The COP1/SPA complex and DET1 function to suppress photomorphogenesis in dark-grown Arabidopsis seedlings. Additionally, they inhibit flowering under non-inductive short-day conditions. The COP1/SPA complex and DET1, as part of the CDD complex, represent distinct high-molecular-weight complexes in Arabidopsis. Here, we provide genetic evidence that these complexes co-act in regulating plant development. We report the isolation of a spa1 enhancer mutation that represents a novel, very weak allele of det1. This det1 ( esp1 ) mutation caused no detectable mutant phenotype in the presence of wild-type SPA1, but showed strongly synergistic genetic interaction with the spa1 mutation in the control of seedling photomorphogenesis, anthocyanin accumulation, plant size as well as flowering time. On the biochemical level, the det1 ( esp1 ) spa1 double mutant showed higher HY5 protein levels than either single mutant or the wild type. The genetic interaction of spa1 and det1 mutations was further confirmed in the spa1 det1-1 double mutant which carries a strong allele of det1. Taken together, these results show that SPA1 and DET1 act together to control photomorphogenesis throughout plant development. Hence, this suggests that COP1/SPA complexes and the CDD complex co-act in controlling the protein stability of COP1/SPA target proteins.