摘要Light signaling precisely controls photo-morphogenic development in plants. PHYTO-CHROME INTERACTING FACTOR 4 and 5 (PIF4 and PIF5) play critical roles in the regulation of this developmental process. In this study, we report CONSTITUTIVELY PHOTOMORPHOGENIC 1 SUPPRESSOR 6 (CSU6) functions as a key regu-lator of light signaling. Loss of CSU6 function largely rescues the cop1-6 constitutively photo-morphogenic phenotype. CSU6 promotes hypo-cotyl growth in the dark, but inhibits hypocotyl elongation in the light. CSU6 not only associates with the promoter regions of PIF4 and PIF5 to in-hibit their expression in the morning, but also di-rectly interacts with both PIF4 and PIF5 to repress their transcriptional activation activity. CSU6 negatively controls a group of PIF4- and PIF5-regulated gene expressions. Mutations in PIF4 and/or PIF5 are epistatic to the loss of CSU6, suggesting that CSU6 acts upstream of PIF4 and PIF5. Taken together, CSU6 promotes light-inhibited hypocotyl elongation by negatively regulating PIF4 and PIF5 transcription and biochemical activity.