Biology Reference
In-Depth Information
Communication Between the Embryo Sac and the
Surrounding Sporophyte May Be Important for Reproductive
Development
In Arabidopsis, the sporophytic and gametophytic tissues are intimately posi-
tioned next to each other within the ovule. Independent studies on Arabidopsis
ovule mutants suggest that the development of the female gametophyte might
require highly synchronized morphogenesis of the maternal sporophyte sur-
rounding the gametophyte [1,35,37]. This notion is exemplified by the fact that
megagametogenesis is largely perturbed in most of the known sporophytic ovule
development mutants. For example, in short integument 1 (sin1) the ovules dis-
play uncoordinated growth patterns of integuments and the nucellus, and embryo
sac development is not initiated [35,71]. In bell1 and aintegumenta mutants,
in which integument morphogenesis and identity are disrupted, embryo sac de-
velopment is arrested [35,37,72,73]. Therefore, early acting sporophytic genes
in the ovule also affect female gametophyte development. On the contrary, in
several mutations where female gametogenesis is completely or partially blocked,
the ovule sporophyte appears morphologically normal. In coa and spl, or female
gametophytic mutations such as hadad and nomega, embryo sac development is
blocked either at the onset or during megagametogenesis, but ovule morphogen-
esis continues normally until anthesis [8,18,38]. It was therefore thought that the
embryo sac does not influence the development of the sporophytic parts of the
surrounding ovule and carpel tissues [2].
Our data clearly demonstrate that in the absence of an embryo sac there was a
predominant transcriptional upregulation of transcription factors, and signaling
molecules in the carpel and the ovule. It is interesting to note that we identified
genes that were previously implicated in gynoecium patterning such as NUBBIN,
SHI and STY2 for their gain of expression in the sporophyte [74-77]. Based on
the proposed functionalities of these and other genes in our dataset, we suggest
that signaling pathways involving auxin and gibberellic acid could possibly be
triggered in the carpel and ovule sporophyte, in the absence of an embryo sac. We
anticipate that sporophytic patterning genes and signaling molecules are under
indirect repressive control by the female gametophyte. Impairment of this signal-
ing cascade leads to deregulation of the sporophytic transcriptome.
Conclusion
Understanding gene expression and regulation during embryo sac development
demands large-scale experimental strategies that subtract the miniature haploid
embryo sac cells from the thousands of surrounding sporophytic cells. We used
