Analysis of the molecular basis of Xanthomonas axonopodis pv. citri pathogenesis in Citrus limon

Abstract

Xanthomonas axonopodis pathovar citri ( Xac ) causes bacterial citrus canker, a serious disease of most citrus species. Xanthomonas campestris pv. campestris ( Xcc ) is the causal agent of black rot disease in cruciferous plants. In Xcc , cell-cell signaling is mediated by diffusible signal factor (DSF). Synthesis of DSF depends on RpfB and RpfF. DSF perception and signal transduction have been suggested to involve a two-component system comprising RpfC and RpfG. It has been proposed that these proteins participate in a signal transduction system linking changes in the environment to the synthesis of DSF and the expression of virulence genes. Although the cluster of the rpf g enes in Xac has synteny with the corresponding cluster in Xcc , two genes ( rpfH and rpfI ) are absent in Xac . To investigate DSF-mediated regulation during Xac - Citrus limon interaction, we constructed two strains of Xac , one with a mutation in the rpfF gene, leading to an inability to produce DSF, and one with a mutation in the rpfC gene leading to an overproduction of DSF. These mutants also show decreased levels of extracellular cyclic β-(1,2)-glucans and decreased production of endoglucanase and protease extracellular enzymes. The Xac DSF-deficient rpfF and the DSF-hyper producing rpfC mutants are both severely compromised in their ability to cause canker symptoms in lemon leaves compared to the wild-type. Here we provide evidence that rpf genes in Xac are involved in controlling virulence factors mediated by DSF.