Key regulator of entry into cell division. Acts as a transcription repressor of E2F target genes, whose activity is required for progress from the G1 to the S phase of the cell cycle. Hyperphosphorylation by CDKA-1 prevents the binding to E2F transcription factors, allowing G1 to S phase transition to operate (PubMed:18064404, PubMed:21444209). Forms a stable complex with E2FA that functions in maintaining cell proliferation through repression of cell differentiation (PubMed:22307083). Plays a central role in the mechanism controlling meristem cell differentiation, cell fate establishment and cell fate maintenance during organogenesis and gametogenesis (PubMed:16377572, PubMed:16815954, PubMed:19359496, PubMed:20525851, PubMed:20585548, PubMed:20683442, PubMed:22595674, PubMed:24285791, PubMed:25303364). Required during lateral organ production (PubMed:20525851). Also involved in controlling asymmetric divisions of stem cells in different stem cell niches (PubMed:22921914, PubMed:23104828, PubMed:24302889). Acts as negative regulator of cell proliferation during leaf and gametophytes development (PubMed:15201912, PubMed:16361519, PubMed:18976913). At later stages of development, restricts the progression through additional endocycles (PubMed:16361519). In the leaf, plays a role in the control of the mesophyll differentiation (PubMed:24118480). Another role is its implication in the regulation of imprinted genes. Acts together with MSI1 to repress the expression of MET1 during gametogenesis. This in turn activates expression of the imprinted genes FIS2 and FWA (PubMed:18700816). Regulates many genes of the polycomb repressive complex 2 (PRC2) (PubMed:18976913, PubMed:19704913, PubMed:20585548). Plays an important role in meiosis affecting different aspects of this complex process (PubMed:21217641). Functions as a positive regulator of the developmental switch from embryonic heterotrophic growth to autotrophic growth (PubMed:21693514). Interaction with mastrevirus RepA or nanovirus Clink protein disrupts the RBR/E2F interaction and releases the transcription of replicative enzymes needed by the virus by increasing the E2F DNA-binding activity (PubMed:16361519). Interacts with the begomovirus replication-associated protein (Rep) (PubMed:10880461), the nanovirus Clink protein (PubMed:17267511), the mastrevirus RepA protein, E2FA, E2FB and E2FC (PubMed:16361519, PubMed:18064404, PubMed:16055635, PubMed:20683442, PubMed:22307083). Interacts with MSI1 through its Domain A (PubMed:18700816). Interacts with ATPK1/S6K1 (PubMed:20683442). Interacts with SCR (PubMed:22921914, PubMed:24302889). Interacts with HAT2 (PubMed:24302889). Interacts with FAMA (PubMed:25303364, PubMed:24571519). Interacts with MYB124 and MYB88 (PubMed:24571519). 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 MYB3R3 in both earlier and later stages of leaves development. Interacts with MYB3R4 only at early stages of leaves development (PubMed:26069325). RBR1 nuclear localization is mediated by the RBR1-ATPK1 interaction. A number of isoforms are produced. According to EST sequences. Expressed in actively dividing cells. Detected in the shoot apical meristem, in young leaf primordia and in both sporophytic tissue and the megagametophyte. Highly expressed before and after fertilization (PubMed:15201912). During meiosis, found to localize along the chromosome axes during late G2/early leptotene. As prophase I progressed, a lower expression is detectable at zygotene then more reduced at pachytene. Not detected at later stages (PubMed:21217641). The polycomb repressive complex 2 (PRC2) containing MSI1, MEA, FIS2 and FIE repress the paternal RBR allele during pollen and seed development (PubMed:18976913). Down-regulated by TCP15 (PubMed:21992944). Highly phosphorylated by CDKA-1 during G1 to S phase transition. Once hyper-phosphorylated, becomes inactive and unable to interact with E2F. Ubiquitinated (Probable). Subject to proteasome-dependent degradation during sucrose starvation (PubMed:21444209). Plants are gametophytic lethals (PubMed:15201912). During meiosis, reduction in the formation of meiotic crossing-overs and a failure of chromosome synapsis, leading to a dramatic reduction in fertility (PubMed:21217641). Belongs to the retinoblastoma protein (RB) family.