Dual role of the Ustilago maydis GATA transcription factor Nit2 in the control ofpathogenic development and nitrogen utilization during biotrophy

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Nitrogen represents the most limiting nutrient in agriculture and it has been shown that biotrophic eukaryotic plant pathogens are specifically adapted to the provision of selected organic nitrogen sources by the host. Successful biotrophic plant pathogens need to establish the provision of assimilates by the colonized host tissue upon penetration, which represent their sole sources of organic carbon and nitrogen. The flexible and efficient utilization of organic carbon and nitrogen sources is the key adaptation of heterotrophic microorganisms to changing environments, which is governed by carbon and nitrogen catabolite repression, CCR and NCR, respectively. Although sensors and regulators of CCR and NCR tend to be conserved in eukaryotic systems, it is only poorly understood how eukaryotic human or plant pathogens adapt to fluctuating availability of nutrients during the progress of infection. Loss of central transcriptional regulators of NCR results in reduced pathogenicity in several (hemi)biotrophic fungal phytopathogens, including the dimorphic basidiomycete Ustilago maydis, the corn smut fungus. U. maydis strains lacking the GATA transcription factor Nit2, a major positive regulator of NCR, exhibit impaired utilization of non-favoured nitrogen sources in saprophytic sporidia as well as hampered activation of filamentous phytopathogenic growth upon host contact, indicating distinct roles for Nit2 at different stages of the U. maydis life cycle. Transcriptome analyses suggest an involvement of Nit2 in the transcriptional network governing the dimorphic switch from saprophytic to pathogenic growth in U. maydis, as well as a role of Nit2 in the control of nitrogen metabolism during biotrophy. The proposed project is set out to unravel the role of Nit2 in the regulation of U. maydis growth and development at different stages of maize infection. First, we aim at resolving the molecular mechanism by which Nit2 integrates into the transcriptional control of early phytopathogenic development by targeted and untargeted approaches. Available data suggests that Nit2 is not just involved in the signalling network that triggers the onset of phytopathogenic growth, but also in the regulation of nitrogen metabolism during biotrophy. In order to better understand the role of Nit2 in host colonization, we will identify Nit2 target genes at different stages of phytopathogenic development in planta. Identification of in planta target genes that respond to the availability of organic nitrogen opens up the potential to investigate (i) the nutritional status of the pathogen in spatiotemporal resolution, (ii) the response of the pathogen to nutrient availability in planta, and (iii) the significance of this response for virulence.