104. Summary
Rosettes of six particles have been visualized
by freeze-fracture in the protoplasmic fracture (PF) faces of:
a) the plasma membrane, b) Golgi cisternae, and c) Golgi-derived
vesicles in mesophyll cells of Zinnia elegans that had
been induced to differentiate synchronously into tracheary elements
in suspension culture. These rosettes have been observed previously
in the PF face of the plasma membranes of a variety of cellulose-synthesizing
cells and are thought to be important in cellulose synthesis.
In Zinnia tracheary elements, the rosettes are localized in the
membrane over regions of secondary wall thickening and are absent
between thickenings. The observation of rosettes in the Golgi
cisternae and vesicles suggests that the Golgi apparatus is responsible
for the selective transport and exocytosis of rosettes in higher
plants, as has been previously indicated in the alga Micrasterias
(GIDDINGS et al. 1980). The data presented indicate that
the Golgi apparatus has a critical role in the control of cell
wall deposition because it is involved not only in the synthesis
and export of matrix components but also in the export of an important
component of the cellulose synthesizing apparatus. The rosettes
are present in the plasma membrane and Golgi vesicles throughout
the enlargement of the secondary thickening, suggesting that new
rosettes must be continually inserted into the membrane to achieve
complete cell wall thickening.