Anthocyanin Biosynthesis and Degradation Mechanisms in Solanaceous Vegetables: A Review

Abstract

Anthocyanins are a group of polyphenolic pigments that are ubiquitously found in the plant kingdom. In plants, anthocyanins play a role not only in reproduction, by attracting pollinators and seed dispersers, but also in protection against various abiotic and biotic stresses. There is accumulating evidence that anthocyanins have health-promoting properties, which makes anthocyanin metabolism an interesting target for breeders and researchers. In this review, the state of the art knowledge concerning anthocyanins in the Solanaceous vegetables, i.e., pepper, tomato, eggplant, and potato, is discussed, including biochemistry and biological function of anthocyanins, as well as their genetic and environmental regulation. Anthocyanin accumulation is determined by the balance between biosynthesis and degradation. Although the anthocyanin biosynthetic pathway has been well-studied in Solanaceous vegetables, more research is needed on the inhibition of biosynthesis and, in particular, the anthocyanin degradation mechanisms if we want to control anthocyanin content of Solanaceous vegetables. In addition, anthocyanin metabolism is distinctly affected by environmental conditions, but the molecular regulation of these effects is poorly understood. Existing knowledge is summarized and current gaps in our understanding are highlighted and discussed, to create opportunities for the development of anthocyanin-rich crops through breeding and environmental management.

Keywords: MYB transcription factor; Solanaceae; anthocyanin biosynthesis; anthocyanin degradation; chemical structure; discoloration; environmental regulation; light dependence.

Figure 1

Example of Solanaceous vegetables rich in anthocyanins. (A) purple pepper fruit, (B) purple eggplant fruit, (C) purple tomato fruit, (D) purple potato tuber, (E) red potato tuber.

Figure 2

General chemical structure of anthocyanidins and the six most common anthocyanidins in Solanaceous vegetables, indicated by “X”.

Figure 3

General structure of the most abundant anthocyanins in Solanaceous vegetables, anthocyanidin-3-(p-coumaroyl-rutinoside)-5-glucoside.

Figure 4

Schematic representation of the anthocyanin biosynthetic pathway. CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3′H, flavonoid 3′-hydroxylase; F3′5′H, flavonoid 3′,5′-hydroxylase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanidin synthase; UFGT, flavonoid 3-O-glucosyltransferase; FLS, flavonol synthase. The “^*” means multiplication.

Figure 5

A simplified model depicting the regulatory mechanism of transcription factors, MYB, bHLH and WD40, that modulate the expression of structural genes of the anthocyanin biosynthetic pathway. (A) Activate regulation of anthocyanin biosynthesis. (B) Repressive regulation of anthocyanin biosynthesis. MYB repressors compete with MYB activators for bHLH JAF13. (C) Repressive regulation of anthocyanin biosynthesis. MYB repressors compete with MYB activators for bHLH AN1. The “ → ” means activation, “—|” means repression and “X” means inactivation.

Figure 6

Anthocyanin accumulation and discoloration profile in pepper fruits of cv. Tequila during fruit development.

Figure 7

A schematic model of transcriptional regulation of vacuolar acidification and color fading in petunia petals under control of the PhPH4-PhAN1-PhAN11-PhPH3 complex.

Figure 8

A model for light-dependent anthocyanin biosynthesis in eggplants (based on Jiang et al., 2016b). The “ → ” means activation, “—|” means repression and “X” means inactivation.