Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE PLANT CELL WALL
1085
turgor
pressure
(B)
(C)
(A)
200 nm
Oriented Cell Wall Deposition Controls Plant Cell Growth
Once a plant cell has left the meristem where it is generated, it can grow dramatically,
commonly by more than a thousand times in volume. The manner of this
expansion determines the final shape of each cell, and hence the final form of the
plant as a whole. Turgor pressure inside the cell drives the expansion, but it is the
behavior of the cell wall that governs its direction and extent. Complex wall-remodeling
activities are required, as well as the deposition of new wall materials.
Because of their crystalline structure, the individual cellulose microfibrils in the
wall are unable to stretch, and this gives them a crucial role in the process. For the
cell wall to stretch or deform, the microfibrils
MBoC6
must
m19.80/19.65
either slide past one another
or become more widely separated, or both. The orientation of the microfibrils in
the innermost layers of the wall governs the direction in which the cell expands.
Cells in plants therefore anticipate their future morphology by controlling the orientation
of the cellulose microfibrils that they deposit in the wall (Figure 19–64).
Unlike most other matrix macromolecules, which are made in the endoplasmic
reticulum and Golgi apparatus and are secreted, cellulose is spun out from
the surface of the cell by a plasma-membrane-bound enzyme complex (cellulose
synthase), which uses as its substrate the sugar nucleotide UDP-glucose supplied
from the cytosol. Each enzyme complex, or rosette, has a sixfold symmetry (see
Figure 19–65) and contains the protein products of three separate cellulose synthase
(CESA) genes. Each CESA protein is essential for the production of a cellulose
microfibril. Three CESA genes are required for primary cell wall synthesis and
a different three for secondary cell wall synthesis.
As they are being synthesized, the nascent cellulose chains assemble into
microfibrils. These are spun out on the extracellular surface of the plasma membrane,
forming a layer, or lamella, in which all the microfibrils have more or less
the same alignment (see Figure 19–63). Each new lamella is deposited internally
to the previous one, so that the wall consists of concentrically arranged lamellae,
with the oldest on the outside. The most recently deposited microfibrils in elongating
cells commonly lie perpendicular to the axis of cell elongation, although
the orientation of the microfibrils in the outer lamellae that were laid down earlier
may be different (see Figure 19–64B and C).
Figure 19–64 Cellulose microfibrils
influence the direction of cell elongation.
(A) The orientation of cellulose microfibrils in
the primary cell wall of an elongating carrot
cell is shown in this electron micrograph of
a shadowed replica from a rapidly frozen
and deep-etched cell wall. The cellulose
microfibrils are aligned parallel to one
another and perpendicular to the axis of
cell elongation. The microfibrils are crosslinked
by, and interwoven with, a complex
web of matrix molecules (compare with
Figure 19–63). (B, C) The cells in (B) and
(C) start off with identical shapes (shown
here as cubes) but with different net
orientations of cellulose microfibrils in their
walls. Although turgor pressure is uniform
in all directions, cell wall loosening allows
each cell to elongate only in a direction
perpendicular to the orientation of the
innermost layer of microfibrils, which have
great tensile strength. Cell expansion
occurs in concert with the insertion of new
wall material. The final shape of an organ,
such as a shoot, is determined in part by
the direction in which its component cells
can expand. (A, courtesy of Brian Wells and
Keith Roberts.)