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Poster: Molecular & Cellular Plant-Microbe Interactions: MPMI


The Type II-dependent secretion of virulence factors is necessary for systemic colonization by the xylem-limited pathogen, Xylella fastidiosa
B. INGEL (1), P. Wang (2) (1) University of California Riverside, U.S.A.; (2) University of California Riverside, U.S.A.

Xylella fastidiosa (Xf) is a xylem-limited, gram-negative bacterium that causes Pierce’s disease (PD) in grapevines. PD symptom progression is highly correlated with systemic colonization of the xylem, which is achieved by bacterial movement via the degradation of pit membranes separating adjacent xylem vessels. Pit membranes are readily dismantled by Xf cell wall-degrading enzymes (CWDEs), three of which have been implicated as virulence factors: a polygalacturonase (PG), and two endoglucanases (EngXCA1 and EngXCA2). As pathogenicity was nullified in a PG-deficient mutant, the PG has been a focal point for Xf inhibition studies. However, PG expression has proved to be difficult, and we have sought alternative CWDE inhibition targets. A knockout mutation of the Type II secretion system (T2SS) ATPase gene (ΔxpsE) resulted in a complete loss of pathogenicity similar to that displayed by the PG-deficient mutant. As Xf lacks a Type III secretion system, it is predicted that virulence factors such as CWDEs are secreted via the T2SS. Secretome analyses of both the Xf wild-type and ΔxpsE mutant strains had distinct differences in protein composition, and an antibody targeting EngXCA2 revealed that secretion of this endoglucanase is Type II-dependent. These results provide clear evidence that Xf relies on the T2SS for secretion of the CWDEs necessary for systemic colonization, and inhibition of this secretion system will significantly reduce Xf pathogenicity in grapevines.