97. Mueller, S. C., Maclachlan, G. A., and
R. M. Brown, Jr. 1983. Cellulose microfibril assembly and orientation
in higher plant cells. Journal of Applied Polymer Science:
Applied Polymer Symposium 37:79-90.
97. Synopsis
Freeze-fractured plasma membranes of seedlings
of Zea mays L., Burpee's Snowcross, and Pisum sativum
L., variety Alaska, contain terminal complex structures and the
impressions of microfibrils from the newest cell wall layer.
Terminal complex subunits are on the exoplasmic fracture (EF)
face, and rosette subunits are on the protoplasmic fracture (PF)
face of the membrane. The association of terminal complexes and
rosettes with microfibril tips and their association with newly
deposited groups of microfibrils is indirect evidence for their
role in microfibril assembly. Microfibril tips associated with
terminal complexes have been observed with the tips all pointing
in the same direction (S. C. Mueller and R. M. Brown. Jr., Planta,
154, 489 (1982)). This asymmetry in the orientation of microfibril
tips was observed when microfibril deposition was very orderly.
Asymmetric patterns of microfibrils around pit fields that are
flow-like in appearance have also been observed. Unidirectionally
oriented microfibril tips and flow-like patterns of microfibrils
were still observed in colchicine-treated cells when microtubules
were depolymerized. However, bands of microfibrils were no longer
observed on the plasma membrane. Microtubules may be responsible
for certain orientations of microfibrils, particularly the formation
of bands of microfibrils in newly deposited wall layers. However,
microfibril orienting mechanisms are more complex, involving factors
still present during colchicine treatment. A marker specific
for newly formed microfibrils could be used to probe the function
of terminal complexes. Since UDP-glucose is thought to be a precursor
of cellulose microfibrils in higher plant cells, EM radioautography
was used to determine the site of incorporation of glucose. However,
under the conditions used, glucose was only incorporated from
UDP-glucose at the surface of cut or damaged pea stem cells, i.e.,
in vitro. Thus, incorporation of glucose from UDP-glucose
was not useful for probing the patterns of cellulose microfibril
synthesis in vivo.