WATERSOAKING FUNCTION(S) OF XCMH1005 ARE REDUNDANTLY ENCODED BY MEMBERS OF THE XANTHOMONAS AVR PTH GENE FAMILY/

Xanthomonas campestris pv. malvacearum strain XcmH1005 is the source o f 6 plasmidborne avr genes that are members of the Xanthomonas avirule nce (avr)/pathogenicity (pth) gene family, but only one of these highl y homologous genes is known to affect its pathogenicity (Yang et al., 1994, Mol. Plant-Microbe Interact. 7:345-355). Since XcmH1005 containe d six additional DNA fragments that hybridized to gene family members, these additional fragments were cloned, characterized, and assayed fo r avirulence and pathogenicity functions, Four of the six fragments ex hibited avirulence function; three are new genes and one (avrBn) is pr eviously described but now recognized as a member of the gene family, Marker-eviction mutagenesis of XcmH1005 and complementation tests reve aled that at least seven of the 10 XcmH1005 avr/pth family members exh ibited pleiotropic pathogenicity (watersoaking) function on cotton, bu t only five avr/pth genes were needed for full watersoaking. The contr ibution of each gene to pathogenicity was additive, and five of the 10 avr/pth genes appeared to be redundant, One XcmH1005 mutant, in which seven members of the gene family were eliminated, was asymptomatic on susceptible cotton and 1,600 times less mutant bacteria were released onto the leaf surface in comparison with the wild-type strain, Howeve r, the in planta growth rate and yield of this mutant on susceptible p lants were identical to those of the wild type, We conclude that patho genicity of XcmH1005 on cotton, but not growth in planta, requires mul tiple specific members of the Xanthomonas avr/pth gene family.