Brassinosteroids(BRs)are a group of naturally occurring plant hormones regulating many cellular and physiological processes during normal growth and development.cog1-2D was identified as an activation-tagged genetic modifier of an intermediate BR receptor mutant,bri1-5.COG1 encodes a Dof-type transcription factor.The dominant single mutants,cog1-2D,showed an elongated hypocotyl phenotype under light conditions.Whereas inducible expression of a dominant-negative form of COG1,COG1-SRDX,with a 12-amino acid repressor sequence fused to COG1 at its C terminus,results in shorten-hypocotyl plants.Two basic helix-loop-helix transcription factors,PIF4 and PIF5,were found to be transcriptionally regulated by COG1.The hypocotyl growth of cog1-2D showed reduced sensitivity to exogenously applied 24-epibrassinolide.COG1-SRDX caused short hypocotyl phenotypes can be rescued by 24-epibrassinolide application,which indicated a role of COG1 in modulating BR homeostasis.BR profile assay showed that BR levels are elevated in cog1-2D seedlings.RNA-seq and qRT-PCR data showed that several BR biosynthetic genes were upregulated in cog1-2D mutants compared with wild type.Genetic analysis and qRT-PCR data indicated that PIF4 and PIF5 are required for COG1 to regulate BR biosynthesis and hypocotyl elongation.ChIP and EMSA analyses demonstrated that COG1 directly binds to the promoter regions of PIF4 and PIF5.PIF4 and PIF5 also directly bind to the promoter regions of two BR biosynthetic genes,CPD and DWF4.Our results demonstrate another transcriptional regulation mechanism through which plant maintains BR homeostasis,which is critical for normal growth and development.