Bacterial leaf streak, caused by Xanthomonas oryzae pv. oryzicola, is an important disease of rice （Oryza sativa）. Genetic determinants （tatABC genes） of the twin-arginine translocation （Tat） pathway from X. oryzae pv. oryzicola strain RsGD42 were cloned and characterized, meanwhile, a tatC disruption mutant was generated. The tatC mutant lacked detectable flagella and was highly impaired in motility and chemotaxis. Furthermore, it was observed that the tatC mutant exhibited a reduced production of extracellular polysaccharide （EPS） and a significant reduction of virulence on adult rice plants compared to wild type strain. However, the tatC mutation in X. oryzae pv. oryzieola strain RsGD42 did not affect the growth rate and the ability to induce hypersensitive response （HR） in nonhost tobacco （Nicotiana tabacum L. cv. Samsun）. In conclusion, the data indicated that the Tat pathway significantly contributed to the virulence of X. oryzae pv. oryzicola.
Ninety one isolates of Xanthomonas oryzae pv. oryzae were collected from different rice- growing regions in China and determined for their virulence on 24 rice near-isogenic lines containing single resistance gene and 2-4 genes: IRBB1 (Xa1), IRBB2 (Xa2), IRBB3 (Xa3), IRBB4 (Xa4), IRBB5 (xa5), IRBB7 (Xa7), IRBB8 (xa8), IRBB10 (Xa10), IRBB11 (Xa11), IRBB13 (xa13), IRBB14 (Xa14), IRBB21 (Xa21), IR24 (Xa18), IRBB50 (Xa4 + xa5), IRBB51 (Xa4 + xa13), IRBB52 (Xa4 + Xa21), IRBB53 (xa5 + xa13), IRBB54 (xa5 + Xa21), IRBB55 (xa13 + Xa21), IRBB56 (Xa4 + xa5 + xa13), IRBB57 (Xa4 + xa5 + Xa21), IRBB58 (Xa4 + xa13 + Xa21), IRBB59 (xa5 + xa13 + Xa21) and IRBB60 (Xa4 + xa5 + xa13 + Xa21). The results showed that most isolates were less virulent on lines with more than one genes pyramided than those with single resistance gene. The isolates tested were more virulent on IR24 and IRBB10, less virulent on IRBB5, IRBB7 and IRBB21. Based on interactions between isolates and rice near-isogenic lines, 7 cultivars with single gene (IRBB5, IRBB4, IRBB3, IRBB14, IRBB2, IRBB1 and IR24) were chosen as the differentials, and the tested isolates were classified into 7 virulence groups. The reaction patterns of the 7 groups in order were: RRRRRRR, RRRRRRS, RRRRRSS, RR/SRRSSS, RRRSSSS, RRSSSSS, RSSSSSS. The virulence frequencies were 7.69, 6.59, 14.29, 12.09, 14.29, 28.57 and 16.48% respectively. The elementary system for races identification has been established in China based on the results. It will be possible to compare with races in other countries, and the results will facilitate the development of rice resistance breeding to bacterial blight in China.
