Melatonin Enhances Cold Tolerance by Regulating Energy and Proline Metabolism in Litchi Fruit

Abstract

Melatonin (MLT) is a vital signaling molecule that regulates multiple physiological processes in higher plants. In the current study, the role of MLT in regulating chilling tolerance and its possible mechanisms in litchi fruit during storage at ambient temperatures after its removal from refrigeration was investigated. The results show that the application of MLT (400 µM, dipping for 20 min) to 'Baitangying' litchi fruit effectively delayed the development of chilling injury (CI) while inhibiting pericarp discoloration, as indicated by higher chromacity values (L*, a*, b*) and anthocyanin levels. MLT treatment suppressed the enhancements of the relative electrical conductivity (REC) and malondialdehyde (MDA) content, which might contribute to the maintenance of membrane integrity in litchi fruit. MLT treatment slowed the decline in cellular energy level, as evidenced by higher adenosine triphosphate (ATP) content and a higher energy charge (EC), which might be ascribed to the increased activities of enzymes associated with energy metabolism including H + -ATPase, Ca 2+ -ATPase, succinate dehydrogenase (SDH), and cytochrome C oxidase (CCO). In addition, MLT treatment resulted in enhanced proline accumulation, which was likely a consequence of the increased activities of ornithine-δ-aminotransferase (OAT) and ∆ 1 -pyrroline-5-carboxylate synthase (P5CS) and the suppressed activity of proline dehydrogenase (PDH). These results suggest that the enhanced chilling tolerance of litchi fruit after MLT treatment might involve the regulation of energy and proline metabolism.Foods 2020, 9, 454 2 of 13 transferred to shelf conditions at ambient temperatures; this is referred to as chilling injury (CI) [4]. Pericarp browning is regarded as the most characteristic CI symptom in chilled litchi fruit and may be the result of cellular de-compartmentalization triggered by membrane spoilage under chilling stress, where the adequate contact of oxidative enzymes and phenol substrates yields brown insoluble pigments [5].Although some safe strategies (such as chitosan-coating [6], modified atmosphere packing [7,8], combined applications of organic acid dipping and foil wrapping [9], L-cysteine application [10], kojic acid treatment [11], and methionine solution immersion [12]) have been demonstrated to ameliorate the chilling stress-induced browning of litchi fruit, there is still an imperative requirement to explore more methods that can reliably enhance the cold tolerance of litchi fruit.Melatonin (N-acetyl-5-methoxytryptamine, MLT) is a ubiquitous bioactive molecule with multiple functions in nature [13]. In higher plants, MLT is extensively distributed in almost all tissues and organs and plays important roles in multifarious physiological processes, including seed germination, floral development, photosynthesis efficiency, maturation and senescence, osmotic adjustment and resistance to numerous environmental stresses [13]. In addition to having plant growth regulator-like functions, MLT has been identified as a powerful quencher o...

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