The Relationship Between The Leaf Lipid Composition Changes And Nitrogen-deficiency Tolerance

Monogalactosyldiacylglycerol(MGDG)and digalactosyldiacylglycerol(DGDG)are important components of plant photosynthesis membrane such as chloroplast membrane and thylakoid membrane.These galactolipids play an important role in maintaining the structure and function of photosynthesis membrane.Membrane stability and integrity are easily damaged by various environmental stresses,and then the membrane function is affected.During the long-term evolution of plants,change of the lipid components especially the galactolipid of chloroplast membrane is an important way for plant adaptation to stress.Changes of membrane lipid composition were induced by nitrogen deficiency.However,it is unclear whether the tolerant ability of different cultivars to low nitrogen stress is related with their membrane lipid synthesis ability and whether can pass the regulation of the synthesis of membrane lipids to improve plant tolerance to low nitrogen..The objective of this study was to evaluate the effects of nitrogen deficiency on membrane lipid composition and content and its relationship with low nitrogen tolerance,which would provide a theoretical basis for improving crop tolerance to low nitrogen and crop breeding.The main results are as follows:1.‘CH58'was used as the experimental material to study the relationship of flag leaf senescence with the changes in membrane lipid composition under nitrogen starvation during the filling stage.Compared with the high nitrogen treatment,the photosynthetic rate,chlorophyll and carotenoid content and total nitrogen content of the leaves decreased significantly under nitrogen starvation.Moreover,the content of starch and soluble sugar increased significantly under low nitrogen stress.The results showed that nitrogen deficiency could affect the synthesis of galactolipids and phospholipids in wheat leaves,and reduced the unsaturated fatty acid and protein-packing density in thylakoids.Membrane fluidity was destroyed in chloroplast,and then the structure and function of the chloroplast membrane were damaged,which resulted in the senescence of wheat leaves.Therefore,changes in membrane lipid composition were closed related with the leaf senescence which was induced by nitrogen deficiency.2.The objective of this part of the study was to evaluate the effects of nitrogen deficiency on membrane lipid composition and content in different nitrogen tolerant wheat(Triticum aestivum L.)cultivars.Two wheat cultivars(Pumai 9,PM9 and Ningmai 14,NM14)were grown in nitrogen deficient hydroponic solution for one month.The results showed that the biomass was more decreased in NM14 than PM9 under nitrogen deplete condition.The chlorophyll content,photochemical efficiency of PSII and photosynthetic rate in PM9 were significantly higher than those of NM14 during nitrogen deficiency.Lipid analyses showed that the levels of MGDG,DGDG and total lipid in the two cultivars were decreased under nitrogen deplete condition.However,the content of DGDG,DGDG/MGDG,total lipid of PM9,the double bond index(DBI)of MGDG,PL and total lipid,protein-packing density in thylakoids in PM9 were significantly higher than those of NM14 after nitrogen-deficiency treatment.Taken together,the changes of lipid components and contents in leaves might play an important role in the response to nitrogen deficiency.3.Transgenic tobacco plants overexpressing rice monogalactosyldiacylglycerol synthase(OsMGD)gene,which encodes the key enzyme for galactolipid biosynthesis,and wild-type tobacco plants were used to investigate the role of synthesis of galactolipids in plant low nitrogen tolerance.We found that the biomass,photosynthetic rate and the chlorophyll,carotenoid,soluble sugar,total nitrogen content in transgenic tobacco plants were significantly higher than that of wild type plant under nitrogen deplete condition.Transmission electron microscopy of the chloroplast showed that the chloroplast was less damaged in the transgenic tobacco under nitrogen deplete condition.These results indicated that the low nitrogen tolerance of transgenic tobacco plants was higher than that of wild type plant.The results also showed that the contents of galactolidpids,phospholipids,total lipid and DGDG/MGDG in leaves of transgenic tobacco plants were significantly higher than those in wild type plant.The protein-packing density in thylakoids in the transgenic tobacco lines was significantly higher than that of the wild-type SR.The results indicated that overexpression of OsMGD tobacco reduced the inhibition of nitrogen deficiency on the growth condition and improved the tolerance of tobacco to nitrogen starvation.In summary,galactolipid plays an important role in the response to nitrogen deficiency.The ability of low tolerance to nitrogen may be related to its ability to maintain membrane lipid synthesis under low nitrogen.Increasing the content of galactolipid could enhance the tolerance of crops to nitrogen deficiency.