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Fungal dimorphism and plant infection during Magnaporthe grisea – rice interaction

Final Report Summary - MAGNAFACT (Fungal dimorphism and plant infection during Magnaporthe grisea - rice interaction)

The rice blast fungus Magnaporthe oryzae is one of the most damaging disease of cultivated rice worldwide and an emerging disease on wheat, impacting on global food security. In order to develop durable and environmentally friendly control methods, it is important to expand our knowledge on the molecular mechanisms underpinning M. oryzae-rice interaction.

RNA-binding proteins play a fundamental role in the control of gene expression at posttranscriptional level and are responsible for regulating essential biological activities. Here, we initiated studies in the post-transcriptional mechanisms that control M. oryzae infectionrelated processes. To this end, we characterised a RNA-binding protein required for full disease symptom production in the rice blast fungus. We found an insertional mutant M35 that showed reduced lesions on leaves and roots. The T-DNA was located in a gene encoding an RRM protein with six RGG tripeptides (RBP35). The RRM domain is widely spread in eukaryotes although only a small fraction has been studied. In humans, it is estimated that about 2% of the total proteome contain at least one RRM (497 gene products out of ~25 000 genes in the human genome). The M. oryzae genome encodes 76 RRM proteins and RBP35 represents the first M. oryzae RRM protein investigated to date. The combination of RRM and RGG modules is found in well characterised RNA-binding proteins with highly diverse functions in human and yeast. Orthologues of RBP35 are found only in filamentous fungi.

Using a combination of cell biology, biochemistry and transcriptomics, we investigated the involvement of RBP35 in M. oryzae full disease symptom production. Notably, we demonstrate that RBP35 interacts in vivo with a highly conserved protein component of the eukaryotic polyadenylation machinery. We show that RBP35 present different diffusional properties in nuclei of distinct fungal structures, and consequently different protein/nucleic acid interactions. Further, we find that RBP35 regulates the length of 3’UTRs of transcripts with developmental and virulence-associated functions. We prove that the ∆rbp35 mutant is affected in the TOR (target of rapamycin) signaling pathway showing significant changes in nitrogen metabolism and protein secretion. We conclude that RBP35 is a novel component of the polyadenylation machinery of M. oryzae required for alternative 3’end processing of transcripts associated with signaling and metabolism. Results indicate that RBP35 acts as a gene-specific polyadenylation factor, ultimately regulating developmental and infectionrelated processes in the rice blast fungus. Nothing it is known about pre-mRNA 3’ end processing in filamentous fungi and our study suggest that their polyadenylation machinery differs from yeast and higher organisms. This study can provide new insights into the evolution of the pre-mRNA maturation and the regulation of gene expression in eukaryotes (PLoS Pathogens 2011, accepted).