Gene-for-gene relationships between rice and diverse avrBs3/pthA avirulence genes in Xanthomonas oryzae pv. oryzae

The interactions between Xanthomonas oryzae pv. oryzae (Xoo) and rice are controlled in a gene-for-gene manner. In this study, a 359 bp DNA fragment of the avrXa3 gene containing three nuclear localization signal (NLS) motifs present in all members of the avrBs3/pthA family was used as a probe to screen a genomic library of the JXOIII strain of Xoo. The results demonstrated that diverse members of the family exist in the pathogen genome. The avrBs3/pthA genes occurred at isolated individual portions or in clusters. The positive avr gene clones were transferred into the virulent recipient PXO99(A). Pathogenicity tests in near isogenic lines of rice confirmed that four resistance (R) genes (Xa2, Xa3, xa5 and xa8) matched the four avr genes (avrXa2, avrXa3, avrxa5 and avrxa8) in the genome of Xoo strain JXOIII. The avrBs3/PthA-like gene (1.7 kb) present in cosmid p54, may specifically interact with the Xa3 gene present in IRBB3, and is designated avr/pthA3. Sequencing indicated that there are only 1.5 copies of the 102 bp repeat unit in avr/pthA3. Alignment of the twelfth and thirteenth amino acids in the repetitive units encoded by this gene with those in other representatives of the AvrBs3 family revealed a unique repeat arrangement which might contribute to variation in the avirulence genes in Xoo. The parental rice line IR24 was found to contain several R genes for resistance to Xoo bacterial blight.