Phosphatidic acid accumulation in response to extended cold water imbibition disrupts membrane structure that inhibits germination of cotton (Gossypium hirsutum L.) seeds

Abstract

This work aimed to understand glycerophospholipid regulation in cotton seeds that imbibed under cold stress and determine how such regulation affects overall germination performance. Lipidomics analysis showed that imbibition under cold stress drastically increase phosphatidic acid (PA) levels, while significantly reducing the content of other membrane lipids including phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and phosphatidylserine in seeds. The observed PA accumulation was associated with the upregulation of genes encoding phospholipases that catalyze PA synthesis through the hydrolysis of other membrane lipids. Alongside modifications in glycerophospholipid contents, the cold-imbibed seeds leaked excessively and germinated poorly, suggesting deleterious effects of cold-induced PA accumulation. Exogenous PA treatment of cotton seeds demonstrated a time- and concentration-dependent inhibition of germination, whereas suppression of phospholipase D activity under cold stress attenuated electrolyte leakage and improved seed germination. Together, these support our hypothesis on the harmful effects of excessive PA on seed germination. PA is conical and has a structural propensity to induce negative curvatures on membranes. Under cold stress, negative curvatures exacerbate cellular leakage which is injurious to germinating seeds. Microscopic analysis of PA-treated plant tissue indicates disruption in membrane structure that increased with increasing concentrations of PA. Based on our findings, we propose that under cold stress, germinating seeds rapidly synthesize PA via the phospholipase pathway to signal distress. With prolonged cold-water imbibition, however, induction of negative curvature due to PA accumulation combined with the breakdown of other membrane lipids disrupts membrane integrity, exacerbating cytoplasmic leakage and causing poor germination in seeds.