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

氮是农业中最具限制性的营养物质，研究表明，生物营养性真核植物病原体特别适应宿主提供的选定有机氮源。成功的生物营养化植物病原体需要在侵入时由定植的寄主组织提供同化物，这是它们有机碳和氮的唯一来源。有机碳源和氮源的灵活高效利用是异养微生物适应环境变化的关键，这一过程分别受碳和氮分解代谢抑制、CCR和NCR控制。尽管CCR和NCR的传感器和调节因子在真核系统中趋于保守，但人们对真核人类或植物病原体在感染过程中如何适应营养物质可用性的波动知之甚少。NCR中心转录调控因子的缺失导致几种（半）生物营养真菌植物病原体的致病性降低，包括二态担子菌黑穗病菌和玉米黑穗病菌。缺乏GATA转录因子Nit2 （NCR的主要正调节因子）的菌株在腐生孢子虫中表现出对非有利氮源的利用受损，并且在与宿主接触时阻碍了丝状植物病原生长的激活，这表明Nit2在U. maydis生命周期的不同阶段具有不同的作用。转录组学分析表明，Nit2参与了调控褐藻从腐生到致病生长二态转换的转录网络，以及Nit2在生物营养过程中控制氮代谢的作用。该项目旨在揭示Nit2在玉米感染不同阶段对美国玉米线虫生长发育的调控作用。首先，我们旨在通过靶向和非靶向方法解决Nit2整合到早期植物病原发育转录控制中的分子机制。现有数据表明，Nit2不仅参与触发植物致病性生长的信号网络，还参与生物营养过程中氮代谢的调节。为了更好地了解Nit2在寄主定殖中的作用，我们将在植物的植物致病发育的不同阶段鉴定Nit2靶基因。在植物中识别对有机氮可用性作出反应的靶基因，开辟了研究以下问题的潜力：(1)在时空分辨率上病原体的营养状况，(2)病原体对植物营养可用性的反应，以及(3)这种反应对毒力的重要性。

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.