Unraveling the Ustilago maydis-maize interaction : insights into nutrient acquisition and effector function

Ustilago maydis is a basidiomycete fungal phytopathogen that causes the corn smut disease in maize. Corn smut poses a significant threat to maize production worldwide highlighting the need for additional solutions to manage the disease. To develop novel disease management strategies, it is essential to understand the pathogen’s virulence mechanisms. We undertook the characterization of two aspects of U.maydis virulence including nutrient uptake and effector function. We generated conditional heme auxotrophic strains and found that the heme auxotroph was unable to acquire heme from its maize host during infection. This discovery was further exploited to repurpose as a fungicide, an existing commercialized herbicide that targets a key enzyme in the heme biosynthesis pathway. We found that at diluted concentrations, this herbicide was able to significantly reduce the virulence of U.maydis in maize seedlings. In addition, we looked at the transcriptomic response of the heme auxotrophic strains to exogenous heme. We found that heme treatment led to widespread upregulation of ergosterol biosynthetic genes in heme-starved cells. The transcriptomic analysis also hinted to the characterization of SREBP (Sre1) and SCAP (Scp1) homologs in U.maydis. We created single knockouts in both genes and characterized their relevance in growth on media containing CoCl₂ or azole where both Δsre1 and Δscp1 showed reduced growth. Finally, we characterized the function of a putative chloroplast-localized effector, Pce3 to the virulence of the U.maydis. Since the deletion of Pce3 in the fungus resulted in only subtle virulence defect in maize, we heterologously overexpressed Pce3 in A.thaliana to further characterize its function. We found that lines overexpressing Pce3 displayed a curly leaf phenotype. An immunoprecipitation experiment using Pce3 as a bait revealed that Pce3 might interact with the DEAD-box RNA helicase, RH3. Surprisingly, the Pce3 overexpression lines displayed some of the phenotypes associated with RH3 dysregulation. Taken together, this study focussing on nutrient acquisition and putative chloroplast effector characterization revealed novel strategies that could be exploited in the future to design novel field fungicide. The exacerbating impact of global warming is intensifying the spread and severity of plant diseases, underscoring the need for effective and sustainable management solutions.