RING-finger-type ubiquitin E3 ligase Constitutively Photomorphogenic 1 (COP1) and floral integrators such as FLOWERING LOCUS T (FT), TWIN SISTER OF FT (TSF) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) have been identified as regulators of stomatal movement. However, little is known about their roles and relationship in dark-induced stomatal closure. Here, we demonstrated that COP1 is required for dark-induced stomatal closure using mutant. The mutant closed stomata in response to exogenous nitric oxide (NO) but not hydrogen peroxide (H O ), and H O but not NO accumulated in in darkness, further indicating that COP1 acts downstream of H O and upstream of NO in dark-induced stomatal closure. Expression of , and in wild-type (WT) plants decreased significantly with dark duration time, but this process was blocked in . Furthermore, , , and mutants accumulated NO and closed stomata faster than WT plants in response to darkness. Altogether, our results indicate that COP1 transduces H O signaling, promotes NO accumulation in guard cells by suppressing , and expression, and consequently leads to stomatal closure in darkness. These findings add new insights into the mechanisms of dark-induced stomatal closure.