Identification of effector candidate genes of Rhizoctonia solani AG-1 IA expressed during infection in Brachypodium distachyon

Rhizoctonia solani is a necrotrophic phytopathogen belonging to basidiomycetes. It causes rice sheath blight which inflicts serious damage in rice production. The infection strategy of this pathogen remains unclear. We previously demonstrated that salicylic acid-induced immunity could block R. solani AG-1 IA infection in both rice and Brachypodium distachyon. R. solani may undergo biotrophic process using effector proteins to suppress host immunity before necrotrophic stage. To identify pathogen genes expressed at the early infection process, here we developed an inoculation method using B. distachyon which enables to sample an increased amount of semi-synchronous infection hyphae. Sixty-one R. solani secretory effector-like protein genes (RsSEPGs) were identified using in silico approach with the publicly available gene annotation of R. solani AG-1 IA genome and our RNA-sequencing results obtained from hyphae grown on agar medium. Expression of RsSEPGs was analyzed at 6, 10, 16, 24, and 32 h after inoculation by a quantitative reverse transcription-polymerase chain reaction and 52 genes could be detected at least on a single time point tested. Their expressions showed phase-specific patterns which were classified into 6 clusters. The 23 RsSEPGs in the cluster 1–3 and 29 RsSEPGs in the cluster 4–6 are expected to be involved in biotrophic and necrotrophic interactions, respectively.