| Summary: | The corn smut fungus, <i>Ustilago maydis</i>, is an excellent model for studying biotrophic plant-pathogen interactions, including nutritional adaptation to the host environment. Iron acquisition during host colonization is a key aspect of microbial pathogenesis yet less is known about this process for fungal pathogens of plants. Monothiol glutaredoxins are central regulators of key cellular functions in fungi, including iron homeostasis, cell wall integrity, and redox status via interactions with transcription factors, iron-sulfur clusters, and glutathione. In this study, the roles of the monothiol glutaredoxin Grx4 in the biology of <i>U. maydis</i> were investigated by constructing strains expressing a conditional allele of <i>grx4</i> under the control of the arabinose-inducible, glucose-repressible promoter <i>P<sub>crg</sub></i><sub>1</sub>. The use of conditional expression was necessary because Grx4 appeared to be essential for <i>U. maydis.</i> Transcriptome and genetic analyses with strains depleted in Grx4 revealed that the protein participates in the regulation of iron acquisition functions and is necessary for the ability of <i>U. maydis</i> to cause disease on maize seedlings. Taken together, this study supports the growing appreciation of monothiol glutaredoxins as key regulators of virulence-related phenotypes in pathogenic fungi.
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