Abstract：The hrp genes of Xanthomonas oryzae pv. oryzicola (Xoc) encode type Ⅲ secretion system (T3SS) and deliver T3SS effectors into host cells to trigger hypersensitive response (HR) on nonhost plants tobacco (Nicotiana benthamiana) and pathogenicity on rice (Oryza sativa). Hpa1 protein triggers HR on tobacco and HpaB is an exit protein for T3SS. However, it is still unclear whether hpa1 and hpaB genes together determine Xoc to cause pathogenicity on rice and to trigger HR on tobacco. In this study, we acquired the hpa1 and hpaB single mutant as well as the hpa1hpaB double mutant. In planta assay showed that the hpa1 mutant reduced the virulence of Xoc on rice, whereas still maintained the capacity of triggering HR on tobacco. Both of the hpaB mutant and the hpa1hpaB double mutant induced HR on tobacco, but abolished pathogenicity on rice. These results indicated that other unknown harpin(s) exist(s) in Xoc, which is (are) HpaB-independent effector(s). Immunobloting assay showed that XopQ1Xoc was not secreted through T3SS when hpaB gene was mutated, suggesting that HpaB control the secretion of T3SS effectors that are considered to be pathogenic factors. The above evidences provide the clues to elucidate novel unknown harpin(s) and to analyze the molecular interaction of HpaB-dependent and HpaB-independent effectors with rice.
