Biology Reference
In-Depth Information
Interesting insights can be gained from the subset of embryo sac expressed
genes (8.6%) that was subtracted for their enriched expression only in the embryo
sac. It was recently reported that 10% to 11% of the pollen transcriptome was
selectively expressed in the pollen, as evident from their absence of expression in
the sporophytic transcriptomes (n = 1,584 in [30] and n = 6,587 in [33]). In a
very similar study [32], it was reported that 9.7% of the 13,977 male gameto-
phytically expressed genes were specific for the male gametophyte. Even though
the complete embryo sac transcriptome is yet to be determined, it appears that the
enriched transcriptome of the embryo sac we report here is similar in size to that
of pollen. Male and female gametophyte enriched transcriptomes appear to be
much larger than the specific transcriptomes of vegetative organs such as leaf and
entire seedlings, which accounted for 2% to 4% of their corresponding complete
transcriptomes [33]. When we compared the genes with enriched expression in
the embryo sac or pollen, the embryo sac appears to harbor more transcriptional
regulators than pollen (8% versus 3%) [30]. However, the pollen transcriptome
exhibited a greater abundance of signaling proteins than the embryo sac (23%
versus 2%). This implies that either the pollen is more active in signaling than
the female gametophyte at the time around fertilization, or that the sensitivity of
detecting signaling genes in the embryo sac will have to be improved in the future
studies. The promise of our approach to deducing genes with enriched expression
was supported by the presence of essential genes that are female gametophyte spe-
cific, such as MEDEA and MYB98 in our dataset [12,22]. Furthermore, temporal
and spatial expression of nine transcripts in this study, and 18 other genes from
previous studies, suggests that the whole dataset of embryo sac expressed genes
may comprise genes that are expressed either in the entire embryo sac or restricted
to a few or single cell types.
A significant fraction of genes were probably undetected by this experiment
for two reasons: relatively similar or higher expression in the maternal sporophytic
tissues; and low level of expression in the embryo sac, similar to most of the
known female gametophytic genes. For example, cell cycle genes are barely repre-
sented among our candidate genes. In contrast, the pollen transcriptome has been
reported to be enriched with several core cell cycle transcripts [33]. Although our
comparative approach is very different from that reported by Pina and coworkers
[33], there could be a large number of cell cycle regulators that are expected to be
expressed during embryo sac development, suggesting a need for improvements
in embryo sac isolation and subsequent transcriptome analysis. Unlike the relative
ease in isolating some embryo sac cell types in maize and wheat, large-scale isola-
tion of the embryo sac cells is not possible in Arabidopsis [58,59]. Following the
work conducted by Yu and coworkers [34], we present here a large-scale study to
explore embryo sac expressed genes in Arabidopsis. If the scale of gene discovery
