Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2008
Publication Date: 12/2/2008
Citation: Thowthampitak, J., Shaffer, B.T., Prathuangwong, S., Loper, J.E. 2008. Role of rpfF in virulence and exoenzyme production of Xanthomonas axonopodis pathovar glycines, the causal agent of bacterial pustule of soybean. Phytopathology. 98(12):1252-1260. Interpretive Summary: Certain bacteria in the genus Xanthomonas are important pathogens of plants. One of these bacteria, Xanthomonas axonopodis pv. glycines causes bacterial pustule of soybean, which is an important disease constraining soybean production in many parts of Asia, including Thailand. This project evaluated 10 strains of this pathogen isolated from various soybean growing regions of Thailand for the production of a chemical compound (called DSF) that is known to be produced by other species of Xanthomonas that cause other plant diseases. We discovered that all ten strains of X. a. glycines produce DSF and have rpfF genes that are known to be required for DSF production. We sequenced the rpfF gene from one strain, and made an rpfF mutant of that strain. The rpfF mutant caused less disease on soybean and also produced reduced levels of enzymes that contribute to plant disease. These results indicate that X. a. glycines produces DSF, which contributes to virulence and enzyme production by this plant pathogen.
Technical Abstract: Ten strains of Xanthomonas axonopodis pv. glycines, the causal agent of bacterial pustule of soybean, obtained from various soybean growing regions of Thailand produced an extracellular diffusible factor (DSF) related to a well-characterized quorum sensing molecule produced by other Xanthomonas spp. Genomic DNA of the ten strains of X. a. glycines contained rpfF, a gene encoding for the biosynthesis of DSF in Xanthomonas campestris pv. campestris. The rpfF gene from one strain of X. a. glycines was fully sequenced, and the 289 aa product is most closely related to RpfF of other Xanthomonas spp. (95-98% identical). Three independently-generated rpfF mutants of X. a. glycines were defective in the production of DSF, as expected if rpfF encodes for the biosynthesis of DSF in X. a. glcyines, as in other Xanthomonas spp. The rpfF mutants exhibited reduced virulence on soybean and produced less than wildtype levels of extracellular polysaccharide; the extracellular enzymes cellulase, protease, endo-ß-1,4-mannanase, and polygalacturonate lyase; and pectolytic activity. Transcripts for three genes that encode for the extracellular enzymes protease, cellulase and pectate lyase were at lower abundance in the rpfF mutants than in the parental strain of X. a. glycines. These results indicate that X. a. glycines produces a diffusible signal related to DSF, which contributes to virulence and exoenzyme production by this phytopathogenic bacterium.