The transcriptional landscape of plant infection by the rice blast fungus Magnaporthe oryzae reveals distinct families of temporally co-regulated and structurally conserved effectors

Abstract

The rice blast fungus Magnaporthe oryzae causes a devastating disease that threatens global rice (Oryza sativa) production. Despite intense study, the biology of plant tissue invasion during blast disease remains poorly understood. Here we report a high resolution, transcriptional profiling study of the entire plant-associated development of the blast fungus. Our analysis revealed major temporal changes in fungal gene expression during plant infection. Pathogen gene expression could be classified into 10 modules of temporally co-expressed genes, providing evidence for the induction of pronounced shifts in primary and secondary metabolism, cell signalling and transcriptional regulation. A set of 863 genes encoding secreted proteins are differentially expressed at specific stages of infection, and 546 genes named MEP (Magnaporthe effector protein) genes were predicted to encode effectors. Computational prediction of structurally-related MEPs, including the MAX effector family, revealed their temporal co-regulation in the same co-expression modules. We characterised 32 MEP genes and demonstrate that Mep effectors are predominantly targeted to the cytoplasm of rice cells via the biotrophic interfacial complex (BIC) and use a common unconventional secretory pathway. Taken together, our study reveals major changes in gene expression associated with blast disease and identifies a diverse repertoire of effectors critical for successful infection.

Rgs1 is a regulator of effector gene expression during plant infection by the rice blast fungus Magnaporthe oryzae

“…We therefore generated a strain of M. oryzae expressing an effector-encoding gene MEP2 , fused to the green fluorescent protein gene (GFP). MEP2 was identified in a recent study that characterized the effector repertoire of M. oryzae based on their differential expression during pathogenesis ( 25 ). MEP2 shows peak expression 48 h after infection based on RNA-seq analysis, corresponding to a time of rapid plant tissue biotrophic colonization by the fungus ( 25 ).…”

“…MEP2 was identified in a recent study that characterized the effector repertoire of M. oryzae based on their differential expression during pathogenesis ( 25 ). MEP2 shows peak expression 48 h after infection based on RNA-seq analysis, corresponding to a time of rapid plant tissue biotrophic colonization by the fungus ( 25 ). We generated a MEP2:GFP gene fusion, transformed it into a M. oryzae wild-type rice pathogenic strain Guy11 and selected a transformant with a single integration of the reporter gene construct.…”

“…The reported roles of Rgs1 are therefore associated with the prepenetration phase of development, prior to plant tissue invasion ( 16 , 17 , 33 ). To investigate the potential role of Rgs1 as an effector regulator, we therefore investigated the temporal expression of RGS1 in publicly available RNA-seq dataset (PRJEB45007) ( 25 ). This showed that RGS1 is expressed in conidia and during the initial stages of appressorium formation, but then very reduced in expression during plant infection.…”

“…A recent transcriptional profiling study has shown that effector gene expression in M. oryzae occurs only during plant infection ( 25 ). Effector genes are temporally regulated during infection with early-acting effectors expressed as soon as 8 h after conidial germination, and large families of structurally conserved effectors, such as the Max effectors ( 26 ), expressed during biotrophic growth of the pathogen, 24 to 48 h after initial infection ( 25 ).…”

“…A recent transcriptional profiling study has shown that effector gene expression in M. oryzae occurs only during plant infection ( 25 ). Effector genes are temporally regulated during infection with early-acting effectors expressed as soon as 8 h after conidial germination, and large families of structurally conserved effectors, such as the Max effectors ( 26 ), expressed during biotrophic growth of the pathogen, 24 to 48 h after initial infection ( 25 ). The specific temporal and spatial expression patterns suggest that M. oryzae effectors must be under very precise transcriptional regulation, but little is known regarding the transcriptional regulators necessary to achieve this complexity of control.…”