Fig. 4
Agroinfiltration of MtLAP1 initiates the anthocyanin gene regulatory network in M. truncatula: proposed model. Upon agroinfiltration of 35S:MtLAP1, MtLAP1 can form an MBW activation complex with constitutively expressed bHLH1 (basic helix-loop-helix clade 1; Medtr8g098275) and WDR (WD-repeat proteins; Medtr3g192840 and/or Medtr7g084810) proteins, which activates expression of bHLH2 (basic helix-loop-helix clade 2; Medtr1g072320; MtTT8). A core MBW complex, containing bHLH2, forms that positively autoregulates bHLH2 expression and activates expression of the anthocyanin biosynthetic genes (represented by DFR; dihydroflavanol 4-reductase) resulting in anthocyanin pigment production. The core MBW also activates expression of MYBrep genes (R2R3-MYB; Medtr4g585530/MtMYB530 and Medtr5g079670/MtMYB2) and R3-MYB (Medtr2g088730/MtMYB730). The inclusion of the MYB repressor into the MBW complex results in the transcriptional repression of target promoters of the core MBW complex (bHLH2, MYBrep and R3-MYB). Feedback inhibition is provided by R3-MYB by inhibiting the formation of new MBW complexes by titrating bHLH1 and bHLH2. Numbers in green are the fold change of expression for each gene significantly differentially expressed (P value <0.05; MtLAP1, bHLH2 = Medtr1g072320/MtTT8; MYBrep = Medtr4g485530; R3-MYB = Medtr2g088730/MtMYB730); upward arrow represents upregulation; 1 Medtr4g585530/MtMYB530 MYBrep; 2 Medtr5g079670/MtMYB2 MYBrep; Asterisk See Fig. 3 for fold change expression of biosynthetic genes