Exogenous γ-aminobutyric acid (GABA) provides a carbon skeleton to promote the accumulation of sugar and unsaturated fatty acids in vegetable soybean seeds

γ-aminobutyric acid (GABA) influences various physiological processes in plants, particularly in carbon and nitrogen metabolism. However, the mechanism underlying carbon (sucrose and unsaturated fatty acid) metabolism in vegetable soybeans was still unknown. In this study, a foliar spray of GABA (10 mM) elevated the level of Ca²⁺ by up-regulating the expression of calmodulin (GmCaM), which increased glutamate decarboxylase (GAD) activity and boosted endogenous GABA content. This, in turn, enhanced the expression of coding genes of GABA transferase (GmGABA-T) and succinic semialdehyde dehydrogenase (GmSSADH), as well as the activity of GABA transferase (GABA-T), activated the GABA shunt to supply carbon to the tricarboxylic acid (TCA) cycle, thus improved carbon metabolism. The gene expression and activity of sucrose metabolism-related enzymes were also enhanced, leading to the increased accumulation of total soluble sugars, sucrose, glucose, etc. Additionally, exogenous GABA treatment elevated the level of unsaturated fatty acids, including omega-3 arachidonic acid, linoleic acid, alpha-linolenic acid, etc. However, these effects were attenuated by 3-mercaplopropionic acid (3-MP), an inhibitor of GABA synthesis. In summary, exogenous GABA provides a carbon skeleton that promotes the accumulation of sugar and unsaturated fatty acids in vegetable soybean seeds. This research provides a valuable theory for further improving the yield and quality of vegetable soybeans.