Plant immunity is complex, and studies of leaf epidermal cells attacked by powdery mildew fungi have been instrumental in revealing how it relies on plant endomembrane trafficking. Immunity against these biotrophic fungi is manifested as cell wall deposits ('papillae') and the hypersensitive reaction (HR), both involving plant endomembrane traffic. Papillae contain extracellular vesicles (EVs), some of which depend on the 'endosomal sorting complexes required for transport' machinery, while others depend on mechanisms yet to be uncovered. Existence of two such EV pathways agrees with several old electron microscopy observations of papillae. Interestingly, resistance protein-activated HR has been shown to be a form of immunity also depending on membrane trafficking, namely the pathway to the vacuole.
Thordal-Christensen et al. (Fri,) studied this question.