Gene and Protein Expression Profiles in Rice Gametes and Zygotes: A Cue for Understanding the Mechanisms of Gametic and Early Zygotic Development in Angiosperms

Abstract

In angiosperms, female gamete differentiation, fertilization, and subsequent zygotic development occur in embryo sacs deeply embedded in the ovaries. Despite their importance in plant reproduction and development, how the egg cell is specialized, fuses with the sperm cell, and converts into an active zygote for early embryogenesis remains unclear. This lack of knowledge is partly attributable to the difficulty of direct analyses of gametes and zygotes in angiosperms. Cell type-specific transcriptomes were obtained by microarray analyses for egg cells, sperm cells and zygotes isolated from rice flowers, and up- or down-regurated genes in zygotes after fertilization were identified as well as genes enriched in male and female gametes. In addition to transcriptome, proteins expressing in egg and sperm cells were globally detected by highly sensitive liquid chromatography coupled with tandem mass spectroscopy technology, and proteins that are specifically or predominantly expressing in gametes were also identified by comparison of protein expression profiles between gametes and somatic cells/pollen grains. Several rice or Arabidopsis lines with mutations in genes identified by these proteome/transcriptome analyses showed clear phenotypic defects in seed set or seed development. These findings suggest that the cell type-specific proteome/transcriptome data for gametes/zygotes are foundational information toward understanding the mechanisms of gametic and early zygotic development in angiosperms.