Delayed activation of the paternal genome during seed development

Little is known about the timing of the maternal-to-zygotic transition duri ng seed development in flowering plants. Because plant embryos can develop from somatic cells or microspores(1), maternal contributions are not consid ered to be crucial in early embryogensis(2). Early-acting embryo-lethal mut ants in Arabidopsis, including emb30/gnom which affects the first zygotic d ivision(3,4) have fuelled the perception that both maternal and paternal ge nomes are active immediately after fertilization. Here we show that none of the paternally inherited alleles of 20 loci that we tested is expressed du ring early seed development in Arabidopsis. For genes that are expressed at later stages, the paternally inherited allele becomes active three to four days after fertilization. The genes that we tested are involved in various processes and distributed throughout the genome, indicating that most, if not all, of the paternal genome may be initially silenced. Our findings are corroborated by genetic studies showing that emb30/gnom has a maternal-eff ect phenotype that is paternally rescuable in addition to its zygotic letha lity. Thus, contrary to previous interpretations, early embryo and endosper m development are mainly under maternal control.