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During early embryogenesis in mammals and higher plants,the maternalto-zygotic transition (MZT) marks the turnover of developmental control from maternal products to de novo zygotic genome transcripts.Intensive studies in animals indicate that early embryonic development is largely maternally controlled.In recent years,the MZT has drawn the attention of botanists,as it is important for understanding the mechanism of embryogenesis and hybrid vigor.In this study,we present a brief overview of some aspects of the MZT in flowering plants.Based on what we have learned from Nicotiana tabacum,we hypothesize that the MZT occurs before zygotic cell division and that the development of the fertilized egg cell in flowering plants can be divided into two phases:the zygote stage,which is mainly controlled maternally,and the one-celled proembryo stage,in which zygotic genome activation (ZGA) occurs and is required for zygote division.