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Xanthomonas campestris pv. malvacearum strain Xcm-H1005 is the source of 6 plasmidborne avr genes that are members of the Xanthomonas avirulence (avr)/ pathogenicity-(pth) gene family, but only one of these highly homologous genes is known to affect its pathogenicity (Yang et al., 1994. Mol. Plant-Microbe Interact. 7:345-355). Since XcmH1005 contained six additional DNA fragments that hybridized to gene family members, these additional fragments were cloned, characterized, and assayed for avirulence and pathogenicity functions. Four of the six fragments exhibited avirulence function; three are new genes and one (avrBn) is previously described but now recognized as a member of the gene family. Marker-eviction mutagenesis of XcmH1005 and complementation tests revealed that at least seven of the 10 XcmH1005 avr/pth family members exhibited pleiotropic pathogenicity (watersoaking) function on cotton, but only five avr/pth genes were needed for full watersoaking. The contribution 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. However, the in planta growth rate and yield of this mutant on susceptible plants were identical to those of the wild type. We conclude that pathogenicity of XcmH1005 on cotton, but not growth in planta, requires multiple specific members of the Xanthomonas avr/pth gene family.
Yinong Yang (Mon,) studied this question.