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Grx4 : a key regulator of secondary metabolism, nitrogen uptake, and iron homeostasis in the corn smut fungus, Ustilago maydis McCotter, Sean Wesley


The corn smut fungus, Ustilago maydis, is the premier basidiomycete model for the study of biotrophic plant-pathogen interactions. In fungi, monothiol glutaredoxins are central regulators of key cellular functions such as iron homeostasis, cell wall integrity and redox status via their interactions with other proteins as well as iron-sulfur clusters and glutathione. In this study I characterized the novel roles of the monothiol glutaredoxin Grx4 in the biology of U. maydis. In addition to its roles identified in other fungi, Grx4 is necessary for normal pathogenesis by U. maydis on its plant host, Zea mays. Mutants expressing a conditional allele of grx4 under the control of the arabinose-induced/glucose-repressed promoter Pcrg, exhibited decreased virulence on maize which correlated with grx4 transcript levels at the time of infection. Additionally, perturbations were detected in homeostasis and perception of the essential nutrients iron and nitrogen following grx4 repression in glucose-containing media. Furthermore, grx4 repression strongly altered secondary metabolism, leading to induction of melanin and itaconic acid biosynthetic genes and accumulation of these metabolites in vitro. A subset of Grx4-regulated multicopper oxidase genes with possible roles in secondary metabolism, (particularly genes potentially involved in melanin biosynthesis) were targeted for further reverse genetic study. This aspect of the work identified the key laccase enzyme required for melanization and laccase activity by U. maydis in a variety of conditions, Lac2. It also identified hitherto unknown roles in virulence for this gene family. Together, these data suggest that glutaredoxins could play important roles in the virulence of plant pathogenic fungi in addition to their established roles as key regulators of fundamental cellular processes. The involvement of Grx4 in the production by U. maydis of industrially relevant secondary metabolites (e.g., itaconic acid) also renders these findings of biotechnological interest.

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