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Plant Physiol, January 2001, Vol. 125, pp. 252-265
Nodal Endoplasmic Reticulum, a Specialized Form of Endoplasmic
Reticulum Found in Gravity-Sensing Root Tip Columella
Cells1
Hui Qiong
Zheng and
L. Andrew
Staehelin*
Department of Molecular, Cellular and Developmental
Biology, University of Colorado, Boulder, Colorado
80309-0347
The endoplasmic reticulum (ER) of columella root cap cells has been
postulated to play a role in gravity sensing. We have re-examined the
ultrastructure of columella cells in tobacco (Nicotiana tabacum) root tips preserved by high-pressure
freezing/freeze-substitution techniques to gain more precise
information about the organization of the ER in such cells. The most
notable findings are: the identification of a specialized form of ER,
termed "nodal ER," which is found exclusively in columella cells;
the demonstration that the bulk of the ER is organized in the form of a
tubular network that is confined to a peripheral layer under the plasma
membrane; and the discovery that this ER-rich peripheral region
excludes Golgi stacks, vacuoles, and amyloplasts but not mitochondria.
Nodal ER domains consist of an approximately 100-nm-diameter central rod composed of oblong subunits to which usually seven sheets of rough
ER are attached along their margins. These domains form patches at the
interface between the peripheral ER network and the ER-free central
region of the cells, and they occupy defined positions within central
and flanking columella cells. Over one-half of the nodal ER domains are
located along the outer tangential walls of the flanking cells.
Cytochalasin D and latrunculin A cause an increase in size and a
decrease in numbers of nodal ER domains. We postulate that the nodal ER
membranes locally modulate the gravisensing signals produced by the
sedimenting amyloplasts, and that the confinement of all ER membranes
to the cell periphery serves to enhance the sedimentability of the
amyloplasts in the central region of columella cells.
1
This work was supported by the National
Aeronautics and Space Administration (grant no. NAG5-3967).
*
Corresponding author; e-mail staeheli{at}spot.colorado.edu; fax
303-492-7744.
© 2001 American Society of Plant Physiologists
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