Xanthomonas oryzae pv. oryzae avirulence genes contribute differently and specifically to pathogen aggressiveness

Genomic copies of three Xanthomonas oryzae pv. oryzae avirulence (avr) gene s, avrXa7, avrXa10, and avrxa5, and four homologous genes, aB3.5, aB3.6, aB 4.3, and aB4.5, were mutagenized individually or in combination to study th e roles of avr genes in one component of pathogen fitness, i.e., aggressive ness or the amount of disease X. oryzae pv. oryzae causes in susceptible ri ce lines. These X. oryzae pv. oryzae genes are members of the highly relate d Xanthomonas avrBs3 gene family. Compared to the wild-type strain, X. oryz ae pv. oryzae strains with mutations in avrXa7, avrxa5, and the four homolo gous genes caused shorter lesions on rice line IR24, which contains no resi stance genes relevant to the wild-type strain. The contribution of each gen e to lesion length varied, with avrXa7 contributing the most and avrXa10 sh owing no measurable effect on aggressiveness. The functional, plasmidborne copies of avrXa7, aB4.5, and avrxa5 restored aggressiveness only to strains with mutations in avrXa7, aB4.5, and avrxa5, respectively. Mutations in av rXa7 were not complemented by plasmids carrying any other avr gene family m embers. These data indicate that some, but not all, avr family members cont ribute to pathogen aggressiveness and that the contributions are quantitati vely different. Furthermore, despite their sequence similarity, the aggress iveness functions of these gene family members are not interchangeable. The results suggest that selection and pyramiding resistance genes can be guid ed by the degree of fitness penalty that is empirically determined in avr g ene mutations.