Regulation Of FAD Gene By Exogenous Salicylic Acid Under Low Temperature In Cucumis Sativus L. Seedlings

The cucumber(Cucmis Sativus L. cv. Zhongnong 31)seedling was used as experimental materials to study the impact of exogenous salicylic acid(SA, 50 μmol·L-1) on expression of fatty acid desaturase genes(FADs) in leaves and roots under low temperature stress(8℃).1. Fatty acid desaturases(FADs) introduce double bonds into the hydrocarbon chains of fatty acids to produce unsaturated fatty acids, and therefore play a critical role in plant development and acclimation to environmental stresses. In this study, 23 full-length FAD genes in cucumber were identified through database searches, including three Cs FAB2 genes, two Cs FAD2 genes, fourteen Cs FAD5 genes, and one gene each for Cs FAD3, Cs FAD4, Cs FAD6 and Cs FAD7. These cucumber FAD genes were distributed on all seven chromosomes and two additional scaffolds. Based on a phylogenetic analysis, the cucumber FAD proteins were clustered into five subfamilies with their counterparts from other plants. Gene structures and protein sequences were considerably conserved in each subfamily. All three Cs FAB2 proteins shared conserved structure with the known plant soluble FAD proteins. The other cucumber FADs belonged to the membrane-bound FADs and contained three highly conserved histidine boxes. Additionally, the putative endoplasmic reticulum retention signal was found at the C-termini of the Cs FAD2 and Cs FAD3 proteins, while the N-termini of Cs FAD4, Cs FAD5, Cs FAD6,Cs FAD7 and three Cs FAB2 s contained a predicted chloroplast signal peptide, which was consistent with their associated metabolic pathways. Furthermore, a gene expression analysis showed that Cs FAD2 and Cs FAD3 were universally expressed in all tested tissues, whereas the other cucumber FAD genes were preferentially expressed in the cotyledons or leaves. The tissue-specific expression patterns of cucumber FAD genes were highly correlated with the differences in the fatty acid compositions of the roots and leaves. Finally, the cucumber FAD genes showed a cold-induced and heat-repressed expression pattern,although with distinct regulatory time courses among the different Cs FAD members, which indicates the potential roles of these FADs in temperature stress resistance in cucumber.2. To explore the effects of SA on fatty acid compositions in cucumber(Cucumis sativus L.)seedlings under low temperature, the changes in total contents of fatty acids and relative content of each composition, as well as the expression levels of fatty acid desaturase(Cs FAD) genes were detected. The results showed that SA application could repress the low temperature-induced increase in malondialdehyde content and relative electrolyte leakages, and maintain the stability of cell membranes.Under low temperature, total contents of fatty acids and relative contents of unsaturated fatty acids were decreased in cucumber leaves and roots, while the ratio of saturated fatty acids increased. Thereby, the unsaturation degree and double bond index of fatty acid were reduced. After SA application, both total contents and unsaturation degree of fatty acid could be restored. Low temperature could induce the expression of Cs FAD genes. After SA application, the induction folds of Cs FAD3 and Cs FAD7 were significantly increased. In summary, in the cucumber seedlings exposed to low temperature, exogenous application of SA could induce the expression of Cs FAD genes to increase the unsaturation degree of fatty acids, thereby enhance the stability of cell membranes, and finally increase the stress tolerance of seedlings.These results suggested that exogenous SA could induce the expression of Cs FADs, increase the membrane lipid UFA content and DBI, which ultimately lead to the low temperature resistance of cucumber seedlings improve.