The Blumeria graminis Effector BgtH12 Suppresses Wheat Immunity and Acts as a Virulence Factor.

Abstract

The obligate biotrophic fungus Blumeria graminis f. sp. tritici (Bgt), the causal agent of wheat powdery mildew, secretes effector proteins to suppress plant immunity and establish infection. Here, we characterize BgtH12, a conserved small secreted protein that is highly expressed during early infection (peaking at 48 hpi). Functional analysis using host induced gene silencing (HIGS) with two independent fragments achieved 54-82% knockdown of BgtH12 transcripts, resulting in significantly reduced fungal sporulation, hyphal length, and branching. Stable transgenic RNAi lines confirmed these findings, with fungal biomass reduced to 0.6-0.7 fold of the wild type control. Conversely, stable overexpression of BgtH12 in wheat enhanced susceptibility, increasing fungal biomass 4 to 7 fold and accelerating hyphal growth. Histological examination revealed that BgtH12 promotes infection by suppressing early host defense responses: overexpression lines exhibited reduced reactive oxygen species (ROS) accumulation at 24 and 48 hpi, while RNAi lines showed enhanced ROS production. In Nicotiana benthamiana, BgtH12 suppressed BAX triggered cell death, and delivery into wheat cells via the type III secretion system (T3SS) attenuated callose deposition and the ROS burst, indicating broad spectrum suppression of pattern triggered immunity. Subcellular localization assays in wheat protoplasts and N. benthamiana leaves showed that BgtH12 associates with the cell periphery. Together, these results establish BgtH12 as a critical virulence factor that suppresses basal immunity to promote Bgt colonization.