The Ca2+ channels CNGC2 and CNGC20 mediate methyl jasmonate–induced calcium signaling and cold tolerance

The phytohormone methyl jasmonate (MeJA) enhances plant cold stress tolerance, but the underlying mechanisms remain elusive. Here, we discovered that MeJA induces a transient Ca2+ influx and elevated cytoplasmic free Ca2+ ([Ca2+]cyt) levels during the watermelon (Citrullus lanatus) response to cold stress. Conversely, silencing jasmonic acid carboxyl methyltransferase (ClJMT), encoding an enzyme that methylates JA to MeJA, led to contrasting effects compared with MeJA application. Upon cold exposure, MeJA rapidly and continuously upregulated 2 Ca2+-permeable channel genes, namely cyclic nucleotide-gated ion channel (ClCNGC) 2 and ClCNGC20. Silencing ClCNGC2 or ClCNGC20 attenuated MeJA-induced Ca2+ influx, [Ca2+]cyt accumulation, C-REPEAT BINDING FACTOR (CBF) pathway activation, and watermelon cold tolerance. Accordingly, ClCNGC2 or ClCNGC20 overexpression increased Ca2+ influx, [Ca2+]cyt levels, and expression of the CBF regulon and improved freezing tolerance in transgenic Arabidopsis thaliana plants. Multiple assays showed that ClCNGC2 and ClCNGC20 do not directly interact. Interestingly, silencing ClCNGC2 or ClCNGC20 abolished MeJA-induced upregulation of ClCNGC20 or ClCNGC2, respectively, in watermelon response to cold, demonstrating their reciprocal activation at the transcriptional level. Collectively, these findings suggest a mutual dependence between ClCNGC2 and ClCNGC20 in mediating MeJA-induced Ca2+ influx followed by [Ca2+]cyt elevation, subsequently activating the CBF pathway and enhancing cold tolerance in plants. This study provides insights into the molecular mechanisms underlying MeJA-mediated plant cold tolerance, holding potential for the breeding or engineering of cold-resistant cucurbit varieties.