Novel ribosomal genes from maize are differentially expressed in the zygotic and somatic cell cycles

We have isolated cDNAs representing more than 50 different genes from libraries of unfertilised egg cells and zygotes of maize, expression of which is up- or downregulated after in vitro fertilisation (IVF). Among the cDNAs isolated are seven which encode proteins that are probably involved in translation and two encoding proteins probably involved in DNA replication. The latter genes are strongly induced on fertilisation. This indicates that zygotic gene activation (ZGA) - the switch from maternal to embryonic control of development - occurs in the zygote shortly after fertilisation in higher plants - earlier than in animal systems so far investigated. Four novel transcripts for ribosomal proteins (S21A, S21B, L39, P0) involved in ribosome biosynthesis and translation were analysed in more detail. The expression of all four genes correlates with cell division activity and is strongly induced during the G1 phase of the somatic cell cycle. A different mode of regulation operates in the first embryonic cell cycle: relatively large amounts of transcript are stored in the unfertilised egg cells, and by 18 h after IVF, two ribosomal genes are induced while a third is downregulated. These results indicate that using the combination of single-cell culture techniques with novel molecular methods, it is possible to isolate and study numerous genes expressed in female gametes and zygotes of higher plants. The detailed analysis of the four ribosomal protein genes demonstrates that the zygotic and somatic cell cycles are differentially regulated.