PAMP-triggered immunity (PTI) is the first line of plant defense against invading organisms. Initiated through the perception of conserved pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide or flagellin, PTI can provide early protection against a broad range of pathogens. Active suppression of PTI by microbial effector proteins, particularly those secreted by the Type III secretion system (T3SS), is a well-known strategy employed by bacterial plant pathogens that enables them to subvert PTI and successfully colonize their hosts. In this study, we demonstrate that the xylem-limited bacterium, Xylella fastidiosa, which lacks a T3SS, utilizes an alternative strategy to delay elicitation of innate immune responses. By decorating its LPS PAMP molecule with a high molecular weight O antigen, this bacterium physically masks itself from early recognition by the grapevine innate immune system. We have elucidated the chemical structure of the O antigen and found that it is primarily an α1,2-linked rhamnan polymer. X. fastidiosa cells lacking O antigen elicited hallmarks of PTI such as ROS production, specifically in the plant xylem tissue compartment, which is comprised primarily of non-living cells. By coupling histological and genome-wide transcriptional profiling, we demonstrate that X. fastidiosa lacking its O antigen shield activates defense-related genes in grapevine. This includes a stronger and more prolonged oxidative burst at concentrations high enough to inhibit pathogen proliferation. To begin exploring translational applications of our findings, we also demonstrate that purified X. fastidiosa LPS elicitor can prime grapevine defenses, thereby conferring host tolerance to subsequent challenge with X. fastidiosa.
Overall design: Grapevine petioles were inoculated with wild type X. fastidiosa, wzy mutant and buffer. For RNAseq analysis petioles (three biological replicates) were collected locally (i.e., at point of inoculation) and distally at six timepoints (0 hours, 8 hours, 24 hours, 48 hours, 1 week, and 2 weeks after inoculation).
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