Xylella Genomics and Bacterial Pathogenicity to Plants

467

449

The rpf gene cluster of Xanthomonas campestris pathovar campestris (Xcc) is required for the pathogenesis of this bacterium to plants. Several rpf genes are involved in the coordinate positive regulation of the production of virulence factors mediated by the small diffusible molecule DSF (for diffusible signal factor). RpfF directs the synthesis of DSF, and a two-component sensory transduction system comprising RpfC and RpfG has been implicated in the perception of the DSF signal and signal transduction. In L medium, rpfF, rpfG, rpfC, and rpfGHC mutants grew as matrixenclosed aggregates, whereas the wild type grew in a dispersed planktonic fashion. Synthesis of the extracellular polysaccharide xanthan was required for aggregate formation. Addition of DSF triggered dispersion of the aggregates formed by the rpfF strain, but not those of rpf strains defective in DSF signal transduction. An extracellular enzyme from Xcc whose synthesis was positively controlled by the DSF͞rpf system could disperse the aggregates produced by all rpf strains. The enzyme was identified as the single endo-␤-1,4-mannanase encoded by the Xcc genome. This enzyme had no detectable activity against soluble xanthan. The endo-␤-1,4-mannanase was required for the full virulence of Xcc to plants. On the basis of this model system, we propose that one role of the ␤-mannanase during disease is to promote transitions from an aggregated or biofilm lifestyle to a planktonic lifestyle in response to the DSF signal.

Section: Discussionmentioning

confidence: 99%

“…The ability of Xcc to incite disease depends on several factors, including the synthesis of extracellular plant cell wall-degrading enzymes and the extracellular polysaccharide (EPS) xanthan (5,6). The rpf genes act to positively regulate the synthesis of extracellular enzymes, EPS, and pathogenicity (5)(6)(7)(8).…”

mentioning

confidence: 99%

Biofilm dispersal inXanthomonas campestrisis controlled by cell–cell signaling and is required for full virulence to plants

Dow

Crossman

Findlay

et al. 2003

467

449

“…campestris (Xcc), which is closely related to X. fastidiosa (13) but is not insect vectored, requires rpfF for synthesis of a diffusible signaling factor (DSF) (14). rpfF encodes a protein similar to enoyl-CoA hydratases that synthesizes DSF (14), an ␣,␤ unsaturated fatty acid (15).…”

mentioning

confidence: 99%

Cell-cell signaling controlsXylella fastidiosainteractions with both insects and plants

Newman

Almeida

Purcell

et al. 2004

263

296

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.

“…For a demonstration of chimera efficacy, we targeted Xylella fastidiosa (Xf), a Gram-negative bacterial pathogen with a wide range of plant hosts of economic importance (5)(6)(7)(8). Xf is vectored primarily by sharpshooter insects and colonizes in the plant xylem (6,7).…”

mentioning

confidence: 99%

“…Xf is vectored primarily by sharpshooter insects and colonizes in the plant xylem (6,7). The Xf subspecies fastidiosa (Xff), as exemplified by the California strain Temecula1, causes Pierce disease (PD) in grape.…”

mentioning

confidence: 99%

An engineered innate immune defense protects grapevines from Pierce disease

Dandekar

Gouran

Ibáñez

et al. 2012

We postulated that a synergistic combination of two innate immune functions, pathogen surface recognition and lysis, in a protein chimera would lead to a robust class of engineered antimicrobial therapeutics for protection against pathogens. In support of our hypothesis, we have engineered such a chimera to protect against the Gram-negative Xylella fastidiosa (Xf), which causes diseases in multiple plants of economic importance. Here we report the design and delivery of this chimera to target the Xf subspecies fastidiosa (Xff), which causes Pierce disease in grapevines and poses a great threat to the wine-growing regions of California. One domain of this chimera is an elastase that recognizes and cleaves MopB, a conserved outer membrane protein of Xff. The second domain is a lytic peptide, cecropin B, which targets conserved lipid moieties and creates pores in the Xff outer membrane. A flexible linker joins the recognition and lysis domains, thereby ensuring correct folding of the individual domains and synergistic combination of their functions. The chimera transgene is fused with an amino-terminal signal sequence to facilitate delivery of the chimera to the plant xylem, the site of Xff colonization. We demonstrate that the protein chimera expressed in the xylem is able to directly target Xff, suppress its growth, and significantly decrease the leaf scorching and xylem clogging commonly associated with Pierce disease in grapevines. We believe that similar strategies involving protein chimeras can be developed to protect against many diseases caused by human and plant pathogens.