Regulation of Anthocyanin Biosynthesis

Regulation of anthocyanin production involves transcriptional activators of the R2R3 MYB and the basic helix-loop-helix (bHLH) (or MYC) types (Table 3.2). This was first revealed by studies of the monocot Z. mays. It was found that the anthocyanin pathway is turned on in this species through the combined action of one member of the COLORED ALEURONEl (C1)/PURPLE PLANT (PL) MYB family and one member of the REDl (R)/BOOSTERl (B) bHLH family. The members of the MYB and bHLH families are functionally redundant, and their specific expression patterns enable spatial and temporal control of anthocyanin biosynthesis. 

Activation of the biosynthetic genes is dependent on direct interaction between the MYB and bHLH TFs within the transcriptional activation complex. The complex binds DNA through discrete cA-elements in the target gene promoters, one that is recognized specifically by the MYB member and one (the ARE) that is recognized by an as yet unidentified protein. The bHLH member functions in part through the ARE, and may be the protein that binds directly to it, or alternatively, interacts with a different protein that binds to it.  

Transcription Factors that Regulate Flavonoid Biosynthetic Genes 

Fragaria x ananasa Anthocyanins and flavonols MYB FaMYBl (repressor) 297 

Ipomoea tricolor Anthocyanins bHLH IVORY SEED 408 

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Except for MYB.Ph2, PI, and P2, the other TFs normally function as regulators of anthocyanin biosynthesis. 

Mathews, H. et al.. Activation tagging in tomato identifies a transcriptional regulator of anthocyanin biosynthesis, modification, and transport. Plant Cell, 15, 1689, 2003. 

As we begin to unravel the molecular mechanisms of the regulation of anthocyanin biosynthesis, we must remember that evolution and selection had significant opportunities to explore new ways to color plants, flowers, and seeds. Model plant systems such as maize, petunia, and Arabidopsis will continue to provide the framework to understand how pigment accumulation is controlled. However, it is important to investigate how nature has exploited variations in these central prototypes to provide the amazing diversity found today in the type and distribution of anthocyanin pigments. 

Deluc L, Bogs J, Walker AR, Ferrier T, Decendit A, Merillon JM, Robinson SP and Barrieu F. 2008. The transcription factor VvMYB5b contributes to the regulation of anthocyanin and proanthocyanin biosynthesis in developing grape berries. Plant Physiol 147 2041-2053. 

The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

R2R3 MYB and HLH transcription factors acting in concert (Table 3.1). More recently, proteins containing conserved WD40 repeats have been implicated in the regulation of flavonoid biosynthesis as well. Thus, it is by variations of the combination of MYB/HLH/WD factors that anthocyanin accumulation is controlled (Table 3.1). 

Stitch K, Eidenberger T, Wurst F, Forkmann G (1992) Flavonol synthase activity and the regulation of flavonol and anthocyanin biosynthesis during flower development in Dianthus caryophyllus. Z Naturforsch 47C 553-560... 

Kobayashi, S. et al., M>>Z)-related genes of the Kyoho grape Vitis labruscana) regulate anthocyanin biosynthesis. Planta, 215, 924, 2002. 

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