Onion AcGSTU27 participated in S-alk(en)ylcysteine sulfoxides (CSOs) biosynthesis to improve abiotic stress tolerance

Glutathione S-transferases (GSTs) serve as multifunctional proteins involved in diverse catalysis and regulatory processes. GSTs catalyze the conjugation of glutathione (GSH) bound to an exogenous substrate and are involved in plant stress response. S-alk(en)ylcysteine sulfoxides (CSOs) are the main bioactive substances of onion. GSTs are potentially responsible for the conversion of GSH into S-(2-carboxypropyl) glutathione, an intermediate in the biosynthesis of CSOs. Here, a GST gene, AcGSTU27, was determined to be participating in CSOs biosynthesis in onion. AcGSTU27 overexpression (OE) increased the CSOs biosynthesis in onion callus. Exogenous GSH promoted the accumulation of CSOs, especially in AcGSTU27-OE lines. Furthermore, abiotic stress promoted CSOs accumulation in onion. Gene expression analysis revealed that AcGSTU27 was significantly up-regulated under abiotic treatment. The CSOs contents of AcGSTU27-OE and wild-type (WT) callus were significantly increased under Cu, Cd, and glufosinate treatments. Moreover, the increase in CSOs levels in AcGSTU27-OE materials was higher than that in the WT. The results reveal that AcGSTU27 participated in CSOs biosynthesis and can respond to abiotic stresses. Ectopic overexpression AcGSTU27 in Arabidopsis promoted the heavy-metal and herbicide tolerance of transgenic lines. AcGSTU27 transgenic plants showed better adaptability to Cu, Cd, and glufosinate treatment with a lower malondialdehyde content and higher antioxidant enzyme activities. Overall, these findings suggest that AcGSTU27 participated in CSOs biosynthesis under abiotic stress in onion. AcGSTU27 also improved plant stress tolerance through antioxidant activity regulation. This work enhances our comprehension of the biosynthetic mechanisms underlying CSOs in onion.