Semester of Graduation

Fall 2021

Degree

Master of Science (MS)

Department

Department of Plant Pathology and Crop Physiology

Document Type

Thesis

Abstract

Taproot decline (TRD) of soybean is caused by Xylaria necrophora. However, the mechanisms by which it infects soybean and produces symptoms are unknown. Symptoms of TRD include foliar interveinal chlorosis and necrosis and root necrosis, yet X. necrophora appears to be restricted to the roots. While Xylaria species are predominantly wood decomposers, a few are pathogens. More importantly, they produce secondary metabolites (SMs) with multiple bioactivities, including phytotoxicity and antimicrobial activity. Preliminary experiments suggested that X. necrophora was producing SMs responsible for TRD symptoms. We evaluated cell-free culture filtrates (CFs) for phytotoxicity on soybeans, testing for changes in chlorophyll content, by infiltrating them into soybean leaves, exposing leaf disks, and by direct exposure to stem cuttings. Results support the hypothesis that TRD symptoms are caused by SMs produced by X. necrophora. CFs were then extracted using hexane, dichloromethane, diethyl ether, and ethylacetate. The fraction extracted with diethyl ether showed the highest phytotoxic activity followed by dichloromethane. Phytotoxicity, however, may not be the only role for these compounds in the etiology of TRD. The high recovery rate of X. necrophora from soybean roots (~80%), suggests that SMs play a role in competition over other soil-inhabiting fungi. Thus, we evaluated the CFs and extracts for antimicrobial activity against 15 fungal pathogens by conducting dual direct interaction assay and an antimicrobial assay using 10% and 50% (v/v) CFs in PDA. Results showed high rates of antimicrobial activity from both assays against the 15 pathogens tested. After further evaluation of bioactive extracts using LC-MS and LC-MS/MS analyses, ten SMs were identified: 1. 18-Deoxy-19,20-epoxycytochalasin Q, 2. 21-O-Deacetylcytochalasin 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 were identified from the antimicrobial fraction and 1, 2, 5, 6, 8, 9, and 10 from the phytotoxic fraction. SMs 1, 2, 5, and 6, were identified from both extracted fractions. SMs capable of causing phytotoxicity and inhibiting a diversity of fungal pathogens suggests an important role for these SMs in the etiology of TRD.

Committee Chair

Doyle, Vinson P.

Available for download on Thursday, August 11, 2022

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