Study On Mechanism Of Rice Adult Plant Resistance To Bacterial Blight

Adult plant resistance (APR) is a type of riesistance expressed only when plant grows to a specific developmental stage. APR has been found in a number of diseases in different plants. Howerver, the actural mechanism about it is not clear to date. In this paper, the mechanism of APR to rice bacterial blight was studied in molecular, physiological and biochemical levels.1. By using 90 primer combinations in DDRT-PCR, gene expression was studied in seedlings and adult plants of oryza sativa Wase Aikoku 3 inocualted with Xanthomonas oryzae pv. oryzae strain T7133 and those treated with water, respectively. Out of the total detected 7000 cDNAs, only 9 were expressed differentialy. 3 cDNAs were cloned and sequenced. By BLAST analysis, the 3 cDNAs had same nucleotide sequnce, which encoded NADH dehyrogenase. By analysis of Northern blot, difference was not detectd among different treatments. Thus, the isolated cDNAs were possible falsive.2. By analysis of cDNA-AFLP with 256 primer combinations, gene expression was studied in seedlings and adult plants of oryza sativa Wase Aikoku 3 inocualted with Xanthomonas oryzae pv. oryzae strain T7133 and those treated with water, respectively. Out of the total detected 16000 cDNAs, 122 differently epressed cDNAs were found to associate with APR. 70 cDNAs were seccessfully cloned and sequnced. By BLAST analysis, 41 cDNAs were homologous to genes with identified functions, which were involved in light perception, expression regulation, and defense response. 29 cDNAs were homologous to genes with no identified functions. By Northern blot or RT-PCR analysis, 11 cDNAs showed diffentialy expression.3. In order to investigate whether APR is related to the genome methylation, the pattern of DNA methylation was studied by useing methylation sensitive amplified polymorphism (MSAP) in seedlings and adult plants of Wase Aikoku 3 inoculated with T7133, water and those untreated, respectively. Among the 2000 detected fragments, methylation polymorphism for 380 were found, of which,8.7% and 0.5 % were induced by seedling and adult plant development, respectively, whereas 1.6% and 0.8% were induced by pathogne in seedlings and adult plants, respectively. Thus, it is likely that APR to rice bacterial blight is related to the methylation of genome at different developmental stages.4. In order to investigate whether the activiation of LTR-retrotransposon was involved in APR to rice bacterial blight, the polymorphism of rice LTR-retrotransposon Tosll was studied by sequence specific amplified polymorphism (SSAP) in seedlings and adult plants of Wase Aikoku 3 inoculatd with T7133, water and those untreated, respectively. Among the 2000 detected Tosl7, polymorphism for 16 were observed, of which, 0.45% and 0.1% were activeted at seedling and adult stage, respectively, and 0.15% and 0.15% were actived by pathogen in seedlings and adult plants, respectively. These results indicated that the activtion of rice LTR-retrotransposon likely associated with the APR to rice bacterial blight.5. By RT-PCR, expression of defense genes PAL LOX PBZ1 PRla and Chtl were analysized in seedlings and adult plants of Wase Aikoku 3 treated with T7133 and water, respectively. Results indicated that PAL was induced to express by pathogen and wound at the adult stage while it only induced by pathogen at the seedling stage, and the expression of PAL was stronger in adult plants than that in seedlings. LOX was induced to express both in seedlings and adult plants by pathogen and it was stronger in adult plants than that in seedlings. Expression of PBZ1 was induced by either pathogen or wound in both seedlings and adult plants and it is earlier and stronger in adult plants than that in seedlings. PRla and Chtl were not expressed.6. The concentration of hydrogen peroxide (H2O2) and the activities of peroxidase (POD), polyphenol oxidase (PPO), superoxide dismutase (SOD) and catalase (CAT) were measured in rice leaves inoculated with Xanthomonas oryzae pv. oryzae T7133 or water at seedling and adult plant stage, respectively. Inoculation with T7133 on leaves of both seedlings and adult plants resulted in an increase in the concentration of H2O2 and the activities of POD, PPO, and SOD. The concentrationof H2O2 and the activities of PPO and SOD were higher in adult plants than in seedlings, whereas the POD activity was lower. The activity of CAT in leaves of both seedlings and adult plants inoculated with T7133 was lower than that in water-treated control, and the adult plants showed much lower activity compared to the seedlings. The results suggest the involvement of H2O2, PPO, SOD, and CAT but not of POD, in adult plant resistance against rice bacterial blight.