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Figure 1. 

The regulatory network of transcription factors (TFs) repressing anthocyanin biosynthesis in horticultural crops. The repressors affect the formation of MBW(①)/other activation complexes (②) to indirectly repress anthocyanin biosynthetic pathway genes (ABPs) or directly inhibit ABPs (③). TFs activating anthocyanin and MYB-bHLH-WD40 (MBW) complex can also induce the expression of repressors to balance the biosynthesis of anthocyanin in horticulture crops. The activation complexes of anthocyanin mainly include MBW-WRKY26^[33] in grape, BBX18-HY5^[34] in pear, and ERF3-MYB114-bHLH3^[55] in pear.

Figure 2. 

Repression motifs and the molecular mechanisms of TFs with repression motifs to repress anthocyanin biosynthesis in horticultural crops. The TFs with bHLH-binding motifs repress anthocyanin biosynthesis by interfering with the formation of MBW complexes. The repression mechanism of the TFs with the C1 motif is unknown. The TFs with EAR motif forming TF-TPL/TPRs-HDAs complexes to inhibit anthocyanin biosynthesis have been identified^[73,74]. In addition, TFs with EAR motif may recruit histone methylation-linked chromatin remodelers (SUVH3 and PKR1) or unknown chromatin remodelers^[58,80] to inhibit anthocyanin biosynthesis. The TFs with TLLLFR motif may interact with TPL/TPRs^[58] or unknown negative regulators to inhibit anthocyanin biosynthesis. The function of unknown repression motifs of TFs in horticultural crops awaits further investigation. MBW is the complex formed by R2R3-MYB, bHLH, and WD40; TPL/TPRs are transcriptional corepressor TOPLESS/TOPLESS-related proteins; HDAs are histone deacetylases.

Figure 3. 

The types and mechanisms of posttranscriptional regulation of transcription factors (TFs) to negatively regulate anthocyanin biosynthesis in horticultural crops. Alternative splicing of anthocyanin-activated TFs and miRNAs-mediated the cleavage of anthocyanin-activated TFs reduce anthocyanin biosynthesis. The translational repression by miRNA in anthocyanin biosynthesis, lncRNAs repressing the biosynthesis of anthocyanins, and circular RNA-mediated regulation of TFs affecting the biosynthesis of anthocyanins in horticulture crops need to be investigated further. AS is alternative splicing; ABPs are anthocyanin biosynthetic pathway genes.

Figure 4. 

The types and mechanisms of post-translation modifications of transcription factors (TFs) to repress anthocyanin biosynthesis in horticultural crops. The top/light green background represents multiple post-translation modifications that may work cooperatively or competitively on the same TFs in horticultural crops. The bottom/light yellow background represents the mechanisms of post-translation modifications of TFs to repress anthocyanin biosynthesis in horticultural crops. ABPs are anthocyanin biosynthetic pathway genes.