Effect of calcium on the virulence of Xylella fastidiosa across genetically diverse subspecies

Abstract

The Gram-negative plant pathogenic bacterium Xylella fastidiosa (Xf) inhabits the xylem vessels of its host, where it forms biofilms and aggregates believed to cause disease symptoms. Although the role of Calcium (Ca) in virulence regulation is well established, it is unclear whether specific virulence mechanisms are conserved across pathogen subspecies. I hypothesize that Ca serves as a universal signal that promotes adhesion and biofilm formation across genetically diverse strains, and that altering host Ca levels impacts disease progression. The main goal of this research was to determine the role of Ca in the virulence traits of a broad range of Xf isolates and to assess how soil Ca amendments influence infection in vivo. A comprehensive screening of 51 strains from four subspecies (fastidiosa, multiplex, sandyi, and pauca) was performed to evaluate their phenotypic responses to Ca. A positive correlation between Ca supplementation and biofilm formation was observed across all strains, suggesting that Ca-enhanced biofilm development is a conserved virulence trait. Conversely, twitching motility assays showed highly variable responses, with some strains exhibiting increased, decreased, or unchanged motility, indicating that motility regulation varies among strains. In Nicotiana tabacum experiments, soil Ca amendments, such as gypsum, were tested to determine whether they intensify Xf infection. The hypothesis was not confirmed due to confounding nutrient imbalances in the growth substrates. The study analyzes limitations and highlights key factors to improve future experiments. Although Ca influences Xf biofilm formation, controlling confounding variables is crucial to assess Ca’s effect on disease severity