Melatonin Promotes the Chilling Tolerance of Cucumber Seedlings by Regulating Antioxidant System and Relieving Photoinhibition

Abstract

Chilling adversely affects the photosynthesis of thermophilic plants, which further leads to a decline in growth and yield. The role of melatonin (MT) in the stress response of plants has been investigated, while the mechanisms by which MT regulates the chilling tolerance of chilling-sensitive cucumber remain unclear. This study demonstrated that MT positively regulated the chilling tolerance of cucumber seedlings and that 1.0 μmol⋅L–1 was the optimum concentration, of which the chilling injury index, electrolyte leakage (EL), and malondialdehyde (MDA) were the lowest, while growth was the highest among all treatments. MT triggered the activity and expression of antioxidant enzymes, which in turn decreased hydrogen peroxide (H2O2) and superoxide anion (O2⋅–) accumulation caused by chilling stress. Meanwhile, MT attenuated the chilling-induced decrease, in the net photosynthetic rate (Pn) and promoted photoprotection for both photosystem II (PSII) and photosystem I (PSI), regarding the higher maximum quantum efficiency of PSII (Fv/Fm), actual photochemical efficiency (ΦPSII), the content of active P700 (ΔI/I0), and photosynthetic electron transport. The proteome analysis and western blot data revealed that MT upregulated the protein levels of PSI reaction center subunits (PsaD, PsaE, PsaF, PsaH, and PsaN), PSII-associated protein PsbA (D1), and ribulose-1,5-bisphosphate carboxylase or oxygenase large subunit (RBCL) and Rubisco activase (RCA). These results suggest that MT enhances the chilling tolerance of cucumber through the activation of antioxidant enzymes and the induction of key PSI-, PSII-related and carbon assimilation genes, which finally alleviates damage to the photosynthetic apparatus and decreases oxidative damage to cucumber seedlings under chilling stress.

Melatonin Rescues Photosynthesis and Triggers Antioxidant Defense Response in Cucumis sativus Plants Challenged by Low Temperature and High Humidity

“…LT and HH damage photosynthesis by affecting energy production. Ninety-five percent of the dry matter content of plants is derived from photosynthesis and forms the basis of crop yield ( Zhang et al, 2021 ). The decline in photosynthetic efficiency is the main reason for the reduction in plant biomass under LT and HH conditions ( Amin et al, 2021 ).…”

“…Chlorophyll is essential for harvesting light during photosynthesis. Both LT and HH severely retard or inhibit the synthesis of chlorophyll, which directly impacts plant growth and yield ( Zhang et al, 2021 ). Cold stress degrades photosynthetic pigments by upregulating chlorophyllase activity and downregulating enzymes involved in chlorophyll biosynthesis, thus directly affecting photosynthesis and chlorophyll fluorescence ( Jahan et al, 2021 ).…”

“…A cucumber ACTIN gene was used for data normalization. Three technical replications were performed, and relative gene expression was calculated using the 2 –ΔΔCt method ( Zhang et al, 2021 ).…”

“…Tropical plants are susceptible to environmental factors such as low temperature (LT) ( Zhao et al, 2017 ) and high humidity (HH) ( Han et al, 2019 ), which hamper plant growth and impose physiological stress. Any compromise of plant growth parameters can eventually lead to food scarcity, as has been observed recently in many plant species in response to global climatic changes ( Han et al, 2019 ; Zhang et al, 2021 ). Weather-related changes and frequent fluctuations in temperature have been shown to affect the growth, development, and biochemical and physiological attributes of crop plants ( Sakina et al, 2019 ).…”

“…MT is a small molecule first discovered in the bovine pineal gland ( Lerner et al, 1958 ). This discovery led to the investigation of the impacts of MT on animal biology, including physiological processes such as circadian rhythm, sleep, body temperature, physical activity, retinal physiology, sexual activity, seasonal reproduction, and the immune system ( Zhang et al, 2021 ). Recently, Erdal (2019) revealed that MT plays a critical role in plants as well.…”