Investigating the biology of plant tissue invasion and cell-to-cell movement by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast, one of the most serious diseases affecting rice production. The fungus is also the causal agent of wheat blast, a disease that now threatens wheat production in South America and South Asia. During plant infection, M. oryzae forms a specialised infection structure called an appressorium. The infection cell generates enormous turgor, focused as mechanical force to breach the rice cuticle. Re-polarisation of the appressorium requires a hetero-oligomeric septin complex that organises a toroidal F-actin network at the base of the appressorium. This allows the fungus to invade epidermal cells and develop biotrophic invasive hyphae. Septin-mediated plant infection is controlled by NADPH oxidase activity and a regulated burst of reactive oxygen species occurs within the appressorium. The process is regulated by a turgor sensing protein kinase, which can sense when optimal appressorium turgor is achieved and the switch to polarised growth is triggered. A pressure-mediated cell cycle checkpoint is also necessary for initiation of septin activation and the re-orientation of the cortical F-actin cytoskeleton. Once tissue is invaded the fungus undergoes differential expression and secretion of a large repertoire of effector proteins that are delivered to the apoplastic space which surrounds invasive hyphae, or directed into plant cells. The fungus also undergoes distinct physiological changes, including activation of enzymes associated with utilisation of a broad spectrum of carbon sources, as we as distinct secondary metabolic pathwys. M. oryzae suppresses plasmodesmatal immunity in order to facilitate its spread from cell-to-cell in plant tissue. This is controlled by a specific MAP kinase signalling pathway and requires septin-dependent hyphal constriction to enable the fungus to spread rapidly in rice tissue.