Brassinosteroid Biosynthesis Is Modulated via a Transcription Factor Cascade of COG1, PIF4, and PIF5.

Brassinosteroids (BRs) are essential phytohormones regulating various developmental and physiological processes during normal growth and development. () was identified as an activation-tagged genetic modifier of , an intermediate BR receptor mutant in Arabidopsis (). encodes a Dof-type transcription factor found previously to act as a negative regulator of the phytochrome signaling pathway. single mutants show an elongated hypocotyl phenotype under light conditions. A loss-of-function mutant or inducible expression of a dominant negative form of in the wild type results in an opposite phenotype. A BR profile assay indicated that BR levels are elevated in seedlings. Quantitative reverse transcription-polymerase chain reaction analyses showed that several key BR biosynthetic genes are significantly up-regulated in compared with those of the wild type. Two basic helix-loop-helix transcription factors, and , were found to be transcriptionally up-regulated in Genetic analysis indicated that PIF4 and PIF5 were required for COG1 to promote BR biosynthesis and hypocotyl elongation. Chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that COG1 binds to the promoter regions of and , and PIF4 and PIF5 bind to the promoter regions of key BR biosynthetic genes, such as and , to directly promote their expression. These results demonstrated that COG1 regulates BR biosynthesis via up-regulating the transcription of and .