A Crucial Role of GA-Regulated Flavonol Biosynthesis in Root Growth of Arabidopsis.

Flavonols have been demonstrated to play a series of important roles in plant growth, development, and communications with other organisms. Flavonol biosynthesis is spatiotemporally regulated by the subgroup 7 R2R3-MYB (SG7 MYB) transcription factors including MYB11/MYB12/MYB111. However, whether SG7-MYBs activity is subject to posttranslational regulation remains unclear. Here, we show that gibberellic acid (GA) inhibits flavonol biosynthesis via DELLA proteins in Arabidopsis. Protein interaction analyses revealed that DELLAs (RGA and GAI) interacted with SG7 MYBs (MYB12 and MYB111) both in vitro and in vivo, leading to enhancing the affinity of MYB binding to the promoter region of the key genes for flavonol biosynthesis, and their transcriptional levels. We also observed that the level of auxin in the root tip was negatively correlated to root flavonol content. Furthermore, genetic assays showed that loss-of-function mutations in MYB12 that is dominantly expressed in roots partially rescued the short root phenotype of the GA deficient mutant ga1-3 by increasing root meristem size and mature cell size. Consistently, exogenous application of the flavonol quercetin recovered root meristem size of myb12 ga1-3 to that of ga1-3. Taken together, our data elucidate a molecular mechanism by which GA promotes root growth via directly reducing flavonol biosynthesis.