Demolish the
wall—but recycle the bricks
Seed storage reserves
are, across the plant kingdom, laid down in a number of different tissues or
organs, with the cotyledons and the triploid endosperm being the most common.
Although their role is to supply carbon and/or nitrogen to the growing
seedling, a wide range of compounds are used as storage materials and very
little goes to waste. This is well illustrated by the work of
Buckeridge et
al., Sao Paulo, Brazil
and Stirling, Scotland (pp. 435–444) on the
cotyledons of
Lupinus angustifolius. The
authors had previously
observed that, during germination, the extensively thickened cotyledonary cell
walls were reduced very markedly in thickness. Much of the thickening of these
walls is due to deposition of
b-galactan
polymers (which also contain some arabinose) and it had already been shown that
the b-galactan is the material mobilized from the
walls during germination. The authors had also observed the de novo synthesis
of a hydrolytic enzyme, an exo-(1®4)-b-galactanase (which breaks down galactans by
‘chewing in’ from the end of the polymer). The authors have now extended these
observations with some elegant experiments using purified enzyme and cell walls
(‘ghosts’) of storage mesophyll cells isolated from imbibed but ungerminated
seeds. Exposure of these ghosts to the enzyme resulted in dramatic loss of
thickening from the cell walls so that they came to resemble the cotyledon cell
walls of germinated seeds. Further, the interaction between the enzyme and the b-galactan substrate was very specific. By
conjugating the purified enzyme to colloidal gold, it was shown in thin
sections of these cells that the enzyme bound only to the b-galactan thickenings and not to the original
pectin components of the primary wall. Here, as the authors state, is a clear
change of function during evolution in which the cell wall has taken on the
role of a storage organelle.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
