The Mechanism Of ?-Tocopherol On Regulating Linoleic Acid In Artemisia Sphaerocephala Kraschen Under Salt Stress

Salt stress is one of the most serious environmental factors limiting the productivity of crop plants,especially in arid and semiarid regions.Under salt stress,?-tocopherol(?-T)and polyunsaturated fatty acid(PUFA)can improve membrane permeability and adjust Na~+level in plant accordingly.Little studies have shown that?-T could regulate the synthesis of linoleic acid(C18:2)to linolenic acid(C18:3)on the endoplasmic reticulum.Artemisia sphaerocephala Kraschen(A.sphaerocephala)is a desert shrub,widely distributed in the arid and semi-arid desert in the northwest of China.It is the pioneer plant to be used for dune rehabilitation.Its seeds and leaves are rich in?-T and C18:2,but the function of them under salt stress and physiological and molecular mechanisms of?-tocopherol regulates C18:2 remains unknown.In this study,A.sphaerocephala was used as the material and Artemisia ordosica Krasch(A.ordosica)as the control,the investigation is designed to explore that the effects of salt stress on?-tocopherol content and fatty acid composition of leaves and seeds,lipid,its fatty acid composition and fatty acid desaturase(FAD)gene expression of leaves and chloroplast;make use of the tobacco with key genes of tocopherol from A.sphaerocephala,the fatty acid composition of leaf and its key FADs gene expression under salt stress has been explored.We attempt to elucidate the physiological role of?-T and C18:2 in the adaptation of A.sphaerocephala to salt stress and the biological regulation mechanism of?-T on C18:2 traits.The main results are as follows:1.A.sphaerocephala has higher Na~+content than A.ordosica under salt stress,while the chlorophyll and biomass were significantly higher,and the electrical conductivity was significantly lower than that of A.ordosica,implying A.sphaerocephala has higher salt tolerance.Under high salt stress,the?-T in leaves of A.sphaerocephala has significantly increased and C18:2 accumulated significantly and persistently,while Na~+,oleic acid(C18:1),C18:3 and jasmonic acid(JA)decreased remarkably.This indicates that high level of?-T probably reduces JA and increases C18:2 contents via inhibiting conversion of C18:2 into C18:3,which improve the permeability and stability of cell membrane as well as higher salt resistance of A.sphaerocephala under high salt stress.2.A.sphaerocephala can maintain its growth under both low salt and high salt stress due to the increase of phosphatidylglycerol(PG)and phosphatidylcholine(PC)content and the increase of the PC/phosphatidylethanolamine(PE)and digalactosyldiacylglycerol(DGDG)/monogalactosyldiacylglycerol(MGDG)ratio,respectively.For the first time,the contribution of the degree of lipid fatty acid unsaturation to leaf fatty acid unsaturation was quantified,and found that contribution of PE and PC lipid unsaturation increases significantly under salt stress,whereas PG increases only under low salt stress.In the late stage of high salt stress,the significant increase of C18:2 in PG,PE and PC is noted,however,C18:3,FAD3 and FAD7/8expression decreases and has negative correlation with?-T.This shows that PG,PE and PC are the essential lipid components contributing to the growth of A.sphaerocephala under salt stress.Its stable unsaturated fatty acid is the result of the increased C18:2 content.This further proves that the high level of?-T inhibits the conversion of C18:2 into C18:3,possibly by inhibiting the expression of FAD3 and FAD7/8.3.The chloroplast fatty acid unsaturation of A.sphaerocephala was higher and more stable than that of A.ordosica,which made the photosynthetic capacity better than A.ordosica under salt stress.Its higher and stable fatty acid unsaturation depends on the increased PG and SQDG unsaturation and the stable PC unsaturation of chloroplast in the early stage of stress,the increase of PC unsaturation and the stability of PG unsaturation of chloroplast in the later stage.At the later stage of severe salt stress,the content of?-T in the chloroplast of A.sphaerocephala increased significantly,and it was significantly positively correlated with the ratio of C18:2/C18:3 in chloroplast PC and PG,and FAD3 gene expression,while was extremely negative correlation with the expression of FAD7/8 gene,indicating that the high content of?-T in the chloroplast of A.sphaerocephala in the late stage of high salt stress may inhibit the expression of FAD7/8 in chloroplast and inhibit the synthesis of C18:2 to C18:3 in PG and PC.4.There were no significant differences in physiological indexes and fatty acid composition between A.sphaerocephala and A.ordosica under normal germination conditions,and the?-T content for A.sphaerocephala is higher than for A.ordosica.Under salt stress,A.sphaerocephala has a higher germination rate,higher total antioxidant capacity and higher C18:2 content than A.ordosica.However,the conductivity and MDA content is less than A.ordosica.This indicates that the higher C18:2 content is the key to maintaining the membrane stability by means of reducing the membrane permeability and thus improving the salt tolerance of A.sphaerocephala under salt stress.5.The overexpression of AsHPT&?-TMT bivalent transgenic tobacco has higher?-T content and salt tolerance than other types of tobacco.Under the salt stress,bivalent transgenic tobacco features the stable membrane unsaturation and higher C18:2,indicating that the stable membrane unsaturation is the result of the high C18:2.The function of higher?-T in regulating C18:2 metabolism may be beyond the role as antioxidants:it inhibits the synthesis of C18:2 to C18:3 by inhibiting the expression of FAD3 gene,thereby increasing the content of C18:2.