Biology Reference
In-Depth Information
Direct relationships between changes of cell shape and morphogenesis of larger structures
are shown most clearly by organisms in which cell movement is not possible; these organ-
isms include some multicellular prokaryotes and all multicellular plants and fungi. Much
of the gain of size of developing plant tissues is achieved by cell enlargement rather than
cell multiplication. One of the simplest ways in which an expanding cell can change shape
is by expanding more in one direction than others ('simplest' in terms of the shape change,
not necessarily simplest in terms of the molecular mechanisms that underlie it). In a tissue
composed of cells that cannot move with respect to one another, a single cell cannot expand
in this way unless it is on the edge of a tissue and expands outwards, but if the behaviour of
cells is regulated so that all expand together, the entire tissue will change shape (
Figure 4.6
).
This system of simultaneous anisotropic cell expansion is used a great deal by growing
plants. At the tip of a root, for example, there is a zone of cell proliferation immediately
behind the protective root cap and, proximal to this, there is a zone of cell expansion. In
the zone of expansion, cell walls are allowed to yield, in a controlled manner, to osmotic pres-
sure inside the cell (Chapter 6). If this cell expansion were isotropic, a broad and relatively
short root would result (
Figure 4.7
). In reality, the cell expansion is anisotropic so that cells
enlarge a great deal along the axis of the root and little in girth. Since cells cannot move
past each other, this section of the root is therefore forced to elongate as a whole and
a long, thin root is the result (
Figure 4.7
). Similar anisotropic cell expansions are seen in
shoots and leaves. Where cell expansion takes place more on one side of a shoot than on
the other, it causes the shoot to bend: this is an important mechanism for organism-scale
morphogenesis in plants.
Anisotropic expansion of cells can be achieved either by distributing the expansion
throughout the cell walls along the elongating axis of the cell ('diffuse expansion') or by
FIGURE 4.6
Simultaneous anisotropic expansion by cells can re-shape an entire tissue.
