Surface α-1,3-Glucan Facilitates Fungal Stealth Infection by Interfering with Innate Immunity in Plants

Plants evoke innate immunity against microbial challenges upon recognition of pathogen-associated molecular patterns (pamps),such as fungal cell wall chitin. nevertheless,pathogens may circumvent the host pamp-triggered immunity. we previously reported that the ascomycete magnaporthe oryzae,a famine-causing rice pathogen,masks cell wall surfaces with α-1,3-glucan during invasion. here,we show that the surface α-1,3-glucan is indispensable for the successful infection of the fungus by interfering with the plant's defense mechanisms. the α-1,3-glucan synthase gene mgags1 was not essential for infectious structure development but was required for infection in m. oryzae. lack or degradation of surface α-1,3-glucan increased fungal susceptibility towards chitinase,suggesting the protective role of α-1,3-glucan against plants' antifungal enzymes during infection. furthermore,rice plants secreting bacterial α-1,3-glucanase (agl-rice) showed strong resistance not only to m. oryzae but also to the phylogenetically distant ascomycete cochlioborus miyabeanus and the polyphagous basidiomycete rhizoctonia solani; the histocytochemical analysis of the latter two revealed that α-1,3-glucan also concealed cell wall chitin in an infection-specific manner. treatment with α-1,3-glucanase in vitro caused fragmentation of infectious hyphae in r. solani but not in m. oryzae or c. miyabeanus,indicating that α-1,3-glucan is also involved in maintaining infectious structures in some fungi. importantly,rapid defense responses were evoked (a few hours after inoculation) in the agl-rice inoculated with m. oryzae,c. miyabeanus and r. solani as well as in non-transgenic rice inoculated with the ags1 mutant. taken together,our results suggest that α-1,3-glucan protected the fungal cell wall from degradative enzymes secreted by plants even from the pre-penetration stage and interfered with the release of pamps to delay innate immune defense responses. because α-1,3-glucan is nondegradable in plants,it is reasonable that many fungal plant pathogens utilize α-1,3-glucan in the innate immune evasion mechanism and some in maintaining the structures. © 2012 fujikawa et al.