Interactions of Xylella fastidiosa and Endophytic Bacteria in Citrus: A Review

Citrus variegated chlorosis (CVC) is a disease of sweet orange [Citrus sinensis (L.)] caused by Xylella fastidiosa subsp. pauca, a phytopathogenic bacterium that has been shown to infect all sweet orange cultivars. Xylella fastidiosa is a fastidious Gram-negative, xylem-limited bacterium which was rapidly disseminated by infected nursery trees and by several xylem-feeding sharpshooter insect vectors. In Brazil, CVC is the most economically important of several plant diseases caused by X. fastidiosa. One factor that may confer apparent resistance to CVC is the endophytic microbial community colonizing individual C. sinensis plants. Endophytes are microorganisms that do not visibly harm the host plant, but can be isolated from the internal tissues of surface-disinfected plants. Furthermore, as they colonize an ecological niche similar to that of certain plant pathogens, they are likely candidates for biocontrol agents. There is evidence that X. fastidiosa interacts with endophytic bacteria present in the xylem of sweet orange, and that these interactions, particularly with Methylobacterium mesophilicum and Curtobacterium flaccumfaciens, may affect disease progress. Studies of endophytic bacterial populations in sweet orange suggest that symptoms of CVC in sweet orange could be influenced by the relative populations of Methylobacterium spp., C. flaccumfaciens and X. fastidiosa subsp. pauca. Symbiotic control is a new strategy that uses symbiotic endophytes as biological control agents to antagonize or displace pathogens. Candidate endophytes for use in symbiotic control of CVC must occupy the xylem of host plants and attach to the precibarium of sharpshooter insects in order to have access to the pathogen. In the present review, we focus on interactions between endophytic bacteria from sweet orange plants and X. fastidiosa subsp. pauca, especially those that could result in some strategy for symbiotic control of CVC. _____________________________________________________________________________________________________________