Barley endosomal MONENSIN SENSITIVITY1 is a target of the powdery mildew effector CSEP0162 and plays a role in plant immunity

Encasements formed around haustoria and biotrophic hyphae as well as
hypersensitive reaction (HR) cell death are essential plant immune
responses to filamentous pathogens. In this study we examine the
components that may contribute to the absence of these responses in
susceptible barley attacked by the powdery mildew fungus. We find that the
effector CSEP0162 from this pathogen targets plant MONENSIN SENSITIVITY1
(MON1), which is important for the fusion of multivesicular bodies to
their target membranes. Overexpression of CSEP0162 and silencing of barley
MON1 both inhibit encasement formation. We find that the Arabidopsis
ecotype No-0 has resistance to powdery mildew, and that this is partially
dependent on MON1. Surprisingly, we find the MON1-dependent resistance in
No-0 not only includes an encasement response, but also an effective HR.
Similarly, silencing of MON1 in barley also blocks Mla3-mediated HR-based
powdery mildew resistance. Our results indicate that MON1 is a vital plant
immunity component, and we speculate that the barley powdery mildew fungus
introduces the effector CSEP0162 to target MON1 and hence reduces
encasement formation and HR.