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Therootmeristem(RM) isafundamental structure that is responsible for postembryonic root growth.TheRMcontainsthequiescent center(QC), stemcells and frequently dividingmeristematic cells, inwhich the timing and the frequency of cell division are tightly regulated. In Arabidopsis thaliana, several gain-of-function analyses have demonstrated that peptide ligands of the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-RELATED (CLE) family are important for maintaining RM size. Here, we demonstrate that a plant U-box E3 ubiquitin ligase, PUB4, is a novel downstreamcomponent of CLV3/CLE signaling in the RM. Mutations in PUB4 reduced the inhibitory effect of exogenous CLV3/CLE peptide on root cell proliferation and columella stem cell maintenance. Moreover, pub4 mutants grown without exogenous CLV3/CLE peptide exhibited characteristic phenotypes in the RM, such as enhanced root growth, increased number of cortex/endodermis stem cells and decreased number of columella layers. Our phenotypic and gene expression analyses indicated that PUB4 promotes expression of a cell cycle regulatory gene, CYCD6;1, and regulates formative periclinal asymmetric cell divisions in endodermis and cortex/ endodermis initial daughters. These data suggest that PUB4 functions asa global regulator of cell proliferation and the timingof asymmetric cell division that are important for final root architecture. [ABSTRACT FROM AUTHOR]