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. 2023 Jan 25;28(3):1175. doi: 10.3390/molecules28031175

Pioneering Metabolomic Studies on Diaporthe eres Species Complex from Fruit Trees in the South-Eastern Poland

Barbara Abramczyk 1,*,, Łukasz Pecio 2,3,, Solomiia Kozachok 2, Mariusz Kowalczyk 2, Anna Marzec-Grządziel 1, Ewa Król 4, Anna Gałązka 1, Wiesław Oleszek 2
Editor: Armando Zarrelli
PMCID: PMC9920373  PMID: 36770841

Abstract

Fungi from the genus Diaporthe have been reported as plant pathogens, endophytes, and saprophytes on a wide range of host plants worldwide. Their precise identification is problematic since many Diaporthe species can colonize a single host plant, whereas the same Diaporthe species can inhabit many hosts. Recently, Diaporthe has been proven to be a rich source of bioactive secondary metabolites. In our initial study, 40 Diaporthe isolates were analyzed for their metabolite production. A total of 153 compounds were identified based on their spectroscopic properties—Ultraviolet-visible and mass spectrometry. From these, 43 fungal metabolites were recognized as potential chemotaxonomic markers, mostly belonging to the drimane sesquiterpenoid-phthalide hybrid class. This group included mainly phytotoxic compounds such as cyclopaldic acid, altiloxin A, B, and their derivatives. To the best of our knowledge, this is the first report on the metabolomic studies on Diaporthe eres species complex from fruit trees in the South-Eastern Poland. The results from our study may provide the basis for the future research on the isolation of identified metabolites and on their bioactive potential for agricultural applications as biopesticides or biofertilizers.

Keywords: Diaporthe eres species complex, fruit plants, chemotaxonomic markers, metabolite profiling

1. Introduction

The genus Diaporthe Nitschke belongs to the family Diaporthaceae, (Diaporthales, Diaporthomycetidae, Sordariomycetes, Pezizomycotina, Ascomycota; (MycoBank. 2022; Species Fungorum. 2022; accessed on 21 December 2022), with the anamorph known as Phomopsis. According to the implementation of “one fungus one name (1F:1N) nomen- clature”, Diaporthe has been adopted over Phomopsis because it was first introduced as this, is encountered commonly in the literature, and represents most species [1]. Diaporthe are present as plant pathogens, endophytes, and saprophytes in a wide range of hosts worldwide [2,3,4,5]. Some pathogenic Diaporthe are responsible for several serious diseases of economically important crops, including fruit plants [4,6,7]. Both sexual and asexual morphs of Diaporthe have been associated with cankers, shoot diebacks, bud and shoot blights, and leave spots of peach caused by Phomopsis amygdali [8], apple by P. mali [9], pear by D. eres [10], plum by D. perniciosa [11], grape by P. viticola, P. fukushii, D. eres [12,13], blueberry by D. australafricana, D. ambigua, D. neotheicola, D. passiflorae [14], and many others. Currently, 1177 names of Diaporthe and 984 of Phomopsis are listed in Index Fungorum (http://www.indexfungorum.org/; accessed on 20 September 2022). Identification of Diaporthe species is complicated and was initially based on morphological features, cultural characteristics, and host affiliation leading to a proliferation of names based on the hosts from which they were isolated [15]. It has been observed that the same Diaporthe species colonizes different hosts, and the co-occurrence of different species is commonly reported in the same host [4,16,17,18,19]. Thus, the identification and description of species based on host association are unreliable within Diaporthe [3,20,21]. Moreover, the identification of Diaporthe based only on morphological features such as the size and shape of ascomata [2] and conidiomata [16] also proved insufficient due to their variability under changing environmental conditions [3]. Currently, the taxonomy of Diaporthe is actively changing, with numerous species being described each year, primarily based on molecular data combined with morphological characterization and host associations [3,10,20,21,22,23,24].

In recent years the genus Diaporthe has been widely used in secondary metabolite study due to their production of a variety of unique low- and high-molecular-weight metabolites with different bioactivities which were recently summarized in the extensive review by Xu et al. [25]. Researchers focused mainly on the endophytic species of Diaporthe, which is reported as one of the most frequently isolated genera among endophytic fungi. Probably the same compounds can be produced by endophytic, saprotrophic, and pathogenic species [26]. Over the past decade, 335 bioactive secondary metabolites have been obtained from known Diaporthe species and from those for which only a generic name has been assigned. [25,27,28] Among bioactive compounds 246 were isolated from Phomopsis, 106 from Diaporthe, and 17 from both species [25,29]. The metabolites produced by this genus include terpenoids, steroids, macrolides, ten-membered lactones, alkaloids, flavonoids, fatty acids, and polyketides, being the main structural type [25].

Although endophytic Diaporthe species have been extensively screened in bioassays for metabolite production, no such information is available for fungi belonging to D. eres species complex isolated from fruit trees in Poland. The literature indicates that the same species of the genus Diaporthe can occur on one or different hosts with different lifestyles [2]. Some Diaporthe species described as endophytes include latent phytopathogens, which asymptomatically colonize various host plants [30]. An example is D. eres, which as a pathogen infects many crops, including orchards, and is often the main cause of serious economic losses worldwide [3,20,21,31]. In Poland, however, the endophytic form of D. eres on Prunus domestica was recorded [32]. In the era of global warming and climate change, we must remember that many species may switch their lifestyles or spread into new regions, where they will come into contact with new potential hosts and will become a dangerous cause of diseases [33,34]. Therefore, the additional data such as metabolite profiles of such important fungi like Diaporthe may be crucial to understand their pathogenicity and switching life mode triggers in future research. Since Diaporthe species are a valuable source of bioactive metabolites, it would be worthwhile to further explore the genus for novel compounds that have a biotechnological potential.

The main objective of our study was metabolite profiling of Diaporthe isolates from various orchard plants of south-eastern Poland. To fully delineate the secondary metabolite profile of any fungus is an ambitious undertaking. Thus, this work is just an initial step toward the further exploration of the novel compounds from Diaporthe and their agricultural or pharmaceutical bioactivities.

2. Results and Discussion

2.1. ITS-Based Fungal Identification

The sequences of the ITS regions were used to identify Diaporthe strains. Closely related species have been received by comparing the obtained sequence data with the NCBI database (https://www.ncbi.nlm.nih.gov; accessed on 19 December 2022). The results indicated that 34 of the tested strains were closely related to D. eres species complex with 100% of similarity, 5 strains with 99.8%, and one strain with 99.6% of similarity (Table 1).

Table 1.

Fungi most closely related to Diaporthe based on ITS sequences using BLASTn analysis.

Isolate Host Plant (Shoot) GenBank Accession No. Closest Related Species Similarity [%] Coverage [%]
260J Malus domestica OK474176 D. eres_HQ533144 100 99.8
269J OK474177 D. eres_KU712214 100 100
1439J OK474180 D. eres_HQ533144 100 100
1597J OK474183 D. eres_MK352454 100 100
3105J OK474188 D. eres_GQ996572 100 100
1046G Pyrus communis OK474190 D. eres_MK352454 100 100
1485G OK474193 D. eres_HQ533144 100 100
1679G OK474196 D. eres_MK352454 100 100
1915G OK474198 D. eres_MK352454 100 100
2201G OK474201 D. eres_MH931269 100 100
336W Prunus cerasus OK474203 D. eres_EU571099 100 99.8
1648W OK474204 D. eres_EU571099 100 99.8
1940W OK474205 D. eres_MW228360 99.6 100
3230W OK474214 D. eres_EU571099 100 99.8
3243W OK474216 D. eres_KX274026 100 100
353S Prunus domestica OK474217 D. eres_GQ996572 100 100
1419S OK474220 D. eres_EU571099 100 99.8
1420S * MW664034 D. eres EU571099 100 99.8
1676S OK474223 D. eres_EU571099 100 100
2027S OK474225 D. eres_EU571099 100 99.8
487CZ Prunus avium OK474227 D. eres_EU571099 100 99.8
1478CZ OK474228 D. eres_EU571099 100 99.8
1701CZ OK474229 D. eres_EU571099 100 99.8
1721CZ OK474230 D. eres_EU571099 100 99.8
1725CZ OK474231 D. eres_EU571099 100 99.8
388ORZ Juglans regia OK474233 D. eres_GQ996572 100 100
404ORZ OK474234 D. eres_KX274026 100 100
1755ORZ OK474236 D. eres_GQ281804 99.8 99.8
2238ORZ OK474237 D. eres_HQ533144 100 99.3
2339ORZ OK474238 D. eres_EU571099 100 100
372L Corylus avellana OK474239 D. eres_HQ533144 100 100
1567L OK474240 D. eres_KX274026 100 100
1569L OK474241 D. eres_MK352454 100 100
1805L OK474246 D. eres_GQ996572 100 100
2245L OK474247 D. eres_EU571099 100 100
3213B Prunus persica OK474250 D. eres_MK352454 99.8 100
3215B OK474251 D. eres_MK352454 99.8 100
3216B OK474252 D. eres_MK352454 99,8 100
3290B OK474253 D. eres_HQ533144 99,8 100
3297B OK474254 D. eres_GQ996572 100 100

* in bold D. eres as endophyte [32].

2.2. Chemical Characterization of Fungal Metabolites

Our study revealed that the D. eres species complex isolated from fruit trees in south-eastern Poland showed high biodiversity in the secondary metabolite production. A total of 153 compounds were found as a result of screening of forty isolates belonging to the Diaporthe eres species complex, based on their spectroscopic properties—UV-Vis and mass spectrometry (Table 2). The identified metabolites mainly included polyketides, pyrones, fatty acids/oxylipins, chromones, sesquiterpenoids, phthalides, and numerous derivatives and hybrids belonging to the preceding groups of compounds. The metabolite profile of the studied isolates belonging to the Diaporthe eres species complex is unique and most of the detected compounds have not been described for Diaporthe species before. Furthermore, as far as we know, our research on the characterization of the metabolite profile of D. eres species complex isolated from orchard plants is pioneering and has not been conducted in Europe or other parts of the world before. There are few publications on Diaporthe (=Phomopsis) from fruit plants, but they focus mainly either on Diaporthe from the one host plant or on the selected group of metabolites produced by Diaporthe [35,36].

Table 2.

Annotation of specific metabolites in the studied Diaporthe isolates using UHPLC-qTOF-MS/MS in the negative (NI) and positive ionization (PI) modes.

No MS-DIAL ID (NI) MS-DIAL ID (PI) Rt (min) UV (nm) Meas. m/z [Adduct Type] Neutral Formula MW Error (ppm) Major Fragments * Putative Metabolite Cmp. Class
1 289 3.69 215, 262 511.1095 [2M − H] C11H12O7 256.21 0.74 255.0510, 211.0614, 181.0519, 135.0448 Islandic acid-II Pyranone (α-pyrone)
2 120 270 3.79 220 197.0809 [M − H2O + H]+ C10H14O5 214.22 0.16 197.0807, 179.0704, 151.0754, 137.0594 Multiplolide A 10-membered lactone
3 1349 579 3.98 215, 282 230.1022 [M + NH3 + H]+ C10H12O5 212.20 0.47 195.0655, 177.0538, 149.0598 Pyrenocine P Pyranone (α-pyrone)
4 107 4.15 215 423.1293 [2M − H] C10H12O5 212.20 2.17 211.0611, 167.0698, 111.0434 4-[5-(1-Hydroxyethyl)furan-2-yl]-4-oxobutanoic acid γ-keto acid
5 1015 2082 4.58 215 332.1707 [M + H]+ C15H25NO7 331.36 −0.97 314.1602, 296.1504, 278.1386, 197.0808, 179.0710, 151.0764 Phomopsolide B derivative tiglic acid amide isomer I Pyranone or furanone
6 108 394 4.70 220, 285 213.0755 [M + H]+ C10H12O5 212.20 −0.71 195.0651, 177.0546, 167.0703, 149.0595, 139.0393 Scirpyrone K Pyranone (α-pyrone)
7 264 4.86 220 253.0354 [M − H] C11H10O7 254.19 −0.09 179.0379, 164.0103 Strobide B (cyclopaldic acid derivative) Phthalide
8 976 4.87 215 657.2882 [2M − H] C15H23NO7 329.35 −0.04 328.1409, 229.0713 Dehydro-phomopsolide B derivative tiglic acid amide Pyranone or furanone
9 174 701 4.99 215, 255 239.0550 [M + H]+ C11H10O6 238.19 0.06 221.0443, 203.0335, 193.0499, 177.0545, 175.0391, 160.0153 Convolvulanic acid A isomer I Phthalide
10 1016 1856 5.10 215 314.1603 [M − H2O + H]+ C15H25NO7 331.36 −1.47 314.1599, 296.1485, 278.1402, 197.0812, 179.0705 Phomopsolide B derivative tiglic acid amide isomer II Pyranone or furanone
11 1014 2087 5.85 215 332.1704 [M + H]+ C15H25NO7 331.36 −1.47 314.1598, 296.1495, 278.1385, 197.0814, 179.0699, 137.0588 Phomopsolide B derivative tiglic acid amide isomer III Pyranone or furanone
12 1403 2841 5.98 220 419.1375 [M − H2O + H]+ C18H28O10S 436.47 −1.08 301.0743, 283.0627, 255.0682, 237.0573, 179.0694 Unidentified
13 173 6.10 220 237.0404 [M − H] C11H10O6 238.19 0.26 165.0137 Convolvulanic acid A isomer II Phthalide
14 1907 2838 6.11 220 419.1372 [M − H2O + H]+ C18H28O10S 436.47 −0.16 301.0743, 283.0627, 255.0682, 237.0573, 179.0694 Unidentified
15 844 2134 6.63 220 334.1862 [M + NH3 + H]+ C15H24O7 316.35 −0.54 299.1498, 281.1382, 213.0779, 181.0863 Dihydrohydroxyphomopsolidone B isomer I Furanone
16 847 1905 6.77 220 317.1595 [M + H]+ C15H24O7 316.35 −0.07 299.1505, 181.0863, 153.0909, 137.0593 Dihydrohydroxyphomopsolidone B isomer II Furanone
17 795 1870 7.06 215 315.1438 [M + H]+ C15H22O7 314.33 0.09 179.0704, 161.0599, 151.0757, 137.0596, 119.0491 Dihydrohydroxyphomopsolide B isomer I Pyranone
18 739 1468 7.10 220 295.0819 [M − H2O + H]+ C14H16O8 312.27 −2.15 277.0702, 249.0749, 221.0445, 193.0485, 161.0601 Isariketide Polyketide
19 482 1042 7.31 220 267.1596 [M − H2O + H]+ C15H24O5 284.35 −1.81 249.1469, 231.1386 Hydroxy-altiloxin A isomer-I Drimane sesquiterpenoid
20 508 1058 7.35 220 269.1024 [M − H2O + H]+ C13H18O7 286.28 0.93 169.0495, 151.0384, 123.0441 Unidentified
21 1404 2839 7.41 220 419.1375 [M − H2O + H]+ C18H28O10S 436.47 −0.62 301.0746, 197.0808, 189.0214, 179.0702 Unidentified
22 488 2085 7.51 220 267.1593 [M − H2O + H]+ C15H24O5 284.35 −0.75 249.1483, 231.1374, 205.1586, 189.1277 Hydroxy-altiloxin A isomer II Drimane
sesquiterpenoid
23 798 2085 7.69 220 332.1708 [M + NH3 + H]+ C15H22O7 314.33 −1.34 297.1332, 279.1237, 179.0702 Dihydrohydroxyphomopsolide B isomer II Pyranone
24 1180 7.96 220 363.0717 [M − H] C17H16O9 364.30 1.25 229.0768, 220.0343, 179.038 5-Hydroxymethylasterric acid Diphenyl ether
25 111 128 8.43 220 177.0546 [M − 2 × H2O + H]+ C10H12O5 212.20 −0.37 177.0546, 149.0595 (3R,4R,4aR,6R)-4,8-Dihydroxy-6,7-epoxy-3,4,4a,5,6,7-hexahydro-1H-2-benzopyran-1-one (isomer) Isocoumarin
26 748 2046 8.68 220 330.1555 [M + NH3 + H]+ C15H20O7 312.32 −2.47 277.1059, 195.0652, 177.0546, 135.0448 Dihydrohydroxyphomopsolide A Pyranone
27 143 529 8.73 225, 340 225.0758 [M + H]+ C11H12O5 224.21 −0.22 207.0649, 163.0751, 147.0437 5,7-Dihydroxy-O-methylmellein Dihydroisocoumarin
28 1407 2837 8.85 220 419.1370 [M − H2O + H]+ C18H28O10S 436.47 0.07 301.0743, 197.0809, 179.0701 Unidentified
29 1311 8.92 220 410.0912 [M − H] C27H13N3O2 411.41 4.90 - Unidentified
30 882 9.14 220 319.1322 [M − H] C15H25ClO5 320.81 −1.33 283.1553, 265.1455, 221.1563, 203.1414, 165.0917 Dihydro-hydroxy-altiloxin B isomer-I Drimane sesquiterpenoid
31 145 533 9.21 220, 310 225.1119 [M + H]+ C12H16O4 224.25 −0.29 179.1066, 165.0912, 147.0800 Phomopsinone A Pyrenocine (α-pyrone)
32 799 1872 9.25 220 315.1449 [M + H]+ C15H22O7 314.33 −5.00 297.1353, 215.0913, 197.0803, 179.0698 Dihydrohydroxyphomopsolide B isomer III Pyranone
33 869 1650 9.48 220 301.1204 [M − H2O + H]+ C15H23ClO5 318.79 −0.90 283.1100, 265.0990, 255.1149, 247.1329 Hydroxy-altiloxin B isomer I Drimane sesquiterpenoid
34 1861 3947 9.55 220 728.3152 [M + H]+ C31H53NO16S 727.82 0.80 648.3593, 338.2323, 219.1743, 201.1635 Restricticin derivative -
35 78 175 9.59 220 183.1014 [M − H2O + H]+ C10H16O4 200.23 1.85 165.0894 Stagonolide C/G Macrolide
36 646 895 9.69 220 255.1596 [M − H2O − CO + H]+ C15H24O6 300.35 −0.38 237.1486, 219.1377, 191.1432,
173.1321, 163.1484
Arecoic acid A/B isomer I Sesquiterpene
37 607 1893 9.97 220 316.1761 [M + NH3 + H]+ C15H22O6 298.33 −2.13 299.1465, 281.1374, 201.1512, 181.0859 Phomopsolidone B Pyranone
38 1205 10.08 220 373.0961 [M − H] C25H14N2O2 374.39 3.9 - Unidentified
39 1312 10.13 220 825.2562 [2M − H] C18H23NO10 413.38 1.77 221.0813, 177.0914 Unidentified
40 279 675 10.20 220 237.1484 [M − H2O + H]+ C14H22O4 254.32 0.87 219.1373, 191.1432, 173.1321, 133.1015 Oblongolide R Naphthofuran (polyketide)
41 515 1063 10.46 220 269.1747 [M − H2O + H]+ C15H26O5 286.36 0.12 251.1639, 233.1531, 215.1428, 205.1584, 187.1479, 177.0905 Cytospolide F/Q/M Nonanolide
42 644 892 10.47 220 255.1588 [M − H2O − CO + H]+ C15H24O6 300.35 0.95 237.1483, 219.1379, 191.1428, 173.1329, 163.1481 Arecoic acid A/B isomer II Sesquiterpene
43 250 10.47 220 251.1287 [M − H] C14H20O4 252.31 −0.07 207.1376, 189.1288, 177.1274, 175.1116 Oblongolide B/C1/E/N isomer I Norsesquiterpene γ-lactones
44 1248 10.59 220 389.0879 [M − H] C19H18O9 390.34 −0.24 220.0367, 192.0386, 189.0538, 179.0348, 149.0242 Cladonioidesin Depside
45 331 10.76 220 207.1014 [M + H]+ C12H14O3 206.24 0.83 189.0910, 174.0675, 161.0961, 146.0722 Phomochromone A Chromone
46 2985 10.76 220 435.1773 [M + H]+ C26H26O6 434.48 5.79 229.0833 Prenylcandidusin C Dibenzofuran
47 1063 11.39 220 338.2335 [M − H] C19H33NO4 339.47 2.4 - Unidentified
48 252 11.43 220 251.1292 [M − H] C14H20O4 252.31 −0.86 189.1290, 187.1132 Oblongolide B/C1/E/N isomer II Norsesquiterpene γ-lactones
49 576 1202 11.52 220 279.1226 [M − H2O + H]+ C15H20O6 296.32 0.34 261.1116, 219.1015, 179.0698, 137.0597 Dihydrophomopsolide A Pyranone
50 870 11.62 220 317.1161 [M − H] C15H23ClO5 318.79 0.00 301.2025 Hydroxy-altiloxin B isomer II Drimane sesquiterpenoid
51 1606 11.64 220 521.2042 [M − H] C26H34O11 522.54 −2.61 283.1554, 265.1450, 193.0503, 163.0398 Hydroxy-altiloxin A—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
52 450 1307 11.74 220 283.0633 [M + H]+ C13H14O5S 282.31 0.61 265.0516, 191.0701, 173.0597, 158.0358, 145.0645 Amycolachromone E Chromone
53 1299 11.88 220 403.1039 [M − H] C20H20O9 404.37 −1.10 279.0507, 235.0608, 220.0358, 163.0409 Unidentified
54 385 871 11.89 220 253.1797 [M − H2O + H]+ C15H26O4 270.37 0.45 235.1690, 217.1590, 189.1639, 151.0756 Dihydro-altiloxin A Drimane sesquiterpenoid
55 881 12.02 220 319.1316 [M − H] C15H25ClO5 320.81 −1.33 283.1542, 265.1466, 247.1318, 185.0803 Dihydro-hydroxy-altiloxin B isomer-II Drimane sesquiterpenoid
56 1008 2071 12.02 220 331.2480 [M + H]+ C18H34O5 330.46 −0.30 313.2382, 295.2276, 277.2165, 259.2058 Trihydroxyoctadecenoic acid isomer I Fatty acid/oxylipin
57 1707 12.18 220 568.1605 [M − H] C26H32ClNO11 569.99 −2.44 317.1153, 281.1381, 263.1287, 250.0345, 236.0203, 206.0449, 191.0228, 174.0153 Hydroxy-altiloxin B—isocyclopaldic acid amide hybrid Drimane sesquiterpenoid—phthalide hybrid
58 676 12.19 220 237.1484 [M + H]+ C14H20O3 236.31 0.51 219.1380, 201.1276, 191.1427, 173.1321, 163.1480 Oblongolide C/D/H/J/P isomer Norsesquiterpene γ-lactones
59 372 12.37 220 211.1332 [M + H]+ C12H18O3 210.27 0.34 193.1211 Unidentified Pyranone
60 298 12.40 220, 255, 290, 340 257.0454 [M − H] C14H10O5 258.23 0.57 215.0346, 213.0537, 187.0382, 171.0446, 159.0441 Alternariol Benzochromenone (coumarin derivative)
61 942 2032 12.40 220 329.2330 [M + H]+ C18H32O5 328.44 −2.28 311.2225, 293.2117, 275.2008 Trihydroxyoctadecadienoic acid isomer I Fatty acid/oxylipin
62 552 1799 12.48 220 312.1447 [M + NH3 + H]+ C15H18O6 294.30 −1.48 195.0647, 177.0546, 135.0445 Phomopsolide A/C Dihydropyranone
63 1664 12.55 220 552.1652 [M − H] C26H32ClNO10 553.99 −2.90 317.1169, 234.0391, 190.0506, 175.0281 Hydroxy-altiloxin B—deoxy-isocyclopaldic acid amide hybrid
64 1543 12.57 220 493.2457 [M − H] C26H38O9 494.58 −2.82 211.0597, 196.0295, 181.0496, 177.0206, 151.0390 Luminacin E1 Sesquiterpenoids
65 354 827 12.60 220 251.1644 [M − H2O + H]+ C15H24O4 268.35 −0.11 233.1530, 205.1593, 187.1488, 145.1006 Altiloxin A Drimane sesquiterpenoid
66 1700 12.69 220 566.1454 [M − H] C26H30ClNO11 567.97 −3.42 317.1166, 301.1204, 281.1374, 248.0193, 204.0305 Hydroxy-altiloxin B—dehydro-isocyclopaldic acid amide hybrid Drimane sesquiterpenoid—phthalide hybrid
67 952 1775 12.84 220 311.2223 [M − H2O + H]+ C18H32O5 328.44 −1.57 293.2118, 275.2001 Trihydroxyoctadecadienoic acid isomer II Fatty acid/oxylipin
68 894 1981 12.99 220 324.2174 [M + H]+ C18H29NO4 323.43 −3.60 306.2070, 288.1961 Bipolamide A Triene amide
69 1009 13.03 220 329.2334 [M − H] C18H34O5 330.46 −0.16 229.1441, 211.1338, 183.1394, 171.1047 Trihydroxyoctadecenoic acid isomer II Fatty acid/oxylipin
70 1846 13.07 220 680.2120 [M − H] C32H40ClNO13 682.11 −0.67 318.0985, 317.1156, 303.0737, 281.1389, 274.1086, 259.0844, 246.1130, 231.0902 Hydroxy-altiloxin B—methyl-salfredin C3 hybrid Drimane sesquiterpenoid—phthalide hybrid
71 1830 13.12 220 329.2327 [M − H] C18H34O5 330.46 2.00 229.1443, 211.1331, 183.1394, 171.1016 Trihydroxyoctadecenoic acid isomer III Fatty acid/oxylipin
72 676 1060 13.14 220 303.1365 [M − H] C15H25ClO4 304.81 1.19 267.1605, 249.1500, 223.1693, 141.0918 Dihydro-altiloxin B Drimane sesquiterpenoid
73 1583 3423 13.21 220 514.3136 [M + H]+ C25H43N3O8 513.63 −2.55 496.3022, 452.2766, 382.2589, 364.2488 Arbumycin Cyclic peptide
74 999 13.31 220 329.2328 [M − H] C18H34O5 330.46 0.75 293.2133, 201.1118, 171.1022, 139.1113 Trihydroxyoctadecenoic acid isomer IV Fatty acid/oxylipin
75 1254 13.44 220 391.1398 [M − H] C20H24O8 392.40 −1.00 - Unidentified
76 1676 3507 13.63 220 555.1639 [M − H] C26H33ClO11 556.99 −0.07 317.1157, 281.1380, 263.1278, 237.0376, 191.0350, 175.0379 Hydroxy-altiloxin B—cyclopolic acid hybrid isomer-I Drimane sesquiterpenoid—phthalide hybrid
77 663 1341 13.70 220 285.1259 [M − H2O + H]+ C15H23ClO4 302.79 −2.32 267.1146, 239.1203, 203.1422, 175.1484 Altiloxin B Drimane sesquiterpenoid
78 780 13.78 220 311.2219 [M − H] C18H32O4 312.44 −0.70 293.2119, 249.1863, 231.1748, 157.0865 Dihydroxyoctadecadienoic acid isomer I Fatty acid/oxylipin
79 1821 3804 13.82 220 652.2162 [M − H] C31H40ClNO12 654.10 0.65 334.0927, 317.1156, 290.1035, 275.0792, 231.0908, 190.0529 Hydroxy-altiloxin B—dihydro-salfredin A7 hybrid Drimane sesquiterpenoid—phthalide hybrid
80 1046 13.91 220 335.0825 [M − H] C16H17ClN2O4 336.77 −4.90 - Unidentified -
81 1833 14.01 220 659.4739 [2M − H] C18H34O5 330.46 0.94 311.2218, 293.2122, 211.1324, 199.1340 Trihydroxyoctadecenoic acid isomer V Fatty acid/oxylipin
82 940 14.13 220 655.4417 [2M − H] C18H32O5 328.44 2.32 309.2066, 291.1962, 227.1285, 209.1176, 197.1180, 185.1179 Trihydroxyoctadecadienoic acid isomer III Fatty acid/oxylipin
83 996 14.21 220 329.2336 [M − H] C18H34O5 330.46 1.05 293.2116, 211.1356, 199.1345, 171.1022 Trihydroxyoctadecenoic acid isomer VI Fatty acid/oxylipin
84 221 14.47 215, 320 193.0856 [M + H]+ C11H12O3 192.21 1.67 175.0754, 147.0805, 132.0577 5-Methylmellein Benzopyran
85 993 14.54 220 329.2328 [M − H] C18H34O5 330.46 1.36 311.2215, 293.2091, 211.1334, 199.1335, 181.1232, 169.1221 Trihydroxyoctadecenoic acid isomer VII Fatty acid/oxylipin
86 299 14.64 220, 290 515.1241 [2M − H] C12H15ClO4 258.70 1.83 213.0685, 183.0586 Acremonisol A Dihydroisocoumarin (aromatic pentaketide)
87 1677 14.69 220 555.1662 [M − H] C26H33ClO11 556.99 −4.20 317.1154, 299.1059, 237.0396, 191.0339, 175.0391 Hydroxy-altiloxin B—cyclopolic acid hybrid isomer-II Drimane sesquiterpenoid—phthalide hybrid
88 1612 14.70 220 523.2178 [M − H] C26H36O11 524.56 −1.17 285.1711, 267.1604, 241.1815, 237.0396, 223.1692, 193.0506 Dihydro-hydroxy-altiloxinA—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
89 828 14.71 220 313.2383 [M − H] C18H34O4 314.46 −0.85 295.2275, 277.2166, 259.2035, 235.2090, 157.0861 Dihydroxyoctadecenoic acid isomer I Fatty acid/oxylipin
90 1663 14.73 220 552.1649 [M − H] C26H32ClNO10 553.99 −1.27 301.1210, 250.0360, 206.0481, 191.0226 Altiloxin B—isocyclopaldic acid amide hybrid isomer-I Drimane sesquiterpenoid—phthalide hybrid
91 1662 14.82 220 552.1644 [M − H] C26H32ClNO10 553.99 −0.37 301.1207, 250.0352, 206.0463, 191.0226 Altiloxin B—isocyclopaldic acid amide hybrid isomer-II Drimane sesquiterpenoid—phthalide hybrid
92 178 711 14.88 220 239.1645 [M + H]+ C14H22O3 238.32 −1.38 221.1529, 193.1580, 175.1481, 135.1176, 119.0851 Penihydrone Cyclic alcohol
93 1720 14.95 220 583.1603 [M − H] C27H33ClO12 585.00 −2.61 317.1161, 281.1405, 263.1272, 221.0447, 189.0188, 167.1104 Hydroxy-altiloxin B—O-methylisocyclopaldic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
94 1785 14.98 220 311.2227 [M − H] C18H32O4 312.44 0.27 293.2108, 275.2017, 249.1856, 235.1706, 195.1390 Dihydroxyoctadecadienoic acid isomer II Fatty acid/oxylipin
95 1661 845 15.43 220 552.1644 [M − H] C26H32ClNO10 553.99 −0.37 301.1218, 250.0359, 206.0457, 191.0226 Altiloxin B—isocyclopaldic acid amide hybrid isomer-III Drimane sesquiterpenoid—phthalide hybrid
96 722 1784 15.53 220 311.2224 [M + H]+ C18H30O4 310.43 −2.30 293.2109, 275.2007, 187.1115, 159.1152 Hydroxyoxooctadecadienoic acid isomer I Fatty acid/oxylipin in
97 1532 15.54 220 489.2122 [M − H] C26H34O9 490.54 1.65 211.0614, 209.0445, 195.0280, 193.0501, 181.0505, 151.0398 Austalide O Meroterpenoid
98 1718 3602 15.54 220 580.1950 [M − H] C28H36ClNO10 582.04 0.86 301.1209, 278.0668, 263.0430, 247.1330 Altiloxin B—O-dimethylisocyclopaldic acid amide hybrid Drimane sesquiterpenoid—phthalide hybrid
99 1505 700 15.69 220 477.2493 [M − H] C26H38O8 478.58 0.19 403.2497, 211.0607, 181.0491, 151.0384 Antroquinonol U Meroterpenoid
100 2684 15.79 220 404.2065 [M + NH3 + H]+ C22H26O6 386.44 0.68 267.1229, 233.0818, 147.0650, 129.0551 Colletofragarone A1 Cyclohexenone
101 1630 3501 15.82 220 536.1675 [M − H] C26H32ClNO9 537.99 −0.59 301.1206, 234.0417, 191.0452, 175.0275 Altiloxin B—deoxy-isocyclopaldic acid amide hybrid isomer-I Drimane sesquiterpenoid—phthalide hybrid
102 260 15.84 220 251.1648 [M − H] C15H24O3 252.35 0.27 207.1738 Deoxy-altiloxin A Drimane sesquiterpenoid
103 1575 16.01 220 507.2226 [M − H] C26H36O10 508.56 1.91 269.1762, 251.1647, 223.1736, 193.0507, 163.0384 Dihydro-altiloxin A—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
104 284 16.01 220 253.1809 [M − H] C15H26O3 254.37 0.07 235.1704, 209.1891, 193.1591, 177.1280 (Deoxy-dihydro-altiloxin A) Diaporol I Drimane sesquiterpenoid
105 1421 16.04 220 293.2112 [M − H2O + H]+ C18H30O4 310.43 −0.25 275.2002, 219.1386, 179.1453, Hydroxyoxooctadecadienoic acid isomer II Fatty acid/oxylipin
106 1656 16.07 220 550.1492 [M − H] C26H30ClNO10 551.97 −1.18 301.1219, 283.1122, 265.1450, 248.0206, 176.0324 Altiloxin B—dehydro-isocyclopaldic acid amide hybrid Drimane sesquiterpenoid—phthalide hybrid
107 1839 3843 16.10 220 664.2158 [M − H] C32H40ClNO12 666.11 1.24 318.0981, 301.1211, 303.0744, 274.1095, 259.0842, 246.1126, 231.0901 Altiloxin B—methyl-Salfredin C3 hybrid Drimane sesquiterpenoid—phthalide hybrid
108 714 1422 16.17 220 293.2113 [M − H2O + H]+ C18H30O4 310.43 −0.58 275.2004, 215.1781, 175.1494, 161.1325 Hydroxyoxooctadecadienoic acid isomer III Fatty acid/oxylipin
109 1567 3318
16.19 220 505.2075 [M − H] C26H34O10 506.54 0.83 267.1603, 249.1496, 223.1703, 193.0513, 163.0392 Altiloxin A—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
110 1782 16.40 220 295.2269 [M − H2O + H]+ C18H32O4 312.44 −0.41 277.2162, 259.2048, 161.1326 Dihydroxyoctadecadienoic acid isomer III Fatty acid/oxylipin
111 1374 2961 16.43 220 433.2588 [M + H]+ C25H36O6 432.55 −0.77 415.2468, 387.2535, 369.2420, 341.2481, 285.1835, 239.1797 Wortmannilactone B/D isomer I Macrolide
112 1631 3502 16.44 220 536.1703 [M − H] C26H32ClNO9 537.99 −1.89 301.1225, 190.0499, 162.0564 Altiloxin B—deoxy-isocyclopaldic acid amide hybrid isomer-II Drimane sesquiterpenoid—phthalide hybrid
113 1814 3741 16.74 220 636.2213 [M − H] C31H40ClNO11 638.10 0.65 334.0929, 301.1205, 290.1025, 275.0799, 231.0904, 190.0511 Altiloxin B—dihydro-salfredin A7 hybrid Drimane sesquiterpenoid—phthalide hybrid
114 1637 3461 16.78 220 539.1686 [M − H] C26H33ClO10 540.99 0.65 301.1213, 265.1457, 221.1567, 193.0505, 175.0396, 163.0394 Altiloxin B—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
115 1669 3460 17.24 220 553.1849 [M − H] C27H35ClO10 555.01 −0.54 301.1214, 283.1080, 207.0668, 175.0394, 147.0429 Altiloxin B—O-methylcyclopolic acid hybrid isomer I Drimane sesquiterpenoid—phthalide hybrid
116 788 1512 17.34 220 295.2272 [M − H2O + H]+ C18H32O4 312.44 −1.37 277.2156, 235.2046, 217.1958, 163.1476 Dihydroxyoctadecadienoic acid isomer IV Fatty acid/oxylipin
117 1668 3459 17.37 220 553.1848 [M − H] C27H35ClO10 555.01 −0.36 301.1202, 283.1101, 207.0664, 175.0398 Altiloxin B—O-methylcyclopolic acid hybrid isomer II Drimane sesquiterpenoid—phthalide hybrid
118 832 1577 17.61 220 297.2429 [M − H2O + H]+ C18H34O4 314.46 −1.52 279.2307, 261.2215, 167.1051 Dihydroxyoctadecenoic acid isomer II Fatty acid/oxylipin
119 2792 17.79 220 415.2115 [M + H]+ C24H30O6 414.49 0.04 281.1393, 135.0814, 119.0854 4-O-methylmelleolide Sesquiterpene
120 715 1758 17.79 220 311.2214 [M + H]+ C18H30O4 310.43 0.92 293.2119, 275.2013, 249.2216, 177.1276 Gallicynoic acid D (Dihydroxyoctadecenynoic acid isomer) Fatty acid/oxylipin
121 783 1526 18.12 220 295.2272 [M − H2O + H]+ C18H32O4 312.44 −2.65 277.2161, 259.2038 Dihydroxyoctadecadienoic acid isomer V Fatty acid/oxylipin
122 1703 18.43 220 567.1631 [M − H] C27H33ClO11 569.00 −0.90 317.1164, 281.1396, 263.1289, 219.1380, 153.0906 Hydroxy-altiloxin B—O-methylcyclopaldic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
123 1375 2960 18.68 220 433.2582 [M + H]+ C25H36O6 432.55 0.61 415.2480, 387.2521, 369.2433, 341.2478, 295.2427, 239.1790 Wortmannilactone B/D isomer II Macrolide
124 1291 2676 18.83 220 403.2477 [M + H]+ C24H34O5 402.52 0.50 385.2376, 367.2280, 357.2441, 339.2318, 321.2214, 311.2386, 283.1698, 237.1647 Macrolactin G/I/K isomer Macrolide
125 775 1504 19.02 220 295.2271 [M − H2O + H]+ C18H32O4 312.44 −1.05 277.2171, 167.1430 Dihydroxyoctadecadienoic acid isomer VI Fatty acid/oxylipin
126 1533 3248 19.05 220 489.2134 [M − H] C26H34O9 490.54 −0.80 251.1649, 237.0400, 191.0350, 163.0390 Deoxy-altiloxin A—cyclopolic acid hybrid Drimane sesquiterpenoid—phthalide hybrid
127 1537 19.26 220 491.2297 [M − H] C26H36O9 492.56 −2.12 253.1811, 235.1703, 191.0360, 163.0401 Deoxy-dihydro-altiloxin A—cyclopolic acid hybrid
(Diaporol I—cyclopolic acid hybrid)
Drimane sesquiterpenoid—phthalide hybrid
128 756 1837 19.27 220 313.2381 [M + H]+ C18H32O4 312.44 −1.81 295.2274, 277.2167, 249.2212, 185.1311, 125.0962 Dihydroxyoctadecadienoic acid isomer VII Fatty acid/oxylipin
129 1323 2824 19.48 220 417.2644 [M + H]+ C25H36O5 416.55 −0.60 399.2534, 371.2595, 353.2478, 325.2533, 297.2572, 239.1786, 197.1328 Macrolactin M Macrolide
130 561 19.67 220 587.4315 [2M − H] C18H30O3 294.43 1.29 275.2015, 195.1361 Oxooctadecadienoic acid I Fatty acid/oxylipin
131 3451 19.79 220 520.3404 [M + H]+ C33H45NO4 519.72 3.34 502.3311, 337.2739, 258.1101, 184.0733 Sespendole Indolosesquiterpene
132 593 1222 20.23 220 279.2324 [M − H2O + H]+ C18H32O3 296.45 1.88 261.2233, 237.1848, 209.1537, 195.1388, 181.1222 Hydroxyoctadecadienoic acid isomer I Fatty acid/oxylipin
133 1464 3051
20.28 220 459.2383 [M − H] C26H36O7 460.56 1.15 385.2379, 379.2274, 357.2444, 195.0297 Tropolactone D Meroterpenoid
134 596 20.75 220 295.2276 [M − H] C18H32O3 296.45 0.91 277.2169 Hydroxyoctadecadienoic acid isomer II Fatty acid/oxylipin
135 1657 21.26 220 551.1692 [M − H] C27H33ClO10 553.00 −0.46 301.1206, 283.1098, 221.1540 Altiloxin B—O-methylcyclopaldic acid hybrid
(Pestalotiopen A)
Drimane sesquiterpenoid—phthalide hybrid
136 560 1491 21.35 220 295.2268 [M + H]+ C18H30O3 294.43 −0.10 277.2170, 235.1703, 179.1434 Hydroxyoctadecatrienoic acid isomer II Fatty acid/oxylipin
137 1757 21.44 220 612.3671 [M + FA - H] C33H49N3O5 567.76 −2.95 228.0640, 168.0426, 122.9844, 93.5690 Unidentified -
138 571 1487 21.58 220 295.2266 [M + H]+ C18H30O3 294.43 0.58 277.2157, 241.1954, 221.1525, 179.1431 Hydroxyoctadecatrienoic acid isomer III Fatty acid/oxylipin
139 626 21.59 220 297.243 [M − H] C18H34O3 298.46 1.74 279.2327 Hydroxyoctadecenoic acid isomer I Fatty acid/oxylipin
140 592 1230 22.24 220 279.2322 [M − H2O + H]+ C18H32O3 296.45 0.53 261.2233, 149.0231 Hydroxyoctadecadienoic acid isomer III Fatty acid/oxylipin
141 591 1587 22.61 220 297.2432 [M + H]+ C18H32O3 296.45 −2.63 279.2305, 251.2366, 183.1373, 169.1578, Hydroxyoctadecadienoic acid isomer IV Fatty acid/oxylipin
142 1541 22.61 220 491.3375 [M + FA - H] C28H46O4 446.66 0.70 427.3211, 425.3062, 409.3098, 407.2971, 391.2990, 281.2488 Stoloniferone N Ergostane steroid
143 598 1572 22.87 220 297.2426 [M + H]+ C18H32O3 296.45 −0.60 279.2305, 251.2366, 183.1373, 169.1578, 141.1270 Hydroxyoctadecadienoic acid isomer V Fatty acid/oxylipin
144 4070 22.90 220 865.4796 [M + H]+ C55H64N2O7 865.11 −1.12 459.2595, 389.2176, 371.2069, 303.1442, 233.1020 Unidentified
145 311 22.96 220 257.2119 [M − H] C15H30O3 258.40 1.23 211.2077, 207.1727, 189.1644 Hydroxypentadecanoic acid Fatty acid/oxylipin
146 4078 23.12 220 865.4789 [M + H]+ C55H64N2O7 865.11 0.30 459.2589, 389.2171, 371.2066, 303.1441, 233.1017 Unidentified -
147 602 1565 23.49 220 297.2424 [M + H]+ C18H32O3 296.45 −0.60 279.2319, 261.2209, 243.2110, 233.2265, 167.1431, 135.1176 Hydroxyoctadecadienoic acid isomer VI Fatty acid/oxylipin
148 403 24.15 220 271.2277 [M − H] C16H32O3 272.42 0.62 225.2225, 223.2062, 197.1894 Hydroxyhexadecanoic acid isomer Fatty acid/oxylipin
149 631 24.81 220 297.2433 [M − H] C18H34O3 298.46 0.73 251.2377, 249.2236 Hydroxyoctadecenoic acid isomer II
150 2637 24.91 220 395.3313 [M + H]+ C28H42O 394.63 −1.16 377.3208, 311.2371, 293.2264, 251.1790, 211.1486, 157.1013 Ergosta-5,7,9(11),22-tetraen-3beta-ol Sterol
151 449 1291 25.58 220 281.2471 [M + H]+ C18H32O2 280.45 1.45 263.2371, 245.2259, 161.1332 (Linoleic acid) octadecadienoic acid isomer Fatty acid
152 1293 2391 25.70 220 357.2997 [M + H]+ C21H40O4 356.54 0.66 339.2895, 283.2635, 265.2525, 247.2422 2,3-Dihydroxypropyl oleate (octadecenoyl)-sn-glycerol) Monoacylglycerol
153 472 1329 26.65 220 283.2635 [M + H]+ C18H34O2 282.46 −1.22 265.2527, 247.2426, 163.1482 Octadecenoic acid isomer Fatty acid/oxylipin

* Numbers in bold represent the base peak.

2.2.1. Polyketides

The UHPLC-HRESIMS analysis of extracts from Diaporthe isolates led to the annotation of several compounds from the polyketide group—pyranones such as, dihydrohydroxyphomopsolide B isomers I–III (17, 23, 32), dihydrophomopsolide A (49), dihydrohydroxyphomopsolide A (26), and furanones such as phomopsolidone B (37) and dihydrohydroxyphomopsolidone B isomers I and II (15, 16) (Figure 1). These compounds were tentatively identified based on the high-resolution mass of the precursor ions and the fragments generated via common fragmentation pathways in positive ionization mode. Namely, the loss of one or two water molecules (−18 Da or 36 Da), followed by the loss of a tiglic acid (2-methylbut-2-enoic acid) residue (-C5H8O2), giving intense fragment ions with m/z 179 or 197 for compounds 17, 23, and 32 ion at m/z 177 for compounds 26 and 49, and m/z 181 for compounds 15, 16, and 37, was observed (Table 2). Phomopsolides are common secondary metabolites derived from Diaporthe [25]. They were initially isolated from Phomopsis oblonga, a fungus that provided some protection against elm bark beetle infestations [37]. They have been proved for their antibacterial activity against Staphylococcus aureus [38]. Moreover, phomopsolide A/C (62), from the endophytic Diaporthe sp. AC1 from Artemisia argyi, was proved to inhibit the growth of Fusarium graminearum, F. moniliforme, Botrytis cinerea, and Verticillium dahliae, indicating that the compound may have a broad spectrum of antifungal activity [29].

Figure 1.

Figure 1

Putative structures of polyketides found in the tested Diaporthe isolates.

2.2.2. Pyrones

Pyrones represent a class of oxygen-based heterocyclic compounds that naturally occur in two isomeric forms as either 2-pyrone (α-pyrone) or 4-pyrone (γ-pyrone). The number 2/4 is assigned based on the position of the carbonyl group relative to the oxygen atom within the ring system [39]. In our study, Diaporthe spp. isolated from fruit trees produced phomopsinone A (31) and pyrenocine P (3) (Figure 2), which belong to the α-pyrones. Their fragmentation spectra showed mainly water (−18 Da) and/or CO losses (−28 Da). However, characteristic UV maxima at around 280 nm indicated α-pyrone structures (Table 2). Previously, phomopsinone A and pyrenocine J-M have been isolated from the endophytic fungus Phomopsis sp. and have shown antifungal, antibacterial, and antialgal activity [27,28]. Phomopsinone A showed very strong antifungal activity against Botrytis cinerea, Pyricularia oryzae, and Septoria tritici. Pyrenocine J-M had strong antibacterial activity especially against the gram-negative bacterium E. coli, since gram-negative bacteria are usually difficult to inhibit. Similarly, all mentioned compounds showed algicidal activity against Chlorella fusca [27,28].

Figure 2.

Figure 2

Putative structures of pyrones found in the tested Diaporthe isolates.

The studied Diaporthe isolates, apart from the metabolite characteristics for the genus Diaporthe, also produced several new bioactive compounds usually present in the other species of fungi but not in Diaporthe [39]. For example, islandic acid-II (1), originally isolated from Penicillium islandicum, in the literature was reported as showing the complete growth inhibition of Yoshida sarcoma tumor cells [40]. Another compound produced by the tested Diaporthe isolates was scirpyrone K (6) (Figure 2). Its fragmentation pathway was very similar to that of compound 3. Previously, it had been isolated from a marine fungus identified as Phialocephala sp. strain FL30r. This compound exhibited weak radical scavenging activity with no cytotoxic activities reported [41].

2.2.3. Oxylipins

Oxylipins constitute a large family of oxidized fatty acids and their derivatives. Bioactive lipid production is widespread among many organisms including filamentous fungi [42]. In many cases, oxylipins have a role in both organismal development and communication with the host on a cellular basis [43,44]. The literature showed that fungal oxylipins are involved in influencing processes in infected host tissues, presumably by mimicking endogenous signal molecules [45,46]. Fungi have the ability to use the host plant’s oxylipin to achieve their own benefits. For example, by increasing the production of toxins, they improve their virulence [45,46], and by increasing sporulation they can accelerate reproduction in the tissues of the host plant [47]. Additional functions of fungal oxypilins have also been reported. They are related to fungal development regulation, metabolism, and host-pathogen interaction [42,48,49]. The synthesis of oxylipins proceeds due to substrates released by phospholipids and acylglycerides such as: oleic, linoleic, linolenic, and arachidonic acids [50,51]. Various reactions occurring in an oxidizing environment, in combination with enzymatic activity, contribute to the formation of various oxylipins from a given fatty acid. [52]. In our study we have tentatively identified thirty-seven oxylipins of predominantly C18 chain (Table 2); among them, trihydroxyoctadecenoic acid isomers I-VII (56, 69, 71, 74, 81, 83, 85) have been found in the tested Diaporthe isolates. It should be mentioned that the differences in fragmentation patterns between structural isomers were minimal and did not allow us to determine the position of double bonds or hydroxyl groups in the analyzed compounds. Previously, similar metabolites have been produced in the tubers of taro (Colocasia antiquorum) as a defense response to inoculation with black rot fungus (Ceratocystis fimbriata) [53]. They were isolated for the first time from the Chinese truffle Tuber indicum [54]. It has been proven, for example, that (9S,12S,13S)-tri-hydroxyoctadeca-10E-enoic acid had antifungal activities against Magnaporthe grisea causing rice blast disease [55], and (13S)-hydroxy-9,11-octadecadienoic acid had nematocidal properties [56].

2.2.4. Chromones

Chromones are naturally occurring phenolic derivatives of chromone (1,4-benzopyrone or 4H-chromen-4-one) and are isomers of coumarin. They are produced abundantly by many genera of plants, being a part of a normal healthy diet and by fungi. This class of compounds is mainly associated with antioxidant, antimicrobial, anticancer, and anti-inflammatory activities [57]. In our study, Diaporthe spp. produced phomochromone A (45) (Figure 3), which can exhibit an antifungal, antibacterial, and algicidal activities, which is supported by the literature. For example, two new chromones, phomochromone A and B, have been isolated from the endophytic fungus Phomopsis spp. from Cistus monspeliensis which showed good antifungal, antibacterial, and algicidal properties towards Septoria tritici, Microbotryum violaceum, Botrytis cinerea, E. coli, Bacillus megaterium, and Chlorella fusca [58]. Amycolachromone E (52) (Figure 3) and the series of other chromone derivatives were isolated from the deep-sea marine actinomycete Amycolatopsis sp. [59].

Figure 3.

Figure 3

Putative structures of chromones found in the tested Diaporthe isolates.

2.2.5. Sesquiterpenoids

Drimane-type sesquiterpenoids are a large group of compounds that have been found in plants and fungi, exhibiting various biological activities [60,61]. During the research conducted by Zang et al. [62] and Chen et al. [63], a variety of new drimane-type metabolites, including diaporols B–I (104), Q, and R, have been isolated from the mangrove endophytic Diaporthe sp. [62,63]. Furthermore, two drimane-type sesquiterpenoids, named altiloxins A (65) and B (77) (Figure 4), showing phytotoxic activity on the lettuce seedlings were obtained from Phoma asparagi [64]. Considering the fragmentation spectra of compounds 65, 77, and 104, in the Diaporthe isolates studied, we determined the presence of a number of their derivatives—dihydro-altiloxin A (54), dihydro-altiloxin B (72), hydroxy-altiloxin A isomers I and II (19, 22), hydroxy-altiloxin B isomers I and II (33, 50), and deoxy-altiloxin A (102).

Figure 4.

Figure 4

Putative structures of sesquiterpenoids found in the tested Diaporthe isolates.

2.2.6. Phthalides

Phthalides are natural substances used in traditional medicine in Asia, Europe, and North America, which can be found both in plants and fungi [65,66,67,68,69]. In our study Diaporthe spp. produced the convolvulanic acid A isomers I–II (9,13), which was previously reported from Phomopsis convolvulus, a host-specific pathogen of field bindweed (Convolvulus arvensis) [66]. This metabolite showed phytotoxic activity against C. arvensis, proving that it could be used as an herbicide to control this weed effectively [66].

2.2.7. Hybrid Compounds

HRESIMS analysis revealed a molecular formula of C27H33ClO10 ([M − H] at m/z 551.1692) for compound 135, suggesting close structural analogy to pestalotiopene A [68]. The structural similarity of both compounds was further corroborated by detecting the same mass fragment at m/z 301.1206 with the characteristic chlorine isotope splitting, corresponding to the altiloxin B part of pestalotiopene A. Previously, drimane sesquiterpene-cyclopaldic acids hybrids, pestalotiopens A and B, were isolated from the mangrove-derived fungus Pestalotiopsis sp. obtained from leaves of the Chinese mangrove Rhizophora mucronate [68]. Pestalotiopen A (135), an altiloxin B—O-methylcyclopaldic acid hybrid, showed moderate antibacterial activity against Enterococcus faecalis [68]. Cyclopaldic acid was also produced by Seiridium cupressi, the pathogen of a canker disease of cypress, showing phytotoxic and antifungal activity [67], and by Coccomyces strobi isolated from needles of Pinus strobus, showing moderate growth inhibition of Microbotryum violaceum (=Ustilago violacea) and weak antibiotic activity against Bacillus subtilis, with no inhibition observed against E. coli at the highest tested concentration [69]. In the search for natural products as an alternative to synthetic pesticides, cyclopaldic acid has been reported to possess insecticidal [70], fungicidal [71], as well as herbicidal [72] activities. Recently, Samperna et al. [73], during the investigation of the effects of cyclopaldic acid in Arabidopsis thaliana plants and protoplasts, showed that this metabolite induced leaf chlorosis, ion leakage, membrane-lipid peroxidation, hydrogen peroxide production, and inhibited root proton extrusion in vivo and plasma membrane H+-ATPase activity in vitro. In our study, we report the presence of over twenty-five compounds, ethers of altiloxin A and its derivatives with cyclopolic acid (51, 88, 103, 109, 126, and 127), and ethers of altiloxin B and its derivatives with either (iso)cyclopaldic acid, cyclopolic acid, or salfredins A7/C3 (57, 63, 66, 70, 76, 79, 87, 90, 91, 93, 95, 98, 101, 106, 107, 112–115, 117, 122, and 135) (Figure 5). The identity of these compounds was tentatively established by the similarity of fragmentation spectra to those of compound 135 (Table 2).

Figure 5.

Figure 5

Putative structures of hybrid sesquiterpenoids-phthalides found in the tested Diaporthe isolates.

2.3. Metabolite-Based Chemotaxonomy

As a preliminary step in multivariate statistical analysis, PCA analysis provided an unsupervised overview of LC-MS fingerprints obtained in both ionization modes (NI and PI). Both NI and PI PCA score plots revealed a close clustering of the QC samples (Figure 6A), indicating that the separation, observed between fungal isolates into two distinct chemotypes was mainly due to biological reasons.

Figure 6.

Figure 6

The score plots of principal component analysis (PCA) in negative ionization mode and positive ionization mode LC-MS data of the tested Diaporthe isolates, where each point represents a single isolate. (A) PCA colored by the host plant. (B) Isolates in the PCA are colored by k–means clustering cluster assignments from negative ionization mode; the elliptic areas represent the 95% confidence regions.

To avoid biased group assignment of the PCA plots, samples were statistically assigned into 2 clusters (chemotypes) based on the k–means clustering algorithm in NI mode, and the groups generated by the k–means clustering algorithm in the negative mode were assigned to the positive mode (Figure 6B). The clustering of the data was easily visualized in both ionization modes, and confirmed by clusters obtained separately by HCA (Figure 7A,B). The first five PCs explained 75.1% of the variance in NI and 75.4% in PI modes, and 57.1% of the total variance was projected in the first two PCs in NI, while 56.0% in PI, which suggested the similar quality of data obtained in both ionization modes. Indeed, the PCA score plots showed similar patterns with specific host plants (understood as metadata) grouped together (pear, sweet cherry, and walnut), while the rest were much more dispersed, and there were no clear associations between the metadata and the groups in the PCA. We decided to use NI mode for further work due to the lower complexity of LC-MS data (high amount of in-source collision-induced dissociation in PI).

Figure 7.

Figure 7

LC-MS-based hierarchical cluster analyses (HCA, Pearson distance, and Ward’s linkage rule) show the tested Diaporthe isolates following differentiation in negative ionization mode (A) and positive ionization mode (B). Letters in isolate names refer to the host plant: J = Apple; L = Hazelnut; B = Peach; G = Pear; S = Plum; W = Sour Cherry; Cz = Sweet Cherry; and orz = Walnut. Numbers 1, 2, and 3 after the underscore refer to individual biological repetitions.

To validate the k–means/HCA model and to identify the features responsible for the classification, we performed a supervised PLS-DA analysis, and overall, 52.1% of the total variance was displayed on the first two principal component axes of the PLS-DA score plot (Figure 8A), with R2X = 0.946 and Q2 = 0.924 calculated from the first three components via a 10-fold cross-validation method, with Q2 as the measured performance. Since PLS-DA tends to overfit data, the model was validated to understand whether the separation is statistically significant or is due to random noise. This hypothesis was tested using the permutation test—separation distance (B/W), with 100 permutations with observed statistics having a p < 0.01 (Figure 8C).

Figure 8.

Figure 8

PLS-DA score plots of clusters in NI-mode-based k–means clustering of the tested Diaporthe isolates; the elliptic areas represent the 95% confidence regions (A); the top 50 features ranked based on scores of VIP, features are numbered based on MS-DIAL ID (see Table 2) (B); and permutation test results of the PLS-DA model (statistical test: separation distance (B/W)), the number of permutations set at 100 (C).

A p-value below 0.01 in 100 permutations means that not even once (<0.01 × 100) did the permutated data yield a better performance (higher B/W) than the original label, suggesting the significant difference between these two clusters.

Potential variables to separate clusters 1 and 2 in the dendrogram were identified as potential biomarkers using VIP values which estimate the importance of each variable in the projection used in a PLS-DA model. The greater-than-one rule is usually considered for detecting the descriptors with the greatest importance in the projection. However, we decided, due to a large number of significant metabolites (>300), to use VIP scores > 1.8 (Figure 8B). The peak intensity ratios were also subjected to an unpaired non-parametric test (Wilcoxon rank-sum test, also known as the Mann–Whitney U test) within MetaboAnalyst, and false discovery rates (FDR < 0.05) were calculated to discover if those features are significantly different between cluster 1 and 2. A large fold change (FC > 10) between the two putative chemotypes was also considered a selection criterion, with FC > 100 indicating the presence/absence of the feature in question. As a result, 43 features meeting these conditions (VIP = 2.20–1.81, FDR adj. p-value = 3.23 × 10−19–4.71 × 10−18, FC = 348–19) were selected (Table 3) for receiver operating characteristic (ROC) analysis in order to assess their potential as chemotaxonomical biomarkers. ROC curves are used to evaluate classification and prediction models in bioinformatics. They are often summarized in a single metric known as area under the curve (AUC), where AUC = 1.0 indicates an excellent classifier and AUC = 0.5 means the classifier has no practical utility [74]. In this regard, we calculated the AUC for each selected candidate biomarker, and the AUC values obtained ranged from 0.972 to 1.000 (Table 3). Furthermore, to consider factors other than genetics, i.e., host plant, year of strain isolation or storage time, a combination of multiple individual markers must be considered into a single multivariate model, providing improved levels of discrimination and confidence. To this end, we applied the PLS-DA model to combine our 43 selected markers to obtain the AUC (Figure 9A), and predicted the classification probability into each chemotype (Figure 9B). The performance of this model was tested using a balanced Monte-Carlo cross-validation procedure, and as a result the average accuracy based on 100 cross-validations was 0.991.

Table 3.

The 43 top ranked features contributing to the group discrimination in PLS-DA and marked as potential biomarkers for the tested Diaporthe isolates from data generated in NI mode.

No. MS-DIAL ID Tentative Identification VIP Score FDR Adj. p-Value Fold Change
(Cluster 2/Cluster 1)
AUC
109 1567 Altiloxin A—cyclopolic acid hybrid 2.20 3.33 × 10−19 128 0.995
33 869 Hydroxy-altiloxin B isomer I 2.15 5.51 × 10−19 51 0.989
114 1637 Altiloxin B—cyclopolic acid hybrid 2.14 3.23 × 10−19 181 0.998
126 1533 Deoxy-altiloxin A—cyclopolic acid hybrid 2.12 3.23 × 10−19 283 0.999
1 289 Islandic acid-II 2.08 3.23 × 10−19 159 0.998
77 663 Altiloxin B 2.07 3.54 × 10−19 121 0.994
76 1676 Hydroxy-altiloxin B—cyclopolic acid hybrid isomer-I 2.05 3.23 × 10−19 178 0.996
133 1464 Tropolactone D 2.03 3.54 × 10−19 264 0.994
93 1720 Hydroxy-altiloxin B—O-methylisocyclopaldic acid hybrid 2.01 4.71 × 10−18 19 0.972
99 1505 Antroquinonol U 2.00 3.54 × 10−19 96 0.994
95 1661 Altiloxin B—isocyclopaldic acid amide hybrid isomer-III 1.99 1.73 × 10−18 348 0.979
41 515 Cytospolide F/Q/M 1.97 1.04 × 10−18 72 0.983
72 676 Dihydro-altiloxin B 1.96 3.23 × 10−19 138 0.996
54 385 Dihydro-altiloxin A 1.95 4.83 × 10−19 93 0.991
22 488 Hydroxy-altiloxin A isomer-II 1.94 7.42 × 10−19 53 0.986
18 739 Isariketide 1.93 3.23 × 10−19 236 0.997
64 1543 Luminacin E1 1.93 3.23 × 10−19 59 0.998
79 1821 Hydroxy-altiloxin B—dihydro-salfredin A7 hybrid 1.93 3.33 × 10−19 143 0.995
113 1814 Altiloxin B—dihydro-salfredin A7 hybrid 1.93 3.23 × 10−19 266 0.996
24 1180 5-Hydroxymethylasterric acid 1.92 3.23 × 10−19 131 0.999
70 1846 Hydroxy-altiloxin B—methyl-salfredin C3 hybrid 1.92 3.23 × 10−19 174 0.996
98 1718 Altiloxin B—O-dimethylisocyclopaldic acid amide hybrid 1.92 7.85 × 10−19 217 0.986
65 354 Altiloxin A 1.91 5.51 × 10−19 82 0.989
30 882 Dihydro-hydroxy-altiloxin B isomer-I 1.91 5.34 × 10−19 54 0.989
57 1707 Hydroxy-altiloxin B—isocyclopaldic acid amide hybrid 1.91 3.23 × 10−19 165 0.997
7 264 Strobide B 1.91 3.23 × 10−19 95 0.997
102 260 Deoxy-altiloxin A 1.90 3.23 × 10−19 135 0.999
127 1537 Diaporol I—cyclopolic acid hybrid 1.90 3.23 × 10−19 114 0.998
44 1248 Cladonioidesin 1.88 3.23 × 10−19 150 1.000
19 482 Hydroxy-altiloxin A isomer-I 1.87 8.10 × 10−19 84 0.985
104 284 Diaporol I 1.87 3.23 × 10−19 98 0.999
51 1606 Hydroxy-altiloxin A—cyclopolic acid hybrid 1.86 3.23 × 10−19 153 0.998
91 1662 Altiloxin B—isocyclopaldic acid amide hybrid isomer-II 1.86 1.06 × 10−18 146 0.983
101 1630 Altiloxin B—deoxy-isocyclopaldic acid amide hybrid isomer-I 1.86 7.65 × 10−19 192 0.986
55 881 Dihydro-hydroxy-altiloxin B isomer-II 1.85 9.38 × 10−19 56 0.984
107 1839 Altiloxin B—methyl-Salfredin C3 hybrid 1.84 9.22 × 10−19 207 0.984
87 1677 Hydroxy-altiloxin B—cyclopolic acid hybrid isomer-II 1.84 1.06 × 10−18 78 0.983
103 1575 Dihydro-altiloxin A—cyclopolic acid hybrid 1.83 3.54 × 10−19 90 0.994
112 1631 Altiloxin B—deoxy-isocyclopaldic acid amide hybrid isomer-II 1.83 6.66 × 10−19 115 0.987
97 1532 Austalide O 1.82 3.67 × 10−19 42 0.993
52 450 Amycolachromone E 1.82 1.61 × 10−18 40 0.980
66 1700 Hydroxy-altiloxin B—dehydro-isocyclopaldic acid amide hybrid 1.81 3.23 × 10−19 103 0.996
80 1046 Unidentified 1.81 5.51 × 10−19 73 0.989

Figure 9.

Figure 9

ROC curve for combined biomarker models (set of 43 metabolites); 100 cross-validations were performed, and the results were averaged to generate the plot (A); The average of predicted class probabilities of each sample across the 100 cross-validations. As the algorithm uses a balanced sub-sampling approach, the classification boundary is located at the center (x = 0.5, the dotted line). The corresponding confusion matrix showed that all isolates were correctly classified in all cases (B).

Hierarchical clustering with a heat map is also shown to easily visualize the concentration variation of the top 100 tentatively identified metabolites (according to t-tests) expressed in the tested Diaporthe isolates (Figure 10). A sharp contrast of their accumulation is observed, while at the same time the samples are clearly grouped by their group membership, determined by HCA and k–means analyses.

Figure 10.

Figure 10

Hierarchical clustering with the heat map generated from the top 100 tentatively identified metabolites present in the tested Diaporthe isolates, according to t-tests, using Pearson distance for similarity measure and Ward’s linkage algorithm for clustering. Clusters were grouped based on the HCA/k–means analyses shown in Figure 6B and Figure 7. Cell colors indicate relative concentration values as high (dark brown) or low (dark blue), with samples in columns and features (MS DIAL ID in NI) in rows. Features from Table 3 are enclosed in red rectangles.

The study on the utilization of metabolites as chemotaxonomic markers for species identification refers to the genus Penicillium, Aspergillus, Fusarium, Alternaria, and the Xylariaceae family [75,76]. However, in the case of Diaporthe, this type of research was limited. In the research conducted by Horn et al. [77,78] on endophytic Phomopsis (=Diaporthe) from woody host, three metabolites named phomodiol, phomopsolide B, and phomopsichalasin were indicated as potential chemotaxonomic markers for this fungi. In addition, Abreu et al. [79] showed that the production of secondary metabolites by Phomopsis and related Diaporthales may be species-specific, indicating the value of utilizing the metabolic analysis in taxonomic research on closely related species.

In our research, isolates belonging to the Diaporthe eres species complex isolated from fruit trees produced 153 metabolites from which 43 were recognized as potential chemotaxonomic markers, mostly belonging to the drimane sesquiterpenoid—phthalide hybrid class. This group included mainly phytotoxic compounds such as cyclopaldic acid and altiloxin A, B and their derivatives. It is noteworthy that during our investigation, the phytotoxic compound cyclopaldic acid was produced not only by the pathogenic Diaporthes species but also by the endophytic D. eres isolate 1420S, previously described by Abramczyk et al. [32] and used in the present study. Following the observations of Graniti et al. [67] and McMullin et al. [69], the production of phytotoxic cyclopaldic acid may be related to Diaporthe changing its lifestyle from endophytic to pathogenic, under favorable conditions. Thus, it is possible that endophytic D. eres isolate 1420S [32], is a weak opportunistic pathogen, switching from an endophytic to a pathogenic phase when the host tissue becomes weakened. This issue requires more advanced research in the future.

3. Materials and Methods

3.1. Chemicals and Reagents

Hypergrade for LC-MS acetonitrile (≥99.9%) and HPLC gradient-grade methanol (≥99.9%) were purchased from Merck (Darmstadt, Germany), LC-MS grade formic acid (98–100%) was purchased from Sigma Aldrich (Steinheim, Germany). A Milli-Q Simplicity 185 water purification system from Millipore (Milford, MA, USA) was used for preparation of ultrapure water (18.2 MΩ·cm).

3.2. Fungal Strains and Culture Conditions

We investigated 40 Diaporthe strains isolated during previous studies from different species of fruit trees growing in south-eastern Poland (Table 1) [4,31]. All axenic cultures were deposited at the Fungal Collection of Phytopathology and Mycology Subdepartment, University of Life Sciences in Lublin (Poland). Thirty-nine came from shoots with visible disease symptoms and one from healthy Prunus domestica as endophyte, described previously by Abramczyk et al. [32]. Diaporthe strains were isolated according to the methodology described by Król [80]. Healthy fragments of the tested plants were properly disinfected by rinsing several times, first in a 10% sodium hypochlorite solution, then in sterile distilled water. After drying, the plant fragments were placed on potato dextrose agar (PDA, Difco) and incubated for 5 days at 25°C, in the dark. When the fungus colonies appeared, pure cultures were prepared according to the methodology described previously [80].

3.3. DNA Extraction, Amplification and Sequencing

Strains were incubated on PDA at 25 °C for 7 days before to DNA extraction. The total genomic DNA was extracted using the FastDNA®SPIN Kit and the FastPrep®Instrument (Qbiogene, Inc., Carlsbad, CA, USA), according to the manufacturer’s protocol. All extracted DNA was stored at −20 °C until use.

The amplification of the fragment of the internal transcribed spacer region (ITS) of the nuclear ribosomal RNA gene, the universal primers ITS1: TCCGTAGGTGAACCTGCGG and ITS4: TCCTCCGCTTATTGATATGC were used [81]. For the amplification of ITS regions, 25 μL of the reaction mixture was prepared, which consisted of the following components: 1 μL of genomic DNA (5 ng/μL), DreamTaq™ Green PCR Master Mix (2×) (Thermo Scientific, Waltham, MA, USA) in a volume of 12.5 μL, primers (10 μM) in a volume of 1 μL each and purified water in a volume of 9.5 μL. The PCR reaction was run under the following conditions: 95 °C for 3 min, followed by 39 cycles of 95 °C for 30 sec, 55 °C for 50 sec, 72 °C for 1 min, and final extension at 72 °C for 10 min. Sequencing of the obtained PCR products was performed in the Genomed S.A. (Warsaw, Poland). The sequence data received were deposited in GenBank (Table 1). The Bionumerics 7.6 (Applied Maths NV., Sint-Martens-Latem, Belgium) and SEED v.2.1.05 (Institute of Microbiology CAS, Prague, Czech Republic) software was used for bioinformatic analyses.

The obtained sequences were blasted against the NCBIs GenBank nucleotide database to determine the closest related species.

3.4. Extraction of Fungal Metabolites

For metabolite extraction, 40 Diaporthe strains were three-point inoculated on 90 mm Petri plates containing PDA, and incubated for 28 days at 23 °C under a 12 h photoperiod, referring to the methodology of Abreu et al. [80], with modifications. Fungal discs (5-mm diameter) were collected in three individual biological repetitions each (n = 3). Each fungal culture (120 total) and three non-inoculated medium samples were freeze-dried (Christ Gamma 1–16 LSC, Martin Christ, Osterode am Harz, Germany) and subsequently ground with a mortar. Dried material (25 mg) was transferred to a 5 mL screw-capped centrifuge tube (Eppendorf, Hamburg, Germany) and added to 2.5 mL of extraction solvent mixture, MeOH/H2O 80:20 (v/v). Samples were then thoroughly vortex-mixed for 1 min and ultrasonicated for 20 min under 4 °C. Samples were centrifuged (18,000× g for 20 min under 4 °C), and the supernatants were transferred to separate vials and analyzed using UHPLC-QTOF HRMS. A QC (Quality Control) sample (aliquot of all samples) was also prepared and injected six times before randomized sample injection for column conditioning and at every forty samples to evaluate the performance of the LC-MS method during the detection.

3.5. UHPLC-QTOF HRMS Profiling

Ultrahigh-performance liquid chromatography-quadrupole time of flight-high-resolution MS (UHPLC-QTOF HRMS) analyses were performed on an Impact II HD mass spectrometer (Bruker, Billerica, USA) coupled to a U-HPLC Ultimate 3000 RSLC system (Thermo Fisher Scientfic, Hemel Hempstead, UK). Five-microliter injections of samples were fed from a thermostatted autosampler (8 °C) onto a CORTECS T3 column (150 mm × 2.1 mm i.d., 2.7 μm, Waters, Milford, USA), equipped with a guard column, and the column was kept at 35 °C. Mobile phases were (A) ultrapure water with 0.1% formic acid (FA), and (B) acetonitrile with 0.1% FA. The flow rate was set at 500 μL/min and the solvent gradient profile was as follows: 0.0–1.0 min, 5% B; 1.0–27.0 min, 5–99% B (concave-shaped gradient—Dionex gradient curve 6); 27.0–30.0 min, 99% B. Between the injections, the column was equilibrated with six volumes of 5% B. Mass detection was performed using an electrospray source in positive ionization (PI) and negative ionization (NI) modes. Ionization spray voltages were set to 4.0 kV (for PI) and 3.0 kV (for NI); dry gas flow was 6 l/min; the dry gas temperature was 200 °C; collision cell transfer time was 90 μs; and nebulizer pressure was 0.7 bar. MS1 and MS/MS data (range 80–1800 m/z) were collected using Bruker DataAnalysis 4.3 software in data-dependent acquisition (DDA) mode—after each full MS1 scan, the two most intense ions were fragmented with collision energies of 20 eV for PI and 30 eV for NI.

3.6. Data Processing and Metabolite Identification

LC-MS raw data were first converted into the ‘Analysis Base File’ (ABF) format [82] using Reifycs Abf (Analysis Base File) Converter (https://www.reifycs.com/AbfConverter/ (accessed on 25 May 2021)) and processed with MS-DIAL (RIKEN, version 4.90) [83]. MS1 and MS2 tolerances were set to 0.01 and 0.05 Da, respectively, in centroid mode for each data set (PI and NI). In PI and NI modes, automatic feature detection was performed between 3.0 and 27.0 min for mass range between 80 and 1800 Da. The minimum peak height intensity was set to 2000 for NI and 3000 for PI modes, respectively; linear-weighted moving average as the smoothing method using 5 scans and peak width 5 scans. Peaks were aligned on a QC reference file with an RT tolerance of 0.10 min and a mass tolerance of 0.015 Da and retained in the feature table if they appeared in at least 3 samples. All peaks detected from non-inoculated medium were removed from the generated matrix if their “Sample average/blank average” ratio was lower than 10, thus removing the background and contaminants and preserving the true biological mass signals from LC-MS data.

The kept significant features were exported to the MS-FINDER program (RIKEN, version 3.52) for in silico-based annotation using the hydrogen rearrangement rules (HRR) scoring system [84]. The MS1 and MS2 tolerances were set to 10 and 25 ppm, respectively, and the isotopic ratio tolerance set to 20%. The formulas were filtered to exclusively contain only C, H, O, N, P, S, and Cl atoms. Selected compounds were searched against the built-in database in the MS-FINDER system: NANPDB (Northern African Natural Products Database), KNApSAcK, COCONUT, T3DB (the toxin and toxin target database), and NPA (Natural Products Atlas), and only structures with a score above 5 were retained for thorough analysis. Fungal metabolites were tentatively identified by their high-resolution mass data, MS/MS fragmentation pattern analysis, UV data, and published literature.

3.7. Multivariate Statistical Analysis

The aligned data table was LOWESS (locally weighted scatterplot smoothing), normalized using the pooled QC samples and exported from MS-DIAL software to comma-separated value (CSV) format prior to analysis using MetaboAnalyst (version 5.0) [85]. The data were filtered by removing variables showing low repeatability among QC samples (RSD > 20%). Two data matrices were constructed, one in PI mode (120 isolates × 3557 metabolites) and the second in NI mode (120 isolates × 1759 metabolites). The samples were then normalized by the sum to account for the effects of sample dilution (different content of culture medium in the samples), data were log10-transformed to correct for heteroscedasticity and Pareto-scaled to reduce the influence of intense peaks, which transformed the data matrix into a more Gaussian-type distribution [86,87]. First, unsupervised principal component analysis (PCA) was used as an exploratory data analysis to provide an overview of LC-MS fingerprints. Unsupervised groups from the PCA were assigned by k–means clustering analysis and confirmed by hierarchical cluster analysis (HCA) performed to obtain a dendrogram of fungal strains according to metabolite profiling (Pearson distance measure, Ward’s clustering algorithm). On the clusters obtained, a partial least squares discriminant analysis (PLS-DA) was conducted using clusters as Y value, and their potential variables were selected based on variable importance in projection (VIP > 1.0) values and false discovery rate (FDR < 0.05) by Wilcoxon rank-sum test.

4. Conclusions

The results of our study demonstrated a rich diversity of metabolites secreted by the tested Diaporth eres species complex. The characterization of these compounds could be the basis for the future research on their isolation and bioactive potential for agricultural applications as biopesticides or biofertilizers.

Furthermore, the future research should include a larger population of Diaporthe from fruit plants from various areas of Poland. It would be worth determining their metabolic profile, then isolating more important compounds to confirm their structure and bioactive properties. In addition, the optimization of culture media and cultivation conditions for producing richer metabolite profiles are necessary for a more conclusive chemical classification of these fungi.

Although the bioactivity of cyclopaldic acid and altiloxins (the main components of the drimane sesquiterpenoid—phthalide hybrids) identified in the present study as potential biomarkers for species belonging to the Diaporthe eres complex is known, as described above, the genes involved in their biosynthesis have not yet been defined. In general, the eukaryotic genes involved in a single metabolic pathway are scattered throughout the genome, whereas the genes required for a fungus to produce a given secondary metabolite are very frequently clustered, adjacent to one another on the chromosome [88]. Such clusters are found in the majority of filamentous fungi and may range from only a few to more than 20 genes [89]. Thus, identifying a biosynthetic gene cluster for the main compounds reported as biomarkers for species from Diaporthe eres complex, could be the next step to supplement the current research by the results relied on the genetic methods used on a larger Diaporthe population.

Over the last decade, multi-locus DNA sequence data and morphological characterization have been extensively used to identify Diaporthe on a species level [3,7,10,20,21,90,91,92]. The gene regions most commonly used for this purpose in Diaporthe are the internal transcribed spacer (ITS), together with translation elongation factor-1α (EF-1α), β-tubulin, partial histone H3 (HIS), and calmodulin (CAL) [3,6,20,21,93,94]. However, they are still limited to those species for which the comparative sequence data have been deposited in the public database. Nevertheless, a multi-locus sequencing should always be used for identification of Diaporthe species [6]. In agreement with the study of Abreu et al. [79] and Horn et al. [77], the metabolite profiling may support phenotypic species recognition in Diaporthe. Thus, when studying closely related species in the Diaporthe eres complex, a holistic approach combining morphological characterization, metabolic profile and multi-locus sequencing for species identification is certainly worth considering [79].

Characterizing metabolites biosynthesized by Diaporthe infecting shoots of fruit trees is vital for the phytotoxic properties and chemotaxonomy. It is also essential to better understand the conditions under which the fungi start producing the toxins and switch their lifestyle from endophytic to pathogenic.

Finally, it is hoped that the results from our initial research will enrich the biodiversity of the chemical compounds of species from Diaporthe eres complex and provide a series of new information for this genus.

Acknowledgments

The Diaporthe isolates used in the present work came from the Fungal Collection of Phytopathology and Mycology Subdepartment, University of Life Sciences in Lublin (Poland).

Author Contributions

Conceptualization, B.A. and Ł.P.; methodology, B.A., Ł.P. and M.K.; validation, B.A. and Ł.P.; formal analysis, B.A., Ł.P., S.K. and A.M.-G.; investigation, B.A., Ł.P. and S.K.; resources, B.A. and Ł.P.; data curation, B.A. and Ł.P.; writing—original draft preparation, B.A. and Ł.P.; writing—review and editing, B.A., Ł.P., S.K. and M.K.; visualization, B.A. and Ł.P.; supervision, E.K., A.G. and W.O.; project administration, B.A.; funding acquisition, B.A.; All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.7371706 (accessed on 28 November 2022).

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Not applicable.

Funding Statement

This research was funded by the National Science Centre (NCN), Poland, the grant number 2016/21/N/NZ9/01526.

Footnotes

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data presented in this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.7371706 (accessed on 28 November 2022).


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