The NAC transcription factor (TF) family is one of the largest plant-specific TF families and its members are involved in the regulation of many vital biological processes during plant growth and development. Recent studies have found that NAC TFs play important roles during the ripening of fleshy fruits and the development of quality attributes. This review focuses on the advances in our understanding of the function of NAC TFs in different fruits and their involvement in the biosynthesis and signal transduction of plant hormones, fruit textural changes, color transformation, accumulation of flavor compounds, seed development and fruit senescence. We discuss the theoretical basis and potential regulatory models for NAC TFs action and provide a comprehensive view of their multiple roles in modulating different aspects of fruit ripening and quality.

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...

SUMMARY Fruit ripening in tomato (Solanum lycopersicum) is the result of selective expression of ripening‐related genes, which are regulated by transcription factors (TFs). The NAC (NAM, ATAF1/2, and CUC2) TF family is one of the largest families of plant‐specific TFs and members are involved in a variety of plant physiological activities, including fruit ripening. Fruit ripening‐associated NAC TFs studied in tomato to date include NAC‐NOR (non‐ripening), SlNOR‐like1 (non‐ripening like1), SlNAC1, and SlNAC4. Considering the large number of NAC genes in the tomato genome, there is little information about the possible roles of other NAC members in fruit ripening, and research on their target genes is lacking. In this study, we characterize SlNAM1, a NAC TF, which positively regulates the initiation of tomato fruit ripening via its regulation of ethylene biosynthesis. The onset of fruit ripening in slnam1‐deficient mutants created by CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats and CRISPR‐associated protein 9) technology was delayed, whereas fruit ripening in OE‐SlNAM1 lines was accelerated compared with the wild type. The results of RNA‐sequencing (RNA‐seq) and promoter analysis suggested that SlNAM1 directly binds to the promoters of two key ethylene biosynthesis genes (1‐aminocyclopropane‐1‐carboxylate synthase: SlACS2 and SlACS4) and activates their expression. This hypothesis was confirmed by electrophoretic mobility shift assays and dual‐luciferase reporter assay. Our findings provide insights into the mechanisms of ethylene production and enrich understanding of the tomato fruit ripening regulatory network.

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