H 2 O 2 and Ca 2+ Signaling Crosstalk Counteracts ABA to Induce Seed Germination.

Seed germination is a critical stage and the first step in the plant's life cycle. H 2 O 2 and Ca 2+ 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 H 2 O 2 and Ca 2+ counteracted abscisic acid (ABA) to induce seed germination in melon and Arabidopsis by modulating ABA and gibberellic acid (GA 3 ) balance. H 2 O 2 treatment induced a Ca 2+ influx in melon seeds accompanied by the upregulation of cyclic nucleotide-gated ion channel (CNGC) 20 , which encodes a plasma membrane Ca 2+ -permeable channel. However, the inhibition of cytoplasmic free Ca 2+ elevation in the melon seeds and Arabidopsis mutant atcngc20 compromised H 2 O 2 -induced germination under ABA stress. CaCl 2 induced H 2 O 2 accumulation accompanied by the upregulation of respiratory burst oxidase homologue (RBOH) D and RBOHF in melon seeds with ABA pretreatment. However, inhibition of H 2 O 2 accumulation in the melon seeds and Arabidopsis mutant at rbohd and at rbohf abolished CaCl 2 -induced germination under ABA stress. The current study reveals a novel mechanism in which H 2 O 2 and Ca 2+ signaling crosstalk offsets ABA to induce seed germination. H 2 O 2 induces Ca 2+ influx, which in turn increases H 2 O 2 accumulation, thus forming a reciprocal positive-regulatory loop to maintain a balance between ABA and GA 3 and promote seed germination under ABA stress.