| Water shortage has become a bottleneck restricting the development of agriculture in China. Drought stress inhibits seed germination and photosynthesis in plants, thus causing seriously crop yields lose. In this paper, the differential response of morphology parameters, physiology parameters, and transcriptome in the process of seed germination between lowland rice(Oryza sativa L.) Yuefu, upland rice IRAT109, and introgression lines to PEG induced water stress were analyzed. On the other hand, upland rice and lowland rice were used to investigate the influence of PEG-induced dehydration stress and exogenous ABA on photosynthetic parameters, chlorophyll fluorescence parameters, and the expression of chloroplast and ABA biosynthesis-related genes. An attempt to understand the mechanisms of seed germination and photosynthetic mechanism under drought stress in upland rice. The results were as follows:PEG stress reduced the seeds germination by influencing the germination vigor, growth of root and bud with the increasing of MDA content. Compared with lowland rice, there was still showing good ability to germination in upland rice owing to the high ratio of GA/ABA, content of IAA and ZR, POD activity, soluble sugar content and a-amylase activity, which activated the star of seed germination and response to drought resistance and supplied energy for germination. So, The germination vigor, germination index, and root length in upland rice were better than that in lowland rice under stress condition.To analyze responses to PEG stress in gene transcriptional level, a large-scale transcriptome analysis were undertaken. We identified 29038 annotated genes and 2 448 new transcripts. 2 410 genes showed difference between upland rice and lowland rice, of which 58.01 percent having higher expression in upland rice than lowland rice. Except for water stress related genes, most of genes that were higher expression in upland rice than in lowland rice were involved in pathway of ABA, GA, starch and sucrose metabolism, and antioxidant reductase. 1 270 differential genes in upland rice between stress and control condition were identified, of which 56.46 percent showing up-regulated under PEG stress. 160 genes were up-regulated by water stress in the process of seed germination of upland rice. According to the results of GO and KEGG, there more genes involving in hormone signal, sugar and starch metabolic, and antioxidant reductase in upland rice under stress, thus upland rice can germinate more better and faster.The effects of short-term polyethylene glycol PEG stress on photosystem II(PSII) of rice seedlings were investigated using chlorophyll(Chl) fluorescence imaging. After exposure to PEG stress, the decrease in φPSII, q P, and ETR and increase in NPQ proceeded from the upper section to the base of the leaf. Chl fluorescence parameters, φPSII, q P, and ETR, were more sensitive to PEG stress in upland rice than in lowland rice. The most sensitive position in the leaf was different between the two ecotypes and was located in the leaf tip in upland rice and in the upper 4.5-6 cm section in lowland rice. Spatial heterogeneity in the top fully expanded leaf for all Chl fluorescence parameters was found under control and stress conditions. These results indicate different leaf anatomy and development between zones in the top fully expanded leaf of the two rice ecotypes. Additionally, the findings suggest a more rapid stress response mechanism in upland rice.Relative water content, transpiration rate, stomatal conductance, intracellular CO2 and net photosynthetic rate decreased under PEG stress. After 24 h stress, the openness of PSII reaction centers and electron transfer rate increased in upland rice, while decreased in lowland rice. As for NPQ, there was continued increase in upland rice under PEG stress, but NPQ decreased after long time stress indicated that it can not work to dissipation of excessive excitation energy into harmless heat. The results suggested that upland rice have more better adjusted ability and stability to long time stress than lowland rice.In order to understand the differential function of exogenous ABA in the regulation of drought tolerance between two rice genotypes, upland rice and lowland rice, photosynthetic parameters, chlorophyll fluorescence parameters, and the expression of chloroplast and ABA biosynthesis-related genes were investigated under 15% polyethylene glycol and exogenous ABA(60 μM) treatments. In both rice lines, most of the photosynthetic parameters, chlorophyll fluorescence parameters, and chloroplast and ABA biosynthesis-related gene transcript levels were rapidly reduced by PEG stress, with the exception of up-regulated levels of Os Psb A, Os NCED3, Os NCED4, and Os ZEP in lowland rice and Os NCED3, Os NCED4, and Os ZEP in upland rice. Moreover, a rapid stress-responsive regulation mechanism was found in upland rice according to the more rapid and strong up-regulation of three ABA biosynthesis-related genes in upland rice than in lowland rice. Under PEG stress, exogenous ABA application significantly enhanced the recovery of the net photosynthetic rate, stomatal conductance, and transpiration rate in upland rice, with increased expression of Os Psb D1, Os Psb D2, Os NCED2, Os NCED3, Os NCED4, and Os NCED5. These data suggest a role for chloroplast and ABA biosynthesis-related genes in photosystem II induction by exogenous ABA in the UR genetic background. |
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