Genome-Wide Identification of the MYB Gene Family in Bougainvillea and Silencing-Based Evidence for the Involvement of BbMYB6 and BbMYB69 in Yellow and Red Bract Coloration

MYB transcription factors (TFs) serve pivotal regulatory functions in plant pigmentation; however, the composition of the MYB family in Bougainvillea spp. and the regulatory mechanisms underlying bract coloration have not yet been systematically examined. Here, 163 BbMYB TFs were detected from the Bougainvillea genome through bioinformatic methods and categorized into three subfamilies: 1R-MYB (13 members), R2R3-MYB (144 members), and 3R-MYB (six members). Phylogenetic analysis further assigned the BbMYB proteins to 15 subgroups. Conserved motif analysis showed that most BbMYB proteins contained conserved motifs at the N-termini and that the R2 and R3 repeat regions of R2R3-MYB proteins collectively possessed five highly conserved tryptophan residues. Gene structure analysis demonstrated that BbMYB genes contained 0 to 12 introns and exhibited conserved intron distribution patterns within the same subgroups. Promoter cis-acting element analysis revealed 54 total elements, classified into four categories: hormone-responsive, stress-responsive, development-related, and light-responsive elements. According to transcriptomic data and reverse transcription quantitative polymerase chain reaction validation, BbMYB6, BbMYB8, and BbMYB10 were significantly upregulated in yellow bracts, whereas BbMYB69, BbMYB89, and BbMYB148 were significantly upregulated in red bracts. Virus-induced gene silencing experiments further demonstrated that silencing BbMYB6 caused fading in yellow bracts and a significant reduction in flavonoid content, whereas silencing BbMYB69 caused fading in red bracts and a significant decrease in betacyanin content, suggesting that these two genes are involved in positively regulating the coloration of yellow and red bracts, respectively. This work comprehensively analyzed the MYB gene family in Bougainvillea, pinpointed essential candidate genes linked to bract coloration, and established a theoretical basis, along with genetic resources, for molecular breeding of bract color in Bougainvillea.