Barley powdery mildew invasion coincides with the dynamic accumulation of leaf apoplastic extracellular vesicles that are associated with host stress response proteins

Abstract

The mutual exchange of extracellular vesicles across kingdom borders is a feature of many plant-microbe interactions. The occurrence and cargos of extracellular vesicles has been studied in several instances, but their dynamics in the course of infection have remained elusive. Here we used two different procedures, differential high-speed centrifugation and polymer-based enrichment, to collect extracellular vesicles from the apoplastic wash fluid of barley (Hordeum vulgare) leaves challenged by its fungal powdery mildew pathogen, Blumeria hordei. Both methods yielded extracellular vesicles of similar quality and morphological characteristics, though the polymer approach was associated with higher reproducibility. We noted that extracellular vesicles derived from the apoplastic wash fluid constitute polydisperse populations that are selectively responsive to leaf infection by B. hordei. Extracellular vesicles of [~]100 nm - 300 nm diameter became progressively more abundant, in particular from 72 hours post inoculation onwards, resulting in a major peak late during fungal infection. Vesicles of [~]300 nm - 500 nm showed similar accumulation dynamics but reached much lower levels, suggesting they might constitute a separate population. Proteome analysis uncovered an enrichment of biotic stress response proteins associated with the extracellular vesicles. The barley t-SNARE protein Ror2, the ortholog of the PEN1 marker protein of extracellular vesicles in Arabidopsis thaliana, accumulates in extracellular vesicles during powdery mildew infection, hence also qualifying as a potential marker protein. Our study serves as a starting point for investigating the role of extracellular vesicles at different stages of plant-microbe interactions.