Ectopic Expression of a R2R3-MYB Transcription Factor Gene LjaMYB12 from Lonicera japonica Increases Flavonoid Accumulation in Arabidopsis thaliana

Lonicera japonica Thunb. is a widely used medicinal plant and is rich in a variety of active ingredients. Flavonoids are one of the important components in L. japonica and their content is an important indicator for evaluating the quality of this herb. To study the regulation of flavonoid biosynthesis in L. japonica, an R2R3-MYB transcription factor gene LjaMYB12 was isolated and characterized. Bioinformatics analysis indicated that LjaMYB12 belonged to the subgroup 7, with a typical R2R3 DNA-binding domain and conserved subgroup 7 motifs. The transcriptional level of LjaMYB12 was proportional to the total flavonoid content during the development of L. japonica flowers. Subcellular localization analysis revealed that LjaMYB12 localized to the nucleus. Transactivation activity assay indicated that LjaMYB12 was a transcriptional activator. Then, ectopic expression of LjaMYB12 in Arabidopsis could increase PAL activity and flavonoid content and promote transcription of a range of flavonoid biosynthetic genes. Interestingly, the fold changes of downstream genes in the flavonoid biosynthetic pathway were significantly higher than that of the upstream genes, which suggested that LjaMYB12 may have different regulatory patterns for the upstream and downstream pathways of flavonoid biosynthesis. The results provided here will effectively facilitate the study of subgroup 7 MYBs and transcriptional regulation of flavonoid biosynthesis in L. japonica.