To cause rice blast disease the filamentous fungus Magnaporthe oryzae secretes a battery of effector proteins into host plant tissue to facilitate infection. Effector-encoding genes are expressed only during plant infection and show very low expression during other developmental stages. How effector gene expression is regulated in such a precise manner during invasive growth by M. oryzae is not known. Here, we report a forward-genetic screen to identify regulators of effector gene expression, based on the selection of mutants that show constitutive effector gene expression. Using this simple screen, we identify Rgs1, a regulator of G-protein signalling (RGS) protein that is necessary for appressorium development, as a novel transcriptional regulator of effector gene expression, which acts prior to plant infection. We show that an N-terminal domain of Rgs1, possessing transactivation activity, is required for effector gene regulation and acts in an RGS-independent manner. Rgs1 controls expression of at least 60 temporally co-regulated effector genes, preventing their transcription during the pre-penetration stage of development prior to plant infection. A regulator of appressorium morphogenesis is therefore also required for orchestration of pathogen gene expression required for invasive growth by M. oryzae during plant infection.
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