Positive Interaction between HO and Ca Mediates Melatonin-Induced CBF Pathway and Cold Tolerance in Watermelon ( L.).

Cold stress is a major environmental factor that detrimentally affects plant growth and development. Melatonin has been shown to confer plant tolerance to cold stress through activating the C-REPEAT BINDING FACTOR (CBF) pathway; however, the underlying modes that enable this function remain obscure. In this study, we investigated the role of HO and Ca signaling in the melatonin-induced CBF pathway and cold tolerance in watermelon ( L.) through pharmacological, physiological, and genetic approaches. According to the results, melatonin induced HO accumulation, which was associated with the upregulation of () during the early response to cold stress in watermelon. Besides, melatonin and HO induced the accumulation of cytoplasmic free Ca ([Ca]) in response to cold. This was associated with the upregulation of () in watermelon. However, blocking of Ca influx channels abolished melatonin- or HO-induced CBF pathway and cold tolerance. Ca also induced expression and HO accumulation in early response to cold stress in watermelon. Inhibition of HO production in watermelon by RBOH inhibitor or in Arabidopsis by knockout compromised melatonin-induced [Ca] accumulation and melatonin- or Ca-induced CBF pathway and cold tolerance. Overall, these findings indicate that melatonin induces -dependent HO generation in early response to cold stress. Increased HO promotes [Ca] accumulation, which in turn induces HO accumulation via , forming a reciprocal positive-regulatory loop that mediates melatonin-induced CBF pathway and subsequent cold tolerance.