Sweet cherry is a valuable non-climacteric fruit with elevated phytonutrients, whose fruit quality attributes are prone to rapid deterioration after harvest, especially peel damage and water loss of stem. Here the metabolic and transcriptional response of exogenous melatonin was assessed in two commercial cultivars of sweet cherry (Santina and Royal Rainier) during cold storage. Gene expression profiling revealed that cuticle composition and water movement may underlie the effect of melatonin in delaying weight loss. An effect of melatonin on total soluble solids and lower respiration rate was observed in both cultivars. Melatonin induces overexpression of genes related to anthocyanin biosynthesis, which correlates with increased anthocyanin levels and changes in skin color (Chroma). Our results indicate that along with modulating antioxidant metabolism, melatonin improves fruit quality traits by triggering a range of metabolic and gene expression changes, which ultimately contribute to extend sweet cherry postharvest storability.Copyright © 2020 Elsevier Ltd. All rights reserved.

Twenty-four edible plants were investigated for the presence of melatonin, heretofore considered to be a molecule found only in the animal kingdom. The amount of melatonin in different plants varied greatly with highest melatonin being present in plants of the rice family. Melatonin was identified by radioimmunoassay and verified by high performance liquid chromatography with fluorescence detection. Feeding a diet containing plant products rich in melatonin to chicks increased radioimmunoassayable levels of melatonin in their blood. Likewise, melatonin extracted from plants inhibited binding of [125I]iodomelatonin to rabbit brain. Thus, melatonin ingested in foodstuffs enters the blood and is capable of binding to melatonin binding sites in the brain of mammals.

Melatonin, a multiple signal molecule, plays important roles in delaying senescence during the development of plants. Because few species have been studied for the effect of exogenous melatonin on anti-aging, the plausible mechanism of melatonin of anti-aging effects on other plant species has remained largely unknown. In the present study, the effects of exogenous melatonin on leaf senescence in kiwifruit were examined during natural aging after melatonin (200 m M) or water (Control) pretreatment. The decreased membrane damage and lower hydrogen peroxide (H2O2) content due to the enhanced scavenging activity of antioxidant enzymes peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) demonstrated that melatonin effectively delayed the aging of kiwifruit leaves. Likewise, owing to up-regulated expression of chlorophyll a/b-binding protein (CAB) gene in the sampled leaves pretreated with melatonin, chlorophyll degradation decreased. Therefore, osmoregulatory substances in sampled leaves accumulated (e.g., soluble sugar and soluble protein) and seedling cell environment stability was maintained. Simultaneously, melatonin decreased H2O2 concentration owing to increased glutathione (GSH) and ascorbate (AsA) content, and the expression levels of glutathione reductase (GR), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) were up-regulated by melatonin application, indicating that the increase of GSH and AsA was attributed to the expression of these genes. In addition, a large amount of flavonoids accumulated in seedlings pretreated with melatonin, and transcript levels of eight genes involved in flavonoid synthesis, including phenylalanine ammonialyase (PAL), cinnamate-4-hydroxymate (C4H), chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), flavonol synthase (FNS), leucoanthocyanin reductase (LAR), anthocyanin reductase (ANR), flavonoid 3-O-glucosyltransferase (UFGT) were enhanced in response to melatonin application. These results indicated that melatonin delayed aging of kiwifruit leaves by activating the antioxidant capacity and enhancing flavonoid biosynthesis. All of these results can provide clear proof that melatonin plays a key roles in delaying leaf senescence.

Anthocyanins are a class of compounds that are widespread in plants, where they provide protection against stresses, and are also beneficial to human health as dietary components. Melatonin application is known to affect anthocyanin production, but the relationship between anthocyanin and melatonin is still unclear. In this study, we analyzed anthocyanin contents and the expression levels of anthocyanin biosynthetic and regulatory genes in tissue cultured plantlets of two Malus crabapple cultivars following various exogenous melatonin treatments under light and dark conditions. The application of exogenous melatonin not only promoted anthocyanin accumulation in leaves, but also increased the contents of flavonols and proanthocyanins (PAs), via a process that was not dependent on light. Quantitative real time PCR (qRT-PCR) analyses indicated that the expression of flavonoid biosynthetic genes, flavonoid related transcription factors and melatonin biosynthetic genes was induced by melatonin. We propose that anthocyanin biosynthesis is regulated by melatonin in crabapple leaves via the expression of flavonoid related transcription factors. This study provides insight into the mechanism of melatonin induction of anthocyanin biosynthesis in woody plants, and suggests that pretreatment with melatonin may represent a cultivation strategy to increase the flavonoid contents of plants.