Cell-cell signaling controlsXylella fastidiosainteractions with both insects and plants

Abstract

Xylella fastidiosa, which causes Pierce's disease of grapevine and other important plant diseases, is a xylem-limited bacterium that depends on insect vectors for transmission. Although many studies have addressed disease symptom development and transmission of the pathogen by vectors, little is known about the bacterial mechanisms driving these processes. Recently available X. fastidiosa genomic sequences and molecular tools have provided new routes for investigation. Here, we show that a diffusible signal molecule is required for biofilm formation in the vector and for vector transmission to plants. We constructed strains of X. fastidiosa mutated in the rpfF gene and determined that they are unable to produce the signal activity. In addition, rpfF mutants are more virulent than the wild type when mechanically inoculated into plants. This signal therefore directs interaction of X. fastidiosa with both its insect vector and plant host. Interestingly, rpfF mutants can still form in planta biofilms, which differ architecturally from biofilms in insects, suggesting that biofilm architecture, rather than a passive response to the environment, is actively determined by X. fastidiosa gene expression. This article reports a cell-cell signaling requirement for vector transmission. Identification of the genes regulated by rpfF should elucidate bacterial factors involved in transmission and biofilm formation in the insect.

Genomic Insights into Xylella fastidiosa Interactions with Plant and Insect Hosts

“…Plant polysaccharides induce phenotypic changes leading to higher degrees of adhesiveness and, consequently, attachment to vectors after acquisition from plants (Killiny and Almeida 2009b). The pattern of gene expression of X. fastidiosa exposed to pectin-supplemented media is also similar to that of X. fastidiosa cells occurring at high cell density (Newman et al 2004;Chatterjee et al 2008b).…”

“…However, many of these early constructs were not stable once introduced in X. fastidiosa chromosome without antibiotic selection, and genetic analysis of knockouts within plants or insects was impossible. Development of tools allowing the stable transformation of X. fastidiosa by homologous recombination through the utilization of plasmids carrying its own chromosomal replication origin (oriC) circumvented this limitation (Monteiro et al 2001;Newman et al 2003Newman et al , 2004. More recently, the pAX1 plasmid series carrying a colE1-like (pMB1) replicon associated with four different antibiotic selection markers allowed the recovery of double recombinants (Matsumoto et al 2009).…”

“…Importantly, X. fastidiosa exhibits the ability to actively move against the flow of xylem fluids in vessels by twitching motility (Meng et al 2005). The apparently high efficiency with which cells of X. fastidiosa move through the orifices of pits (Newman et al 2004), which are apparently enlarged due to the action of extracellular enzymes secreted by this pathogen (Perez-Donoso et al 2010), may be due at least in part to its ability to move along surfaces by retraction of the type IV pili. X. fastidiosa also produces relatively short type I pili, encoded by fimA, that apparently act to restrict motility (De La Fuente et al 2007).…”

“…It is also important to note that rpfF mutants of X. fastidiosa that do not produce diffusible signaling factor (DSF) express pglA and genes encoding certain other extracellular enzymes at a higher level than that of the wild-type strain (Chatterjee et al 2008b;Wang et al 2012). Given that rpfF mutants are hyper-virulent to grape (Newman, et al 2004), it is reasonable to assume that the elevated expression of these cell wall-degrading enzymes in this mutant background could have contributed to at least some of the enhanced virulence that they exhibit.…”

“…contains 12 genes, 9 of which are conserved in X. fastidiosa. In both genera, rpfF produces DSF, which diffuses freely through cell membranes and is sensed by a two-component transmembrane receptor RpfC and RpfG (Barber et al 1997;Newman et al 2004), but many of the genes in this operon remain functionally uncharacterized.…”