Positive Interaction between H<sub<2</sub<O<sub<2</sub< and Ca<sup<2+</sup< Mediates Melatonin-Induced CBF Pathway and Cold Tolerance in Watermelon (<i<Citrullus lanatus</i< 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....
Ausführliche Beschreibung

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 H<sub<2</sub<O<sub<2</sub< and Ca<sup<2+</sup< signaling in the melatonin-induced CBF pathway and cold tolerance in watermelon (<i<Citrullus lanatus</i< L.) through pharmacological, physiological, and genetic approaches. According to the results, melatonin induced H<sub<2</sub<O<sub<2</sub< accumulation, which was associated with the upregulation of <i<respiratory burst oxidase homolog D</i< (<i<ClRBOHD</i<) during the early response to cold stress in watermelon. Besides, melatonin and H<sub<2</sub<O<sub<2</sub< induced the accumulation of cytoplasmic free Ca<sup<2+</sup< ([Ca<sup<2+</sup<]<sub<cyt</sub<) in response to cold. This was associated with the upregulation of <i<cyclic nucleotide-gated ion channel 2</i< (<i<ClCNGC2</i<) in watermelon. However, blocking of Ca<sup<2+</sup< influx channels abolished melatonin- or H<sub<2</sub<O<sub<2</sub<-induced CBF pathway and cold tolerance. Ca<sup<2+</sup< also induced <i<ClRBOHD</i< expression and H<sub<2</sub<O<sub<2</sub< accumulation in early response to cold stress in watermelon. Inhibition of H<sub<2</sub<O<sub<2</sub< production in watermelon by RBOH inhibitor or in Arabidopsis by <i<AtRBOHD</i< knockout compromised melatonin-induced [Ca<sup<2+</sup<]<sub<cyt</sub< accumulation and melatonin- or Ca<sup<2+</sup<-induced CBF pathway and cold tolerance. Overall, these findings indicate that melatonin induces <i<RBOHD</i<-dependent H<sub<2</sub<O<sub<2</sub< generation in early response to cold stress. Increased H<sub<2</sub<O<sub<2</sub< promotes [Ca<sup<2+</sup<]<sub<cyt</sub< accumulation, which in turn induces H<sub<2</sub<O<sub<2</sub< accumulation via <i<RBOHD</i<, forming a reciprocal positive-regulatory loop that mediates melatonin-induced CBF pathway and subsequent cold tolerance. Ausführliche Beschreibung