The Ustilago maydis transcription factor Nit2 controls nitrate assimilation during biotrophy and adjusts organic nitrogen metabolism in maize leaves under N limitation

Little is known on how filamentous phytopathogens adapt nitrogen acquisition upon nitrogen limitation in the host plant. In previous work, we have shown that the transcription factor Nit2 plays a major role for the utilization of non-favored nitrogen sources like nitrate, minor amino acids or nucleobases in saprotrophic sporidia of the basidiomycete corn smut fungus Ustilago maydis .

Here, we employed Δnit2 mutants in the natural FB1 x FB2 background to identify Nit2 target genes during biotrophy and investigated the impact of Nit2 on the physiology of leaf galls in nitrogen replete versus nitrogen limited host plants. Comparative RNA-Seq analysis of galls caused by the U. maydis Δnit2 mutant and by the wild type, revealed that about one third of Nit2 targets during biotrophy are involved in nitrogen metabolism and transport. Induction of the nitrate assimilation cluster was completely dependent on Nit2, which lead to substantial accumulation of nitrate and reduced accumulation of the nitrogen-rich phloem transport amino acids asparagine and glutamine in Δnit2 leaf galls compared to wild type galls under nitrogen limitation, while diminished asparagine contents were the only physiological difference in Δnit2 leaf galls in nitrogen replete control plants. Since total protein content in galls and pathogenicity were comparable between fungal genotypes in both nitrogen regimes, the findings of our physiological and transcriptomic analysis demonstrate that nitrate utilization is dispensable for Ustilago maydis during biotrophy and can likely be actively compensated by increased utilization of abundant organic nitrogen sources, like asparagine, GABA and glutamine in a partially Nit2-dependent fashion.