FaTRAB1, a bZIP transcription factor, enhances anthocyanin biosynthesis in strawberry leaves via tissue-specific regulation

The bZIP transcription factor FaTRAB1 has been extensively studied as a central regulator of plant adaptation to environmental stresses. However, its role in modulating anthocyanin biosynthesis remains unclear. This study reveals a novel function of FaTRAB1 in regulating anthocyanin accumulation in both strawberry leaves (Fragaria vesca 'Ruegen') and fruits (Fragaria × ananassa 'Benihoppe'). FaTRAB1 is a nucleocytoplasmic-localized protein, exhibiting relatively higher expression in leaves and small green fruits. Stable overexpression of FaTRAB1 in 'Ruegen' strawberry significantly enhanced anthocyanin levels in young, mature, and old leaves. Mass spectrometry and HPLC analysis identified three major anthocyanins in mature strawberry leaves: cyanidin-3-O-glucoside (C3G) and cyanidin-3,5-O-diglucoside (C3,5dG) (78.91%), and peonidin-3,5-O-diglucoside (Pg3,5dG, 12.6%), with pelargonidin-3-O-glucoside (Pg3G, 8.49%) as a minor component. Protein interaction assays demonstrated that FaTRAB1 likely competes with FabHLH3 for binding to FaMYB10 and FaTTG1, forming a novel regulatory complex that coordinately activates the promoter of anthocyanin structural genes (FaF3'H, FaANS, FaUFGT, and FaOMT). This mechanism preferentially promotes the accumulation of cyanidin derivatives (C3,5dG and C3G) in leaves, resulting in reddish-purple pigmentation, while Pg3,5dG and trace Pg3G contribute secondarily. In contrast, strawberry fruits predominantly accumulate Pg3G. Intriguingly, FaTRAB1 may slightly suppress FaMYB10/FaTTG1-mediated biosynthesis of Pg3G and cyanidin-based anthocyanins in fruits. Instead, it primarily enhances C3G accumulation through synergistic interaction with FaF3'H, leading to distinct pigmentation patterns between leaves and fruits. These findings elucidate the tissue-specific regulatory role of FaTRAB1 in strawberry anthocyanin accumulation, providing critical insights into organ-specific coloration mechanisms. This work establishes a theoretical foundation for improving strawberry color quality through targeted genetic manipulation.