Transcription Factor VvbHLH137 Positively Regulates Anthocyanin Accumulation in Grape (Vitis vinifera)

Grape (Vitis vinifera) is a popular fruit with a rich color, favorable taste, and high nutritional quality. The formation of the color of its berries is primarily determined by anthocyanin composition and concentration. Basic helix–loop–helix proteins (bHLHs) serve as critical modulators of anthocyanin synthesis, yet many bHLHs in grape have not been systematically studied and remain uncharacterized. In this study, we tracked and detected berry components in ‘Moldova’ grapes during three developmental stages using UPLC-MS/MS and identified malvidin derivatives as the primary main anthocyanins. Our transcriptome sequencing analysis revealed 40 genes and several transcription factors (TFs) involved in anthocyanin pathways and berry coloration, with VvCHS2 (Vitvi05g01044) showing the highest expression. Among TFs, six bHLH candidates were identified, and VvbHLH137 was determined to positively regulate anthocyanin synthesis. The over-expression of VvbHLH137 in Arabidopsis thaliana significantly augmented the anthocyanin content. In addition, VvbHLH137 was found to form interactions with VvMYB15, VvMYB44, and VvMYB306 to impact anthocyanin accumulation. It also directedly stimulates VvDFR and VvF3H transcription via binding to their promoters. These findings provide insights into anthocyanin synthesis in grapes and support molecular breeding efforts for grape cultivars with enhanced coloration.