Membrane lipid remodeling under salt stress is associated with salt tolerance in lettuce

Salt stress is one of the most important abiotic stresses limiting crop production in the salinity affected growing regions. Lettuce (Lactuca sativa L.) is sensitive to salinity. Understanding the physiological and biochemical mechanisms involved in salt tolerance would facilitate developing strategies to breed lettuce cultivars with improved salt tolerance. Membrane lipid remodeling has been shown to play important roles in adaptation and tolerance to salt stress in several crop species; however, such information is lacking for lettuce. Accordingly, we conducted a controlled growth chamber experiment to investigate membrane lipid remodeling under salt stress using four pairs of lettuce genotypes - each pair consisting of a salt-tolerant and a salt-sensitive genotype from the same horticultural type.

Based on the 15 lipid classes analyzed, the lettuce leaf lipidome consisted of 79% phospholipids, 20% galactolipids and ~ 1% triacylglycerols (TAGs). Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were the predominant phospholipids accounting for 36% and 34% of the total phospholipids, respectively. Under salt stress, both content and unsaturation levels of galactolipids and TAGs increased, whereas content and unsaturation levels of phospholipids, except phosphatidylglycerol (PG) decreased. DGDG (digalactosyldiacylglycerol):MGDG (monogalactosyldiacylglycerol) ratio remained stable under salt stress, whereas PC:PE ratio increased. The changes in the PC:PE ratio had a significant correlation with changes in fresh weight (FW) indicating that PC: PE ratio had a role in salt tolerance. Correlation and regression analyses indicated significant relationships of changes in FW with changes in the content of the lipids PG, PC, phosphatidic acid (PA), TAG 16:0, TAG 18:2, TAG 18:1, and TAG 18:0 due to salt stress, suggesting that these lipid classes were involved in salt tolerance in lettuce in the present study.

This study demonstrates that lettuce undergoes significant membrane lipid remodeling under salt stress, and these changes contribute to salt tolerance. Using lipid profiles from control plants and salt-induced reductions in FW, we developed regression models which could potentially be used to predict FW losses due to salt stress based on MGDG and DGDG content in lettuce grown under control or normal conditions. These findings highlight key lipid traits that may be used in breeding or screening strategies to improve salt tolerance in lettuce.