Title: The diffusible factor (DF) signaling in the phytopathogen Xanthomonas campestris pv. campestris

Presenter: Prof. HE Yawen

University: Shanghai Jiao Tong University

Time: 13:30-16:30, April 13, 2012

Venue: Room A202, Institute of Microbiology, Chinese Academy of Sciences

Abstract: One of the characteristic features of the genus Xanthomonas is the production of yellow, membrane-bound pigments called xanthomonadins. The chemical structure of xanthomonadin I, the major pigment formed by Xanthomonas juglandis XJ103, has been characterized as 17-(4-bromo-3-methoxyphenyl)-17-bromo-heptadeca -2,4,6,8,10,12,14,16-octaenoic acid. Xanthomonadins are found to function in protection of bacteria against photobiological damages and are needed for epiphytic survival and host infection. In addition, xanthomonadins were found to protect egg-phophatidylcholine lipsomes against peroxidation damage. Our recent results showed that xanthomonadin-deficient Xanthomonas campestris pv. campestris (Xcc) strains were less virulent and more sensitive to exogenous H2O2 than the wild-type strain. These findings have established the role of xanthomonadins in maintaining the ecological fitness and virulence of Xanthomonas species.

Xcc is the causal agent of black rot of crucifers, which is possibly the most important disease of crucifers worldwide. A diffusible factor (DF) was found to be implicated in the regulation of production of xanthomonadin and extracellular polysaccharide (EPS) in Xcc strain B24. It was originally thought that DF of Xcc might be a butyrolactone, and thus operate as a dedicated signal molecule modulating gene expression. Recently, DF was purified from Xcc strains 8004 and XC1 and chemically characterized as 3-hydroxybenzoic acid (3-HBA). Genetic and biochemical analysis showed that 3-HBA was associated with bacterial survival, cell viability, H2O2 resistance, and systemic infections. xanB2 (Xcc4014 in Xcc strain ATCC33913) is required for DF biosynthesis. Numerous tested bacterial species from the genus of Xanthomonas, Xyllela, Xylophilus, and Burkholderia also display 3-HBA-like activity, and an even wider range of bacterial genomes contain homologs of xanB2, suggesting that the biosynthesis and biological functions of 3-HBA are widely conserved in bacteria.

These findings presented many intriguing questions for further investigations. For example, little is known how DF is synthesized in Xcc; although DF synthase XanB2 was functionally predicted to be pteridine-dependent deoxygenase, little is known about its catalytic mechanisms and enzymatic properties; it remains unclear how DF is implicated in diverse biological functions, in particular, xanthomonadin biosynthesis. These questions will be discussed on the basis of our latest results.