Melatonin-mediated low-temperature tolerance of cucumber seedlings requires Ca/CPKs signaling pathway.

Melatonin (Mel) is recognized as a prominent plant growth regulator. This study investigated the alleviating effect of Mel pretreatment on growth inhibition caused by low-temperature (LT) stress (10 °C/6 °C) in cucumber seedlings and explored the role of the Ca/Calcium-dependent protein kinases (CPKs) signaling pathway in Mel-regulated LT tolerance. The main results are as follows: compared to LT treatment alone, 100 μM Mel increased both the content of Ca (highest about 42.01%) and the expression levels of Ca transporter and cyclic nucleotide-gated channel (CNGC) genes under LT. Similarly, Mel enhanced the content of CPKs (highest about 27.49%) and the expression levels of CPKs family genes in cucumber leaves under LT. Additionally, pretreatment with 100 μM Mel for three days strengthened the antioxidant defense and photosynthesis of seedlings under LT. Genes in the ICE-CBF-COR pathway and the MAPK cascade were upregulated by Mel, with maximum upregulations reaching approximately 2.5-fold and 1.9-fold, respectively, thus conferring LT tolerance to cucumber seedlings. However, the above beneficial effects of Mel were weakened by co-treatment with calcium signaling blockers (LaCl or EGTA) or CPKs inhibitors (TFP or W-7), suggesting that the Ca/CPKs pathway is involved in the Mel-mediated regulation of LT tolerance. In conclusion, this study revealed that Mel can alleviate growth inhibition in cucumber seedlings under LT stress and demonstrated that the Ca/CPKs signaling pathway is crucial for the Mel-mediated enhancement of LT tolerance. The findings hold promise for providing theoretical insights into the application of Mel in agricultural production and for investigating its underlying mechanisms of action.