Molecular and cellular analyses of pathogenicity and host specificity in rice blast disease

Abstract

Rice (Oryza sativa L.) production worldwide is constrained by rice blast disease caused by the ascomycetous fungus Magnaporthe oryzae. Rice blast has become a model system for the study of fungal plant diseases based on its global relevance to agriculture and on our ability to apply molecular genetic and genomic analyses to both the pathogen and the plant. We have applied molecular and cellular analyses to understand critical processes in the M. oryzae disease cycle. The dark melanin pigment produced by the fungus is critical for the function of its specialized appressorial cell, which punches the leaf surface by generating the highest pressure known in any biological system, estimated at 80 times the atmospheric pressure. Without melanin, the fungus can neither generate this pressure nor puncture the plant surface and disease does not occur. M. oryzae genome sequencing identified a cluster of melanin biosynthesis genes that included an attractive candidate for the transcription factor that regulates melanin biosynthesis in appressoria. We report the structural and functional characterization of this putative transcription factor, although its role remains elusive. Host cellular responses after appressorial penetration are equally important in determining if disease will occur. We have characterized the cellular response of one rice variety to a compatible fungal strain (causes disease), an incompatible strain (fails to cause disease due to specific triggering of rice defenses) and a non-host strain (causes disease in barley but not in rice). Distinctive fungal and rice cellular responses correlated with the outcome of each particular pathogen-strain rice interaction. We report contrasting responses in two rice leaf sheath assays that are amenable to live cell microscopy, as well as a novel cellular response of crystalline aggregations deposited inside the host cell under appressoria on the leaf surface. Our studies have important implications for future analyses of pathogenicity and host specificity in rice blast disease.