A novel role of the SQDG synthase gene in Eucommia ulmoides SQD2 in drought tolerance: Lipid remodeling, chloroplast protection, and antioxidant regulation in transgenic tobaccos

Sulfoquinovosyldiacylglycerol (SQDG) is a key anionic lipid of thylakoid membranes, but its contribution to drought tolerance remains poorly understood. In this study, the Eucommia ulmoides SQDG synthase gene EuSQD2 was functionally characterized and heterologously expressed in tobacco. EuSQD2 showed preferential expression in young leaves and encoded a conserved SQDG synthase. Transgenic tobacco plants overexpressing EuSQD2 exhibited enhanced drought tolerance, as evidenced by reduced wilting, lower reactive oxygen species accumulation, improved photosynthetic performance, higher water use efficiency, and increased relative water content. Lipidomic analyses revealed substantial SQDG accumulation accompanied by coordinated remodeling of membrane lipids, including elevated phosphatidylglycerol and phosphatidylethanolamine levels. Ultrastructural observations demonstrated that EuSQD2 overexpression effectively preserved chloroplast and thylakoid integrity under drought stress. Moreover, transgenic plants displayed optimized stomatal regulation, enhanced proline accumulation, reduced lipid peroxidation, and increased antioxidant enzyme activities. Transcriptomic analyses indicated enrichment of pathways related to photosynthesis, lipid metabolism, antioxidant defense, and stomatal regulation. These results demonstrate that EuSQD2 enhances drought tolerance by promoting SQDG-mediated membrane stabilization and coordinated physiological and molecular adjustments.