Lipid composition remodeling and storage lipid conversion play a critical role in salt tolerance in alfalfa (Medicago sativa L.) leaves

Alfalfa is the most important forage internationally due to its high yield and nutritional value, but its production is severely limited by soil salinization. A better understanding of alfalfa's salt response mechanism is needed to provide guidance for breeding salt tolerant varieties. Lipidomic profiling of the salt-tolerant alfalfa cultivar Medicago sativa cv. Zhongmu-4 was performed after salt treatment. Of the 275 molecular species of lipid iden-tified, 171 had altered levels after salt treatment. The levels of phospholipids, glycolipids, fatty acids, and sphingolipids decreased whereas lysophospholipids and glycerides accumulated in the salt treated plants. Spe-cifically, higher PC/PE and DGDG/MGDG ratios and increased levels of FA 18:3 were observed, suggesting that changes in bilayer membrane structure integrity and fluidity maintenance occur in alfalfa leaves during salt stress. An accumulation of DAG (36:6, 36:5) and TAG (54:8, 54:9, 54:7, 54:6, 52:5, 52:6) was found, consistent with their potential role as storage substrates for membrane regeneration under salt stress. Changes in PI, LPC (16:0, 18:3, 18:2, 18:0), LPE (16:0, 18:2), and Cer (42:1;4 O, 34:1;4 O, 40:1;4 O) suggest these compounds may play a role in signal transduction under salt stress. The levels of PE, DGDG, and TAG were highly correlated (r> 0.8) with changes in salt-related physiological parameters. In addition, the apparent remodeling of lipid profiles was supported by complementary changes in gene expression, including DGD2, SQD2, FAD2, DGK, DGAT, and TAGL, that can be important candidate genes for alfalfa salt tolerance improvement. The detailed description of the changes occurring in lipid profiles and gene expression in alfalfa under salt stress provided by this study implicate lipid remodeling as an important feature of the salt-stress response, and provides a basis for the genetic improvement of alfalfa salt tolerance.