| Monogalactosyldiacylglycerol(MGDG) and digalactosyldiacylglycerol(DGDG) are important parts of plant photosynthetic membrane, especially for the thylakoid membrane.Combining with their special physical and chemical properties, MGDG and DGDG are thought to play a key role in maintaining the integrity of membrane structure and function.Monogalactosyldiacylglycerol synthase(MGD) is a key enzyme for the biosynthesis of MGDG, and also is the key enzyme for the formation of photosynthetic membrane. The up-regulating of the MGD expression can increase the MGDG synthesis rate and thus improve the tolerance of plants under stress conditions. Stress factors will first attack cell membrane, and damage its structure and function. Therefore, plants can alter the membrane lipid content and composition autonomously to enhance its ability to survive in stressful environment in their life history. In this study, rice(Oryza sativa L. cv.Nipponbare) was used as materials, we identified the type of MGD according to changes in the expression patterns of MGD genes under different growth conditions, and studied the effect of salt stress on membrane lipid content and composition. Meanwhile, we investigated the effect of gibberellin(GA3) pretreatment on alleviation of salt stress damage in plants. We also studied the different responses between roots and leaves of membrane lipids to Al stress in roots and leaves, and which could provide a theoretical basis for improving the stress tolerance in rice. The main results are as follows:1. The rice seedlings were cultured for 24 h in nutrient solution in different treatments(100 μM Na Cl, 10 μM GA3, 100 μM ABA and 10 μM Me JA) respectively, and were sampled at different times. We found that the expression levels of Os MGD02 g and Os MGD09 g genes decreased significantly with processing time, and the expression level was lower than control after 24 h. While the change of expression of Os MGD08 g genewas fluctuant, and had a significant increase under the treatment, so we supposed that Os MGD08 g gene might be encode the type B MGD. GA3 can upregulate the expression of these three genes better than others.2. The rice seedlings were cultured for 3 d in nutrient solution with 100 μM Na Cl, and one set of the seedings was pretreated with 10 μM GA3 for 24 h. We found that biomass and chlorophyll content were significantly lower than control, the content of galactolipids and phospholipids were also reduced under salt stress. And salt stress also caused the increase of the saturated fatty acids and the decrease of the unsaturated fatty acids of membrane lipids(especially DGDG). Exogenous GA3 can significantly mitigate the inhibition of salt stress on plants and improve the survival ability of rice seedlings in salt stress condition.3. After exposure to 100 μM Al(p H 4.5) for 48 hours, the biomass and chlorophyll content was decreased significantly. Meanwhile, Al treatment also caused an increase in electrolyte leakage. The lipid analysis showed that the levels of galactolipids were decreased under Al treatment in both roots and leaves. Additionally, the contents of phospholipids showed opposite change in roots and leaves under Al stress, which exhibited a significant reduction in leaves, and the other showed a significant increase in roots. Al also caused a decrease in membrane fluidity since the double bond index(DBI)in galactolipids and phospholipids were decreased.Taken together, different environmental conditions will affect the expression of MGD genes in rice, and different gene types had different responses. The content and composition of membrane lipids changed under salt stress, while exogenous application of GA3 can increase the synthesis of MGDG to maintain the stability of the cell membrane by inducing the upregulation of MGD gene, so as to enhance salt tolerance in rice. There are different changes of phospholipids in rice roots and leaves, which showed that the different tissues and organs of rice had various response mechanisms in response to Al toxicity. |
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