Figure 8.

Model of the Anthocyanin Gene Regulation Network in Eudicots.

(A) During noninductive conditions, active MYB repressors (typically R2R3-MYB) are expressed at high levels, and WDR proteins and bHLH1 (Ph-JAF13/At-EGL3 clade bHLH) are constitutively expressed. Repressor MBW complexes may form, titrating bHLH and WDR proteins. MYB repressors are in excess of R2R3-MYB activators, allowing recruitment of the repressor to target genes by MBW complexes containing an R2R3-MYB activator. Transcription is actively repressed through repression motifs (e.g., EAR domain).

(B) (1) Anthocyanin synthesis is initiated in stressed leaves or developing flowers/fruit by activating the expression of the R2R3-MYB activator. (2) The R2R3-MYB activator, WDR, and bHLH1 (Ph-JAF13/At-EGL3 clade bHLH) proteins form an MBW activation complex that (3) activates the expression of bHLH2 (Ph-AN1/At-TT8 clade bHLH). (4) A core MBW activation complex containing bHLH2 proteins forms; this activates the expression of bHLH2 (reinforcement) and activates the expression of the anthocyanin biosynthesis genes (5), ultimately resulting in anthocyanin accumulation. The MBW complex also activates the expression of the MYB repressor (typically R2R3-MYB) (6), although this activation appears to be overruled by environmental signals in vegetative tissues. The inclusion of the repressor into the MBW complex (7) results in active transcriptional repression of target promoters of the core MBW complex (anthocyanin biosynthetic genes, such as DFR, bHLH2, R3-MYB, and MYB repressor). Feedback inhibition is provided by R3-MYB repressors; these are activated by the MBW complex (8) and inhibit the formation of new MBW complexes (9) by titrating bHLH factors. (10) The R3-MYB repressors and WDR proteins are capable of intercellular movement, which may contribute to pigmentation patterning.