Doctor of Philosophy / Department of Plant Pathology / Richard B. Todd / Fungi can be found living in a range of environments, including soil and the ocean, and as pathogens of plants and animals. The ability of fungi to adapt to diverse and changing environments is dependent on their ability to sense and respond to an array of signals, including the presence or absence of nitrogen nutrients. Fungi can utilize a diverse array of nitrogen nutrients and do so in a regulated and preferential manner. When preferred nitrogen nutrients such as ammonium and glutamine are present (nitrogen sufficiency), genes required for the utilization of alternative nitrogen sources are not expressed. In the absence of a preferred nitrogen source (nitrogen limitation) the genes for utilization of alternative nitrogen sources are transcriptionally derepressed and can be induced by the presence of a particular nitrogen nutrient, such as nitrate or proline. In the absence of any nitrogen nutrient (nitrogen starvation) the expression of some genes is further elevated. In filamentous fungi the expression of genes required for the utilization of nitrogen nutrients is coordinated by the orthologs of the conserved Aspergillus nidulans GATA transcription factor AreA, which activates transcription of nitrogen utilization genes. AreA activity is controlled by autogenous transcriptional activation, mRNA transcript stability, regulated nucleo-cytoplasmic distribution, and interactions with NmrA, AreB and TamA. The combined effect of these regulatory mechanisms generally results in AreA being inactive during nitrogen sufficiency and active during nitrogen limitation and nitrogen starvation. However, during nitrogen sufficiency AreA remains active at the promoters of some genes, including gdhA, which encodes the key nitrogen assimilation enzyme NADP-dependent glutamate dehydrogenase. In this work we have used both classical genetics and next generation sequencing approaches to examine regulated gene expression and how AreA activity is modulated, primarily during nitrogen sufficiency. We have studied regulation of gdhA to characterize how AreA evades nitrogen metabolite repression. We identify leucine biosynthesis as being a key regulatory signal involved in gdhA expression and characterize the genes required for leucine biosynthesis. We also show that TamA regulates the gdhA promoter by direct DNA binding, which requires interaction with AreA. We have also characterized repression of AreA to identify a potential mode of NmrA corepressor action. Finally, we have characterized the AreA nuclear export signal and explored mechanisms that control regulated nuclear export of AreA.

Nous avons étudié la voie MAPK Mps1/Slt2 du champignon pathogène du riz Magnaporthe Oryzae. Chez la levure, Slt2 active les facteurs de transcription Rlm1, Swi4 et Swi6. Nous avons identifié le gène orthologue de ScSWI4 chez M. oryzae et étudié les rôles de Mps1, Rlm1, Swi4 et Swi6 en comparant les phénotypes de leurs mutants nuls. Δmps1, Δswi4, Δswi6 ont le même défaut de mélanisation, tandis que Δmps1 et Δrlm1 sont déficients pour la sporulation. L'absence d'hyphes aériens et d'hydrophobicité du mycélium sont des phénotypes spécifiques de Δmps1, tandis qu'une réduction de croissance est associée spécifiquement à Δswi4 et Δswi6. Aucun de ces mutants ne présente une hypersensibilité aux enzymes de dégradation de la paroi (EDPs) et aux inhibiteurs de la paroi (aculeacine A, nikkomycin Z, calcofluor white : CFW). La pathogénie de Δswi4 est réduite, tandis que celle de Δswi6 est similaire à la souche sauvage. Δmps1 et Δrlm1 sont non pathogènes. La transcriptomique comparative de la souche sauvage, Δmps1, Δrlm1 et Δswi4 montre qu'il n'existe qu'un petit nombre de gènes sur- ou sous- exprimés chez ces mutants, le nombre maximal étant observée entre Δmps1 et Δswi4. Le gène AGS1 encodant l'alpha-1, 3-glucane synthase, une cible supposée de Mps1, n'est ni sur- ni sous-exprimé chez ces mutants. La souche sauvage et Δmps1 ont la même sensibilité aux EDPs et au CFW à pH6. Cependant, à pH5, la souche sauvage devient résistante aux EDPs et au CFW à pH6. Cependant, à pH5, la souche sauvage devient résistante aux EDPs et au CFW, tandis que Δmps1 perd cette résistance, suggérant qu'elle nécessite Mps1. Notre étude montre que la voie MAPK Mps1 joue un rôle important dans plusieurs processus biologiques de M. oryzae et précise quels sont les cibles / We have studied the Mps1/Slt2 MAPK signalling pathway in the rice blast fungus Magnaporthe oryzae. In yeast, Slt2 activates the transcription factors Rlm1, Swi4 and Swi6. We have identified the M. oryzae gene orthologous to ScSWI4 and studied the roles of Mps1, Rlm1, Swi4 and Swi6 in M. oryzae by comparing the phenotypes of their null mutants. Δmps1, Δswi4, Δswi6 displayed the same defects in melanisation, while Δmps1 and Δrlm1 are defective in sporulation. Loss of aerial hyphae formation and mycelium hydrophobicity are phenotypes specific of Δmps1, while reduced growth is a specific phenotype of Δswi4 and Δswi6. None of these mutants displayed an increased sensitivity against cell wall degrading enzymes (CWDEs) and cell wall inhibitors (aculeacine A, nikkomycin Z, calcofluor white : CFW). Pathogenicity was reduced in Δswi4, while Δswi6 was as pathogenic as WT. Δmps1 and Δrlm1 were non-pathogenic. Comparative transcriptomic of WT, Δmps1, Δrlm1 and Δswi4 highlighted only a limited number of genes up- and down-regulated in these mutants, with the largest number observed between Δmps1 and Δswi4. The alpha-1,3-glucan synthase encoding gene AGS1, a suggested Mps1 target, was not up- nor down-regulated in these mutants. WT and Δmps1 have a similar sensitivity to CWDE and CFW at pH6. However, at pH5, WT displays a resistance to CWDEs and CFW, while Δmps1 loses this pH5 induced resistance, suggesting it requires the Mps1 pathway. This work shows that Mps1 MAPK pathway has an important role in different M. oryzae biological processes and provides new insights on its transcriptional targets