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

Grape (<i>Vitis vinifera</i>) 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 <i>VvCHS2</i> (<i>Vitvi05g01044</i>) showing the highest expression. Among TFs, six bHLH candidates were identified, and <i>VvbHLH137</i> was determined to positively regulate anthocyanin synthesis. The over-expression of <i>VvbHLH137</i> in Arabidopsis thaliana significantly augmented the anthocyanin content. In addition, <i>VvbHLH137</i> was found to form interactions with <i>VvMYB15</i>, <i>VvMYB44</i>, and <i>VvMYB306</i> to impact anthocyanin accumulation. It also directedly stimulates <i>VvDFR</i> and <i>VvF3H</i> 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.