Melatonin Improves Drought Stress Tolerance by Remodeling Lipid Metabolism in Setaria italica L.

Membrane lipid remodeling represents a crucial adaptive mechanism for plants in response to drought stress. This study investigated the regulatory influence of melatonin on the photosynthetic attributes, oxidative damage, and lipid metabolism of foxtail millet seedlings subjected to drought stress, with particular emphasis on alterations in lipid composition and fatty acid unsaturation. The findings indicated that melatonin treatment markedly enhanced the drought tolerance of foxtail millet seedlings, resulting in increases in chlorophyll content, net photosynthetic rate, and total dry weight by 51.2%, 39.8%, and 51.1%, respectively. Melatonin increased the levels of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylcholine (PC), while promoting the accumulation of unsaturated fatty acid (18:3) and leading to an increase in the double bond index (DBI). Concurrently, there were significant alterations in the expression of genes associated with glycolipid and phospholipid biosynthesis, aligning with the observed changes in lipid components. These findings indicate that melatonin potentially enhances the drought tolerance of foxtail millet seedlings through the regulation of lipid metabolic reprogramming. This process involves an increase in the content of unsaturated fatty acids and an optimization of the lipid unsaturation index, which collectively contribute to the greater stability, fluidity, and integrity of cellular membranes.