Molecular mechanism for the interaction between gibberellin and brassinosteroid signaling pathways in Arabidopsis

[EN] Plant development is modulated by the convergence of multiple environmental and endogenous signals, and the mechanisms that allow the integration of different signaling pathways is currently being unveiled. A paradigmatic case is the concurrence of brassinosteroid (BR) and gibberellin (GA) signaling in the control of cell expansion during photomorphogenesis, which is supported by physiological observations in several plants but for which no molecular mechanism has been proposed. In this work, we show that the integration of these two signaling pathways occurs through the physical interaction between the DELLA protein GAI, which is a major negative regulator of the GA pathway, and BRASSINAZOLE RESISTANT1 (BZR1), a transcription factor that broadly regulates gene expression in response to BRs. We provide biochemical evidence, both in vitro and in vivo, indicating that GAI inactivates the transcriptional regulatory activity of BZR1 upon their interaction by inhibiting the ability of BZR1 to bind to target promoters. The physiological relevance of this interaction was confirmed by the observation that the dominant gai-1 allele interferes with BR-regulated gene expression, whereas the bzr1-1D allele displays enhanced resistance to DELLA accumulation during hypocotyl elongation. Because DELLA proteins mediate the response to multiple environmental signals, our results provide an initial molecular framework for the integration with BRs of additional pathways that control plant development. We thank the Nottingham Arabidopsis Stock Centre, Tai-ping Sun, Zhi-Yong Wang, Yanhai Yin, Ana Cano-Delgado, Luis Lopez-Molina, and Francois Parcy for providing seeds or reagents; Laura Garcia-Carcel and Gaston Pizzio for help in the early stages of this work; and Salome Prat for fruitful discussions, sharing unpublished results, and careful reading of the manuscript. J.G.-B. holds a Consejo Superior de Investigaciones Cientificas Fellowship of the Joint Admissions Exercise Predoctoral Program. E. G. M. is recipient of a postdoctoral "Juan de la Cierva" contract from the Spanish Ministry of Science and Innovation. A. L. was supported in part by a fellowship of the Fondo per gli Investimenti della Ricerca di Base Progetto Giovani of the Italian Ministry of Education, University, and Research. Work in the authors' laboratory was funded by Spanish Ministry of Science and Innovation Grants BIO2007-60923, BIO2010-15071, and CSD2007-00057 and by Generalitat Valenciana Grants ACO