Investigating Regulation of effector genes expression in the rice blast fungus Magnaporthe oryzae

Abstract

To cause rice blast disease, the filamentous fungus Magnaporthe oryzae, secretes a battery of effector proteins into host plant tissue to facility infection. Most of the known effectors are only expressed in plant tissue during invasive growth and have very low expression in conidia and mycelium. Effector-encoding genes are also temporally co-expressed during infection. Little is known, however, concerning the transcriptional regulation of effector gene expression in M. oryzae. In this thesis, a genetic screen was used to identify mutants that show constitutive expression of effector genes in conidia and mycelium of M. oryzae. The identification of a series of Constitutive Effector Regulator (CER) mutants that were carried out based on their de-repression of MEP2, BAS1, or AVR-PIK gene expression during vegetative growth of the fungus. By using whole-genome sequencing, genetic complementation, and bulked segregant analysis, evidence suggests that Rgs1 acts as a dominant transcriptional repressor of MEP2 transcription in M. oryzae. RNA-seq analysis showed that loss of RGS1 function leads to de-repression of a group of temporally co-regulated effector genes in conidia and mycelium. The deletion of RGS1 causes reduced virulence, altered pigmentation, and inappropriate appressorium formation on non-inductive surfaces. The N-terminal domain of the Rgs1 protein in M. oryzae showed potential to act as a transcription factor based on a yeast transactivation assay. These results suggest that Rgs1 acts as a transcriptional repressor of late-acting effector genes during the pre-penetration stage of development, prior to plant infection.