DELLA-GAF1 Complex Is a Main Component in Gibberellin Feedback Regulation of GA20 Oxidase 2.

Gibberellins (GAs) are phytohormones that regulate many aspects of plant growth and development, including germination, elongation, flowering, and floral development. Negative feedback regulation contributes to homeostasis of the GA level. DELLAs are negative regulators of GA signaling and are rapidly degraded in the presence of GAs. DELLAs regulate many target genes, including AtGA20ox2 in Arabidopsis (Arabidopsis thaliana), encoding the GA-biosynthetic enzyme GA 20-oxidase. As DELLAs do not have an apparent DNA-binding motif, transcription factors that act in association with DELLA are necessary for regulating the target genes. Previous studies have identified GAI-ASSOCIATED FACTOR1 (GAF1) as such a DELLA interactor, with which DELLAs act as coactivators, and AtGA20ox2 was identified as a target gene of the DELLA-GAF1 complex. In this study, electrophoretic mobility shift and chromatin immunoprecipitation assays showed that four GAF1-binding sites exist in the AtGA20ox2 promoter. Using transgenic plants, we further evaluated the contribution of the DELLA-GAF1 complex to GA feedback regulation. Mutations in four GAF1-binding sites abolished the negative feedback of AtGA20ox2 in transgenic plants. Our results showed that GAF1-binding sites are necessary for GA feedback regulation of AtGA20ox2, suggesting that the DELLA-GAF1 complex is a main component of the GA feedback regulation of AtGA20ox2.