H2O2 and Ca2+ Signaling Crosstalk Counteracts ABA to Induce Seed Germination

Abstract

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 <i>Arabidopsis</i> by modulating ABA and gibberellic acid (GA₃) balance. H₂O₂ treatment induced a Ca²⁺ influx in melon seeds accompanied by the upregulation of <i>cyclic nucleotide-gated ion channel</i><i>(CNGC) 20</i>, which encodes a plasma membrane Ca²⁺-permeable channel. However, the inhibition of cytoplasmic free Ca²⁺ elevation in the melon seeds and <i>Arabidopsis</i> mutant <i>atcngc20</i> compromised H₂O₂-induced germination under ABA stress. CaCl₂ induced H₂O₂ accumulation accompanied by the upregulation of <i>respiratory burst oxidase homologue</i><i>(RBOH) D</i> and <i>RBOHF</i> in melon seeds with ABA pretreatment. However, inhibition of H₂O₂ accumulation in the melon seeds and <i>Arabidopsis</i> mutant <i>at</i><i>rbohd</i> and <i>at</i><i>rbohf</i> 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.