Identification and characterization of genes unique to systemic <i>Xanthomonas</i> pathogens
Suppression Subtractive Hybridization (SSH) was used to identify genes present in the systemic pathogen Xanthomonas campestris pv. campestris (XCC), and missing from the non systemic pathogen Xanthomonas campestris pv. armoraciae (XCA). Among the DNA fragments unique to XCC was Xcc2109, one of eight putative avr genes identified in the XCC528T genome (NC 003902). Individual and sequential deletion and/or insertion mutations of all eight putative XCC avr gene loci were made, but no change in pathogenicity was observed in any combination of avr mutations, including a strain with all eight avr genes deleted. However, insertion or deletion mutants at the Xcc2109 locus lost avirulence (i.e., became virulent) on Brassica juncea, a race-determining differential host. The Xcc2109 ORF as annotated was cloned and found to be nonfunctional. A longer gene, encompassing Xcc2109 and here designated avrXccFM, was defined and found to confer avirulence to Xcc2109 mutant strains and to two additional wild type strains, thereby changing their designation. Resistance in B. juncea to XCC528T strains carrying avrXccFM occurs without a typical HR, but with a vascular HR (VHR).
A similar but modified SSH was performed between the two systemic pathogens Xanthomonas phaseoli (XP) and Xanthomonas axonopodis pv. phaseoli var. fuscans (XAP), and the non systemic Xanthomonas axonopodis pv. alfalfae (XAA). Among other gene fragments, a large number of transposable elements as well as gene fragments of unknown function were obtained. Five genes also present in other bacterial pathogens were mutagenized in XP. None of the mutant strains obtained exhibited any effect in virulence except for a pthA homolog, a gene earlier identified in XP and included in the functional study. Complementation assays were inconclusive although in detached leaves transient expression of the pthA gene homolog obtained from an XPF cosmid library, induced a phenotype similar to common bacterial blight of bean.
0307: Molecular biology