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Natural products and molecular genetics underlying the antifungal activity of endophytic microbes

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Title: Natural products and molecular genetics underlying the antifungal activity of endophytic microbes
Author: Mousa, Walaa
Department: Department of Plant Agriculture
Program: Plant Agriculture
Advisor: Raizada, Manish
Abstract: Microbes are robust and promiscuous machines for the biosynthesis of antimicrobial compounds which combat serious crop and human pathogens. A special subset of microbes that inhabit internal plant tissues without causing disease are referred to as endophytes. Endophytes can protect their hosts against pathogens. I hypothesized that plants which grow without synthetic pesticides, including the wild and ancient relatives of modern crops, and the marginalized crops grown by subsistence farmers, host endophytes that have co-evolved to combat host-specific pathogens. To test this hypothesis, I explored endophytes within the ancient Afro-Indian crop finger millet, and diverse maize/teosinte genotypes from the Americas, for anti-fungal activity against Fusarium graminearum. F. graminearum leads to devastating diseases in cereals including maize and wheat and is associated with accumulation of mycotoxins including deoxynivalenol (DON). I have identified fungal and bacterial endophytes, their secreted natural products and/or genes with anti-Fusarium activity from both maize and finger millet. I have shown that some of these endophytes can efficiently suppress F. graminearum in planta and dramatically reduce DON during seed storage when introduced into modern maize and wheat. The most exciting discovery of my research is that an endophytic bacterium (strain M6, Enterobacter sp.), isolated from the roots of finger millet, builds a remarkable physical barrier consisting of bacterial micro-colonies that protect the host against pathogen invasion. M6 creates an unusual root hair-endophyte stacking (RHESt) formation that prevents entry and/or traps the pathogen which is then killed. Tn5 mutant analysis demonstrated that the endophyte kills the fungal pathogen by using a c-di-GMP-dependent signaling network and diverse fungicides including phenazine. The endophyte has evolved an epistatic regulatory interaction to suppress an antibiotic released by Fusarium which would otherwise inhibit phenazine release into the RHESt. The end-result of this remarkable physico-chemical barrier is a reduction in levels of the mycotoxin DON, thus potentially protecting millions of subsistence farmers and their livestock. To the best of my knowledge, RHESt represents a novel plant defence mechanism and suggests the value of exploring the microbiomes of the world's ancient, orphan crops as source of endophytes with antimicrobial activity.
Date: 2016
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