The Membrane Lipid Changes In Carica Papaya In Response To Low Temperature

Temperature is one of the major environmental factors that limit the growth and geographical distribution of plants.When the plants are subjected to temperature stress,the membrane lipid desaturation level will be adjusted accordingly.The higher the unsaturation level of plant fatty acid,the higher the membrane fluidity.As a plant originating from tropical and subtropical regions,papaya is cold-sensitive.However,the underlining mechanisms remains elusive.In this study we compared the cold and freezing sensitivity of several papaya cultivars,and measured the changes of fatty acid desaturation levels.The main results are as follows:Different papaya cultivars were subjected to low temperature treatment as the method below:28?,0.25?/h down to 10?,keep at 10? for 2 days,then down to 0?,-1? And-2? at a rate of 0.21?/h.The relative conductivity,chlorophyll content and semi-lethal temperature were measured respectively.Based on the result of chlorophyll content we get that the Zhongbai low temperature tolerance is stronger than Zhonghuang.Based on the result of relative conductivity we found that low temperature tolerance from high to low is:Zhongbai>Qingpi>Market>Sunrise>Zhonghuang>Gatamala;and their semi-lethal temperature for Zhongbai to Gatamala is-1.23?,-0.1?,0.5?.1.5?,-0.35?,0.2?,and-0.35?,respectively.In addition,we extracted total leaf lipids,used thin layer chromatography(TLC)to isolated individual lipid species,and quantitated the fatty acid composition by gas chromatography(GC).We found that linolenic acid content in Zhongbai and Zhonghuang was close to 60%,followed by palmitic acid content(16%?18%).After low temperature treatment,the linolenic acid content in Zhongbai decreased,the palmitic acid content increased;for Zhonghuang,the palmitic acid content decreased and linolenic acid content increased.After thin layer chromatography separation,total membrane lipids were separated,their relative abundance from high to low is:DGDG>MGDG>PE>PC+PS>PA>unknown.The MGDG is enriched with linolenic acid(-90%,mol%),after low temperature treatment its content declines in both Zhongbai and Zhonghuang cultivars;for DGDG the linoleum acid content reached?60%,after cold treatment for 2d,linolenic acid content increased in Zhongbai and decreased in Zhonghuang.Then,we calculated the double bond index(DBI)of total lipids as well as individual polar lipids.The DBI of total lipids fromZhongbai was slightly higher than that of Zhonghuang.However,low temperature treatment at 10? for 2 days did not significantly affect DBI value in both Zhongbai and Zhonghuang cultivars.The DBI of DGDG was about 2.1,and the DBI value in Zhongbai was marginally lower than that of Zhonghuang.After cold treatment the DBI value was decreased in zhongbai and increased in Zhonghuang.At last,we performed semi-quantitative RT-PCR to quantitate the gene expression which are involved in lipid biosynthesis or unsaturation,including Papaya-FAD2,Papaya-FAD4,Papaya-FAD5,Papaya-FAD6,Papaya-FAD 7,Papaya-FAD8,Papaya-DGD,Papaya-MGD,Papaya-PDAT1 and Papaya-DGAT1.The results showed that Papaya-FAD2,Papaya-FAD7 and Papaya-FAD8 was not affected by low temperature treatment;Papaya-FAD4,Papaya-FAD5,Papaya-DGD and Papaya-MGD were significantly up-regulated after 2 days of low temperature treatment at 10?;The expression of Papaya-FAD6 gene in Zhongbai was slightly decreased,and remained same in Zhonghuang.The expression level of Papaya-PDAT1 gene in Zhongbai was significantly higher than that of Zhongbai,2d cold treatment significant stimulate its expression level;in contrast,Papaya-DGAT2 expression level in Zhonghuang is higher than that of Zhongbai,low temperature treatment at 10? did not significantly affect its expression level.