Mepiquat chloride priming confers the ability of cotton seed to tolerate salt by promoting ABA-operated GABA signaling control of the ascorbate–glutathione cycle

Abstract Background Ensuring that seeds germinate and emerge normally is a prerequisite for cotton production, esp. in areas with salinized soil. Priming with mepiquat chloride (MC) can promote seed germination and root growth under salt stress, but its mechanism has not been fully elucidated. In this study, physiological and biochemical experiments revealed that MC-priming promotes the tolerance of cotton seeds to salt stress by increasing the ability of antioxidant enzymes related to the ascorbate–glutathione (AsA-GSH) cycle to scavenge reactive oxygen species (ROS). Results Results revealed that treatment with inhibitors of abscisic acid (ABA) and γ-aminobutyric acid (GABA) biosynthesis reduced the positive effects of MC-priming. Similarly, MC-priming increased the contents of ABA and GABA under salt stress by stimulating the expression levels of GhNCED2 and GhGAD4 and the activity of calmodulin-binding (CML) glutamate decarboxylase (GAD). Further analysis showed that an inhibitor of ABA synthesis reduced the positive impacts of MC-priming on the content of GABA under salt stress, but the content of ABA was not affected by the GABA synthesis inhibitor. Furthermore, a multi-omics analysis revealed that MC-priming increased the abundance and phosphorylation levels of the proteins related to ABA signaling, CML, and Ca2+ channels/transporters in the MC-primed treatments, which resulted in increased oscillations in Ca2+ in the MC-primed cotton seeds under salt stress. Conclusion In summary, these results demonstrate that MC-mediated ABA signaling operates upstream of the GABA synthesis generated by GAD by activating the oscillations of Ca2+ and then enhancing activity of the AsA-GSH cycle, which ensures that cotton seeds are tolerant to salt stress.