Seed germination is a critical stage and the first step in the plant's life cycle. H₂O₂ and Ca²⁺ act as important signal molecules in regulating plant growth and development and in providing defense against numerous stresses; however, their crosstalk in modulating seed germination remains largely unaddressed. In the current study, we report that H₂O₂ and Ca²⁺ counteracted abscisic acid (ABA) to induce seed germination in melon and Arabidopsis by modulating ABA and gibberellic acid (GA₃) balance. H₂O₂ treatment induced a Ca²⁺ influx in melon seeds accompanied by the upregulation of cyclic nucleotide-gated ion channel(CNGC) 20, which encodes a plasma membrane Ca²⁺-permeable channel. However, the inhibition of cytoplasmic free Ca²⁺ elevation in the melon seeds and Arabidopsis mutant atcngc20 compromised H₂O₂-induced germination under ABA stress. CaCl₂ induced H₂O₂ accumulation accompanied by the upregulation of respiratory burst oxidase homologue(RBOH) D and RBOHF in melon seeds with ABA pretreatment. However, inhibition of H₂O₂ accumulation in the melon seeds and Arabidopsis mutant atrbohd and atrbohf abolished CaCl₂-induced germination under ABA stress. The current study reveals a novel mechanism in which H₂O₂ and Ca²⁺ signaling crosstalk offsets ABA to induce seed germination. H₂O₂ induces Ca²⁺ influx, which in turn increases H₂O₂ accumulation, thus forming a reciprocal positive-regulatory loop to maintain a balance between ABA and GA₃ and promote seed germination under ABA stress.

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