Analysis Of Molecular Pathways Mediated By Nonhost Resistance Gene To Bacterial Leaf Streak And Identification Of Bacterial Blight Resistant Lines And QTLs

Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc) are two highly related pathogens of rice that cause bacterial leaf blight (BB) and bacterial leaf streak (BLS), respectively, which constrain rice production in much of Asia and parts of Africa. Developing resistant cultivars has been the most efficitive approach to control BB, however, most single resistance genes have limited value in breeding programs because of their narrow-spectrum of resistance to the races of the pathogen. By contrast, there is little progress in breeding varieties resistant to Xoc since BLS resistance in rice was a quantitative trait and only a few quantitative resistance loci have been identified so far. With the development of molecular technique, cloned non-host resistance gene will provide an alternative strategy for breeding program.In this study, we studied the genome-wide gene responses differentially regulated during incompatible and compatible interactions between nonhost gene Rxo1 and Xoc in a near isogenic pair of transgenic and nontransgenic lines in the 9804 background by Affymetrix GeneChip, and analyzed the expression patterns of the genes perhaps involved in the resistant response. Besides, the introgression lines with broas spectrum and some quantitative resistance loci resistant to BB were identified using 8 non-target trait populations. The main results are as follows:Firstly, Rxo1, a nonhost resistance gene coloned from maize, was introduced into a rice cultivar by agrobacterial-mediated transformation. The transgenic rice plants exhibited a typical HR to Xoc and deposition of lignin, callose, and phenols; activities of peroxidase andβ-1, 3-glucanase, and some endogenous phytohormones like ABA were enhanced in the transgenic rice lines after inoculation.Secondly, a microarray experiment was performed to reveal the moleculat mechanisms underlying HR of eice to Xoc mediated by Rxo1 using a pair of transgenic and non-transgenic rice lines at 36 and 48 hours post inoculation (hpi). Some 175 and 379 differentially regulated genes (DRGs) were detected in 9804-Rxo1 and 9804 at 36 hours post-inoculation (hpi), respectively: 92.00% of the DRGs were up-regulated in inoculated 9804-Rxo1 and 48.22% of the DRGs were sorted as defense-related genes. Some genes encoding lipoxygenase 2.1, pathogenesis-related protein, terpene synthase family, transcription factors AP2 domain containing protein, myb-like DNA-binding domain containing protein, and C2H2-type zinc-finger transcription factors and Six HR-related peroxidase (POD) genes were significantly up-regulated in inoculated 9804-Rxo1.Whereas, 2450 and 1950 DRGs in 9804-Rxo1 and 9804 were identified at 48hpi: Rxo1 appeared to function in the very early step of the interaction between rice and Xoc, and could specifically activate large numbers of genes involved in signaling pathways leading to HR and some basal defensive pathways such as SA and ET pathways. Rxo1 was able to induce a unique group of WRKY TF genes and a large set of genes encoding SAPK, PPR and RRM proteins that may involved in the defence response.Thirdly, the expression profile of SAPKs involved in the resistant sponses mediated by R gene Xa23 and non-host R gene Rxo1 were analysised. Four OsSAPKs were up-regulated after Xoc inoculation in 9804-Rxo1 compared with the recipients. Interestingly, Xa23 also induced four OsSAPKs'up-regulated expression after inoculation with bacterial blight pathoge. We speculated some genes in SAPK familu might incolved in the resistant response by ABA pathway.Finally, eight non-target trait populations with traits of drought tolerance, salinity tolerance, high yielding or submergence tolerance were used to screen the resistant lines resistant to bacterial blight. Nine lines highly resistant to all of the 14 Xoo strains and 11 lines resistant to 11 strains were selected. Sixty-five QTLs or resistant to BB were identified in 8 populations. Among them, 25 were detected in more than 3 populations or resistant to more than 3 strains. Nine of the 25 had positive additive effect of resistance (AER); another 9 QTLs had negative AER to the strains, indicating the resistance was derived from the recurrent parent or from the donors for each of the 18 QTLs. While the origin of resistance for the other 7 QTLs depent on the strains. Moreover, 13 major QTLs with broad resistance spectrum including RM224/RM123, RM488, RM428, RM217, RM106, and RM258 were found.In this study, we primarily elucidated the nonhost resistance gene Rxo1 could trigger the pathogen-specific HR in heterologous rice, which ultimately leading to a localized programmed cell death which exhibits the characteristics consistent with those mediated by host resistance genes, and found some genes which might involved in the resistant response. Moreover, some resistant rice lines and QTLs were identified, that will provide elite lines resistant to BB and useful information for rice breeding program.