Abstract
Gibberellins (GAs) are phytohormones that regulate many aspects of plant development. DELLA proteins are GA signaling repressors whose degradation is promoted by GA. GAF1 was isolated as a transcription factor that interacts with DELLA protein, GAI. DELLA proteins lack a clear DNA binding domain. However, DELLA proteins appear to be involved in the regulation of gene expression because they function as a transcriptional activator in yeast and plants. DELLA proteins accumulate in the nucleus when GA levels decreased. Therefore, we predicted that DELLA proteins participate in the transcriptional regulation of GA biosynthetic genes that are controlled by GA feedback mechanism. Transient assay showed that GAF1-DELLA complex activates GA 20-oxidase gene. BiFC assay showed that the interaction between GAF1 and DELLA protein disappears in plant cell when treated with GA. These results suggested that the transcriptional activation potential of GAF1 is controlled by endogenous GA levels. In this study, we attempted to specify the GAF1 binding site in the promoter of GA 20-oxidase and investigate the mechanism of transcriptional regulation by GAF1 complex.
