Essential protein required for ovules fertilization (PubMed:15634699, PubMed:19171893). Component of the origin recognition complex (ORC) that binds origins of replication. It has a role in both chromosomal replication and mating type transcriptional silencing. Binds to the ARS consensus sequence (ACS) of origins of replication (By similarity). H3K4me3 effector that regulates positively the transcription of a subset of genes (PubMed:19171893). Component of the origin recognition complex (ORC) composed of at least ORC1 (ORC1A or ORC1B), ORC2, ORC3, ORC4, ORC5 and ORC6. ORC is regulated in a cell-cycle and development dependent manner. It is sequentially assembled at the exit from anaphase of mitosis and disassembled as cells enter S phase. Interacts directly with ORC2 and ORC5 (PubMed:15358564, PubMed:16179646). Binds mostly unmodified histone H3, and, with lower efficiency, H3K4me1 H3K4me2 and H3K4me3 (PubMed:19171893, PubMed:26876097). Follow a cell-cycle regulation with a peak at the G1/S-phase (PubMed:16179646). Mostly expressed in flower buds, and, to a lower exent, in roots, leaves and stems (PubMed:16179646, PubMed:15358564). Restricted to proliferating cells. First active during embryogenesis, but inactive in mature embryos. Expressed in shoot and root apical meristems. Later observed in lateral root primordia and meristems. Detected in young flower buds, developing anthers and mature pollen. Regulated by E2F (PubMed:16179646, PubMed:16126853). Accumulates rapidly after cell cycle reactivation by sucrose addition following cell cycle arrest mediated by sucrose deprivation (PubMed:16179646, PubMed:15358564). Lethal (PubMed:19171893). Unfertilized ovules but normal pollen tube attraction (PubMed:15634699). Belongs to the ORC1 family.