93.Abstract
Cells of maize (Zea mays L.) seedling
that are not fixed or cryoprotected contain the impressions of
cellulose microfibrils on freeze-fractured plasma membranes.
Impressions of the most recently deposited microfibrils have terminal
complexes associated with them (see preceding paper). The orientations
of microtubules in cytoplasmic fractures are parallel to the newest
microfibrils observed on adjacent plasma membrane fractures.
Small groups of microfibrils, distinguished from the next older
layer by their new orientation, are sometimes observed directly
adjacent and parallel to individual microtubules. Whereas microtubules
are parallel to microfibril orientations which vary from transverse
to occasionally longitudinal, microfilaments are parallel to the
longitudinal cell axis. After colchicine treatment, cytoplasmic
microtubules are absent, as are the bands of microfibrils that
are observed on the membrane of control cells. Parallel orientations
of microfibrils and normal pitfield outlines are often still observed
after colchicine treatment. However, on some membranes, multidirectionally-oriented
microfibril tips occur, associated with perturbations of microfibril
orientation and rounded pit-field outlines. In ethylene-treated
cells, some membranes have microfibril tips oriented in only one
direction in new layers of longitudinal microfibrils. On other
membranes, longitudinal bands of microfibril tips are oriented
in opposing directions. We propose that after colchicine treatment,
the patterns of microfibrils reflect an orientation mechanism
which has been uncoupled from the influence of microtubules but
which is still under some other form of cellular control. We
propose that membrane flow could orient the lateral movement
of synthesis complexes in the membrane and that microtubules modulate
this movement, apparently organizing microfibrils into parallel
bands in newly-forming layers.