Abstract
The zygotic embryos of angiosperms develop buried deep within seeds and surrounded by two main extra-embryonic tissues: the maternally derived seed coat tissues and the zygotic endosperm. Generally, these tissues are considered to play an important role in nurturing the developing embryo by acting as conduits for maternally derived nutrients. They are also critical for key seed traits (dormancy establishment and control, longevity, and physical resistance) and thus for seed and seedling survival. However, recent studies have highlighted the fact that extra-embryonic tissues in the seed also physically and metabolically limit embryonic development and that unique mechanisms may have evolved to overcome specific developmental and genetic constraints associated with the seed habit in angiosperms. The aim of this review is to illustrate how these studies have begun to reveal the highly complex physical and physiological relationship between extra-embryonic tissues and the developing embryo. Where possible I focus on Arabidopsis because of space constraints, but other systems will be cited where relevant.
Highlights
The multicellular female gametophyte, situated within the ovule, represents the starting point for angiosperm seed development
It appears that cytoplasmic contact between maternal tissues, the endosperm, and the embryo are lost soon after fertilization[2], meaning that molecular transport between the three tissues must occur via the transcellular pathway, involving transmembrane transport in and out of cells and diffusion across the cell wall compartment
And consistent with this analogy, as in the placenta, the angiosperm endosperm, which contains both male and female genomes owing to double fertilization, appears to be a focus for parental conflicts over resource allocation, in outcrossing or partially outcrossing plants[13]
Summary
The multicellular female gametophyte, situated within the ovule, represents the starting point for angiosperm seed development. This might explain why the embryos of mutants defective in endosperm cellularization (including those lacking FIS-PRC2 activity) fail to develop past the heart stage[36] It could provide an additional explanation for the link between seed growth and endosperm cellularization since, by increasing embryo sink strength, cellularization could deplete nutrients available for uptake by the endosperm. In Arabidopsis, the endosperm is a largely ephemeral tissue that breaks down almost completely during seed development The importance of this process is borne out by the phenotypes associated with loss of function of the endosperm-specific RETARDED GROWTH OF EMBRYO1/ZHOUPI (ZOU) protein. Sheath deposition at the embryo surface depends on the signaling pathway involved in cuticle integrity monitoring (described above), again indicating a complex dialogue between the embryo and endosperm[62]
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