| Xanthomonas oryzae pv. oryzae is the causal agent of bacterial blight of rice. The type III secretion system (TTSS) of X. oryzae pv. oryzae has a pivotal role in the pathogenesis of the bacterium. In this study, role of hrpF , which encodes a component of the TTSS, was studied. The hypothesis that variable TTSS proteins are involved in adaptation to different plant species was examined by testing the ability of hrpF from different species to complement the corresponding gene in X. oryzae pv. oryzae. The hrpF genes from X. campestris pv. vesicatoria, X. campestris pv. campestris, and X. axonopodis pv. citri were each capable of restoring virulence to the hrpF mutant. No evidence was found for hrpF as a host specialization factor in the systems tested. In contrast to X. campestris pv. vesicatoria hrpF mutant, hrpF mutants of X. oryzae pv. oryzae, with either avrXa10 or avrXa7, elicited hypersensitive reactions in rice cultivars with the corresponding R-genes. A double hrpF-hpa1 mutant also elicited a Xa10-dependent resistance reaction. Loss of hrpF and hpa1 may reduce delivery of TTSS effectors. However, the mutations did not completely prevent the delivery of AvrBs3/PthA-like effector proteins into the host cells.; X. oryzae pv. oryzae secretes multiple AvrBs3/PthA proteins into plant cells through the TTSS. The avrBs3/pthA gene family members encode effectors with three nuclear localization signals and a transcription activation domain, and both features are necessary for virulent function. Differential rice gene expression upon bacterial challenge was examined by using microarray and northern hybridization methods. X. oryzae pv. oryzae ingress was found to increase the expression of numerous rice genes. The induction of a set of genes was disease-specific, and the induction of the gene encoding general transcription factor IIA gamma subunit on chromosome I (TFIIAgamma1) was strainspecific. The bacterial gene controlling TFIIAgamma1 expression was cloned from X. oryzae pv. oryzae strain pXO99A and named pthXo7. The sequence analysis revealed that pthXo7 was a novel member of avrBs3/pthA gene family and encoded a central repeat domain consisting of 21.5 repeat sequences. pthXo7 contributed to the virulence of pXO99A. |
