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 (GA3) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca2+ and redox signaling. Melatonin pretreatment induced Ca2+ efflux accompanied by an up-regulation of vacuolar H+/Ca2+ antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca2+ efflux. Inhibition of Ca2+ efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H2O2 accumulation, and H2O2 pretreatment decreased ABA/GA3 ratio and promoted seed germination under ABA stress. However, complete inhibition of H2O2 accumulation abolished melatonin-induced ABA and GA3 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 Ca2+ efflux and H2O2 accumulation, which, in turn, regulate ABA and GA3 balance by promoting ABA catabolism and/or GA3 biosynthesis.

中文翻译：

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褪黑激素通过调节 Ca2+ 外流和 H2O2 积累来拮抗 ABA 作用以促进种子萌发

种子萌发是植物生命周期中的一个重要阶段，对植物的建立有很大贡献。褪黑激素已被证明可以在各种环境压力下促进种子萌发；然而，该机制在很大程度上仍未得到充分探索。在这里，我们报道了褪黑激素通过调节 ABA 和赤霉酸 (GA3) 平衡来拮抗脱落酸 (ABA) 以促进种子萌发。转录组学分析表明，褪黑激素的这种作用与 Ca2+ 和氧化还原信号有关。褪黑激素预处理诱导 Ca2+ 流出，伴随着液泡 H+/Ca2+ 逆向转运蛋白 3 (CAX3) 的上调。拟南芥中的 AtCAX3 缺失表现出减少的 Ca2+ 流出。在 ABA 胁迫下，对甜瓜和拟南芥突变体 AtCAX3 种子中 Ca2+ 外排的抑制会损害褪黑激素诱导的萌发。褪黑激素增加了 H2O2 的积累，H2O2 预处理降低了 ABA/GA3 比，促进了 ABA 胁迫下的种子萌发。然而，完全抑制 H2O2 积累消除了褪黑激素诱导的 ABA 和 GA3 平衡以及种子发芽。我们的研究揭示了一种新的调节机制，其中褪黑激素对抗 ABA 以诱导种子萌发，主要涉及 CAX3 介导的 Ca2+ 外流和 H2O2 积累，进而通过促进 ABA 分解代谢和/或 GA3 生物合成来调节 ABA 和 GA3 平衡。