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Plant growth and development are coordinated by several groups of small-molecule hormones, including brassinosteroids (BRs) and gibberellins (GAs). Physiological and molecular studies have suggested the existence of crosstalk between BR and GA signaling. We report that BZR1, a key transcription factor activated by BR signaling, interacts in vitro and in vivo with REPRESSOR OF ga1-3 (RGA), a member of the DELLA family of transcriptional regulators that inhibits the GA signaling pathway in Arabidopsis thaliana. Genetic analyses of plants with mutations in the genes encoding RGA and BZR1 revealed that RGA suppressed root and hypocotyl elongation of the gain-of-function mutant bzr1-1D. Ectopic expression of proteins of the DELLA family reduced the abundance and transcriptional activity of BZR1. Reporter gene analyses further indicated that BZR1 and RGA antagonize each other’s transcriptional activity. Our data indicated that BZR1 and RGA served as positive and negative regulators, respectively, of both the BR and the GA signaling pathways and establish DELLAs as mediators of signaling crosstalk between BRs and GAs in controlling cell elongation and regulation of plant growth.