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 alpha-1,3-glucan during invasion. Here, we show that the surface alpha-1,3-glucan is indispensable for the successful infection of the fungus by interfering with the plant's defense mechanisms. The alpha-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 alpha-1,3-glucan increased fungal susceptibility towards chitinase, suggesting the protective role of alpha-1,3-glucan against plants' antifungal enzymes during infection. Furthermore, rice plants secreting bacterial alpha-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 alpha-1,3-glucan also concealed cell wall chitin in an infection-specific manner. Treatment with alpha-1,3-glucanase in vitro caused fragmentation of infectious hyphae in R. solani but not in M. oryzae or C. miyabeanus, indicating that alpha-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 alpha-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 alpha-1,3-glucan is nondegradable in plants, it is reasonable that many fungal plant pathogens utilize alpha-1,3-glucan in the innate immune evasion mechanism and some in maintaining the structures.