Abstract
Melatonin (MT; N-acetyI-5-methoxytryptamine) is an amine hormone involved in abiotic stress resistance. Previous studies have confirmed that melatonin can promote seed germination, mediate physiological regulation mechanisms, and stimulate crop growth under stress. However, the osmotic regulation mechanism by which exogenous melatonin mediates salt tolerance in cotton is still largely unknown. To investigate the effect of salt stress on melatonin concentration in germinating cotton seeds, we analyzed melatonin content over time during seed germination under different treatments. Melatonin content reached its minimum at day 6, while cotton germination rates peaked at day 6, indicating melatonin content and seed germination are correlated. Then we investigated the effects of 10-100 μM melatonin treatments on membrane lipid peroxides and osmotic adjustment substances during cotton seed germination under salt stress. Salt stress led to electrolyte leakage (EL) as well as accumulations of hydrogen peroxide (H2O2), malondialdehyde (MDA), organic osmotic substances (i.e., proline, soluble sugars), and inorganic osmotic substances (i.e., Na+, Cl-). Meanwhile, the contents of melatonin, soluble proteins, and K+ as well as the K+/Na+ balance decreased, indicating that salt stress inhibited melatonin synthesis and damaged cellular membranes, seriously affecting seed germination. However, melatonin pretreatment at different concentrations alleviated the adverse effects of salt stress on cotton seeds and reduced EL as well as the contents of H2O2, MDA, Na+, and Cl-. The exogenous application of melatonin also promoted melatonin, soluble sugar, soluble proteins, proline, and K+/Na+ contents under salt stress. These results demonstrate that supplemental melatonin can effectively ameliorate the repression of cotton seed germination by enhancing osmotic regulating substances and adjusting ion homeostasis under salt stress. Thus, melatonin may potentially be used to protect cotton seeds from salt stress, with the 20 μM melatonin treatment most effectively promoting cotton seed germination and improving salt stress tolerance.
Highlights
Salt stress is a critical environmental factor that limits the agricultural productivity, survival, and geographical distributions of plants [1]
The exogenous application of melatonin promoted melatonin, soluble sugar, soluble proteins, proline, and K+/Na+ contents under salt stress. These results demonstrate that supplemental melatonin can effectively ameliorate the repression of cotton seed germination by enhancing osmotic regulating substances and adjusting ion homeostasis under salt stress
Exogenous melatonin promotes cotton seed germination under salt stress Seed germination is the most important stage in the life course of seeds, and it provides the nutritional basis for the growth and development of seeds into seedlings
Summary
Salt stress is a critical environmental factor that limits the agricultural productivity, survival, and geographical distributions of plants [1]. The oxidative reaction of free radicals in membrane lipids leads to the accumulation of more reactive oxygen radicals and hydrogen peroxide in plants, which causes cross-linking polymerization of proteins, nucleic acids, and other biomolecules as well as damage to the membrane system, which in turn increases malondialdehyde (MDA) content and electrolyte leakage (EL) as well as lipid peroxidation. Plants mainly resist salt stress damage through a series of physiological activities that includes osmotic regulation, ion transport, and hormone content change [6]. Osmotic regulators in plants mainly include organic osmotic regulators (proline, soluble sugar, soluble protein) and inorganic osmotic regulators (Na+, K+, Cl-), which can together increase cell fluid concentration and reduce osmotic potential as well as maintain intracellular homeostasis and enhance plant resistance to salt stress [7]
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