# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 2 Main Physiological and Biochemical Changes Are Regulated by Ripening Genes
### Colour Changes during Ripening
Many fruits are enriched with anthocyanins, such as cyanindin-3-glucoside (C3G), which is the predominant anthocyanin in Chinese bayberry [35], mulberry, apple peel [36; 37; 38] and peach skin [39], and other predominant anthocyanins such as pelargonidin in strawberry [40], cyanidin and delphinidin in grape skin [41] and cyanidin-3-rutinoside in cherry [42]. Transcriptional regulation of the downstream structural genes (such as chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3\({}^{\prime}\)H), flavonol synthase (FLS), dihydroflavonol 4-reductase (DFR), flavone 3'-\(O\)-methyltransferasease 1 (OMT1) and anthocyanin synthase (ANS)) governs the three main branches that lead to the production of the different flavonoid pigments in many plant species [43] (Figure 2). Expression of many of the anthocyanin biosynthetic genes is regulated by a conserved mechanism involving a complex comprised of three types of TFs: MYB + bHLH (basic helix-loop-helix) + WD40 (WD40 repeat protein) (MBW) [45; 46]. There are many different types of MYBs, including transcriptional activators, and some, such as the R3-MYB protein MYBL2 or the R2R3-MYB protein MYB27, possess a C-terminal EAR motif and act as repressors [47]. External colour change mechanisms are not universal, however, and ripe 'Hayward' kivirit remain green, or greenish brown externally and light green internally, whereas 'Hort16A' kiwritunt turn gold internally [48]. Fruit inner pericarp of _Actimidia chinensis_ cv. Hort2D has red flesh, because AcMYBF110 is thought to activate the transcription of DFR, ANS and F3 GT1 [49]. In addition, upregulation of _MYBA1-1_ and _MYB5-1_ by low temperature enhances anthocyanin accumulation in 'Hongyang' kiwifruit by transcriptional activation of ANS1, ANS2, DFR1, DFR2 and UFGT2 [50]. Also, different mechanisms are not mutually exclusive, and several can operate in the same fruit. For example, during ripening of some apples, the chloroplasts in the peel cells lose chlorophyll and accumulate carotenoids, and sometimes also anthocyanins, although some varieties remain green [38; 51]. Tomatoes normally lose chlorophyll and accumulate carotenoids, but _\"greenflesh\"_ mutant fruit retain some thylakoids and chlorophylls, and at the same time, they accumulate carotenoids, and consequently, the fruit appear a dirty-brown colour [52]. Furthermore, tomatoes can also accumulate anthocyanins as well as carotenoids if they are manipulated to express the  
Figure 2: The anthocyanins biosynthetic pathway and variations in different fruits. This figure is redrawn and modified from [PERSON] et al. [44]. The flavonol biosynthetic pathway is illustrated, showing enzymatic steps, and the responsible enzyme is indicated in red. CHI, chalcone isomerase; CHS, chalcone synthase; F3H, flavanone 3-hydroxylase; F3\({}^{\prime}\)H, flavonoid 3’-hydroxy; F3\({}^{\prime}\)5\({}^{\prime}\)H, flavonoid 3’,5’-hydroxylase; FLS, flavonol synthase; DFR, dihydroflavonol reductase; OMT1, flavone 3’-\(O\)-methyltransferasease 1.  
necessary genes [53]. Strawberries, black grapes and Chinese bayberry are examples of fruits that lose chlorophyll and normally only accumulate anthocyanins [40; 54; 55].