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Hypoxia tolerance is a virulence component in the colonization of maize seeds by Aspergillus flavus

Subbaiah Chalivendra: Louisiana State University

<div><em>Aspergillus flavus</em> is an opportunistic pathogen that contaminates corn and key oilseed crops with carcinogenic aflatoxins (AFs). As an obligate aerobe, <em>A. flavus</em> is severely limited in its growth, sporulation and AF synthesis under hypoxia and becomes dormant under anoxia when grown <em>in vitro</em>. Yet, the fungus successfully invades the severely hypoxic corn seeds (3% O<sub>2</sub> in the embryo and ~0% in the endosperm) and produces abundant AF, implicating hypoxia tolerance as a key virulence factor. Understanding the basis of this adaptability is important to identify new targets for limiting AF-contamination of the food chain. My presentation will focus on the role of MedA, a transcription factor, in hypoxic tolerance. A <em>medA</em> deletion mutant in <em>A. flavus</em> showed fewer conidia as expected from MedA’s role in conidiogenesis. The mutant also made less AF than the wild type but showed superior growth <em>in vitro</em> and on intact corn seeds. The greater vegetative growth of the mutant was correlated with impaired biofilm formation and in turn, enhanced fungicide sensitivity under hypoxia. This suggests a role for MedA in biofilm formation, a hypoxic structure that protects the fungus from adverse conditions including antifungals and anoxic injury, and is consistent with a report that <em>A. flavus</em> forms biofilm at the surface of anoxic seed tissues only in a resistant hybrid. Based on these new insights, we are exploring potent host-induced gene silencing strategies for AF-mitigation in corn.</div>