HO and Ca Signaling Crosstalk Counteracts ABA to Induce Seed Germination

Seed germination is a critical stage and the first step in the plant's life cycle. HO and Ca 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 HO and Ca counteracted abscisic acid (ABA) to induce seed germination in melon and by modulating ABA and gibberellic acid (GA) balance. HO treatment induced a Ca influx in melon seeds accompanied by the upregulation of , which encodes a plasma membrane Ca-permeable channel. However, the inhibition of cytoplasmic free Ca elevation in the melon seeds and mutant compromised HO-induced germination under ABA stress. CaCl induced HO accumulation accompanied by the upregulation of and in melon seeds with ABA pretreatment. However, inhibition of HO accumulation in the melon seeds and mutant and abolished CaCl-induced germination under ABA stress. The current study reveals a novel mechanism in which HO and Ca signaling crosstalk offsets ABA to induce seed germination. HO induces Ca influx, which in turn increases HO accumulation, thus forming a reciprocal positive-regulatory loop to maintain a balance between ABA and GA and promote seed germination under ABA stress.