BZR1 Interacts with HY5 to Mediate Brassinosteroid- and Light-Regulated Cotyledon Opening in Arabidopsis in Darkness

Light and brassinosteroid (BR) are two central stimuli that regulate plant photomorphogenesis. Although previous phenotypic and physiological studies have implied possible interactions between BR and light in regulating photomorphogenesis, the underlying molecular mechanism(s) remain largely unknown. In the present study, we identified a physical connection between the BR and light signaling pathways, which was mediated by the BR-regulated transcription factor BZR1 and light-regulated transcription factor HY5 in Arabidopsis thaliana. Genetic evidence showed that the gain-of-function bzr1-1D mutant in the BR signaling pathway and loss-of-function hy5-215 mutant in the light signaling pathway exhibited closed cotyledons under BR-deficient and dark-grown conditions and both bzr1-1D and hy5-215 mutants were able to suppress the cotyledon opening phenotype of the BR-insensitive mutants bri1-5 and bin2-1. Biochemical studies demonstrated that BZR1 interacts with HY5 both in vitro and in vivo and ectopic expression of HY5 considerably reduces the accumulation of BZR1 protein. In addition, HY5 specifically interacts with the dephosphorylated form of BZR1 and attenuates BZR1's transcriptional activity in regulating its target genes related to cotyledon opening. Our study provides a molecular framework for coordination of BR and light signals in regulating cotyledon opening, an important process in photomorphogenesis in plants.