Interesting developmental similarities exist between the embryonic proto-derm and the outside cell layer of the second product of double fertilization in angiosperms, the endosperm. Although some debate still exists regarding the evolutionary origin of the angiosperm endosperm (Friedman 2001; Friedman and Floyd 2001; Baroux et al. 2002), the most generally accepted current hypothesis is that it is a sexualised homologue of nutritive tissues which develop from the megagametophyte, for example in the gymnosperms. In the endosperm of many angiosperms, the outside cell-layer has specific cellular modifications, and a different developmental fate to other endosperm cells (Olsen 2004). In Arabidopsis the outer cell layer of the endosperm is maintained intact until seed maturity, whilst internal cells are broken down and used to support the developing embryo. The outer endosperm cells may play a role in taking up nutrients from the neighbouring endodermis of the seed coat. In monocotyledonous plants, for example the Gramineae, this specialisation of the outer endosperm cell layer is more extreme with the development of the highly specialised aleurone cell-layer surrounding the persistent starchy endosperm. The developmental ontogeny of the outer layer of the endosperm is rather different from that of the protoderm, in that the endosperm initially develops as a syncitial monolayer of nuclei lining the endosperm cavity. Upon cellularisation these nuclei become the progenitors of the aleurone, but also of all internal endosperm cells, to which they give rise by periclinal divisions (Olsen 2001). Several recent studies have shown that, as for the embryonic protoderm, the specification of aleurone cell fate is dictated largely by outside position (Geisler-Lee and Gallie 2005; Gruis et al. 2006).
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