Brassinosteroid, gibberellin and phytochrome impinge on a common transcription module in Arabidopsis

Brassinosteroid and gibberellin promote many similar developmental responses in plants; however, their relationship remains unclear. Here we show that BR and GA act interdependently through a direct interaction between the BR-activated BZR1 and GA-inactivated DELLA transcription regulators. GA promotion of cell elongation required BR signalling, whereas BR or active BZR1 suppressed the GA-deficient dwarf phenotype. DELLAs directly interacted with BZR1 and inhibited BZR1–DNA binding both in vitro and in vivo. Genome-wide analysis defined a BZR1-dependent GA-regulated transcriptome, which is enriched with light-regulated genes and genes involved in cell wall synthesis and photosynthesis/chloroplast function. GA promotion of hypocotyl elongation requires both BZR1 and the phytochrome-interacting factors (PIFs), as well as their common downstream targets encoding the PRE-family helix–loop–helix factors. The results demonstrate that GA releases DELLA-mediated inhibition of BZR1, and that the DELLA–BZR1–PIF4 interaction defines a core transcription module that mediates coordinated growth regulation by GA, BR and light signals. Wang and colleagues show that, in Arabidopsis thaliana, brassinosteroid and light-dependent transcription factors are required for giberellin effects on hypocotyl elongation, by modulating transcription of giberellin-induced genes involved in cell wall synthesis and photosynthesis. Conversely, giberellin relieves the brassinosteroid component BZR1 from inhibition by DELLA proteins.