The life cycle of higher plants starts from the zygotic activation after fertilization, where the precise mechanism remains largely unknown.We performed a large scale genetic screening in Arabidopsis to identify zygotic lethal mutants, designated as embryonic factor (fac).Among the 29 mutants obtained that belongs to at least xx loci, embryo developments were arrested either at zygote, post-cell wall thickening or elongated zygote stages, One of the fac mutants, fac6, with a typical mendelian inheritance, showed enlarged nuclei and a cell cycle arrest at anaphase in both embryo and endosperm.The G2-M transition marker cyclinB1;1::GUS was abundantly accumulated in fac6 embryo sac.FAC6 gene, as cloned by map-based approach, encodes a highly conserved APC complex protein.Expression analyses showed that FAC6 has a dynamic expression pattern before and after fertilization, providing genetic evidence that cell cycle initiation, with equal maternal and paternal contributions in embryo development, is crucial for zygotic activation in plants.