Antimicrobial and phytotoxic secondary metabolites produced by Xylaria necrophora, an emerging pathogen of soybean, play key roles in infection biology

Abstract

Xylaria species are recognized globally given their common occurrence as wood-degrading saprophytes in forest ecosystems. They are known for their ability to produce secondary metabolites with diverse bioactivity. A few are pathogens, but Xylaria necrophora is the only species known to be a pathogen of an annual crop, causing taproot decline (TRD) on soybean [Glycine max (L) Merr.]. Recent work determined that culture filtrates produced by X. necrophora are phytotoxic and likely responsible for the foliar symptoms of the disease. We demonstrate that the foliar symptoms may be the result of root inhibition as culture filtrates also stop root development. Xylaria necrophora also produces antimicrobial secondary metabolites (SMs) that likely mediate interactions with other soil microbes to set the stage for plant infection. Bioassay-guided fractionation and extracted fractions from cell-free culture filtrates (CFs) led to the identification of SMs using LC-MS and LC-MS/MS analyses: 1. 18-Deoxy-19,20-epoxycytochalasin Q, 2. 19,20-epoxycytochalasin Q, 3. 5-(1-Hydroxybutyl)-6-(hydroxymethyl)-2H-pyran-2-one, 4. 6-[(1R)-1-Hydroxypentyl]-4-methoxy-2H-pyran-2-one, 5-6. Cytochalasin C and D, 7. Xylopimarane, 8. Hirsutatin A, 9. Xylaric acid C and 10. Zygosporin E. SMs 1-7 presented antimicrobial activity against fungi and 1, 2, 5, 6, 8, 9, and 10 were phytotoxic to soybean. SMs 1, 2, 5, and 6, were both phytotoxic and antimicrobial. This is the first report identifying SMs produced by X. necrophora. SMs capable of both causing phytotoxicity and inhibiting a diversity of fungal pathogens suggests an important role for these SMs in the etiology of TRD.