The XadA trimeric autotransporter adhesins in Xylella fastidiosa differentially contribute to cell aggregation, biofilm formation, insect transmission and virulence to plants

Surface adhesion strategies are widely employed by bacterial pathogens during establishment and systemic spread in their host. A variety of cell surface appendages such as pili, fimbriae and afimbrial adhesins are involved in these processes. The phytopathogen employs several of these structures for efficient colonization of its insect and plant hosts. Among the adhesins encoded in the genome, three afimbrial adhesins, XadA1, Hsf/XadA2, and XadA3, are predicted to be trimeric autotransporters with a C-terminal YadA-anchor membrane domain. We analyzed the individual contributions of XadA1, XadA2, and XadA3 to various cellular behaviors both and . Using isogenic mutants, we found that cell-cell aggregation and biofilm formation were severely impaired in the absence of XadA3. No significant reduction of cell-surface attachment was found with any mutant under flow conditions. Acquisition by insect vectors and transmission to grapevines were reduced in the XadA3 deletion mutant. While the XadA3 mutant was hypervirulent in grapevines, XadA1 or XadA2 deletion mutants conferred lower disease severity than the wild-type strain. This insight of the importance of these adhesive proteins and their individual contributions to different aspects of biology should guide new approaches to reduce pathogen transmission and disease development.