Target of rapamycin coordinates auxin are involved in exogenous melatonin regulated low temperature tolerance in cucumber seedlings.

Low temperature (LT) is an important environmental factor affecting the growth and yield of plants. Melatonin (MT) can effectively enhance the LT tolerance of cucumber. This study found that LT stress induced the expression of Comt1 (caffeic acid O-methyltransferase 1), with the highest expression being about 2-times that of the control. Meanwhile, the content of MT was found to be roughly 63.16% of that in the control samples. Compared with LT treatment alone, exogenous MT pretreatment upregulated the expression levels of TOR (Target of rapamycin), PIN1 (Pin-formed 1), and YUC4 (YUCCA 4), with maximum upregulations reaching approximately 66.67%, 79.32%, and 42.86%, respectively. These results suggest that MT may modulate the tolerance of cucumber seedlings to LT stress by regulating the expression of TOR, PIN1, and YUC4. In addition, co-treatment with AZD-8055 (a TOR inhibitor) or NPA (N-1-naphthylphthalamic acid, an auxin polar transport inhibitor) and MT attenuated MT-induced resistance to LT stress, leading to higher levels of reactive oxygen species (ROS), reduced antioxidant defense capacity, and increased damage to the membrane system in cucumber seedlings. Concurrently, the content of osmoregulatory substances and the photosynthesis decreased. These results demonstrate that both TOR and auxin were required for MT to alleviate LT-induced damage in cucumber. In summary, the present study demonstrates that TOR and auxin signaling synergistically contribute to alleviating LT damage in cucumber seedlings by exogenous MT. These findings help us understand the function of MT and provide insights into the regulatory network of MT that regulates the LT tolerance of plants.

Melatonin-mediated low-temperature tolerance of cucumber seedlings requires Ca2+/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 Ca2+/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 Ca2+ (highest about 42.01%) and the expression levels of Ca2+ 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 (LaCl3 or EGTA) or CPKs inhibitors (TFP or W-7), suggesting that the Ca2+/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 Ca2+/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.