Abstract
Pierce's disease (PD) of grapevines is caused by Xylella fastidiosa (Xf), a xylem-limited gamma-proteobacterium that is responsible for several economically important crop diseases. The occlusion of xylem elements and interference with water transport by Xf and its associated biofilm have been posited as the main cause of PD symptom development; however, Xf virulence mechanisms have not been described. Analysis of the Xf secretome revealed a putative lipase/esterase (LesA) that was abundantly secreted in bacterial culture supernatant and was characterized as a protein ortholog of the cell wall-degrading enzyme LipA of Xanthomonas strains. LesA was secreted by Xf and associated with a biofilm filamentous network. Additional proteomic analysis revealed its abundant presence in outer membrane vesicles (OMVs). Accumulation of LesA in leaf regions associated positively with PD symptoms and inversely with bacterial titer. The lipase/esterase also elicited a hypersensitive response in grapevine. Xf lesA mutants were significantly deficient for virulence when mechanically inoculated into grapevines. We propose that Xf pathogenesis is caused by LesA secretion mediated by OMV cargos and that its release and accumulation in leaf margins leads to early stages of observed PD symptoms.