Melatonin antagonizes ABA action to promote seed germination by regulating Ca 2+ efflux and H 2 O 2 accumulation

Seed germination is a vital stage in the plant life-cycle that greatly contributes to plant establishment. Melatonin has been shown to promote seed germination under various environmental stresses; however, the mechanism remains largely underexplored. Here, we reported that melatonin antagonized abscisic acid (ABA) to promote seed germination by regulating ABA and gibberellic acid (GA ) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca and redox signaling. Melatonin pretreatment induced Ca efflux accompanied by an up-regulation of vacuolar H+/Ca antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca efflux. Inhibition of Ca efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H O accumulation, and H O pretreatment decreased ABA/GA ratio and promoted seed germination under ABA stress. However, complete inhibition of H O accumulation abolished melatonin-induced ABA and GA balance and seed germination. Our study reveals a novel regulatory mechanism in which melatonin counteracts ABA to induce seed germination that essentially involves CAX3-mediated Ca efflux and H O accumulation, which, in turn, regulate ABA and GA balance by promoting ABA catabolism and/or GA biosynthesis.