Gene expression profiles in rice gametes and zygotes: identification of gamete-enriched genes and up- or down-regulated genes in zygotes after fertilization

In angiosperms, fertilization and subsequent zygotic development occur in embryo sacs deeply embedded in the ovaries; therefore, these processes are poorly elucidated. In this study, microarray-based transcriptome analyses were conducted on rice sperm cells, egg cells, and zygotes isolated from flowers to identify candidate genes involved in gametic and/or early zygotic development. Cell type-specific transcriptomes were obtained, and up- or down-regulated genes in zygotes after fertilization were identified, in addition to genes enriched in male and female gametes. A total of 325 putatively up-regulated and 94 putatively down-regulated genes in zygotes were obtained. Interestingly, several genes encoding homeobox proteins or transcription factors were identified as highly up-regulated genes after fertilization, and the gene ontology for up-regulated genes was highly enriched in functions related to chromatin/DNA organization and assembly. Because a gene encoding methyltransferase 1 was identified as a highly up-regulated gene in zygotes after fertilization, the effect of an inhibitor of this enzyme on zygote development was monitored. The inhibitor appeared partially to affect polarity or division asymmetry in rice zygotes, but it did not block normal embryo generation.