Biology Reference
In-Depth Information
Discussion
A Comparative Genetic Subtraction Approach Identifies
Embryo Sac Expressed Candidate Genes
The female gametophyte or the embryo sac develops from a single functional me-
gaspore cell through a series of highly choreographed free-nuclear mitotic divisions
[1,2]. Understanding the molecular pathways that govern embryo sac development
and function, as well as subsequent seed development, has important implica-
tions for both basic plant developmental biology and plant breeding. Despite the
possible involvement of a few thousands of genes in this essential developmental
pathway, only a few more than 100 genes have been identified by loss-of-function
mutations, and most of them have not been studied in detail [14]. In the present
study we provide an alternative strategy to identify genes that are expressed in
the embryo sac of A. thaliana, namely comparative whole-genome transcriptional
profiling by microarray, which led to a candidate dataset of 1,260 genes.
Our approach, similar to that employed by Yu and coworkers [34], is differ-
ent from that used in previously reported whole-genome transcriptional profiling
experiments (for example, pollen transcriptome [33] and whole flower and silique
transcriptome [29]) in that we deduced the transcriptome of the few-celled female
gametophyte by simple genetic subtraction using a mutant that lacks an embryo
sac. Putative embryo sac expressed candidates included a significant number of
genes that are involved in transcriptional regulation, signaling, translational regu-
lation, protein degradation, transport and metabolism, and a majority of genes
that were not identified in previous studies. Similar to previous transcriptional
profiling reports, the largest functional category of embryo sac expressed genes was
plant metabolism [29,52,56]. Percentages of genes classified into transcriptional
regulation and signaling were comparable across embryo sac and pollen expressed
transcriptomes (about 6% to 10%) and, interestingly, these categories are larger
in both gametophytic transcriptomes than the general sporophytic transcriptomes
such as leaf, stem, and root [28]. In a much larger dataset of pollen samples, Pina
and colleagues [33] reported a little over 16% of pollen expressed genes as part of
the signaling category. It is possible that the mature pollen transcriptome is more
active in terms of signal transduction processes than that of the embryo sac, given
its role during polarized tip growth through the female reproductive tract, and the
gametic interaction at fertilization (for review [57]). We could not compare other
functional categories across other organ-specific transcriptome datasets because
the methods employed for functional classification were very different. Briefly,
our work provides novel data for organ specific expression in Arabidopsis and,
in particular, it illustrates the similarities and dissimilarities between male and
female gametophytic expression.
