Transcription factor that binds 5'-AACGG-3' motifs in gene promoters (PubMed:21862669). Involved in the regulation of cytokinesis, probably via the activation of several G2/M phase-specific genes transcription (e.g. KNOLLE) (PubMed:17287251, PubMed:21862669, PubMed:25806785). Required for the maintenance of diploidy (PubMed:21862669). Involved in transcription regulation during induced endoreduplication at the powdery mildew (e.g. G.orontii) infection site, thus promoting G.orontii growth and reproduction. Component of a DREAM-like complex which modulates a variety of developmentally regulated genes and of the mitotic genes in proliferating and differentiated cells. Associates with CDKA-1, RBR1 and E2FB, but not with E2FC, in proliferating cells, at early stages of leaves development. Additional isoforms seem to exist. Expressed in roots, cotyledons and leaves, especially in vascular tissues, and in flowers. Expressed in the root quiescent center (PubMed:15937229). Accumulates in division zone of primary root tips and emerging lateral roots. Also present in floral organs in young flower buds. Weakly and uniformly present in the developing embryo and maternal tissues (PubMed:17287251). Expressed specifically in proliferating stage of leaves (PubMed:26069325). Slightly induced by salicylic acid (SA) (PubMed:16463103). Expressed in a cell cycle-dependent manner, with highest levels 2 hours before the peak of mitotic index in cells synchronized by aphidicolin. Activated by CYCB1 (PubMed:17287251). Accumulates at powdery mildew (e.g. G.orontii) infected cells. In EDC plants, defects of cytokinesis (PubMed:25806785). The double mutant myb3r1 myb3r4 often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues (e.g. in embryo during the first or second division after fertilization, in stomata guard mother cell) and several pleiotropic developmental defects, and associated with the selective reduction of several G2/M phase-specific genes transcript levels (e.g. CYCB2, CDC20.1 and KNOLLE). Hypersensitivity to caffeine, an inhibitor of cytokinesis (PubMed:17287251, PubMed:21862669). Impaired powdery mildew (e.g. G.orontii)-induced endoreduplication. Reduced G.orontii growth and reproduction leading to an enhanced resistance to powdery mildew (PubMed:20018666).