Key regulator for endosperm and embryo nuclear divisions. Additional isoforms seem to exist. Expressed in cotyledons, stems, veins of sepals and stigmas, and actively dividing tissues such as shoot apical meristem, root tip and emerging trus leaves. Weak expression in petals and anthers, and not detected in mature leaves. In seeds, expressed in both the endosperm and embryo. The C-terminus is essential for the function of the protein and it contains two conserved motifs (289-300) and (365-389) of unknown function present in orthologous proteins from various eukaryotes. Aborted seed development. Embryos arrested at the globular to heart stage transition and defective endosperm development with multi-nucleolated endosperm cells. At least nine differentially spliced transcript isoforms are produced. Full-length isoforms, but not the truncated ones lacking the C-terminal domain, are able to rescue disruption mutants (PubMed:22991160).