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 ₃ ) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca ²⁺ and redox signaling. Melatonin pretreatment induced Ca ²⁺ efflux accompanied by an up-regulation of vacuolar H+/Ca ²⁺ antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca ²⁺ efflux. Inhibition of Ca ²⁺ efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H ₂ O ₂ accumulation, and H ₂ O ₂ pretreatment decreased ABA/GA ₃ ratio and promoted seed germination under ABA stress. However, complete inhibition of H ₂ O ₂ accumulation abolished melatonin-induced ABA and GA ₃ 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 ²⁺ efflux and H ₂ O ₂ accumulation, which, in turn, regulate ABA and GA ₃ balance by promoting ABA catabolism and/or GA ₃ biosynthesis.
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