engleski

A transcriptomics approach to study female gametophyte functions in flowering seed plants

The female gametophyte (FG or embryo sac) of most flowering seed plants (angiosperms) consists of the haploid cells of the egg apparatus (egg cell and two synergids), a diploid central cell as well as three antipodal cells. In Arabidopsis, the antipodals degenerate, while they undergo endoreduplication and form a cluster of 20-40 cells in cereals. All FG cells are derived from one megaspore after meiosis. A number of functions have been addressed to the FG that include pollen tube guidance and discharge, translocation of the two sperm cells towards the female gametes (egg and central cell), regulation of the double fertilization process, fusion of gamete nuclei, prevention of polyspermy, maternal control of embryo and endosperm development as well as the induction of seed development after fertilization. In stark contrast to our knowledge about these processes at the morphological level, little is known about the molecules and mechanistic pathways regulating the various processes. Until now, most of our knowledge about FG development and function has been generated through genetic screenings and a small number of genes have been identified that are required for FG development and maternal control of seed development. We have performed an alternative transcriptomics based approach: cell-type-specific cDNA libraries have been generated from the individual cells of the FG before and after fertilization followed by differential screening of these libraries using methods such as classical plaque screening, SSH, RFDD-PCR, random EST sequencing and microarrays to identify and clone genes differentially and specifically expressed in the individual cells of the FG (Dresselhaus, Curr. Opin. Plant Biol., 2006). Bioinformatics was applied to select candidate genes for functional studies. Interestingly, a large number of specifically expressed genes seem to be involved in cell-cell communication events, establishment of polarity and cell identity, gene regulation and cell cycle control. A surprisingly high number of specifically expressed genes encode proteins involved in the regulation of protein degradation indicating that post-transcriptional regulation events are important for FG function. We will present results from bioinformatic analyses as well as functional studies of small secreted extracellular peptide ligands as candidate molecules for mediating cell identity and for short range pollen tube guidance as well as sperm cell discharge. Proteins involved in downstream signaling include, for example, Armadillo-repeat and BTB/POZ-domain proteins.

female gametophyte; zygotic embryo; egg cell

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