Reinforced
cells do not prevent breakout
We have noted
before that, in some hard-seeded species, the endosperm cells in mature seeds
are extensively thickened with mannans and/or galactomannans and that enzymic
hydrolysis of these walls recycles monosaccharides for seedling growth.
Further, since the endosperm surrounds the embryo, these cells form a barrier
to germination and it is not clear whether the hydrolysis of the cell wall
polymers is a prerequisite for germination. This is the question addressed by Gong
et al., Guelph,
Canada (pp.
1165–1173). They have looked at the timing of radicle emergence and its
relationship to the activity of endo-β-mannanase in several species in
which the endosperm cell walls are thickened in this way. The taxonomic range
of the species investigated was very broad but the picture that emerges is
consistent across the range: germination, as indicated by radicle emergence, is
not dependent on endo-β-mannanase activity. Instead, the hydrolysis of the
cell walls is a post-germination event. How then does the radicle break out?
Microscopic examination of the germinating seeds revealed two mechanisms. In
most species studied, including fenugreek (Trigonella foenum-graecum),
carob (Ceratonia siliqua) and coffee (Coffea arabica), there is a
region of the endosperm, located under the micropyle and over or surrounding
the tip of the radicle, where the cell walls are not thickened. It is here that
the emerging radicle breaks out. In the second mechanism, seen in date (Phoenix
dactylifera), there is a ring of unthickened cells, underlying the
operculum in the enclosing testa; the endosperm fractures round this ring,
allowing the emerging radicle to push its way through the endosperm as if
removing a lid. Both mechanisms imply exquisite positional controls of the
endosperm cell wall thickening during seed development and provide further
examples of the amazing range of variation on the basic angiosperm lifestyle.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
