Melatonin antagonizes ABA action to promote seed germination by regulating Ca efflux and HO 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 HO accumulation, and HO pretreatment decreased ABA/GA ratio and promoted seed germination under ABA stress. However, complete inhibition of HO 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 HO accumulation, which, in turn, regulate ABA and GA balance by promoting ABA catabolism and/or GA biosynthesis.