The Mechanism Of The Involvement Of Leaf Membrane Lipids In Response To Nitrogen Deficiency In Wheat

Nitrogen is one of the essential nutrients for plants,and more than half of the nitrogen in the leaves is located in photosynthetic apparatus.Plants generally show retarded growth,yellow leaves,accelerated senescence and reduced yield under nitrogen-deficient condition.Therefore,improving the tolerance of plants to nitrogen deficiency is an urgent problem to be solved.In plants,lipids are the major component of cellular biological membranes,the galactolipids monogalactosyldiacylglycerol(MGDG)and digalactosyldiacylglycerol(DGDG)are primary constituents of photosynthetic membranes lipids in chloroplast,which play crucial roles in adaption to stressed environmental conditions.The objective of this study is to investigate the mechanism of membrane lipids involved in the tolerance of wheat in response to nitrogen deficiency.The main results are as follows:To clarify the influence of abiotic stresses on membrane lipid and fatty acid changes,data of membrane lipid and fatty acid contents under different abiotic stresses were collected and analysed.The results showed that the content of MGDG,DGDG,phosphatidylglycerol(PG)and the total lipid in leaves were all decreased under abiotic stresses.Meanwhile,the fatty acid composition of total lipid altered variously under different stress conditions.The contents of 16-carbon fatty acids were increased,while the contents of most 18-carbon fatty acids were decreased.These results indicate that abiotic stresses could affect the contents of membrane lipids and fatty acids.The subgroup analysis of membrane lipids and fatty acids under different abiotic stresses revealed that nitrogen deficiency can significantly affect membrane lipid content and fatty acid content.Furthermore,we found that the contents of MGDG and DGDG were decreased under all stress types.The results illustrate that abiotic stresses such as nitrogen deficiency have significant effects on membrane lipid and fatty acid contents,and the changes in membrane lipids and fatty acid compositions may play a very important role in response to abiotic stresses.In this study,twenty-four wheat cultivars were used to analyze the effects of nitrogen deficiency on wheat growth,physiological parameters and leaf fatty acid contents.Through Pearson's correlation analysis and principal component analysis,the responses of 24 wheat cultivars to nitrogen-deficient stress were evaluated.The results showed that there were significant differences in the tolerance of different wheat cultivars to nitrogen deficiency.Based on our results,Yaomai16,Yunhan805,Yumai 18-99 and Xiaoyan6 were considered to be nitrogen-deficient tolerant cultivars,Yumai58,Aikang58,Xinong223 and Shaanmai139 were considered to be nitrogen-deficient sensitive cultivars.In addition,we found that palmitic acid(16:0)was negatively correlated with chlorophyll content and positively correlated with carbohydrates,while linolenic acid(18:3)and double bond index(DBI)were positively correlated with the biomass and chlorophyll content under nitrogen deficiency.The above results indicate that in addition to biomass and chlorophyll content,the changes of 18:3 and DBI may be related to the tolerance of nitrogen deficient stress in different wheat cultivars.The growth status,photosynthetic ability,chlorophyll content,membrane damage and the expression of membrane lipid fatty acid desaturase genes were determined in wheat cultivars with different tolerance to nitrogen deficiency.The results showed that nitrogen-deficient tolerant wheat cultivars maintained high photo synthetic rate,chlorophyll content and PSII activity(Fv/Fm,ETR and ?PS?)compared with nitrogen-deficient sensitive wheat cultivars under nitrogen deficiency,and the cell membrane damage and thylakoid membrane damage were less affected.At the same time,the content of MDA and the degree of membrane lipid peroxidation in tolerant cultivars were significantly lower than that of sensitive cultivars under nitrogen-deficient treatment.The expression levels of membrane lipid fatty acid desaturases in tolerant cultivars were significantly increased under nitrogen-deficient treatment.These results indicate that tolerant cultivars could up-regulate the expression of key membrane lipid fatty acid desaturase genes,affect the composition of membrane fatty acids,thus reduce membrane damage,and maintain higher photo synthetic ability,therefore,improving the tolerance to nitrogen deficiency in wheat.Nitrogen-deficient tolerant wheat cultivars had lower carbohydrate accumulation compared with nitrogen-deficient sensitive wheat cultivars under low nitrogen condition.In addition,the triose phosphate transporter(TPT)and sucrose transporter(SUT)expression levels in tolerant cultivar was 49.3%and 77%higher than that of sensitive cultivar,respectively.After nitrogen deficiency treatment,the expression of starch synthase(SSI)increased in both cultivars,but the increase was greater in sensitive cultivars than that of tolerant one.Under low nitrogen conditions,the contents of MGDG,DGDG,phosphatidylcholine(PC)and PG and the index of membrane lipid unsaturated fatty acids in the leaves of tolerant wheat cultivar were higher than those of sensitive cultivar.Moreover,the expression levels of membrane lipid metabolism-related genes(GPAT,KCS,CER2,MGD1,MGD2 and MGD3)in Xiaoyan6 were higher than those of Aikang58.These results indicate that membrane lipids can improve the nitrogen-deficient tolerance in wheat by promoting photoassimilates transportation and reducing the accumulation of starch in chloroplast,and alleviate the inhibition of nitrogen deficiency on photosynthesis,thus improve nitrogen deficiency tolerance in plant.This study showed that the response of plant membrane lipid and fatty acid content to abiotic stress,and elucidated the association between membrane fatty acid regulation ability and nitrogen deficiency tolerance of different wheat cultivars under nitrogen deficiency,high 18:3 and DBI were beneficial to the improvement of the tolerance to nitrogen deficiency,and further clarified membrane lipids remodeling was beneficial to maintain the integrity of membrane and photo synthetic capacity under nitrogen deficiency,promoting the photoassimilate transportation,thererfore,improving the plant tolerance to nitrogen deficiency.This study provides great reference value for improving nitrogen deficiency stress resistance in plants.