Cullin-RING E3 ligases （CRLs） regulate different aspects of plant development and are activated by modification of their cullin subunit with the ubiquitin-like protein NEDD8 （NEural precursor cell expressed Developmentally Down-regulated 8） （neddylation） and deactivated by NEDD8 removal （deneddylation）. The CONSTITUTIVELY PHOTOMORPHOGENIC9 （COP9） signalosome （CSN） acts as a molecular switch of CRLs activity by reverting their neddylation status, but its contribution to embryonic and early seedling development remains poorly characterized. Here, we analyzed the phenotypic defects of csn mutants and monitored the cullin deneddylation/neddylation ratio during embryonic and early seedling development. We show that while csn mutants can complete embryogenesis （albeit at a slower pace than wildtype） and are able to germinate （albeit at a reduced rate）, they progressively lose meristem activity upon germination until they become unable to sustain growth. We also show that the majority of cullin proteins are progressively neddylated during the late stages of seed maturation and become deneddylated upon seed germination. This developmentally regulated shift in the cullin neddylation status is absent in csn mutants. We conclude that the CSN and its cullin deneddylation activity are required to sustain postembryonic meristem function in Arabidopsis.