The roles of NAC transcription factors in regulating fruit ripening

Fruit ripening is a key stage in the growth and development of fruit trees. During ripening, fruit becomes soft and the levels of sugars, aroma, and pigments increase. The ripening of a fleshy fruit is not only closely related to the formation of fruit quality, but also to the storability after harvest. The fruit ripening is a genetically programmed and highly coordinated process involved in huge changes in a numbers of gene expressions. Besides the critical role of phytohormones, transcription factors have been known to be involved in fruit ripening. Among the numerous transcription factor families in plants, NAC (NAM, ATAF1/2, and CUC2) transcription factor is one of the largest specific transcription factor families, which consists of a highly conserved N-terminal domain and a highly variable Cterminal transcriptional regulatory domain. Wide varieties of the NAC genes are important for plants response to biotic and abiotic stresses, such as response to light and temperature, salt and drought stress, pathogenic bacteria stress. The NACs have been shown to be involved in the regulation of a numerous ripening related processes in diverse species including tomato, apple, strawberry, banana, peach, litchi, citrus and so on. Increasing evidences have confirmed the key function of NACs in the regulation of fruit ripening including fruit softening, chlorophyll degradation, carotenoid metabolism, biosynthesis of flavonoids and anthocyanins, sugar accumulation, biosynthesis of aroma compounds etc. In the present paper, we reviewed the protein structure, number of the members and the phylogenetic tree of NACs of the different fruit crops, and the function of NAC transcription factors in fruit ripening. Some members of NAC family have been identified and characterized in model plants like Arabidopsis, petunia and to-bacco, as well as fruit crops like apple, citrus, and Pyrus spp. The extensive investigation aided by the availability of several complete plant genomic sequences has identified 253 NAC genes in apple, 185 in pear, 181 in banana, 145 in sweet orange, 70 in grape (Vitis vinifera) which makes them be one of the largest family of TFs in plants. Typically, NAC proteins share a well conserved N-terminal NAC domain and a diversified C- terminal transcription regulatory region. However, a few variations in the structure have also been identified. These structure variations might contribute to the different function among the members. The NAC TFs are associated with the fruit softening by modulating cell wall remodeling proteins including xylosidase, pectate lyase, polygalacturonase, endo-1,4-beta-glucanase, expansin, arabino galactan-proteins. Recent evidence emphasizes the important role of NAC in regulation the biosynthesis of anthocyanins mainly by targeting the promotor of MYBs. Two apple NACs, MdNAC52 and MdNAC42, and a peach NAC (BL) have been shown to promote the anthocyanin biosynthesis by enhancing the expression of the key MYB members. In litchi, a developmental upregulated NAC, LcNAC13, enhance the accumulation of anthocyanins by repressing the expression of the LcR1MYB1. Although the NACs have been shown play critical role in the leaf degreening during senescence, their roles in chlorophyll loss in the fruits and how it works are still largely unclear. But studies have implicated the NAC genes in the regulation of carotenoid biosynthesis by promoting the expression of key synthetic enzyme genes like phytoene desaturase, ζ-carotene desaturase, lycopene cyclase-e, carotene hydroxylase-b. The NACs was found to regulate the biosynthesis of aroma compounds by directly binding to the promotor sequence of ester biosynthesis key alcohol acyltransferase (AAT) gene. The overexpression of apple MdNAC5 enhanced the production of esters through promoting the expression of the MdAAT1. In studies of strawberry and kiwifruit, the NACs were found to regulate the expression of terpene volatility biosynthesis pathway genes such as eugenol synthase 2, nerolidol synthase 1, terpene synthase 1. Phytohormones are essential regulating signals during fruit development and ripening with ethylene and ABA as the critical hormones. The mumerous evidence have suggested that the NACs be involved in fruit ripening regulation through modulating the ethylene biosynthesis and signal transduction. In kiwifruit, AdNAC6/7 enhance ethylene production and fruit ripening by binding the promotor of key ethylene biosynthesis genes, 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and ACC oxidase. The NACs have also been reported to be involved in the biosynthesis and/or signaling of ABA. The expressions of peach NACs were upregulated in response to exogenous ABA. The downregulation of strawberry NAC retarded the expression of ABA biosynthesis key gene FaNCED2/3/5 and FaZEP (zeaxanthin epoxidase). These results indicated that a interaction between NAC transcription factors and phytohormone mainly ethylene and abscisicaid might co-regulate fruit ripening. Hopefully, our paper would provide references for genetic improvement and the development of techniques to manipulate fruit ripening.