Melatonin Application Improves Salt Tolerance of Alfalfa (Medicago sativa L.) by Enhancing Antioxidant Capacity

Abstract

Alfalfa (Medicago sativa L.) is an important and widely cultivated forage grass. The productivity and forage quality of alfalfa are severely affected by salt stress. Melatonin is a bioactive molecule with versatile physiological functions and plays important roles in response to various biotic and abiotic stresses. Melatonin has been proven efficient in improving alfalfa drought and waterlogging tolerance in recent studies. In our reports, we applied melatonin exogenously to explore the effects of melatonin on alfalfa growth and salt resistance. The results demonstrated that melatonin application promoted alfalfa seed germination and seedling growth, and reduced oxidative damage under salt stress. Further application research found that melatonin alleviated salt injury in alfalfa plants under salt stress. The electrolyte leakage, malondialdehyde (MDA) content and H 2 O 2 content were significantly reduced, and the activities of catalase (CAT), peroxidase (POD), and Cu/Zn superoxide dismutase (Cu/Zn-SOD) were increased with melatonin pretreatment compared to control plants under salt stress with the upregulation of genes related to melatonin and antioxidant enzymes biosynthesis. Melatonin was also involved in reducing Na + accumulation in alfalfa plants. Our study indicates that melatonin plays a primary role as an antioxidant in scavenging H 2 O 2 and enhancing activities of antioxidant enzymes to improve the salt tolerance of alfalfa plants.Plants 2020, 9, 220 2 of 17 world's irrigated land affected by saline or sodic globally [7,8]. Salinity has become one of the most important environmental stress factors impairing worldwide agricultural productivity. Under salinity conditions, the seed germination, growth and development processes of alfalfa are inhibited and they finally impair the herbage yield, as well as the forage quality [9]. Breeding new alfalfa cultivars with high salt tolerance is always needed. Genetic engineering and conventional breeding have been proven efficient for improving salt tolerance of various plant species [10][11][12][13]. However, they are time-consuming and complicated. Exogenous application of certain plant growth regulators such as phytohormone and other small molecules has been proven efficient at overcoming the harmful effects of salt stress on plants [14][15][16][17]. Moreover, plant growth regulators in low concentrations always play a role in plants, and are cost-effective. Foliar spraying of salicylic acid (SA) on faba bean inhibits Na + accumulation and lipid peroxidation, improving the antioxidant resistance and finally mitigating the damage caused by salinity [18]. Exogenously applied poly-γ-glutamic acid on wheat maintains the Na + and K + homeostasis and enhances antioxidant capacity by alleviating salinity damage under salt stress conditions [19]. Exogenous spermidine application to salt-stressed cucumber improves the photosynthetic capacity and the activity of key enzymes for CO 2 fixation by regulating the expression of related genes, and tolerance to salinity is thus...

Reactive Oxygen Species and Antioxidant Defense in Plants under Abiotic Stress: Revisiting the Crucial Role of a Universal Defense Regulator

“…Medicago sativa 250 mM NaCl; 2 w Melatonin increased the activities of CAT, POX, and Cu/Zn-SOD. [210] Cucumis sativus 150 mM NaCl; 3 d Melatonin increased CAT, SOD, POD, and APX by 23%, 29%, 15%, and 16%, respectively.…”

The application of artificial neural networks in modeling and predicting the effects of melatonin on morphological responses of citrus to drought stress

“…results, previous studies revealed that the exogenous application of melatonin can improve morphological responses under drought stress in different plants such as tobacco [12], Camellia sinensis [13], cucumber [14], soybean [15], alfalfa [16], rice [17], maize [18], cotton [19], grape [20], and kiwi [21]. It seems that melatonin as a novel plant growth regulator, independently of auxin signaling, plays a pivotal role in shoot and root growth and development [10,11].…”

“…Therefore, it is necessary to apply exogenous melatonin for coping adverse environmental conditions because, sometimes, the endogenous level of melatonin is insufficient for this aim. Several studies have shown that the application of exogenous melatonin could significantly improve stress tolerance in different plants such as tobacco [12], Camellia sinensis [13], cucumber [14], soybean [15], alfalfa [16], rice [17], maize [18], cotton [19], grape [20], and kiwi [21]. Therefore, applying melatonin can be considered as a short-term and promising approach for alleviating drought stress in various economically important plants such as citrus.…”

Morphological, Physiological, and Genetic Responses to Salt Stress in Alfalfa: A Review

“…Under salt stress, the unsaturated fatty acids of plant membranes are decomposed to malondialdehyde (MDA), and the rate of lipid peroxidation in terms of MDA can be used as a biochemical indicator to evaluate salt-tolerant cultivars [30,64]. The salt tolerance of alfalfa is also improved by melatonin application, which acts as an antioxidant in scavenging H 2 O 2 and enhances antioxidant enzymes' activities [65]. Under salt stress, tolerant alfalfa shows less severe cell membrane damage and lower accumulation of reactive oxygen species (ROS) than salt sensitive types [66].…”