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Plant phytochromes are well-studied photoreceptors that sense red and far-red light, regulating photomorpho- genic development. Molecular signaling mechanisms of phytochrome A (phyA) and phyB largely overlap, especially in regulation of PHYTOCHROME-INTERACTING FACTORs (PIFs) and E3 ligase complexes composed of CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSORs OF phyA-105 (SPAs). However, the differences in their molecular signaling mechanisms remain unclear. Constitutively active mutants of phyB (YVB) and NLS-fused phyA (YVA:NLS) mediate light-independent seedling development, leading to constitutive photomorphogenic (cop) phenotypes in their transgenic Arabidopsis plants. Interestingly, YVB interacted with PIF3 independently of light, but YVA showed little interaction. In this study, we investigated distinct signaling mechanisms underlying the similar cop phenotypes given by YVB and YVA:NLS. Our findings indicated that YVA efficiently inactivate the COP1/SPA complex, leading to accumulation of ELONGATED HYPOCOTYL 5 (HY5) and subsequent expression of its target genes HY5 and HYH. YVB induced light-independent PIF3 and PIF1 degra- dation, in addition to HY5 accumulation. Moreover, co-expression of PIF3 in the YVB plant significantly attenuated the cop phenotypes, but minimal effects were observed in the YVA:NLS plant. In particular, PIF3 negatively regulated the interaction between YVB and COP1, which decreased HY5 accumulation in the YVB plant co-expressing PIF3. Furthermore, when transferred from light to dark, PIF3 was highly accumulated in phyB-5, whereas HY5 is degraded faster in phyA-201 compared to that in Ler. Collectively, our results suggest HY5 accumulation as the molecular bases for the cop phenotypes and also indicate that phyB is more important for regulating PIF3, whereas phyA effectively inactivates the COP1/SPA complex relative to PIF3 degradation.