Antifungal Activity Against Plant Pathogens of Metabolites from the Endophytic Fungus Cladosporium cladosporioides

Abstract

Bioassay-guided fractionation of Cladosporium cladosporioides (Fresen.) de Vries extracts led to the isolation of four compounds, including cladosporin, 1, isocladosporin, 2, 5′-hydroxyasperentin, 3, and cladosporin-8-methyl ether, 4. An additional compound 5′,6-diacetyl cladosporin, 5, was synthesized by acetylation of compound 3. Compounds 1-5 were evaluated for antifungal activity against plant pathogens. Phomopsis viticola was the most sensitive fungus to the tested compounds. At 30 μM, compound 1 exhibited 92.7%, 90.1%, 95.4% and 79.9% growth inhibition against Colletotrichum acutatum, Co. fragariae, Co. gloeosporioides and Phomopsis viticola, respectively. Compound 2 showed 50.4%, 60.2% and 83.0% growth inhibition at 30 μM against Co. fragariae, Co. gloeosporioides and P. viticola, respectively. Compounds 3 and 4 were isolated for the first time from Cladosporium cladosporioides. Moreover, the identification of essential structural features of the cladosporin nuclei has also been evaluated. These structures provide new templates for the potential treatment and management of plant diseases.

Introduction

Strawberry anthracnose, a serious disease in many parts of the world, is caused by filamentous fungi of the genus Colletotrichum. Colletotrichum species are defined as destructive pathogens that cause significant economic damage to crops worldwide. The three most common Colletotrichum species on strawberry (Fragaria xananassa Duchesne) are Colletotrichum fragariae A. N. Brooks, Co. acutatum J. H. Simmonds, and Co. gloeosporioides (Penz.) Penz. & Sacc., which may cause anthracnose disease either a singular organism or in combination.^{1, 2} Strawberry anthracnose can be devastating since other plant parts may be infected in addition to the fruit. This infection results in millions of dollars in crop loss each year.³ Phomopsis viticola Sacc. is another common plant pathogen that causes severe diseases in grapes such as phomopsis cane and leaf spot disease. Phomopsis obscurans causes leaf blight of the cultivated strawberry and can also infect foliage, runners, petioles, and fruit with a dark brown center surrounded by light-brown rings with purplish halos.^{4, 5}

Increasing incidence of chemical resistance in fungal pathogens and potential environmental and mammalian toxicities, often caused by the application of conventional fungicides, are factors that drive a need to search for new safe plant protectants.⁶ It is particularly desirable to evaluate biologically active natural products possessing new chemical classes that might function by different modes of action than existing fungicides, thus avoiding problems of cross-resistance to current chemical classes. In terms of the availability of the starting materials, endophytic fungi provide a rich source of numerous agrochemical agents.⁷ In the course of discovery of new pest-control agents from fungi as alternatives to synthetic molecules, 40 fungal crude extracts were screened using a direct-bioautography coupled with Colletotrichum as the detection method. Acetone extracts of Cl. cladosporioides (Fresen.) de Vries showed the most promising activity against the three Colletotrichum species and was selected for further in depth studies. Cl. cladosporioides is a very common saprophytic fungus but it is also a pathogen of many different host plants. It can be isolated from many sources including air, soil, textiles and several other substrates.⁸ Cl. cladosporioides has been reported to be very effective in both prevention and biocontrol agent of the apple scab fugus (Venturia inaequalis).⁹

The aim of this study was to isolate and evaluate the antifungal activity of Cladosporium metabolites against Colletotrichum species and Phomopsis species to develop potential agrochemical leads for disease control. This is the first report on the antifungal activity of Cladosporium metabolites against these fungus species. Moreover, the identification of essential structural features of the metabolites was also studied.

Materials and Methods

General experimental procedures

¹H and ¹³C NMR spectra were obtained on a Bruker model AMX 500 NMR spectrometers with standard pulse sequences, operating at 500 MHz in ¹H and 125 MHz in ¹³C. The chemical shift values were reported in parts per million units (ppm) from trimethylsilane (TMS) using known solvent chemical shifts. Coupling constants were recorded in Hertz (Hz). Standard pulse sequences were used for COSY, HMQC, HMBC, TOCSY, NOESY and DEPT. High-resolution mass spectra (HRMS) were measured on a Micromass Q-Tof Micro mass spectrometer with a lock spray source. Column chromatography was carried out on a 25 cm × 10 cm i.d., silica gel 60 column silica gel (70-230 mesh, Merck) and Sephadex LH-20 (Mitsubishi Kagaku, Tokyo, Japan). TLC (silica gel 60 F254) was used to monitor fractions from column chromatography. Preparative TLC (Analtech, Newark, DE) was carried out on 20 × 20 cm, 1 mm thick, silica gel 60 PF 254+366 plates. Visualization of the TLC plates was achieved with a UV lamp (λ_max 254 and 365 nm) and modified anisaldehyde spray reagent (EtOH: acetic acid: anisaldehyde: sulfuric acid, 85:9:1:5). HPLC analyses were performed on a Waters LC Module I equipped with a UV detector 486 utilizing the Millennium 32 Chromatography Manager software (Waters, Milford, MA). The ODS column used was a 250 × 10 mm i.d., 5 μm, Phenomenex Luna C18 column. All HPLC solvents were HPLC grade (Fisher), filtered through appropriate membranes (water through 0.45 μm and organic solvents through 0.22 μm filters) and sparged prior to and during analysis with nitrogen at a flow rate of 50 mL/min. Chemicals for pharmacological studies were obtained from Sigma-Aldrich.

Fungal material

Cladosporium cladosporioides fungus was collected in Tifton, Georgia, in 1978, lyophilized, and stored at −20 °C. The fungus was plated out on potato-dextrose agar, which was maintained at 24 °C until discrete fungal colonies appeared. Then 50 mL of potato-dextrose broth was inoculated with the fungus spores and incubated for 2 weeks in stationary phase at 24 °C. The fungus was subsequently seeded onto a shredded wheat medium consisting of 100 g of shredded wheat, 200 mL of low-pH mycological broth, 40 g of yeast extract, and 400 g of sucrose in a 2.0 L Fernbach flask (15 flasks were used) followed by incubation for 22 days at 24 °C.¹⁰

Extraction and isolation

Following incubation, 300 mL of acetone was added to each flask (15 flasks were used), and the fungus and the substrate were homogenized. The suspension was filtered and the filtrate was concentrated under vacuum at 40 °C to yield water fraction. The water fraction was then extracted with EtOAc (500 mL × 3). The combined EtOAc extracts were dried over anhydrous Na₂SO₄ and concentrated under a vacuum. The EtOAc extract (21 g) was chromatographed on a 25 cm × 10 cm i.d., 70-230 mesh, silica gel 60 column, with stepwise elution with hexanes, ethyl acetate and methanol, to yield fractions A-G. Bioautography-guided bioassay showed that fractions C, D and E exhibited antifungal activity and were selected for further bioassay-guided isolation. Fraction C was purified by crystallization from hexanes/EtOAc (1:1) to give 1 (5.04 g). Compound 2 (8.3 mg) was isolated by a 250 × 10 mm i.d., 5 μm, Phenomenex Luna C18 HPLC column using MeOH/H₂O gradient elution. Fraction D was subjected to fractionation over a 40 cm × 2 cm i.d., Sephadex LH-20 CC eluted with CH₂Cl₂/MeOH (1:1) to afford 128 sub-fractions. Sub-fractions 72-128 were combined to afford compound 3 (117 mg). Sub-fractions 31-71 were combined and chromatographed on a 55 mm × 21 mm i.d., silica Biotage SNAP Cartridge (Biotage, Charlotte, NC) using a CHCl₃/MeOH gradient to afford 4 (3 mg). Compound 3 (50 mg) was reacted with 2 mL acetic anhydride and 2 mL pyridine for 24 h at room temperature and purified by preparative TLC (petroleum ether/EtOAc 1:1) and dried under nitrogen to give compound 5 (4.5 mg).

Biological assay

Direct-bioautography assay

Bioautography procedures were described in our previous studies.^{11, 12} The acetone extract of Cl. cladosporioides was applied at 80 and 160 μg/spot in chloroform onto a silica plate. Technical fungicide grade standards benomyl, cyprodinil, azoxystrobin, and captan (Chem Service Inc., West Chester, PA) were used as positive controls at 2 mM in 2 μL of 95% ethanol.

Micro-dilution broth assay

A standardized 96-well micro-dilution broth assay developed by Wedge and Kuhajek¹³ was used to evaluate the antifungal activity of pure compounds from Cl. cladosporioides that were identified as active by bioautography.

Strains of Co. acutatum, Co. fragariae, Co. gloeosporioides, Botrytis cinerea Pers.:Fr, Fusarium oxysporum Schlechtend:Fr, P. obscurans (Ellis and Everh.) B. Sutton, and P. viticola Sacc., were used to evaluate the antifungal activity of the tested compounds using in vitro micro-dilution broth assay. Each fungus was challenged in a dose-response format using tested compounds where the final treatment concentrations were 0.3, 3.0 and 30.0 μM. Technical grade commercial fungicides captan and azoxystrobin, which represent two different modes of actions, were used as positive fungicide standards. Each compound was evaluated in duplicate and the experiment was performed three times in time. Mean absorbance and standard errors were used to evaluate fungal growth after 48 and 72 h, except for P. obscurans and P. viticola (120 and 144 h).

Results and Discussion

Bioassay-guided fractionation of Cl. cladosporioides crude extracts (20 g) led to the isolation of four compounds, including cladosporin, 1, isocladosporin, 2, 5′-hydroxyasperentin, 3, and cladosporin-8-methyl ether, 4. A synthesized compound, 5′, 6-diacetyl cladosporin, 5, was also prepared. The structures of these compounds were established by 1D and 2D NMR spectroscopic analysis, mass spectrometric (ESI-MS) data, X-ray crystallography, as well as comparison with the previous literatures values.^14-16 All compounds were further evaluated for their antifungal activity against seven plant pathogens using an in vitro micro-dilution broth assay. In the micro-dilution broth assay, cladosporin, 1, also named asperentin, caused 92.7% growth inhibition of Co. acutatum, 90.1% of Co. fragariae, and 95.4% of Co. gloeosporioides at 30 μM. Compound 1 is a promising compound compared to the standard fungicide azoxystrobin, only caused 40.5% growth inhibition of Co. acutatum and 58.9% of Co. fragariae, respectively (Figure 2). Co. acutatum in genetically insensitive to the benzimidazole class of fungicides and activity of cladosporin in this species indicates that its mode of action is different that that of the benzimidazoles. ^{17, 18} As shown in Figure 3, cladosporin shows significant antifungal selectivity against P. viticola and P. obscurans at 30 μM. Although this is the first report of compounds 3 and 4 from Cl. cladosporioides, they were previously isolated from Aspergillus flavus ¹⁴, Chaetomium globosum ¹⁹ and Eurotium repens ²⁰. The micro-dilution broth assay result demonstrated that 2 exhibited moderate antifungal activity with 50.4% growth inhibition of Co. fragariae, 60.2% of Co. gloeosporioides at 48 h (Figures 2B and 2C) and good antifungal selectivity against Phomopsis species with 80.3% growth inhibition of P. viticola and 22.5% of P. obscurans at 120 h (Figures 3A and 3B). As shown in Figures 3A and 3B, compounds 3 and 5 show no antifungal activity against the three Colletotrichum species, but showed good selectivity over P. viticola (53.9% and 79.4%) and P. obscurans (25.6% and 10.3%). Compound 4 had no antifungal activity against the tested fungi.

Figure 2.

Mean fungal growth inhibition (%) of Colletotrichum acutatum (A), Colletotrichum fragariae (B) and Colletotrichum gloeosporioides (C) after exposure to compounds 1, 2, 3 and 5 using a dose-response format at 48 h. Fungicide standard: captan and azoxystrobin.

Figure 3.

Mean fungal growth inhibition (%) of Phomopsis viticola (A) and Phomopsis obscurans (B) after exposure to compounds 1, 2, 3 and 5 using a dose-response format at 120 h. Fungicide standard: captan and azoxystrobin.

An overall evaluation of the relationship between the structures and antifungal activity of the compounds at 30 μM suggested several essential positions that might be responsible for their antifungal activity (Table 1). The absolute configuration of C-6′ in the structure of 1 and 2 influences antifungal activity of the parent compound. R configuration of C-6′ in structure 2 greatly decreased antifungal activity against Colletotrichum species, but slightly increased the antifungal activity against Phomopsis species. Comparing the structures of 1 and 3, introduction of one hydroxyl group at C-5′ position resulted in complete loss of the antifungal activity against Colletotrichum species and decreased the selectivity against Phomopsis species; this indicated the importance of maintaining an unsubstituted C-5′ for antifungal activity. By comparing the structures 1 and 4, the replacement of hydroxyl group with the methoxy group at C-8 caused broad loss of the antifungal activity against all the tested fungi, which indicated this position might be the active site where hydrogen bonds are formed. Comparing compounds 3 and 5, the replacement of hydrogen of hydroxyl group at C-6 and the hydrogen at C-5′ with acetyl groups greatly increased the selectivity towards the two Phomopsis species.

Table 1.

Overall Fungal Growth Inhibition (%) of Compounds 1, 2, 3, 4 and 5 Against Plant Pathogens at 30 μM

Compounds	Co. acutatum	Co. fragariae	Co. gloeosporioides	P. viticola	P. obscurans
1	92.7%	90.1%	95.4%	79.9%	22.1%
2	38.3%	50.4%	60.2%	83.0%	22.5%
3	NA	NA	NA	53.9%	25.6%
4	NA	NA	NA	35.1%	NA
5	NA	NA	NA	79.4%	10.3%

In summary, fungi provide an abundant source of natural products that may have potential agricultural, environmental, and pharmaceutical use. In particular, Cl. cladosporioides provides a good source of natural cladosporin with a yield of 24% and should be considered as an important source of this metabolite by pharmaceutical and agrochemical companies. Compounds 1, 2, 3 and 5 have potential utility as leads in the development of antifungal agrochemicals against certain plant pathogens. Compounds 1 was tested to show specific antifungal, antibacterial and antitumor properties, as well as insecticidal activity in previous studies.^{16, 20-23} Compound 1 was also reported to be a plant growth regulator (PGR) inhibiting etiolated wheat coleoptiles but did not affect the growth of greenhouse-grown tobacco or corn.²⁴ Many commercial fungicides, such as the triazole class, show fungicidal, PGR activity, and pharmaceutical applications. ²⁵ However, commercial growers have learned to effectively use these compounds to their advantage and apply one chemical agent for both disease control and plant dwarfing. Fortunately, there are usually significant dose-dependent differences between fungicides and PGR effects in plants. However, overuse of this fungicide class can cause excessive dwarfing of some greenhouse crops plants (poinsettia). In the present study, compound 1 was evaluated for the first time against the filamentous fungal plant pathogens used in our micro-dilution broth assay and showed promising fungal growth inhibition. Moreover, the differences in activity indicated that the S configuration of C-6′, the openness of C-5′, the hydroxyl group at C-8, and the introduction of functional groups at C-6 influence the antifungal properties of these compounds. However, some literatures reported that Cl. Cladosporioides could cause allergies and inflammation in sensitive patients at high concentration ^26-29 but they did not test cladosporin itself or the other metabolites. So further toxicity study on animals is needed for those metabolites to either prove this assumption or not.

Figure 1.

Compounds isolated from Cladosporium cladosporioides.

Abbreviations Used

Co.

Colletotrichum

Cl.

Cladosporium