Transcription factor that binds 5'-AACGG-3' motifs in gene promoters (By similarity). Transcription repressor that regulates organ growth. Binds to the promoters of G2/M-specific genes and to E2F target genes to prevent their expression in post-mitotic cells and to restrict the time window of their expression in proliferating cells (PubMed:26069325). 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 RBR1 in both earlier and later stages of leaves development. Interacts with CDKA-1 and E2FC, but not with E2FB, at later stages of leaves development. Expressed both in proliferating and maturing stages of leaves. Slightly induced by ethylene, auxin (IAA), jasmonic acid (JA) and salicylic acid (SA). In double mutant myb3r3 myb3r5 and triple mutant myb3r1 myb3r3 myb3r5, up-regulation of many G2/M-specific genes leading to larger seeds, organs and embryos due to overproliferation and ectopic cell divisions.