Abstract
Melatonin (MT) and nitric oxide (NO) are two multifunctional signaling molecules that are involved in the response of plants to abiotic stresses. However, how MT and NO synergize in response to cold stress affecting plants is still not clear. In this study, we found that endogenous MT accumulation under cold stress was positively correlated with cold tolerance in different varieties of cucumber seedlings. The data presented here also provide evidence that endogenous NO is involved in the response to cold stress. About 100 μM MT significantly increased the nitrate reductase (NR) activity, NR-relative messenger RNA (mRNA) expression, and endogenous NO accumulation in cucumber seedlings. However, 75 μM sodium nitroprusside (SNP, a NO donor) showed no significant effect on the relative mRNA expression of tryptophan decarboxylase (TDC), tryptamine-5-hydroxylase (T5H), serotonin-N-acetyltransferase (SNAT), or acetylserotonin O-methyltransferase (ASMT), the key genes for MT synthesis and endogenous MT levels. Compared with H2O treatment, both MT and SNP decreased electrolyte leakage (EL), malondialdehyde (MDA), and reactive oxygen species (ROS) accumulation by activating the antioxidant system and consequently mitigated cold damage in cucumber seedlings. MT and SNP also enhanced photosynthetic carbon assimilation, which was mainly attributed to an increase in the activity and mRNA expression of the key enzymes in the Calvin–Benson cycle. Simultaneously, MT- and SNP-induced photoprotection for both photosystem II (PSII) and photosystem I (PSI) in cucumber seedlings, by stimulating the PsbA (D1) protein repair pathway and ferredoxin-mediated NADP+ photoreduction, respectively. Moreover, exogenous MT and SNP markedly upregulated the expression of chilling response genes, such as inducer of CBF expression (ICE1), C-repeat-binding factor (CBF1), and cold-responsive (COR47). MT-induced cold tolerance was suppressed by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a specific scavenger of NO). However, p-chlorophenylalanine (p-CPA, a MT synthesis inhibitor) did not affect NO-induced cold tolerance. Thus, novel results suggest that NO acts as a downstream signal in the MT-induced plant tolerance to cold stress.
Highlights
Cucumbers (Cucumis sativus L.) are sensitive to cold stress and are mainly cultivated through the winter in solar greenhouses in northern China; they often encounter cold stress
We first determined whether endogenous MT levels are related to cold tolerance by comparing the changes in endogenous MT, the activity and relative messenger RNA (mRNA) expression of MT synthase enzymes in response to cold stress, and the Fv/Fm, ΦPSII, MDA content, electrolyte leakage (EL), and chilling injury index (CI) in different varieties of cucumber
The increases in MT content and the activities and relative mRNA expression of tryptophan decarboxylase (TDC) and acetylserotonin O-methyltransferase (ASMT) were most significant after 12 h at 5◦C, but an increase in 2-hydroxymelatonin was most significant after the exposure of seedlings to cold stress for 6 h
Summary
Cucumbers (Cucumis sativus L.) are sensitive to cold stress and are mainly cultivated through the winter in solar greenhouses in northern China; they often encounter cold stress. It has been reported that plants can improve their cold tolerance to ensure growth and development mainly through stimulating antioxidant system activity and osmotic adjusting ability (Pan et al, 2020). Almost all studies consider that MT plays a critical role in ROS scavenging, either as a stress-induced agent or a protective molecule (Arnao and Hernandez-Ruiz, 2015; Erland et al, 2018; Martinez et al, 2018; Li et al, 2021). Some evidence has indicated that MT, acting as a phytohormone-like molecule or secondary messenger, participates in many signaling events, including the response to abiotic stress in plants (Yan et al, 2019). MT appears to act as a key molecule in the plant immune response, together with other well-known molecules, such as nitric oxide (NO), and hormones, such as JA and SA (Arnao and Hernandez-Ruiz, 2018)
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