Xylella fastidiosa is a xylem-limited, insect transmitted bacterium that causes fatal diseases in economically important crops, most notably, Pierce’s disease of grapevine. Due to its unique dual lifestyle, X. fastidiosa utilizes different mechanisms to facilitate colonization of its plant and insect hosts. While research has focused heavily on the role of surface-associated proteins, we establish the major cell surface carbohydrate, lipopolysaccharide (LPS), as an important bacterial virulence factor and modulator of innate immunity in grapevine.
We focused our attention on the terminal O antigen moiety of the LPS molecule, which serves as a key point of contact between potential hosts and the environment. Biochemical and structural analysis identified the major O antigen polysaccharide as a linear α1-2 linked rhamnan polymer. By targeting an O antigen polymerase (Wzy), we found that loss of the rhamnose-rich O antigen considerably altered bacterial adhesion dynamics in the blue-green sharpshooter vector. Scanning electron microscopy confirmed that this defect in initial attachment compromised subsequent biofilm formation within vector foreguts, thus impairing pathogen acquisition. During the plant host-pathogen interaction, successful pathogens must overcome plant immune responses to establish and cause disease. Due to the fact that X. fastidiosa lacks the quintessential type III secretion system, and its arsenal of effectors, we sought to determine the contribution of O antigen to this complex biological process. Using RNAseq, we found that in intact cells, O antigen functions as a shield; this allows X. fastidiosa to delay early recognition by the grapevine immune system, thus promoting bacterial survival and the establishment of infection.
The results presented in this dissertation provide a comprehensive understanding of the composition and structure of this major cell surface polysaccharide and offer valuable insight into its diverse functions. Specifically, the studies detailing the implication of O antigen in the evasion of basal defense responses in grapevine present a novel and unprecedented mechanism employed by this unique xylem-dwelling bacterial pathogen.
