In maize (Zea mays L.) and pine (Pinus
taeda L.) seedlings, cellulose microfibril impressions are
present on freeze-fractured plasma membranes. It has been proposed
that impressions of newly synthesized microfibrils are a record
of the movement of terminal synthesizing complexes through the
plasma membrane (Mueller and Brown, 1980, J. Cell Biol.
S4, 315-326). The association of terminal complexes with the
ends of microfibril impressions or with the ends of microfibrils
torn through the membrane indicates the orientation of microfibril
tips. Unidirectionally-oriented microfibril tips (all pointing
in the same direction) are associated with the organized deposition
of parallel arrays of microfibrils. Multidirectionally-oriented
microfibril tips were observed in a cell in which microfibril
deposition was unusually disorganized. Microfibril patterns around
pit fields are asymmetric and resemble flow patterns. Unidirectionally
oriented tears are associated with these microfibrils. Although
microfibril orientations are deflected around pit fields, the
main axis of microfibril orientation is maintained across the
surface of the cell. The hypothesis is proposed that the interaction
of a flowing plasma membrane with microfibril synthesizing complexes
in the plane of the membrane may result in unidirectional deposition
and asymmetric microfibril impressions around pit fields.
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