101. Summary
A brief history of the literature dealing
with cellulose microfibril assembly is presented, and a current
summary of cellulose microfibril synthesizing complexes among
eukaryotic cells is given. Terminal complexes heretofore not
described include the following: linear terminal complexes (TCs)
with three rows in Eremosphaera, Microdictyon, and
Chaetomorpha; globular terminal complexes in Ophioglossum,
Psilotum, Equisetum, and Ginkgo. Cellulose
microfibril assembly in Acetobacter xylinum is very briefly
described and compared with the process among eukaryotic cells.
Particular emphasis on structures which may be involved in the
spatial control of cellulose synthesis is given. Among these
are cytoplasmic structures such as microtubules and microfilaments.
Microfilament structures are shown to clearly surround individual
microtubules which lie adjacent to the plasma membrane. Using
freeze-fracture techniques, these labile associations have been
shown for the first time. Microfibril orientation may be mediated
through an interaction of cortical microtubules in association
with microfilaments. A review of Mueller and Brown's membrane
flow model for microfibril orientation is presented. Cellulose
terminal complex clustering and its role in gravitropic response
is covered. Definitive membrane changes with TC clustering/disaggregetion
and intramembranous particle frequencies, occur within 12 min.
following gravistimulation. These differences are pronounced
in the cells from upper and lower hemicylinders of rapidly frozen
tissue which was studied by the freeze-fracture method. A hypothesis
for cellulose microfibril interaction in controlling the constraint
of the growth axis is presented, and the supporting data for terminal
complex clustering/disaggregation as well as fluorescent brightener
inhibition of the gravitropic response support this hypothesis.
The onset and regulation of cellulose microfibril assembly is
presented for synchronized protoplasts generated by Boergesenia,
using inhibitors of transcription and translation. These results
suggest dynamic turnover of terminal complex subunits during the
assembly of the cellulose microfibril. This study is concluded
with a brief discussion of possible phylogenetic trends in the
evolution of cellulose synthesis. A principal underlying theme
is that the specific arrangement and consolidation of the terminal
complex subunits determine to a large degree the size and shape
of the microfibril, its crystallinity, as well as intramicrofibrillar
associations. Three basic types of TCs appear among all eukaryotic
cells studied so far, namely, the rosette, the globular, and the
linear complex.