MYB-Mediated Regulation of Anthocyanin Biosynthesis

Abstract

Anthocyanins are natural water-soluble pigments that are important in plants because they endow a variety of colors to vegetative tissues and reproductive plant organs, mainly ranging from red to purple and blue. The colors regulated by anthocyanins give plants different visual effects through different biosynthetic pathways that provide pigmentation for flowers, fruits and seeds to attract pollinators and seed dispersers. The biosynthesis of anthocyanins is genetically determined by structural and regulatory genes. MYB (v-myb avian myeloblastosis viral oncogene homolog) proteins are important transcriptional regulators that play important roles in the regulation of plant secondary metabolism. MYB transcription factors (TFs) occupy a dominant position in the regulatory network of anthocyanin biosynthesis. The TF conserved binding motifs can be combined with other TFs to regulate the enrichment and sedimentation of anthocyanins. In this study, the regulation of anthocyanin biosynthetic mechanisms of MYB-TFs are discussed. The role of the environment in the control of the anthocyanin biosynthesis network is summarized, the complex formation of anthocyanins and the mechanism of environment-induced anthocyanin synthesis are analyzed. Some prospects for MYB-TF to modulate the comprehensive regulation of anthocyanins are put forward, to provide a more relevant basis for further research in this field, and to guide the directed genetic modification of anthocyanins for the improvement of crops for food quality, nutrition and human health.

Keywords: MBW complexes; MYB; anthocyanin; environment; negative regulation; positive regulation; transcription factor.

Figure 1

A proposed phylogenetic classification of MYB (v-myb avian myeloblastosis viral oncogene homolog) transcription factors (TFs) (arrows indicate the predicted direction of evolution [52,53]. Different plant protein types vary depending on the number of adjacent MYB duplications (R).

Figure 2

Model of the anthocyanin biosynthetic pathway. Enzymes are indicated in bold uppercase letters [23,26,65]. PAL, Phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; 4CL, Cinnamic acid-4-hydroxylase; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, Flavanone 3-hydroxylase; F3′H, Flavonoid 3′-hydroxylase; F3′5′H, Flavonoid 3′,5′-hydroxylase; ANS, Anthocyanidin synthase; LDOX, Leucoanthocyanidin dioxygenase; UFGT, UDP flavonoid 3-O-glucosyltransferase; UGT, UDP -glucosyltransferase; GST, Glutathione S-transferase; FLS, Flavonol synthase; ANR, Anthocyanidin reductase; EBGs, early biosynthetic gene; LBGs, late biosynthetic gene.

Figure 3

Phylogenetic tree of the regulation of anthocyanin biosynthesis by MYB genes. The tree was constructed based on the entire protein sequences using MEGA 6 software. Blue represents the antagonistic effects of MYB factors in the anthocyanin biosynthetic pathway, and red represents the MYBs as activators to promote the biosynthesis of anthocyanin.

Figure 4

Phylogenetic relationships of conserved protein motifs in MYB transcription factor repressors related to anthocyanin biosynthesis [74]. (A) The phylogenetic tree was constructed based on the entire protein sequences using MEGA 6 software. Accession numbers are listed in Table 1. (B) The motif composition on every branch of the phylogenetic tree is represented by colored boxes. (C) The sequence for motifs R1-2, C1-5 identified by MEME.

Figure 5

Proposed molecular mechanisms of MYB transcription factor in the regulation of expression of target genes [148]. (A) Anthocyanin synthesis is initiated in binding directly to target gene by the MYB activator. (B) The MYB activator, WD40 repeat (WDR), and bHLH1 proteins form an MBW activation complex that activated structural genes and promoted the accumulation of anthocyanin. (C) The MYB activator, WDR, and bHLH1 proteins form an MBW activation complex that activated the expression of bHLH2 and structural genes and promote the accumulation of anthocyanin. (D) MYB repressors are activated by the MBW complex, that binding to the promoter of the terminal structural gene in the anthocyanin biosynthesis pathway. (E) MYB repressors could inhibit anthocyanin biosynthesis by incorporating into the MBW activation complex as a corepressor or by binding to the promoter of target genes directly. (F) Feedback inhibition is provided by R3-MYB repressors; these are activated by the MBW complex and inhibit the formation of new MBW complexes by titrating bHLH factors.

Figure 6

MYB transcription factors regulates the action network of anthocyanin in plants’ internal and external environment. Under light conditions, anthocyanin biosynthesis is initiated in developing leaves or developing flowers/fruits by PHY photoreceptors (1) binding directly to MYB and bHLH. At the same time, CRY (2) and UVR8 (3) photoreceptors could inhibit (4) COP1 factor. In the dark, COP1 degraded MYB transcription factors that reduce the expression of genes activating anthocyanin biosynthesis (5), ultimately inhibiting anthocyanin accumulation. Under cold stress, C repeated binding factor (CBF factor) (6) activated anthocyanin biosynthesis through physical interaction with MYB and bHLH. At the same time, HY5 factor (7) also induces anthocyanin accumulation so that the anthocyanin structural gene could be up-regulated under low temperature under light conditions to activate the downstream network of anthocyanins. Under drought conditions, HAT1 factor (8) negatively regulates anthocyanin accumulation by binding to MBW protein complexes, while in the presence of the plant hormone ABA, ABA could inhibit the physical interaction between HAT1 and MYB75 and activate MYB75 to induce anthocyanin accumulation. When the MYB target sequence was methylated (9), anthocyanin accumulation was also inhibited. Under light conditions, MPK4 (10) interacts with MYB and bHLH and phosphorylates MYB, then participating in the accumulation of light-induced anthocyanins.