Cell cycle regulatory genes from maize are differentially controlled during fertilization and first embryonic cell division

Regulation of early embryo development in higher plants is not understood. As one approach to gain insight into the mechanisms that govern zygotic embryogenesis, we studied the regulation of zygotic cell division by analyzing expression of cell cycle regulatory genes. Transcripts of cdc2 and cyclin genes as well as of a histone gene from maize were detected in single cells by employing recently developed techniques which allow fertilization of isolated gametes and zygote culture in vitro. We analyzed gene expression in sperm cells, in the egg cell and in other cells present in the embryo sac, i.e. synergids, central cell and antipodal cells. Moreover, in vitro gamete fusion enabled us to examine gene expression in the zygote at defined time points from fertilization to the two-celled embryo stage. Whereas the cdc2 gene is expressed constitutively in all cells of the embryo sac, in sperm cells, and throughout zygote development, the cyclin genes display cell-specific expression in the embryo sac and differential expression during zygote development. All three cyclins examined are transcribed de novo after fertilization, indicating a different level of embryonic cell cycle regulation in plants than what is commonly found in animals where early embryonic cell divisions are driven by maternally supplied cyclin mRNA.