (MCP)-1, interleukin (IL)-6, and leptin. It also induces the TRPV1-dependent secretion of insulin and antihyperglycemic hormone glucagon. The potential beneficial effects of capsaicin on cardiovascular and gastroprotective systems are exhibited through the TRPV1-mediated release of neurotransmitter calcitonin gene related peptide (CGRP). Capsaicin exerts its anticancer activity via the activation of cAMP-activated protein kinase, peroxisome proliferator-activated receptor gamma (PPARγ)-induced apoptosis, down-regulation of signal transducer and activator of transcription 3 (STAT3) target gene B-cell lymphoma 2 (Bcl-2), cell-cycle arrest by inhibiting cyclin-dependent kinases (CDK2, CDK4 and CDK6), modulation of the human epithelial growth factor receptor 2 (HER2) pathway and p27 expression, down-regulation of p38mitogen-activated protein kinase (MAPK), protein kinase B (PKB or AKT), and focal adhesion kinase (FAK) activation, and degradation of hypoxia inducible factor 1α [88]. An endophytic fungal strain A. alternata isolated from fruits of C. annuum has also been found to produce capsaicin [44].

2.1.5. Homoharringtonine (HHT)

Valuable anticancer alkaloid homoharringtonine (HHT) for the first time has been isolated from the bark and leaves of threatened medicinal tree Cephalotaxus harringtonia [89]. It acts as a translation inhibitor during G₁ and G₂ phases of cell division. In 2012, homoharringtonine was approved for the treatment of chronic myeloid leukemia under generic name omacetaxine mepeosuccinate by the Food and Drug Administration of the USA [90]. Hu et al. in 2016 screened 213 fungal strains isolated from the bark of Cephalotaxus hainanensis for the HHT biosynthesis ability and found that Alternaria tenuissima was a stable HHT-producing endophyte [45].

2.1.6. Huperzine A

A tropical medicinal moss Huperzia serrata is clinically used for the treatment of Alzheimer’s disease [54]. Its biologically active alkaloid huperzine A (HupA) acts as a strong acetylcholinesterase inhibitor (AChEI), which is a class of medication that improves the level of neurotransmitters in the brain and is hoped to be a potential treatment for Alzheimer’s disease. Li et al. in 2007 first isolated an HupA-producing endophytic fungus Acremonium from H. serrata [46]. For now, 12 different HupA-producing endophytic fungal species from H. serrata and three from Phlegmariurus spp. have been reported by different workers (Table 1). Interestingly, many endophytic fungi have been shown to synthesize novel AChEIs in their metabolite extracts [91].

2.1.7. Peimisine and Imperialine-3β-D-glucoside

Fritillaria, a traditional medicinal plant, is among the most widely used antitussive and expectorant drugs. The principal bioactive constituents of Bulbus Fritillaria cirrhosa are steroidal alkaloids peimisine and imperialine-3β-D-glucoside [57]. Fusarium spp. isolated from Fritillaria unibracteata var. wabensis have also produced peimisine and imperialine-3β-D-glucoside [58,59].

2.1.8. Piperine

Anti-inflammatory and anticancer alkaloid piperine is found in the fruits of Piper longum and Piper nigrum and responsible for their pungent taste. Piperine enhances hepatic-oxidized glutathione and decreases renal glutathione concentration and renal glutathione reductase activity, showing its antidiabetic activity. Piperine decreases liver marker enzymes activity, inhibits lipopolysaccharide-induced expression of interferon regulatory factor, reduces the activation of STAT1, and inhibits the release of Th-2-mediated cytokines indicating its anti-inflammatory activity. Piperine expresses its anticancer activity through the following mechanisms: activates caspase-3 and caspase-9, cleaves poly(ADP-ribose) polymerase (PARP), decreases Bcl-2 protein expression and increases Bax protein, reduces the expression of phosphorylated STAT3 and nuclear factor kappa B (NF-kB) transcription factors, blocks extracellular signal-regulated kinase (ERK1/2), p38 MAPK, and AKT signaling pathways, and suppresses epidermal growth factor (EGF)-induced matrix metalloproteinase (MMP)-9 expression [92]. It has bioavailability-enhancing ability for certain drugs and nutrients. It has also been extracted from the cultures of endophytic fungi Periconia sp., C. gloeosporioides, and Mycosphaerella sp. isolated from Piper spp. [60,61,62]. Recently, piperine production has also been reported from endophytic Phomopsis sp. from Oryza sativa [63].

2.1.9. Quinine

The stem bark and roots of the Cinchona spp. are well-established sources of quinine. It has been used as the only effective medication for malaria for centuries until the development of synthetic antimalarial drugs in 1940s. Quinine functions as an antimalarial by acting as an intra-erythrocytic schizonticide and also as gametocytocidal for Plasmodium malariae and Plasmodium vivax but not for Plasmodium falciparum [93]. One of the earliest reports regarding the endophytic fungi-based synthesis of quinine was published in 2002 [94]. Maehara and co-workers have found 21 endophytic fungal strains of Cinchona ledgeriana positive for quinine synthesis and identified them as strains of Arthrinium, Fomitopsis, Diaporthe, Penicillium, Phomopsis, and Schizophyllum [64]. Similarly, Hidayat et al. reported seven different strains of three Fusarium species capable of producing quinine (Table 1) [65].

2.1.10. Rohitukine

Rohitukine, a lead for the semisynthetic potential anticancer drugs flavopiridol (Sanofi-Aventis, Paris, France) and P-276-00 (Piramal Healthcare Ltd., Mumbai, India), is mainly isolated from the bark of Dysoxylum binectariferum. However, the removal of bark poses a threat to the survival of the source medicinal plant. Rohitukine exhibits anticancer activity through the up-regulation of p53 and caspase-9 and down-regulation of Bcl-2 protein [95]. In 2012, Kumara and his group isolated an endophyte Fusarium proliferatum from D. binectariferum that produces host-derived rohitukine [66]. Later, rohitukine-producing other species of Fusarium (Table 1) were also recovered from D. binectariferum and Amoora rohituka [67].

2.1.11. Sanguinarine

Sanguinarine (SA), a toxic benzophenanthridine alkaloid found in the root of Sanguinaria canadensis and leaves of Macleaya cordata, recently gained attention for its cytotoxic and anticancer activities [68]. It suppresses NF-κB activation and induces a rapid apoptotic response via glutathione depletion, and mitochondrial damage [96]. It exhibits cytotoxicity via affecting the Na⁺-K⁺-ATPase transmembrane protein, which regulates the MAPK pathway, production of reactive oxygen species (ROS), and intracellular calcium level [28]. It also inhibits microtubule polymerization and specifically induces DNA damage in cancer cells. SA has also been produced by endophytic F. proliferatum isolated from leaves of M. cordata [68].

2.1.12. Solamargine

A well-known medicinal plant Solanum nigrum shows anticancer, antioxidant, antimicrobial, hepatoprotective, anti-inflammatory, antipyretic, and diuretic properties due to its flavonoid and steroidal alkaloid contents. Its dominant steroidal alkaloid solamargine has exhibited potent anticancer activity against a wide range of cancer cell lines [97]. Solamargine may induce cell apoptosis via modulating the expression of TNF receptors (TNFRs), down-regulating Bcl-2 and Bcl-xL, increasing caspase-3 activity, and causing DNA damage [98]. Interestingly, an endophytic A. flavus isolated from its stem produced more solamargine than the host callus culture [70].

2.1.13. Swainsonine

Swainsonine is an indolizidine alkaloid found in ‘locoweeds’, including Swainsona canescens, Astragalus, and Oxytropis. It alters glycoprotein processing by inhibiting α-mannosidase and mannosidase II and causes lysosomal storage disease [15]. Research has shown that in Astragalus, Oxytropis, and Swainsona species, swainsonine is produced by endophytic fungi in genera Embellisia and Undifilum [71,72,74]. Interestingly, in earlier research, plants of Astragalus and Oxytropis without endophytes were found to be swainsonine-free [99].

2.1.14. Vinblastine and Vincristine

Madagascar periwinkle (Catharanthus roseus) is a primary source of well-known anticancer terpenoid indole alkaloids vinblastine and vincristine. They are the second most used class of anticancer drugs in chemotherapy regimens of various malignancies such as acute lymphoblastic leukemia and nephroblastoma [100]. Alkaloid vincristine interferes with spindle formation, intracellular transport, and angiogenesis in tumor cells without affecting normal cells. For the first time in 1998, Guo et al. reported the isolation of vinblastine from an endophytic fungus Alternaria sp., residing in C. roseus [75]. The endophytic fungi Fusarium oxysporum, Talaromyces radicus, and Eutypella spp. from C. roseus produced both vinblastine and vincristine [76,77,78,79].

2.1.15. Vincamine

Indole alkaloid vincamine is one of the most important constituents of Vinca minor and Nerium indicum (apocynaceae) and is used in treating various cerebrovascular disorders such as hypertension, chronic ischemic stroke, and vascular dementia [101]. In 2011, Yin and Sun reported a vincamine-producing endophyte from the host V. minor [81].

2.2. Plant-Derived Coumarins (Benzopyrones) from Fungal Endophytes

Coumarins have been routinely employed as herbal remedies since the onset of herbal medicine. It was first isolated as a natural product from seeds of Dipteryx odorata (Coumarouna odorata) [102]. A total of seven medicinally important specific plant-derived coumarins are produced by fungal endophytes (Table 2).

Table 2.

Plant-derived coumarins produced by endophytic fungi.

Plant-Derived Coumarins	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Bergapten, Meranzin	Antioxidant, psoriasis treatment	Balanites aegyptiaca, Citrus bergamia, Grapefruit peel	Penicillium sp.	Avicennia	[103]
			Botryodiplodia theobromae	Dracaena draco	[104]
Isofraxidin	Anticancer, anti-obesity, cardioprotective, neuroprotective, hyper pigmentation	Acanthopanax senticosus, Sarcandra glabra	Annulohypoxylon bovei var. microspora	Cinnamomum sp.	[105,106]
Marmesin	Anticancer, antihelmintic, antioxidant, antisyphilitic, purgative	Ammi majus, Balanites aegyptiaca	Fusarium sp.	Mangrove	[107]
Mellein	Antibacterial, antifungal, antihepatitis c, larvicidal, phytotoxic	Alibertia macrophylla, Litsea akoensis, Garcinia bancana, Moringa oleifera, Stevia lucida	Septoria nodorum	Conifer	[108]
			Penicillium janczewskii	Prumnopitys andina	[109]
			Botryosphaeria mamane	Anonymous	[110]
			A xylariaceous fungus	Sapindus saponaria	[111]
			Annulohypoxylon bovei var. microspora	Cinnamomum sp.	[106]
			Penicillium sp., Xylaria sp.	Alibertia macrophylla, Piper aduncum	[112]
			Annulohypoxylon squamulosum	Cinnamomum sp.	[113]
			Nigrospora sp.	Moringa oleifera	[114]
			Arthrinium (Apiospora montagnei)	Anonymous	[115]
			Xylaria sp.	Garcinia sp.	[116]
			Pezicula sp.	Forsythia viridissima	[117]
			Xylaria cubensis	Litsea akoensis	[118]
Scopoletin, Umbelliferone	Antifungal, antioxidant, anti-inflammatory	Artemisia scoparia, Scopolia carniolica (syn. Scopolia japonica), Viburnum prunifolium	Penicillium sp.	Avicennia	[103]

2.2.1. Bergapten and Meranzin

Furocoumarin bergapten (5-methoxypsoralen) from Citrus bergamia and Balanites aegyptiaca is a potential photosensitizing drug in the oral photochemotherapy of psoriasis. Bergapten forms a stable combination with pyrimidine bases causing DNA damage and phosphatase and tensin homolog (PTEN)-mediated induced autophagy, indicating anticancer activity [119]. Meranzin exhibits an antidepressant effect through regulation of the α2-adrenoceptor [120]. Meranzin along with bergapten is also found in grapefruit peels [121]. Both of the compounds are also produced by endophytic fungi Penicillium sp., Botryodiplodia theobromae, and Alternaria brassicae [103,104].

2.2.2. Isofraxidin

Isofraxidin is a coumarin compound produced in the Siberian ginseng (Acanthopanax senticosus or Eleutherococcus senticosus) and Apium graveolens. Isofraxidin mainly regulates lipid metabolism and protects from related disorders by reducing triglyceride accumulation, TNF-α release, and ROS activation, enhancing the phosphorylation of AMPKα and acetyl coenzyme A carboxylase (ACC). It also reduces hepatic expression of fatty acid synthase (FAS) and 3-hydroxyl-3-methylglutaryl-CoA synthase 2 (HMGC), inhibiting lipogenesis. Additionally, isofraxidin shows anti-inflammatory activity by significantly depleting infiltrating inflammatory cells (F4/80+ Kupffer cells, and CD68+ macrophages) and inflammatory cytokines (TNF-α and IL-6) in liver cells. Moreover, the anti-inflammatory activity of isofraxidin is correlated with the down-regulation of toll-like receptor 4 (TLR4) and NF-κB expression [122]. Isofraxidin bioactivity as a potent hyperpigmentation agent is exerted by increased melanin synthesis via stimulated tyrosinase activity, increased expression of tyrosinase, and melanogenesis regulator microphthalmia-associated transcription factor (MITF) in melanocytes [123]. The cytotoxic effects of isofraxidin on cancer cells is exerted via inhibition of AKT kinase and increase in caspase-3, caspase-9, and Bax/Bcl-2 levels. Isofraxidin has also shown anti-hypertension effects via inhibiting the activity of angiotensin I converting enzyme (ACE). Isofraxidin protects axons and dendrites against amyloid β (Aβ 25–35) and inhibits neuron-degenerating enzyme monoamine oxidase B [124].

2.2.3. Marmesin

Marmesin (furanocoumarins) was first reported from fruits of Ammi majus and later from Balanites aegyptiaca, which is a folkloric medicinal plant with purgative, antihelmintic, and antisyphilitic properties [119,125]. It is also synthesized by endophytic Fusarium sp. isolated from a mangrove plant [107].

2.2.4. Mellein

Dihydroisocoumarin mellein derives its name from a strain of Aspergillus melleus, which is the first reported source of mellein [126]. Later, this compound was found in plants such as Moringa and Stevia [127,128]. Mellein has exhibited antimicrobial and anti-schistosomiasis activities [114,115]. Mellein has been reported from endophytic fungal species Septoria nodorum in 1995 by Findlay et al. followed by dozens of similar reports as listed in Table 2 [108].

2.2.5. Scopoletin and Umbelliferone

Scopoletin (6-methoxy-7-hydroxycoumarin) is a coumarin with antifungal, anti-acetylcholinesterase (AChE), and antitumor properties. Scopoletin inhibits cancer cell proliferation by inducing apoptosis via reducing the protein content and decreasing the acid phosphatase (ACP) activity level [129,130]. Umbelliferone (7-hydroxycoumarin), distributed within the Rutaceae and Apiaceae (Umbelliferae) families, is a fluorescing compound and used as a sunscreen agent. It shows antioxidant, anti-inflammatory, anti-hyperglycemic, anti-tumor, and antimicrobial activities. Umbelliferone exhibits anticancer activity via inducing apoptosis and cell cycle arrest [131].

2.3. Plant-Derived Flavonoids from Fungal Endophytes

Flavonoids are pigments of edible plants consisting of two benzene rings at either side of a three-carbon ring. Multiple substitutions in this basic structure produce several classes of derivatives, such as flavones, isoflavones, flavonols, flavanones, catechins, and anthocyanins. We found 12 different biologically active plant-derived flavonoids (Table 3) recovered from fungal endophytes and important flavonoids are described below.

Table 3.

Plant-derived flavonoids produced by endophytic fungi.

Plant-Derived Flavonoids	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Apigenin	Antibacterial, anticancer, antioxidant, antihyperglycaemic, lipid peroxidation, sedative, thyroid dysfunction	Cajanus cajan, Cephalotaxus harringtonia, Matricaria chamomilla, vegetables	Colletotrichum sp.	Ginkgo biloba	[132,133,134]
			Chaetomium globosum	Cajanus cajan	[135]
			Paraconiothyriu mvariabile	Cephalotaxus harringtonia	[136]
Cajanol	Anticancer, antimicrobial, antiplasmodial	Cajanus cajan	Hypocrealixii	Cajanus cajan	[137]
Chalcone	Antibacterial, antifungal, antitumor, anti-inflammatory	Cleistocalyx operculatus, Members of Leguminosae, Asteraceae, Moraceae	Ceriporia lacerata	Cleistocalyx operculatus (syns. Eugenia operculata, Syzygium operculatum)	[138]
Chrysin	Antiaging, anticonvulsant, antidiabetic, anti-inflammatory, antimicrobial, anxiolytic, hepatoprotective	Passiflora incarnata	Alternaria alternata, Colletotrichum capsici, Colletotrichum taiwanense	Passiflora incarnata	[139]
Curcumin	Anti-inflammatory, antioxidant, antitumor	Curcuma spp.	Chaetomium globosum	Curcuma wenyujin	[140]
			Anonymous	Curcuma wenyujin	[141]
Kaempferol	Antibacterial, antidiabetic, anti-inflammatory, antioxidant, antitumor	Fruits, vegetables, medicinal herbs	Annulohypoxylonboveri var. microspora, Annulohypoxylon squamulosum	Cinnamomum sp.	[106,113]
			Fusarium chlamydosporum	Tylophora indica	[142]
			Mucor fragilis	Podophyllum hexandrum	[143]
Luteolin	Anti-inflammatory, antioxidant, immunomodulatory	Fruits, vegetables, medicinal herbs	Annulohypoxylon boveri var. microspora	Cinnamomum sp.	[106]
			Aspergillus fumigatus	Cajanus cajan	[144]
Quercetin	Anticancer, anti-inflammatory antioxidant	Fruits, vegetables	Aspergillus nidulans, Aspergillus oryzae	Ginkgo biloba	[145]
			Annulohypoxylon squamulosum	Cinnamomum sp.	[113]
			Nigrospora oryzae	Loranthus micranthus	[146]
Rotenone	Insecticide, pesticide, piscicide	Derris elliptica	Penicillium sp.	Derris elliptica	[147]
Rutin	Antioxidant, cardioprotective, neuroprotective	Aegle marmelos Ginkgo biloba, Nerium oleander, Pteris multifida, fruits, vegetables	Anonymous	Pteris multifida	[148]
			Chaetomium sp.	Nerium oleander	[149]
			Xylaria sp.	Ginkgo biloba	[150]
			Aspergillus flavus	Aegle marmelos	[151]
Silymarin	Anticancer, antioxidant, anti-inflammatory, cardioprotective, hepatoprotective	Silybum marianum	Aspergillus iizukae	Silybum marianum	[152]
Vitexin	Antioxidant, antitumor, neuroprotective	Cajanus cajan, Vitex agnus-castus	Colletotrichum sp.	Ginkgo biloba	[134]
			Dichotomopilus funicola	Cajanus cajan	[153]

2.3.1. Apigenin

Apigenin, amply present in Matricaria spp. and vegetables, has anti-inflammatory, antioxidant, and anticancer properties. Apigenin inhibits the proliferation of malignant cancer cells causing G₂-M arrest by inhibition of the mitotic kinase activity of p34^cdc2 and perturbation of cyclin B1 levels [132,154]. It is also a ligand for the central benzodiazepine receptors exerting anxiolytic and sedative effects [155]. Apigenin activates different anti-inflammatory pathways, including p38/MAPK and phosphatidylinositol 3-kinase (PI3K)/AKT, to exert its anti-inflammatory effect. Further, it prevents the IκB degradation and nuclear translocation of the NF-κB, and reduces cyclooxygenase (COX)-2 activity. Additionally, apigenin up-regulates the expression of anti-oxidant enzymes such as glutathione (GSH)-synthase, catalase, and superoxide dismutase (SOD) to counteract cellular oxidative stress. Its neuroprotective effect is exhibited by the lowering of β-amyloids and restoring the ERK/cyclic AMP response element-binding protein (CREB)/ brain-derived neurotrophic factor (BDNF) pathway [156]. Apigenin also regulates hyperglycemia, thyroid dysfunction, and lipid peroxidation [133]. A fungal endophyte Chaetomium globosum, isolated from Cajanus cajan, produced apigenin with good antioxidant activities [135]. Its glycosidic derivatives, namely apigenin-5-O-α-L-rhamnopyranosyl-(1→3)-β-D-glucopyranoside and euryanoside (apigenin-5-O-α-L-rhamnopyranosyl-(1→2)-(6″-O-acetyl)-β-Dglucopyranoside), were detected in Paraconiothyrium variabile, which is an endophytic fungus in the Japanese plum yew (Cephalotaxus harringtonia) from which these compounds had previously been reported [136].

2.3.2. Cajanol

Cajanol (phytoalexin) is an isoflavone from roots of C. cajan displaying anticancer, antimicrobial, and antiplasmodial activities. Cajanol arrests the cell cycle in the G₂/M phase and induces apoptosis via the ROS-mediated mitochondria-dependent pathway [157]. Endophytic strains of Hypocrea lixii from roots of C. cajan have also been reported to produce cajanol in aqueous cultures with anticancer activity [137].

2.3.3. Chrysin

The flavonoid chrysin is found in leaves of P. incarnata and synthesized by foliar endophytes A. alternata, Colletotrichum capsici, and Colletotrichum taiwanense. It has shown promising biological activities, including antibacterial, anti-inflammatory, antidiabetic, anxiolytic, hepatoprotective, anti-aging, and anticancer effects [139].

2.3.4. Curcumin

Curcumin is the major active principal of Curcuma spp. It shows strong anti-inflammatory and antioxidant activities via the downregulation of COX-2, lipoxygenase, TNF-α, IL-1, -2, -6, -8, and Janus kinases. Curcumin anticancer activity involves cell cycle arrest via inhibition of cyclin D1 and CDK4, and induction of apoptotic signals via the up-regulation of Fas, FasL, and DR₅ expression, p-53 mediated activation of caspase, and inhibition of TNF-α-induced activation of NF-κB [158]. A recent report has suggested curcumin as a potent epigenetic modulator with activities like inhibition of DNA methyltransferases (DNMTs), regulation of histone acetyltransferases (HATs) and histone deacetylases (HDACs), regulation of microRNAs (miRNA). It also interacts with DNA and transcription factors [159]. Curcumin has been isolated from fungal endophytes Chaetomium globosum and an unidentified isolate [140,141].

2.3.5. Kaempferol

Kaempferol is a potent antioxidant, anticancer, cardioprotective, neuroprotective, hepatoprotective, and antidiabetic compound found in fruits and vegetables. It blocks the expression of inflammatory cytokines (IL-1B and TNF-α), COX-2 protein, and inducible NO synthase (iNOS). Kaempferol inhibits various cancer cells by arresting cell cycle at the G₂/M phase, targeting several signaling pathways (MAPK/ERK and PI3K/AKT) that are essential for the survival of cancer cells and modulating expression of epithelial-mesenchymal transition (EMT)-related markers. It prevents the EGF-induced activation of activator protein 1 (AP-1) and NF-κB, and phosphorylation of AKT. It enhances cyclin-dependent kinase inhibitor 1A (CDKN1A) levels via the reduced expression of c-Myc and enhanced level of p53 protein [160,161]. Kaempferol is thereby used as a potent chemopreventive agent in cancer treatment. Endophytic Fusarium chlamydosporum as well as its host Tylophora indica both can produce keampferol [142]. It is also produced by some other endophytes (Table 3).

2.3.6. Luteolin

Luteolin is a plant metabolite with reputed antioxidant, anti-inflammatory, anticancer, and antidiabetic properties. Its anticancer property is manifested via cell cycle arrest in S phase, modulation in ROS levels, inhibition of topoisomerases type I and II, reduction of NF-kB and AP-1 activity, stabilization of p53, and inhibition of PI3K, STAT3, insulin-like growth factor 1 receptor (IGF1R), and HER2 [162]. Luteolin is a better inhibitor of alpha-glucosidase than the widely prescribed drug acarbose, suggesting its role in reducing high blood sugar levels [163]. It has also been reported as a secondary metabolite of some endophytic fungal strains (Table 3).

2.3.7. Quercetin

Quercetin is a red pigment with antioxidant, anti-inflammatory, anticancer, antiviral, antidiabetic, cardiovascular, and neuroprotective properties that is widely distributed in plants. Quercetin causes cell cycle arrest in the S phase and activates apoptosis in cancer cells [164]. Quercetin along with vitamin C may be used for the prevention of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2/COVID-19) in high-risk populations [165]. Its antidiabetic activity is exhibited by induced insulin sensitivity and glucose metabolism. The cardiovascular effects of quercetin are exerted by its inhibitory effect on the angiotensin-converting enzyme and by activating Na+-K+-2Cl−cotransporter 1 (NKCC1) in renal epithelial cells [166]. The mechanism behind the neuroprotective effect of quercetin may involve mitigating oxidative stress via induction of nuclear erythroid 2-related factor 2 (Nrf2)/ antioxidant response element (ARE) and antioxidant paraoxonase 2 (PON2) [167]. Recently, three quercetin compounds were extracted from the endophytic fungus Nigrospora oryzae isolated from leaves of the Nigerian mistletoe Loranthus micranthus [146].

2.3.8. Rutin

Rutin, a glycoside of the flavonoid quercetin with powerful antioxidant, anti-inflammatory, anticancer, and promising neuroprotective properties is found in vegetables and fruits. The anti-inflammatory and antioxidant activities of rutin involves inhibition of expression of COX-2 and iNOS via inhibition of p38 MAPK and c-Jun N-terminal kinase (JNK). Rutin decreases Bcl-2 expression, Bcl-2/Bax ratio, MYCN mRNA levels, and the secretion of TNF-α, demonstrating its anticancer property [168]. Its neuroprotective mechanisms include reduction of pro-inflammatory cytokines, improved antioxidant enzyme activities, activation of MAPK cascade, up-regulation of the ion transport and antiapoptotic genes, and restoration of the activities of mitochondrial complex enzymes [169]. Rutin has also been produced by several endophytic fungi, as listed in Table 3.

2.3.9. Silymarin

Silymarin, a bioactive natural compound found in fruits of milk thistle (Silybum marianum), has cardioprotective, hepatoprotective, antioxidant, immunomodulatory, anti-inflammatory, antihepatitic, and antimetastatic activities [170]. The hepatoprotective property of silymarin is accomplished via an increase in glutathione level, inhibition of lipid peroxidation, activation of antioxidant defense, and translational activities in hepatic cells [171]. Its anticancer activity is related to the modulation of NF-kB, suppression of EGFR-MAPK/ERK1/2 and IGF1R signaling, up-regulation of tumor-suppressor genes p53 and p21^CIP1. Similarly, its antiangiogenic activity is linked to suppression of both vascular endothelial growth factor (VEGF) and MMP-2 [172,173]. Endophytic silymarin for the first time has been reported from the strains of Aspergillus iizukae isolated from the leaves and stems of S. marianum [152].

2.3.10. Vitexin

C-glycosyl flavonoid vitexin has recently received increased attention due to its wide range of pharmacological effects including anticancer (as hypoxia inducible factor (HIF)-1α inhibitor), analgesic (via targeting TRPV1), antioxidant, hypotensive, neuroprotective, and antidiabetic effects [174]. It is synthesized by plants such as C. cajan, Ficus deltoidea, Passiflora incarnata, Vitex agnus-castus, and endophytic fungi such as Colletotrichum sp. from G. biloba [134] and Dichotomopilus funicola of C. cajan [153].

2.4. Plant-Derived Lignans from Fungal Endophytes

Lignans are secondary metabolites with a plethora of biological activities, making them noteworthy in several lines of research. Out of a total of seven medicinally important plant-derived lignans that have been secreted by endophytes (Table 4), one is described below.

Table 4.

Plant-derived lignans produced by endophytic fungi.

Plant-Derived Lignans	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Coniferin	Antidiabetic	Angelica archangelica, Conifers	Members of xylariaceae	Angelica archangelica	[175]
Phillyrin	Antioxidant, antidiabetic, anti-inflammatory, anti-obesity, antipyretic	Forsythia suspensa, Phyllirea	Colletotrichum gloeosporioides	Forsythia suspensa	[176,177]
Podophyllotoxin	Antitumor, antivirus	Diphylleia sp., Dysosma sp., Juniperus sp., Podophyllum spp.	Alternaria sp., Penicillium spp.	Podophyllum hexandrum	[178]
			Monilia sp., Penicillium sp.	Dysosma veitchii	[178]
			Penicillium sp.	Diphylleia sinensis	[178]
			Penicillium implicatum	Diphylleia sinensis	[179]
			Alternaria sp.	Juniperus vulgaris	[180]
			Phialocephala fortinii	Podophyllum peltatum	[181]
			Trametes hirsuta	Podophyllum hexandrum	[182]
			Alternaria neesex	Podophyllum hexandrum	[183]
			Fusarium oxysporum	Juniperus recurva	[184]
			Aspergillus fumigatus	Juniperus communis	[185]
			Fusarium solani	Podophyllum hexandrum	[186]
			Mucor fragilis	Podophyllum hexandrum	[143]
			Phialocephala podophylli	Podophyllum peltatum	[187]
			Alternaria tenuissima	Podophyllum emodi	[188]
			Fusarium sp.	Dysosma versipellis	[189]
Sesamin, Syringaresinol, Ketopinoresinol	Antioxidant, anti-inflammatory	Cinnamomum cassia	Annulohypoxylon ilanense	Cinnamomum sp.	[190]
Syringin	Antidiabetic	Syringa vulgaris, Eleutherococcus senticosus, Magnolia sieboldii, Musa paradisiaca	Members of xylariaceae	Syringa vulgaris	[175,191]

Podophyllotoxin

Podophyllotoxin (podofilox), an aryl tetralin lactone lignan of medicinal plant Podophyllum sp., is an important anticancer and antiviral agent. It is also found in Diphylleia, Dysosma, and Juniperus. It has been used as the lead for the chemical synthesis of the many useful anticancer drugs such as etoposide, teniposide, and etopophos phosphate [184,185]. Podophyllotoxin is an anti-tubulin agent that destabilizes microtubules. Its derivatives inhibit the topoisomerase II enzyme, which is required to unwind the double helix of DNA, preventing mitosis in late S/early G₂ phase [192]. For the first time, Yang et al. in 2003 reported the podophyllotoxin producing endophytic fungi (Table 4) from P. hexandrum, Diphylleia sinensis, and Dysosma veitchii [178]. Puri et al. in 2006 reported a fungal endophyte Trametes hirsuta from rhizomes of P. hexandrum that efficiently produces podophyllotoxin and other related aryl tetralin lignans with potent anticancer properties [182]. Later, several other endophytic fungi such as Phialocephala fortinii isolated from the rhizomes of Podophyllum peltatum, Alternaria sp. isolated from Juniperus vulgaris, and P. hexandrum, F. oxysporum isolated from Juniperus recurva, and A. fumigatus isolated from Juniperus communis have been reported as alternative sources for podophyllotoxin [180,181,183,184,185]. Recently, A. tenuissima, a fungal endophyte from the roots of Podophyllum emodi, and Fusarium sp. from Dysosma versipellis showed the presence of podophyllotoxin in their secondary metabolite analysis [188,189]. In total, podophyllotoxin has been isolated from 17 endophytic fungal species collected from 10 different host plant species, as listed in Table 4.

2.5. Plant-Derived Saponins from Fungal Endophytes

Saponins are known to occur in many taxonomically unrelated plants, but there is evidence that they are also produced by endophytic fungi (Table 5). We found three specific and several other plant-derived saponins that have been reported from endophytic fungi.

Table 5.

Plant-derived saponins produced by endophytic fungi.

Plant-Derived Saponins	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Diosgenin	Anti-inflammatory, antitumor, cardiovascular protection	Dioscorea spp.	Paecilomyces sp.	Paris polyphylla var. yunnanensis	[193]
			Cephalosporium sp.	Paris polyphylla var. yunnanensis	[194]
			Fusarium sp.	Dioscorea nipponica	[195]
Ginsenoside	Anti-inflammatory, antioxidation, antitumor	Panax	Camarosporium sp., Dictyochaeta sp., Penicillium sp.	Aralia elata	[196]
			Aspergillus sp., Fusarium sp., Verticillium sp.	Panax ginseng	[197]
			Aspergillus sp., Fusarium sp.	Panax notoginseng	[198]
Gymnemagenin	Antidiabetic	Gymnema sylvestre	Penicillium oxalicum	Gymnema sylvestre	[199]
Other saponins	Cardiovascular disease	Gynostemma pentaphyllum, Manilkara zapota, Sapindus sp., Saponaria sp.	Aspergillus niger, F. oxysporum	Crotalaria pallida	[200]
			Alternaria alternata, Aspergillus niger, Penicillium sp.	Loranthus sp.	[201]
			Alternaria alternata, Aspergillus flavus, Aspergillus niger, Colletotrichum gleosporioides, Trichoderma sp.	Tabebuia argentea	[202]
			Aspergillus sp.	Salvadora oleoides	[203]
			Aspergillus sp.	Justicia beddomei	[204]
			Cochliobolus lunatus (anamorph Curvularia lunata)	Boswellia ovalifoliolata	[205]
			Monochaetia karstenii (syn. Pestalotiopsis maculans), Phyllosticta sp.	Shorea thumbuggaia	[205]
			Aspergillus neoniveus (syn. Fennellia nivea)	Typhonium divaricatum	[206]
			Alternaria alternata, Aspergillus flavus, Aspergillus niger, Cladosporium sp., Penicillium sp., Phomopsis sp., Trichoderma sp.	Aegle marmelos	[207]
			Aspergillus niger, Aspergillus sp., Aspergillus terreus, Aspergillus tubingensis, Coprinopsis cinerea, Curvularia lunata, Fusarium sp.	Eugenia jambolana	[208]
			Aspergillus awamori, Colletotrichum gleosporioides	Rauwolfia serpentina	[209]

Diosgenin

The anti-inflammatory and anticancer agent diosgenin is primarily obtained from Dioscorea zingiberensis. Its anti-inflammatory activity is exerted via reduction in the levels of several inflammatory mediators, including NO and IL-1 and -6, inhibition of the MAPK/AKT/NF-κB signaling pathway, and ROS production. Diosgenin anticancer effects have been linked to p53 activation, immune modulation, cell cycle arrest, modulation of caspase-3 activity, and activation of STAT3 signaling pathway [210]. Considering the depleting natural populations and requirement of a long period of rhizome maturation of its primary source Dioscorea zingiberensis, endophytes might be suitable alternatives to produce diosgenin. Zhou et al. in 2004 first reported Paecilomyces sp. residing in Paris polyphylla var. yunnanensis as an adiosgenin-producing endophytic fungus [193]. Later, an endophytic strain of Fusarium sp. from Dioscorea nipponica was also been reported for enhanced production of diosgenin in its liquid cultures when supplemented with the rhizome extract of its host plant [195].

2.6. Plant-Derived Terpenes from Fungal Endophytes

Table 6 lists 17 specific plant-derived terpenes produced by fungal endophytes with some of them detailed below.

Table 6.

Plant-derived terpenes produced by endophytic fungi.

Plant-Derived Terpenes	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
Agathic acid	Abortifacient, anti-inflammatory, anticancer, trypanocidal	Agathis spp., Copaifera spp., Juniperus osteosperma	Botryosphaeria sp.	Maytenus hookeri	[211,212,213]
			Bionectria sp.	Raphia taedigera	[214]
			Fusarium sp.	Santalum album	[215]
Artemisinin	Antimalarial	Artemisia spp.	Anonymous	Artemisia indica	[216]
Asiaticoside	Antidermatitic, anti-inflammatory, antioxidant, immunomodulatory	Centella asiatica	Colletotrichum gloeosporioides	Centella asiatica	[217]
Azadirachtin	Hepatoprotective, insecticidal	Azadirachta indica	Penicillium (Eupenicillium) parvum	Azadirachta indica	[218]
Bilobalide	Neuroprotective,	Ginkgo biloba	Pestalotiopsis uvicola	Ginkgo biloba	[219]
Borneol	Antiapoptotic, anti-inflammatory, antioxidant, neuroprotective	Cinnamomum camphora var. borneol	Cochliobolus nisikadoi	Cinnamomum camphora var. borneol	[220]
Camphor	Antimicrobial, topical skin preparations	Cinnamomum camphora	Nodulisporium sp.	Lagerstroemia loudoni	[221]
Cineole (Eucalyptol)	Antimicrobial, respiratory illness	Eucalyptus spp.	Hypoxylon sp., Nodulisporium sp.	Persea indica	[222]
			Nodulisporium sp.	Lagerstroemia loudoni	[221]
			Nodulisporium sp.	Thelypteris angustifolia	[223]
			Nodulisporium sp.	Cassia fistula	[224]
			Annulohypoxylon sp.	Neolitsea pulchella	[225]
Dihydrocumambrin	Antibacterial, cytotoxic	Glebionis coronaria (syn. Chrysanthemum coronarium)	Botryodiplodia theobromae	Dracaena draco	[104]
Ginkgolide	Antiallergic, anti-inflammatory	Ginkgo biloba	Fusarium oxysporum	Ginkgo biloba	[226]
Isocupressic acid	Abortifacient	Conifers	Botryosphaeria sp.	Maytenus hookeri	[212,213]
Loliolide	Herbivore resistance	Lolium perenne	Annulohypoxylon ilanense	Cinnamomum sp.	[227]
Taxane (other than taxol)	Anticancer	Taxus spp.	Alternaria, Aspergillus, Beauveria, Epicoccum, Fusarium, Gelasinospora, Geotrichum, Phoma, Phomopsis	Taxus baccata	[228]
			Cladosporium langeronii, Phomopsis sp.	Wollemia nobilis	[229]
Taxol	Anticancer	Taxus brevifolia	Taxomyces andreanae	Taxus brevifolia	[11]
			Taxomyces sp.	Taxus yunnanensis	[230]
			Pestalotiopsis microspora	Taxodium distichum	[231]
			Alternaria sp., Pestalotiopsis microspora	Taxus cuspidata	[232]
			Fusarium lateritium, Monochaetia sp., Pestalotia bicilia	Taxus baccata	[232]
			Pithomyces sp.	Taxus sumatrana	[232]
			Pestalotiopsis microspora	Taxus wallichiana	[233]
			Pestalotiopsis guepinii	Wollemia nobilis	[234]
			Periconia sp.	Torreyagrandifolia	[235]
			Seimatoantlerium nepalense	Taxus wallichiana	[236]
			Alternaria sp., Pestalosiopsis sp.	Ginkgo biloba	[237]
			Penicillium raistrickii	Taxus brevifolia	[238]
			Tubercularia sp.	Taxus chinensis var. mairei	[239]
			Stegolerium kukenani	Kukenan tepuis, Roraima	[240]
			Taxomyces sp.	Taxus sp.	[241]
			Sporormia minima, Trichothecium sp.	Taxus wallichiana	[242]
			Nodulisporium sylviforme	Taxus cuspidata	[243]
			Anonymous	Taxus chinensis var. mairei	[244]
			Botrytis sp.	Taxus chinensis var. mairei	[245]
			Penicillium sp.	Taxus yunnanensis	[246]
			Fusarium mairei	Rhizophora annamalayana	[247]
			Phyllosticta sp.	Ocimum basilicum	[248]
			Alternaria alternata, Ectostromasp., Fusarium mairei, Ozoniumsp., Papulaspora sp.	Taxus chinensis var. mairei	[249]
			Fusarium solani	Taxus celebica	[250]
			Pestalotiopsis pauciseta	Cardiospermum helicacabum	[251]
			Bartalinia robillardoides	Aegle marmelos	[252]
			Colletotrichum gloeosporioides	Justicia gendarussa	[253]
			Fusarium sp.	Taxus wallichiana	[254]
			Phyllosticta citricarpa	Citrus medica	[255]
			Phyllosticta melochiae	Melochia corchorifolia	[256]
			Phyllosticta spinarum	Cupressus sp.	[257]
			Fusarium arthrosporioides	Taxus cuspidata	[258]
			Aspergillus fumigatus	Podocarpus sp.	[259]
			Botryodiplodia theobromae	Taxus baccata	[260]
			Botrytis sp.	Taxus cuspidata	[261]
			Fusarium solani	Taxus chinensis	[262]
			Chaetomella raphigera	Terminalia arjuna	[263]
			Pestalotiopsis terminaliae	Terminalia arjuna	[264]
			Phomopsis sp.	Ginkgo biloba	[265]
			Phomopsis sp.	Larix leptolepis	[265]
			Phomopsis sp.	Taxus cuspidata	[265]
			Phyllosticta dioscoreae	Hibiscus rosa-sinensis	[266]
			Aspergillus sp., Ceratobasidium sp., Cladosporium tenuissimum, Coniothyrium diplodiella, Epacris sp., Fusarium solani, Metarhizium anisopliae, Paraconiothyrium brasiliense, Pezicula sp., Phomopsis sp. Sordaria sp., Trichoderma sp., Xylaria sp.	Taxus chinensis	[267]
			Mucor rouxianus	Taxus chinensis	[268]
			Colletotrichum gloeosporioides	Plumeria acutifolia	[269]
			Gliocladium sp.	Taxus baccata	[270]
			Pestalotiopsis sp.	Catharanthus roseus	[271]
			Aspergillus candidus, Cladosporium cladosporioides	Taxus media	[272,273]
			Aspergillus niger var. taxi	Taxus cuspidata	[274]
			Mucor sp.	Taxus chinensis var. mairei	[275]
			Pestalotiopsis neglecta, Pestalotiopsis versicolor	Taxus cuspidata	[276]
			Pestalotiopsis pauciseta	Tabebuia pentaphylla	[277]
			Lasiodiplodia theobromae	Morinda citrifolia	[278]
			Acremonium sp., Botryosphaeria sp., Fusarium sp., Gyromitra sp., Nigrosporasp., Penicillium sp.	Taxus globosa	[279]
			Paraconiothyrium sp.	Taxus media	[280]
			Didymostilbe sp.	Taxus chinensis var. mairei	[281]
			Stemphylium sedicola	Taxus baccata	[282]
			Colletotrichum gloeosporioides	Tectona grandis	[283]
			Perenniporia tephropora	Taxus chinensis var. mairei	[284]
			Colletotrichum gloeosporioides, Fusarium proliferatum, Guignardia mangiferae	Taxus media	[285]
			Phoma betae	Ginkgo biloba	[286]
			Alternaria sp.	Corylus avellana	[287]
			Colletotrichum gloeosporioides	Moringa oleifera	[288]
			Penicillium sp.	Taxus chinensis	[289]
Penicillium aurantiogriseum	Corylus avellana	[290]
Toosendanin	Anticancer, antifeedant	Melia azedarach	Anonymous	Melia azedarach	[291,292]
Withanolide	Anticancer, cardiovascular disease, Alzheimer’s disease treatment	Withaniasp.	Taleromyces pinophilus	Withania somnifera	[293]
Xanthatin	Antitumor	Xanthium spp.	Paecilomyces sp.	Panax ginseng	[294,295]

2.6.1. Artemisinin

Asian plant Artemisia annua (sweet wormwood) has been in use for the treatment of fever since more than 2000 years. In 1971, Artemisinin, a sesquiterpene lactone with endoperoxide trioxane moiety, was isolated from the A. annua as its active antimalarial principle by Tu Youyou [296]. According to a WHO report, over 2010–2017, about 2.74 billion artemisinin-based combination therapies (ACTs) have been administered globally [297]. Growing evidence revealed that artemisinin and its derivatives have many more biological activities including anti-inflammatory, immunoregulatory, and anticancer activities without any risk of drug-resistant development [298]. Its antimalarial parasite activity is mediated by ROS generation, causing protein damage and compromising parasite proteasome function, inducing the endoplasmic reticulum (ER) stress response [299,300]. Iron (heme), which is a prerequisite for cancer cells multiplication, also activates an endoperoxide bond of artemisinin, creating cytotoxic/cancer-killing carbon-centered free radicals. As an alternative source, Huang et al. isolated artemisinin from an anonymous fungal isolate of Artemisia indica [216].

2.6.2. Bilobalide and Ginkgolides

Bilobalide and ginkgolides, two main terpenoids found in the leaves and bark of G. biloba, are accountable for the therapeutic implication of its whole extract [301]. Ginkgo products, including EGb-761 registered as a phytomedicine in Europe, are now among the best-selling drugs in the world with US$ 1.26 billion worldwide sales in 2012 [302]. Widely consumed bilobalide (sesquiterpene) has neuroprotective, anti-inflammatory, and analgesic potential and inhibits the diffuse pneumonia caused by Pneumocystis carinii [303,304]. Bilobalide has recently been found to be an antagonistic allosteric modulator of the γ-aminobutyric acid A receptors (GABA_ARs), linking its role in improving cognitive and memory functioning domain in impaired persons [305]. Recently, Pestalotiopsis uvicola, a foliar endophyte of G. biloba, has been reported to produce bilobalide [219]. Similarly, ginkgolides are considered as possible drugs based on their key antagonistic effects on the platelet-activating factor (PAF), neuroprotective effects, and protective effects in cardio-cerebral ischemia reperfusion injuries mediated via regulation of TNF-related weak inducer of apoptosis (TWEAK)/ fibroblast growth factor-inducible molecule 14 (Fn14) signaling pathway [301,306,307]. Ginkgolides have also been found in the fermentation products of an endophytic strain of F. oxysporum recovered from the root bark of G. biloba [226].

2.6.3. Paclitaxel

Paclitaxel (PTX) is a highly functionalized diterpenoid taxane family compound with a four-membered oxetane ring and a C-13 ester side chain. It is used as a basic chemotherapy drug to treat several cancer types and was first extracted from medicinal plant Pacific yew (Taxus brevifolia) in 1971 [308]. Paclitaxel binds to the tubulin protein of mitotic spindles, making them nonfunctional. The stabilization of microtubules arrest mitosis in the M phase causes the reversal of cell cycle to the G₀ phase and induces apoptosis [309]. Two decades after the discovery of paclitaxel ‘taxol’, the US FDA approved it for treating ovarian cancer in 1992 with its commercial sales reaching over $3 billion in 2004 [308]. T. andreanae from Taxus spp. was the very first endophyte reported to produce paclitaxel, taxol [11]. The above revolutionary discovery was followed by similar findings from 83 different endophytic fungal species isolated from 35 different host plant species including Taxus and non-Taxus species, as listed in Table 6.

2.6.4. Toosendanin (TSN)

Triterpenoid toosendanin (TSN) is a main bioactive component of fruits and bark of traditional anthelmintic and insecticidal plants Melia azedarach and Melia toosendan. Toosendanin has antibotulinum (inhibits the botulinum neurotoxin interaction with the SNARE protein), anti-influenza (alters nuclear localization of viral polymerase PA protein), anticancer, anti-inflammatory, and analgesic (selective presynaptic blocker) efficacy [310,311]. Possible actions of TSN as an antitumor drug against a variety of cancer types involve inhibition of STAT3, an emerging target for cancer therapy, induction of estrogen receptor β (ERβ) and p53 proteins, and activation of the mitochondrial apoptotic pathway [312,313,314]. Three unidentified endophytic fungal strains in M. azedarach have been reported to produce toosendanin [291,292].

2.6.5. Xanthatin

Xanthatin, a natural sesquiterpene lactone of Xanthium spp., has significant antimicrobial, trypanocidal, and antitumor activities. Xanthatin exerts its trypanocidal activity by inhibiting both prostaglandin E2 (PGE2) synthesis and 5-lipoxygenase activity, thereby avoiding unwanted inflammation commonly observed in trypanosomiasis. It also permanently inhibits the parasite-specific trypanothione reductase [294]. Xanthatin induces cell cycle arrest at the G₂/M checkpoint and apoptosis via disrupting the NF-κB pathway [315]. It is also synthesized by endophytic Paecilomyces sp. from Panax ginseng [295].

2.7. Plant-Derived Quinones and Xanthones from Fungal Endophytes

Table 7 lists 20 different plant-derived quinones and xanthones that were reported from fungal endophytes with some important quinones described below.

Table 7.

Plant-derived quinones and xanthones produced by endophytic fungi.

Plant-Derived Quinones and Xanthones	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
1,7-dihydroxyxanthone	Antioxidant	Weddellina squamulosa	Penicillium sp.	Avicennia	[103]
Anthraquinone	Anticancer, antioxidant, laxative	Digitalis viridiflora, Rumex spp.	Coniothyriumsp.	Salsola oppostifolia	[316]
			Aspergillus fumigatus	Rumex nepalensis, Rumex hastatus	[317]
Emodin	Antibacterial, anti-inflammatory, antitumor, immunosuppressive	Hypericum perforatum, Polygonum cuspidatum, Rheum spp.	Penicillium janthinellum	Melia azedarach	[318,319]
			Talaromyces sp.	Kandelia candel	[320]
			Aspergillus versicolor	Halimeda opuntia	[321]
			Fusarium solani	Rheum palmatum	[322]
			Eurotium chevalieri	Mangrove	[323]
			Alternaria alternata	Hypericum perforatum	[324]
Eugenitin	Glucoamylase activation	Syzygium aromaticum	Dothideomycetes sp.	Leea rubra	[325]
Hypericin	Anti-depressant, antimicrobial, antiretroviral	Hypericum perforatum	Chaetomium globosum	Hypericum perforatum	[326]
			Thielavia subthermophila	Hypericum perforatum	[327]
			Epicoccum nigrum	Hypericum perforatum	[324]
Lapachol	Anticancer, antimicrobial, antiviral, anti-inflammatory, antiparasitic	Tabebuia avellanedae	Alternaria sp., Alternaria alternata, Penicillium sp.	Tabebuia argentea	[202]
			Aspergillus niger	Tabebuia argentea	[328]
Lawsone	Cytotoxic	Lawsonia inermis	Gibberella moniliformis	Lawsonia inermis	[329]
Pachybasin, Phomarin	Antibacterial, antiviral, bioagricultural agent	Digitalis spp., Isoplexis isabelliana	Phoma sorghina	Tithonia diversifolia	[330]
			Coniothyrium sp.	Salsola oppostifolia	[316]
Physcion (Parietin)	Antibiotics, antifungals, cytotoxic	Hopea hainanensis, Rheum officinale	Aspergillus fumigatus	Cynodon dactylon	[331]
			Pleospora sp.	Imperata cylindrical	[332]
			Penicillium sp.	Hopea hainanensis	[333]
			Aspergillus terreus	Opuntia ficus-indica	[334]
			Cercosporella sp.	Schisandra chinensis	[335]
			Eurotiumchevalieri	Mangrove	[323]
Pinselin (Cassiollin)	Cytotoxic	Cassia occidentalis	Aspergillus sydowii	Scapania ciliata	[336,337]
			Phomopsis sp.	Paris polyphylla var. yunnanensis	[338]
			Phomopsis amygdali	Paris axialis	[339]
			Penicillium sp.	Sonneratia apetala	[340]
Plumbagin	Anticancer	Plumbago zeylanica	Cladosporium delicatulum	Terminalia pallida	[341]
Questin	Antioxidant, allelopathic, herbicide	Leea rubra	Dothideomycete	Leea rubra	[325]
			Eurotium rubrum	Hibiscus tiliaceus	[342]
			Eurotium cristatum	Sargassum thunbergii	[343]
			Aspergillus sp.	Pleioblastus amarus	[344]
			Phoma sp.	Phragmites communis	[345]
			Eurotium chevalieri	Mangrove	[323]
Questinol	Anti-inflammatory, antibacterial	Cassia spp., Polygonum spp.	Eurotium rubrum	Hibiscus tiliaceus	[342]
			Penicillium glabrum	Punica granatum	[346]
			Eurotium chevalieri	Mangrove	[323]
Quinizarin	Cytotoxicity, antibacterial	Rubia tinctorum	Epicoccum nigrum	Entada abyssinica	[347]
Rhein	Anticancer, anti-inflammatory, antimicrobial, antioxidant, hepatoprotective, nephroprotective	Rheum palmatum	Fusarium solani	Rheum palmatum	[322]
Shikonin	Anti-inflammatory, anti-HIV, antimicrobial,	Lithospermum erythrorhizon	Anonymous	Mammillaria hahniana	[348]
			Fusarium tricinctum	Lithospermum officinale	[349]
Sterequinone C	Anti-inflammatory	Stereospermum spp.	Penicillium sp.	Avicennia	[103]
Tanshinone	Antibacterial, antifungal, anti-inflammatory, antihypertensive, antitumor	Salvia spp.	Paecilomyces sp.	Panax ginseng	[295]
			Emericella foeniculicola	Salvia spp.	[350]
			Trichoderma atroviride	Salvia miltiorrhiza	[351]
			Phoma glomerata	Salvia miltiorrhiza	[352]
			Alternaria sp.	Salvia miltiorrhiza	[353]
Torreyanic acid	Anticancer, cytotoxic	Torreya taxifolia	Pestalotiopsis microspora	Torreya taxifolia	[354]

2.7.1. Hypericin

Hypericin (naphthodianthrone) is a Hypericum perforatum-derived antidepressive, antineoplastic, antitumor, antiviral, and photosensitizer compound. Hypericin exerts its antidepressant activity by the inhibition of serotonin, norepinephrine, and dopamine reuptake, increases in IL-6 activity, and the agonist action of sigma receptors [355]. Due to preferential accumulation in neoplastic cells, hypericin can be used in photodynamic diagnosis as an effective fluorescence marker for tumor detection and visualization. Light-activated hypericin is used as a strong pro-oxidant agent in photodynamic therapy to induce the apoptosis, necrosis, or autophagy of cancer cells due to its high affinity for neoplastic cells [356]. It prevents the uncoating of the HIV by stabilizing its capsid and suppresses the release of reverse transcriptase. Later, endophytic fungi C. globosum, Thielavia subthermophila, and Epicoccum nigrum isolated from H. perforatum were also found to produce hypericin [324,326,327].

2.7.2. Pachybasin

Pachybasin (anthraquinone) with antimicrobial and antiviral properties was isolated from Digitalis lanata [357]. Later, pachybasin was also isolated from Phoma sorghina, an endophyte of Tithonia diversifolia, and Coniothyrium sp., an endophyte of Salsola oppostifolia [316,330].

2.7.3. Pinselin (Cassiollin)

Immunosuppressive and anticancer xanthone pinselin was initially characterized from a strain of Penicillium amarum, but later found to be identical to cassiollin reported from Cassia occidentalis [336]. Later, plant-derived pinselin was reported from endophytic Phomopsis sp. isolated from P. polyphylla var. yunnanensis, Aspergillus sydowii isolated from the liverwort Scapania ciliata, and Penicillium sp. isolated from the leaves of Sonneratia apetala [337,338,340].

2.7.4. Plumbagin and Shikonin

Plumbagin and shikonin are anticancer naphthoquinones found in Plumbago and Lithospermum, respectively. Both can induce in vitro mammalian topoisomerase II-mediated DNA cleavage [358]. The mechanisms underlying the potential antitumor effects of plumbagin involve increased oxidative stress, caspase activity, loss of mitochondrial membrane potential, induction of cytochrome c release, FasL expression, and high Bax levels via activation of the JNK pathway, down-regulation of expression of NF-κB, suppressed TNF-α-induced phosphorylation of p65 and IκB kinase (IKK), degradation of IκBα, and blocking STAT3/ polo-like kinase 1 (PLK1)/AKT signaling [359,360]. Shikonin can induce apoptosis also via ROS generation and the down-regulation of AKT and receptor interacting protein 1 (RIP1)/NF-κB activity [361]. Both the compounds have also been produced by endophytic fungi, as listed in Table 7.

2.7.5. Rhein

Rheum palmatum is a highly regarded traditional medicinal plant with cathartic, hepatoprotective, nephroprotective, antimicrobial, anti-inflammatory, anticancer, and antiaging properties. The dominant biologically active constituents in the medicinal roots of Rheum are anthraquinones rhein, emodin, and physcion. The hepatoprotective activity of rhein is exerted by its lipid lowering, anti-obesity, anti-inflammatory, and anti-oxidant actions. Rhein also suppresses the expression of alpha-smooth muscle actin (α-SMA) and transforming growth factor-beta (TGF-β), which are indicative of decreased hepatic stellate cell and myofibroblast activation [362]. Nephroprotective properties of rhein arise from its anti-inflammatory action along with the suppression of α-SMA, TGF-β, and fibronectin expression. The anti-inflammatory activity of rhein involves inhibition of the NF-κB pathway, which plays a role in the production of many pro-inflammatory cytokines [363]. The mechanism of rhein anticancer activity involves the inhibition of NF-κB, MAPK, and PI3K/AKT pathways, eventually regulating cell cycle, angiogenesis, and apoptosis [364,365]. Endophytic F. solani from the roots of R. palmatum also produced the host-derived compounds rhein and emodin [322].

2.7.6. Tanshinones

Diterpenoid quinine metabolite tanshinones (tanshinone I, tanshinone IIA, tanshinone IIB, isotanshinone I, and cryptotanshinone), found in the roots of Salvia spp., are considered to be potent anticancer, antiatherosclerosis, antihypertensive, and neuroprotective agents. Tanshinones’ antitumor mechanism involves the inhibition of DNA duplication, cell cycle arrest, regulation of oxidative stress, and reduction of the mitochondrial membrane potential and PTEN-mediated inhibition of the PI3K/AKT pathway to induce apoptosis. Tanshinone I inhibits tumor angiogenesis by the phosphorylation of STAT3 at Tyr705 and hypoxia-induced HIF-1α accumulation in neoplastic cells. The cardiovascular protective effect of tanshinones is exerted by the inhibition of myocardial apoptosis, cardiac fibrosis, atherosclerosis, oxidized low-density lipoprotein (ox-LDL) uptake, thrombin activation, and thrombosis. Tanshinones exhibit significant neuroprotective effects in various neurodegenerative diseases by selectively suppressing pro-inflammatory gene expression in activated microglia, protecting neurons from the neurotoxicity of Aβ, and down-regulating the expression of phosphorylated tau [366]. Endophytic fungi Phoma glomerata and Alternaria sp. residing in the roots of Salvia miltiorrhiza produced tanshinones [352,353]. It has also been secreted by endophytic fungi from Panex (Table 7). Interestingly, elicitors from the endophytic fungi T. atroviride and C. globosum promoted the biosynthesis of tanshinones via enhanced expression of related genes in hairy roots of S. miltiorrhiza [367,368].

2.8. Miscellaneous Plant-Derived Compounds from Fungal Endophytes

In this section, we grouped diverse classes of compounds such as phenolics, phytoalexins and acids with a total number reaching up to 17 (Table 8).

Table 8.

Miscellaneous plant-derived compounds produced by endophytic fungi.

Plant-Derived Compounds	Activities/Applications	Plant Source	Endophytic Source	Host Plant	References
3-Nitropropionic acid (beta-Nitropropionic acid)	Antimycobacterial, nematicidal, succinate dehydrogenase inhibitor	Astragalus falcatus, Coronilla viminalis, Hippocrepis sp., Lotus, Scorpiurus sp., Securigera sp.	Melanconium betulinum	Birches	[369,370]
			Phomopsis phaseoli (syn. Diaporthe phaseolorum)	Rainforest tree	[369]
			Phomopsis spp.	Costus sp.	[370]
			Phomopsis longicolla	Trichilia elegans	[371]
Asarone (Phenyl propane)	Antimicrobial	Cinnamomum camphora	Muscodor tigerii	Cinnamomum camphora	[372]
Azelaic acid (Saturated dicarboxylic acid)	Antimicrobial, anti-inflammatory, anticancer	Wheat, rye, barley	Plectosphaerella cucumerina	Cynanchum auriculatum	[373]
			Aspergillus unguis	Enteromorpha sp.	[374]
Cajaninstilbene acid	Antioxidant, anti-inflammatory, hypoglycemic, neuroprotective	Cajanus cajan	Cytonaema sp.	Quercus sp.	[375]
			Alternaria, Fusarium oxysporum, Fusarium solani, Fusarium proliferatum, Neonectria macrodidym	Cajanus cajan	[376]
Chlorogenic acid (5-O-caffeoylquinic acid) (Cinnamate conjugates)	Antimicrobial, antioxidant, antitumor, immunomodulatory, antiviral	Arnica spp., Arctium lappa, Coffea canephora, Chrysanthemum coronarium, Schefflera heptaphylla	Anonymous	Artemisia indica	[216]
			Sordariomycete sp.	Eucommia ulmoides	[377]
Eugenol (Phenyl propane)	Antimicrobial	Syzygium aromaticum	Alternaria sp.	Rosa damascaena	[378]
			Annulohypoxylon stygium	Anonymous	[379]
			Rhizopus oryzae	Holarrhena pubescens	[380]
			Diaporthe sp., Neopestalotiopsis sp.	Cinnamomum loureiroi	[381]
Digoxin	Cardiac, anticancer	Digitalis lanata	Anonymous	Digitalis lanata	[382]
Forskolin	Antiglaucoma, anti-HIV, antitumor	Coleus forskohlii	Rhizoctonia bataticola	Coleus forskohlii	[383]
Harpagide (Iridoid glycosides)	Anticancer, anti-inflammatory, Leishmanicidal	Harpagophytum procumbens	Alternaria alternata	Scrophularia ningpoensis	[384,385]
Myrtucommulones (Lactone)	Anticancer, anti-inflammatory	Myrtus communis	Neofusicoccum australe (teleomorph Botryosphaeria australis)	Myrtus communis	[386]
Naphthalene (Aromatic hydrocarbon)	Antibacterial, insect repellent	Ancistrocladus tectorius	Muscodor vitigenus	Paullinia paullinioides	[387,388]
			Nodulisporium sp.	Erica arborea	[389]
			Phoma herbarum	Aegle marmelos	[390]
Panaxynol or Falcarinol or Carotatoxin (Polyacetylene)	Anticancer	Panax ginseng, Falcaria vulgaris, Daucus carota, Hedera spp.	Paecilomyces sp.	Panax ginseng	[295]
Resveratrol (Stilbene polyphenol)	Antioxidant, anticancer, epigenetic modulation	Vitis spp.	Alternaria sp., Aspergillus sp., Botryosphaeria sp., Cephalosporium sp., Geotrichum sp., Mucor sp., Penicillium sp.	Polygonum cuspidatum, Vitis quinquangularis, Vitis vinifera	[391]
Salidroside, p-tyrosol	Antioxidant, antihypoxic, adaptogenic	Rhodiola rosea	Phialocephala fortinii	Rhodiola sp.	[392]
Salvianolic acid (Polyphenol)	Antioxidant, cardiovascular, cerebrovascular diseases	Salvia miltiorrhiza	Phoma glomerata	Salvia miltiorrhiza	[352]
Tocopherol (Phenol)	Anti-influenza, antioxidant	Ribes sp.	Aspergillus fumigatus	Cynodon dactylon	[393]

2.8.1. Cajaninstilbene Acid (CSA)

The major active constituent of leaves of therapeutic pigeon pea extract is cajaninstilbene acid (CSA), which is a low-molecular weight compound containing two benzene rings joined by a molecule of ethylene. Pharmacological studies have shown that CSA exhibits antioxidant, anti-inflammatory, analgesic, and neuroprotective effects. Its cytoprotective effects against oxidative stress is exhibited by inducing the Nrf2-dependent antioxidant pathway and gene expression of heme oxygenase-1 (HO-1), NAD(P)H quinone oxidoreductase 1 (NQO1), and glutamate–cysteine ligase modifier subunits by activation of PI3K/AKT, ERK, and JNK signaling pathways [394]. The anti-inflammatory activity of CSA is associated with the inhibition of NF-κB and MAPK pathways [395]. In a study, CSA attenuated the impairment of learning and memory induced by Aβ (1–42) oligomers by stimulating Aβ clearance and inhibiting microglial activation and astrocyte reactivity in the hippocampus. It also decreased the high levels of Glu but increased the low levels of GABA. In addition, CSA inhibited the excessive expression of GluN2B-containing N-methyl-D-aspartate receptors (NMDARs) and up-regulated the downstream protein kinase A (PKA)/CREB/ BDNF/ tropomyosin receptor kinases (TrkB) signaling pathway. The above findings imply that CSA could be a potential neuroprotective agent at the early stage of Alzheimer’s disease [396]. CSA has also been produced by pigeon pea endophytic fungi Alternaria, Fusarium spp., and Neonectria macrodidym [137,376].

2.8.2. Digoxin

The glycoside digoxin from Digitalis spp. has been reported to be cardiotonic and is widely used in the treatment of various heart disorders such as atrial fibrillation, atrial flutter, and heart failure. Digoxin induces an increase in intracellular sodium followed by calcium in the heart by reversibly inhibiting the activity of the myocardial Na+-K+-ATPase pump, leading to an increased force of myocardial contraction and cardiac output. By stimulating the parasympathetic nerve, it slows electrical conduction in the AV node by increasing the refractory period of cardiac myocytes; therefore, it decreases the ventricular response and heart rate. Overall, the stroke volume is increased while the heart rate is decreased, resulting in a net increase in blood pressure [397]. Crude extracts of fungal cultures isolated from Digitalis lanata also showed the production of digoxin [382].

2.8.3. Forskolin (Coleonol)

The roots of Indian Coleus (Coleus forskohlii) contain a biologically active labdane diterpene compound forskolin with antiglaucoma, anti-HIV, and antitumor activities. Other approved and potential applications of forskolin range from the treatment of hypertension and heart failure to lipolysis and body weight control [398]. Forskolin activates a variety of adenylate cyclase systems to increase the cellular concentrations of cyclic AMP, which is an important second messenger necessary to elicit cAMP-dependent physiological responses [399]. An endophytic fungus Rhizoctonia bataticola isolated from C. forskohlii was found to synthesize forskolin [383].

2.8.4. Salidroside and p-Tyrosol (Aglycone of Salidroside)

Rhodiola rosea, a traditional medicinal herb used as stimulant and antidepressant, has great pharmaceutical value including antioxidant, antihypoxic, adaptogenic, cardiovascular, and neuroprotective properties. Its active principals are phenolics (salidroside and p-tyrosol) and glycosides (rosavins) [400]. Salidroside and p-tyrosol inhibit the hypoxia-induced endocytosis of pulmonary Na,K-ATPase via the inhibition of the ROS– AMPK–protein kinase Cζ (PKCζ) pathway, signifying the use of Rhodiola as a popular folk medicine for high-altitude illness [401]. The mechanisms underlying the potential neuroprotective effects of salidroside involve the regulation of oxidative stress response, inflammation, apoptosis, hypothalamus–pituitary–adrenal axis, neurotransmission, neural regeneration, and the cholinergic system [402]. Cui et al. isolated four endophytic fungi from different species of Rhodiola that could produce salidroside and p-tyrosol and characterized P. fortinii as the most capable and stable producer [392].

3. Avenues and Challenges in Application of Endophyte as Alternative Sources of Plant-Derived Natural Compounds

The success of natural products in drug discovery lies in their enormous structural diversity, diverse pharmacological activities, safety, and inherent binding capacity with other biomolecules [2,9,298]. Reports regarding the biosynthesis of plant-derived natural compounds from endophytic fungi coupled with recent dynamic progress in fermentation, extraction, purification, characterization, and bioassay techniques have enabled us to rapidly characterize valuable novel natural products and access earlier inaccessible endophytic resource [403,404]. Generally, the fermentation process for fungi is short, simple, and economically feasible with a great degree of flexibility for modulation by feeding precursors, elicitors, special enzymes, and modifiers for the efficient enhanced production of bioactive compounds. Endophytes can uniquely biotransform original plant-derived bioactive compounds to their more efficient derivatives, leading to structural and functional diversification [77,136,146]. These studies have evidenced the incredible manipulability of fungal secondary metabolism. There are cases where endophytes up-regulated the synthesis of host compounds and the expression of related genes in the plant host. Hence, each report of the biosynthesis of plant-derived natural compounds from fungal endophytes clearly presents a hopeful way for the efficient and specific production of valuable bioactive natural compounds using endophytes as stable and smart “bio-laboratories”.

However, this approach needs to overcome certain challenges. First, there is an ongoing search for highly productive endophytic fungi for desired plant-derived compounds followed by their strain improvement through epigenetic modulations, mutations, and genetic engineering to make them suitable for industrial applications. Furthermore, we need to elucidate the complete biosynthesis route including all the enzymes and related genes involved through ‘omics’—genomics, transcriptomics, proteomics, and metabolomics—to regulate and manipulate the biosynthesis process for improved productivity [405,406,407]. Alternatively, the identified biosynthetic pathway of the bioactive compounds can be assembled and mimicked in convenient systems, offering an approach to produce target compounds with ease. Second, we need to know more about the roles of host plant–endophyte interactions, requirements of plant niche, and identities of specific signals/elicitors in the synthesis and induction of host-derived natural compounds by endophytes under the OSMAC strategy to overcome the problem of low yield and attenuation, the major challenges for commercial success of this novel approach [406,408,409,410]. The reasons for the attenuation of products have been attributed to the lack of apparent signals/molecules arising from host–endophyte and/or endophyte–peer endophytes interactions in axenic monocultures, resulting in the switching off of genes [86]. However, characterization of the specific nature of assumed activator signals/molecules remains to be done. Third, this area needs collaborations between scientists working in this area and in the pharmaceutical industry for the successful industrial scale production of pharmaceutically valuable compound/leads [411]. The pharmaceutical industry must prioritize their endeavors toward the endophyte-dependent biosynthesis of plant-derived natural compounds.

4. Conclusions

After screening a large spectrum of articles dedicated to endophyte research, natural product drug discovery, combinatorial chemistry, genomics, metabolomics ethnobotany, modern medicine, and multidisciplinary science, we curated 101 specific plant-derived medicinal compounds efficiently biosynthesized by hundreds of endophytic fungi. Nonetheless, the exciting progress that has been made in the field of functional genomics, genome mining and genome scanning, fermentation technology, green combinatorial chemistry, and systems biology might remove the roadblocks in the way of commercial success of this innovative approach [282,412,413]. In conclusion, the pursuit of the idea of endophyte-dependent enhanced in vivo and in vitro production of plant-derived valuable metabolites is of prime importance for the pharmaceutical industries, for the health care systems, and for a “green drug revolution”.

Author Contributions

Conceptualization, A.S., D.K.S. and S.K.G.; Writing—Original Draft Preparation, A.S. and D.K.S.; Writing—Review and Editing, D.K.S., S.K.G., J.F.W. and R.N.K.; Supervision, S.K.G. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by CSIR, New Delhi and SERB, New Delhi, India (EEQ/2016/000555 and EEQ/2020/000485). RNK acknowledges funding support from SERB, New Delhi, India (EEQ /2020/000549). J.F.W. was supported by the Rutgers Agricultural Experiment Station and USDA NIFA Multi-State Project W-4147.

Data Availability Statement

Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.