Melatonin triggers tissue-specific changes in anthocyanin and hormonal contents during postharvest decay of Angeleno plums

Fruit-specific effects of tryptophan and melatonin as active components to extend the functionality of red fruits during post-harvest processing

Preserving quality attributes in the distribution chain is a challenging task, particularly in fruits with a brief shelf life. The application of melatonin in cherries, raspberries, strawberries and blueberries stored at room temperature was evaluated, as well as the effects of its precursor (tryptophan) to determine their specificity and interchangeable feasibility for post-harvest applications. The results demonstrated that melatonin is effective in all tested fruits, reducing deterioration rate and its severity, preserving fruit firmness and reducing darkening and weight loss. Furthermore, tryptophan applications incremented melatonin contents in strawberries and blueberries and delayed decay in both fruits. Melatonin reduced postharvest losses in all studied fruits related to its antisenescent properties, while the beneficial impact of tryptophan in extending shelf life was fruit-specific and appeared to be partly mediated by melatonin. Melatonin and tryptophan must be considered as active components of new formulations for extending the shelf life of red fruits during post-harvest processing.

The role of melatonin in delaying senescence and maintaining quality in postharvest horticultural products

The postharvest lifespan of horticultural products is closely related to loss of nutritional quality, accompanied by a rapid decline in shelf life, commercial value, and marketability. Melatonin (MT) application not only maintains quality but also delays senescence in horticultural products. This paper reviews biosynthesis and metabolism of endogenous MT, summarizes significant effects of exogenous MT application on postharvest horticultural products, examines regulatory mechanisms of MT-mediated effects, and provides an integrated review for understanding the positive role of MT in senescence delay and quality maintenance. As a multifunctional molecule, MT coordinates other signal molecules, such as ABA, ETH, JA, SA, NO, and Ca2+, to regulate postharvest ripening and senescence. Several metabolic pathways are involved in regulation of MT during postharvest senescence, including synthesis and signal transduction of plant hormones, redox homeostasis, energy metabolism, carbohydrate metabolism, and degradation of pigment and cell wall components. Moreover, MT regulates expression of genes related to plant hormones, antioxidant systems, energy generation, fruit firmness and colour, membrane integrity, and carbohydrate storage. Consequently, MT could become an emerging and eco-friendly preservative to extend shelf life and maintain postharvest quality of horticultural products.

Views and perspectives on the indoleamines serotonin and melatonin in plants: past, present and future

In the decades since their discovery in plants in the mid-to-late 1900s, melatonin (N-acetyl-5-methoxytryptamine) and serotonin (5-methoxytryptamine) have been established as their own class of phytohormone and have become popular targets for examination and study as stress ameliorating compounds. The indoleamines play roles across the plant life cycle from reproduction to morphogenesis and plant environmental perception. There is growing interest in harnessing the power of these plant neurotransmitters in applied and agricultural settings, particularly as we face increasingly volatile climates for food production; however, there is still a lot to learn about the mechanisms of indoleamine action in plants. A recent explosion of interest in these compounds has led to exponential growth in the field of melatonin research in particular. This concept paper aims to summarize the current status of indoleamine research and highlight some emerging trends.

Mechanism of induced soluble sugar accumulation and organic acid reduction in plum fruits by application of melatonin

Melatonin (MT) can improve plant resistance and fruit quality. The mechanism by which MT affects soluble sugar and organic acids accumulation in drupe fruits is not clear. In this study, 100 µmol/L MT was sprayed on the leaves of plum trees at the second stage of rapid fruit expansion (90 and 97 d after flowering), and the effects of MT on plum fruit quality and its effects on the soluble sugar-organic acid metabolism were investigated. At 28 d after MT treatment (at maturity), the longitudinal diameter, fruit weight, and vitamin C content of plum fruits were increased by 5.05%, 12.93%, and 56.09%, respectively, compared to the control. MT caused significant increase in the total soluble solids content and decreased the titratable acid content. MT increased the contents of total soluble sugar, sucrose, sorbitol, and citric acid after 21 and 28 days of treatment, while decreasing the contents of fructose, malic acid, quinic acid, and tartaric acid after 28 days of treatment. Additionally, MT increased the activities of sucrose synthase (catabolism direction), sucrose phosphate synthase, glucokinase, fructokinase, sorbitol oxidase, and NADP+-malic enzyme, and decreased the activities of soluble acid converting enzyme, cell wall insoluble converting enzyme, NAD+-sorbitol dehydrogenase, and NAD+-malic dehydrogenase after 21 or 28 days of treatment. Moreover, the differentially expressed genes (DEGs) after 21 and 28 days of treatment were accelerated starch and sucrose metabolism, galactose metabolism, fructose and mannose metabolism, as well as glycolysis, gluconeogenesis, and pentose phosphate metabolism pathways. In conclusion, exogenous MT increases soluble sugar content and decreases organic acid content in plum fruits by regulating various soluble sugar-organic acid metabolic pathways, thereby improving the fruit quality.

Exogenous Melatonin Enhances Dihydrochalcone Accumulation in Lithocarpus litseifolius Leaves via Regulating Hormonal Crosstalk and Transcriptional Profiling

Dihydrochalcones (DHCs) constitute a specific class of flavonoids widely known for their various health-related advantages. Melatonin (MLT) has received attention worldwide as a master regulator in plants, but its roles in DHC accumulation remain unclear. Herein, the elicitation impacts of MLT on DHC biosynthesis were examined in Lithocarpus litseifolius, a valuable medicinal plant famous for its sweet flavor and anti-diabetes effect. Compared to the control, the foliar application of MLT significantly increased total flavonoid and DHC (phlorizin, trilobatin, and phloretin) levels in L. litseifolius leaves, especially when 100 μM MLT was utilized for 14 days. Moreover, antioxidant enzyme activities were boosted after MLT treatments, resulting in a decrease in the levels of intracellular reactive oxygen species. Remarkably, MLT triggered the biosynthesis of numerous phytohormones linked to secondary metabolism (salicylic acid, methyl jasmonic acid (MeJA), and ethylene), while reducing free JA contents in L. litseifolius. Additionally, the flavonoid biosynthetic enzyme activities were enhanced by the MLT in leaves. Multiple differentially expressed genes (DEGs) in RNA-seq might play a crucial role in MLT-elicited pathways, particularly those associated with the antioxidant system (SOD, CAT, and POD), transcription factor regulation (MYBs and bHLHs), and DHC metabolism (4CL, C4H, UGT71K1, and UGT88A1). As a result, MLT enhanced DHC accumulation in L. litseifolius leaves, primarily by modulating the antioxidant activity and co-regulating the physiological, hormonal, and transcriptional pathways of DHC metabolism.

Nanovehicles for melatonin: a new journey for agriculture

The important role of melatonin in plant growth and metabolism together with recent advances in the potential use of nanomaterials have opened up interesting applications in agriculture. Various nanovehicles have been explored as melatonin carriers in animals, and it is now important to explore their application in plants. Recent findings have substantiated the use of silicon and chitosan nanoparticles (NPs) in targeting melatonin to plant tissues. Although melatonin is an amphipathic molecule, nanocarriers can accelerate its uptake and transport to various plant organs, thereby relieving stress and improving plant shelf-life in the post-harvest stages. We review the scope and biosafety concerns of various nanomaterials to devise novel methods for melatonin application in crops and post-harvest products.

Dissecting postharvest chilling injuries in pome and stone fruit through integrated omics

Lowering the storage temperature is an effective method to extend the postharvest and shelf life of fruits. Nevertheless, this technique often leads to physiological disorders, commonly known as chilling injuries. Apples and pears are susceptible to chilling injuries, among which superficial scald is the most economically relevant. Superficial scald is due to necrotic lesions of the first layers of hypodermis manifested through skin browning. In peaches and nectarines, chilling injuries are characterized by internal symptoms, such as mealiness. Fruits with these aesthetic or compositional/structural defects are not suitable for fresh consumption. Genetic variation is a key factor in determining fruit susceptibility to chilling injuries; however, physiological, or technical aspects such as harvest maturity and storage conditions also play a role. Multi-omics approaches have been used to provide an integrated explanation of chilling injury development. Metabolomics in pome fruits specifically targets the identification of ethylene, phenols, lipids, and oxidation products. Genomics and transcriptomics have revealed interesting connections with metabolomic datasets, pinpointing specific genes linked to cold stress, wax synthesis, farnesene metabolism, and the metabolic pathways of ascorbate and glutathione. When applied to Prunus species, these cutting-edge approaches have uncovered that the development of mealiness symptoms is linked to ethylene signaling, cell wall synthesis, lipid metabolism, cold stress genes, and increased DNA methylation levels. Emphasizing the findings from multi-omics studies, this review reports how the integration of omics datasets can provide new insights into understanding of chilling injury development. This new information is essential for successfully creating more resilient fruit varieties and developing novel postharvest strategies.

Active coatings based on oxidized chitin nanocrystals and silk fibroins for the control of anthracnose in 'Hass' avocados

Postharvest avocado losses are mainly due to anthracnose disease caused by Colletotrichum gloeosporioides. Chemical fungicides are effective, but their negative effects on health and the environment have led to the search for sustainable alternatives such as biopolymer-based coatings and natural compounds. Therefore, chitin nanocrystals (NCChit) were extracted using a sustainable deep eutectic solvent (DES) and chemically modified into oxidized chitin nanocrystals (O-NCChit) or deacetylated chitin nanocrystals (D-NCChit) to modulate and increase the charge surface density and the dispersibility of the crystals. The modified NCChits were dispersed with silk fibroins (SF), essential oil (EO), melatonin (MT) and/or phenylalanine (Phe) to elaborate active coatings. Antioxidant and antifungal in vitro analyses showed that the O-NCChit/SF-based coating had the best performance. In addition, in vivo tests were carried out through the artificial inoculation of C. gloeosporioides on coated avocados. O-NCChit/SF/MT-based coatings reduced the severity of anthracnose by 45 %, the same effect as the chemical fungicide (Prochloraz®). Moreover, avocado quality parameters during cold storage and the shelf-life period were also evaluated, where nonsignificant differences were observed. Therefore, this study demonstrates the great potential of O-NCChit and SF in combination with active compounds for the control of anthracnose in 'Hass' avocados.

Experimental approaches in studying active biomolecules modulating fruit ripening: Melatonin as a case study

Phytohormones are naturally occurring small organic molecules found at low concentrations in plants. They perform essential functions in growth and developmental processes, from organ initiation to senescence, including fruit ripening. These regulatory molecules are studied using different experimental approaches, such as performing exogenous applications, evaluating endogenous levels, and/or obtaining genetically modified lines. Here, we discuss the advantages and limitations of current experimental approaches used to study active biomolecules modulating fruit ripening, focusing on melatonin. Although melatonin has been implicated in fruit ripening in several model fruit crops, current knowledge is affected by the different experimental approaches used, which have given different and sometimes even contradictory results. The methods of application and the doses used have produced different results in studies based on exogenous applications, while different measurement methods and ways of expressing results explain most of the variability in studies using correlative analyses. Furthermore, studies on genetically modified crops have focused on tomato (Solanum lycopersicum L.) plants only. However, TILLING and CRISPR methodologies are becoming essential tools to complement the results from the experimental approaches described above. This will not only help the scientific community better understand the role of melatonin in modulating fruit ripening, but it will also help develop technological advances to improve fruit yield and quality in major crops. The combination of various experimental approaches will undoubtedly lead to a complete understanding of the function of melatonin in fruit ripening in the near future, so that this knowledge can be effectively transferred to the field.

Crosstalk between melatonin and reactive oxygen species in fruits and vegetables post-harvest preservation: An update

Fruits and vegetables contain numerous nutrients, such as vitamins, minerals, phenolic compounds, and dietary fibers. They reduce the incidence of cardiovascular diseases and the risk of certain chronic diseases, and improve the antioxidant and anti-inflammatory capacity. Moreover, melatonin was found in various fruits and vegetables species. Melatonin acts as a multifunctional compound to participate in various physiological processes. In recent years, many advances have been found that melatonin is also appraised as a key modulator on the fruits and vegetables post-harvest preservation. Fruits and vegetables post-harvest usually elicit reactive oxygen species (ROS) generation and accumulation. Excess ROS stimulate cell damage, protein structure destruction, and tissue aging, and thereby reducing their quality. Numerous studies find that exogenous application of melatonin modulates ROS homeostasis by regulating the antioxidant enzymes and non-enzymatic antioxidants systems. Further evidences reveal that melatonin often interacts with hormones and other signaling molecules, such as ROS, nitric oxide (NO), hydrogen sulfide (H₂S), and etc. Among these 'new' molecules, crosstalks of melatonin and ROS, especially the H₂O₂ produced by RBOHs, are provided in fruits and vegetables post-harvest preservation in this review. It will provide reference for complicated integration of both melatonin and ROS as signal molecules in future study.