Biology Reference
In-Depth Information
microtubules determine the direction of cellulose microfibrils is to be retained, then there
must at the very least be a powerful feedback system in which each component shapes the
other.
Detailed observations on the rates of elongation and radial expansion of cells, and on the
orientations of microtubules and microfibrils as cells move through the growth zone of the
maize root, suggest another problem; although the alignment of the fibres is compatible
with the model, changes in the alignment lag behind changes in the rate of elongation.
10
This observation casts doubt on the whole idea that changes in the direction of microfibil
deposition regulate elongation. It may be that other changes in the cell wall, connected
with the degree and strength of cross-linking, are the real determinants of the direction in
which the wall will yield to turgor pressure. Some secreted enzymes, such as expansins,
promote the creep between components of the cell wall and therefore allow the wall to stretch
in response to applied force.
30
It may be that tightly regulated expression or secretion of these
enzymes plays a major role in controlling the rates of cell expansion.
FOCUSED CELL GROWTH: ROOT HAIRS, POLLEN TUBES
AND TRICHOMES
In focused cell growth, expansion takes place from only one surface of the cell and the
other walls remain as they are. This mode of elongation seems not to be used to drive the
expansion of whole tissues, at least not in angiosperms and gymnosperms.
)
It is used
instead to produce long, thin cell processes such as root hairs that increase the surface area
available for absorption of nutrients from the soil. These processes grow from their tips, in
a manner morphologically analogous to the growth of neuronal processes in animals
(Chapters 5 and 8). In an expanding root hair, the microtubule cytoskeleton is aligned
approximately longitudinally with respect to the axis of elongation,
31
an orientation that
emphasizes that focused growth is different from diffuse elongation. The microtubule cyto-
skeleton seems to play a comparatively minor role in the growth of root hair tips: depolyme-
rizing microtubules of root hair cells in A. thaliana using oryzalin does not block root hair
growth.
31
It does, however, perturb the directionality of growth so that normally straight
root hairs become wavy and some root hairs develop multiple growing tips.
Actin microfilaments are found in a similar orientation as microtubules. Actin also forms
a diffuse network at the growing tip.
32
There are some differences in the organization of
microfilaments in growing root hairs of different species. Root hairs of angiosperms tend
to have arrays that run approximately longitudinally whereas rhizoids of bryophytes,
A
which are essentially similar tissues, have arrays that are obviously helical.
33
Again, it is
possible that helices are present in both cases, but that their pitch is very different. Whatever
their precise arrangement, microfilaments are vital to the process of tip growth. If they are
depolymerized with cytochalasins or latrunculins, root hair extension fails.
31,34
)
Angiosperms are flowering plants and gymnosperms are coniferous plants and their relatives; together
they constitute the 'higher' plants.
A
Bryophytes are mosses and liverworts.
