Abstract: Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xooc) cause bacterial blight (BB) and bacterial leaf streak (BLS) in rice (Oryza sativa)，respectively, which constrain production of this staple crop in China and the world. According to bioinformatics of the genome sequence of Xoo and other recent reports, there are 15-30 avrBs3/PthA (avr/pth) genes in Xoo and Xooc. Recent research results suggest that avr/pth genes are not only involved in virulence when there are no corresponding R gene in rice, but also in suppressing plant innate immunity for hypersensitive response, and in avirulence when there are matching R genes in rice. Despite of avr/pth genes existing Xooc, no R genes were found for BLS in rice, assuming that there are defense inhibitors in Xooc. Members of avr/pth family genes are remarkably similar. However, the main difference is in the number and apparent nature of near-identical 102-base-pair direct repeats in the central portion of the coding sequence. In this review, we postulated that the evolution of the avr/pth genes might occur from the simple to the complex. This may elucidate the pathogenicity differentiation of Xoo and Xooc in rice.

Key words: Xanthomonas oryzae pv. oryzae, Xanthomonas oryzae pv. oryzicola, avrBs3/PthA, Pathogenicity differentiation