Melatonin regulates tomato cold tolerance through SlPMTRs-SlCaM6-SlICE1 signaling cascade

Cold stress severely impairs plants' growth and productivity by inducing oxidative damage and disrupting cellular signaling. While phytomelatonin (MT) enhances cold tolerance, the role of its specific receptors and the signaling transduction pathways remains unclear. This study has demonstrated that the phytomelatonin receptors SlPMTR1/2 are essential for phytomelatonin protection against cold injury in tomatoes when treated at 4°C for 48 h. Upon sensing 10 μM melatonin, SlPMTR1/2 mitigated oxidative damage through enhancing activities of antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase), thereby reducing levels of reactive oxygen species and malondialdehyde accumulation. Meanwhile, SlPMTR1/2 physically interacted with calmodulin SlCaM6, recruiting it to the plasma membrane and reducing its nuclear localization. This sequestration alleviated SlCaM6's inhibition of the transcription factor SlICE1 in the nucleus. Consequently, released SlICE1 activated the expression of SlCBF1 and downstream COR genes. Furthermore, SlCBF1 directly upregulated the expression of cyclic nucleotide-gated channels 2 (SlCNGC2), promoting extracellular Ca²⁺ influx upon cold shock-a response amplified by MT in a SlPMTR1/2-dependent manner. This enhanced Ca²⁺ signaling reinforces cold tolerance. Collectively, we have unveiled a dual-pathway signaling cascade where SlPMTR1/2 orchestrated tomato cold adaptation by enhancing antioxidant enzyme activities and interacting with SlCaM6 to activate the SlICE1-SlCBF1/SlCNGC2 transcriptional module to amplify Ca²⁺-mediated cold responses.