Dynamics of cell wall components of Magnaporthe grisea during infectious structure development

Oligosaccharides derived from cell wall of fungal pathogens induce host primary immune responses. To understand fungal strategies circumventing the host plant immune responses, cell wall polysaccharide localization was investigated using fluorescent labels during infectious structure differentiation in the rice blast fungus Magnaporthe grísea, cc-1,3glucan was labelled only on appressoria developing on plastic surfaces, whereas it was detected on both germ tubes and appressoria on plant surfaces. Chitin, chitosan and ß-1,3-glucan were detected on germ tubes and appressoria regardless of the substrate. Major polysaccharides labelled at accessible surface of infectious hyphae were α-1,3-glucan and chitosan, but after enzymatic digestion of cc-1,3-glucan, β-1,3-glucan and chitin became detectable. Immunoelectron microscopic analysis showed a-1,3-glucan and β-1,3-glucan intermixed in the cell wall of infectious hyphae; however, a-1,3-glucan tended to be distributed farther from the fungal cell membrane. The fungal cell wall became more tolerant to chitinase digestion upon accumulation of α-1,3-glucan. Accumulation of α-1,3-glucan was dependent on the Mps1 MAP kinase pathway, which was activated by a plant wax derivative, 1,16-hexadecanediol. Taken together, α-1,3-glucan spatially and functionally masks β-1,3glucan and chitin in the cell wall of infectious hyphae. Thus, a dynamic change of composition of cell wall polysaccharides occurs during plant infection in M. grisea.