Biology Reference
In-Depth Information
Reorientation of existing microtubules would require their release from the plasma
membrane, and this presumably requires the activation of phospholipase D.
19
Failure to
release microtubules, as when plants are treated with the phospholipase D inhibitor
1-butanol, would prevent inappropriately aligned microtubules from correcting themselves,
and would therefore account for the disorganized cytoskeleton in such plants.
19
If phospho-
lipase D is controlled by plant hormones, as it is by growth factor pathways in animals,
26
then
one of the actions of the hormones may be to provide a controlled release of microtubules
from the cortex to allow their repositioning. What is missing from this account is the mech-
anism that actually performs reorientation. If microtubules with the 'new' orientation were
somehow fixed firmly in the cell, then association with them, via cross-linking proteins,
may be enough to reorientate released microtubules that had the old orientation. This mech-
anism cannot work if all microtubules are equal, though, because the more common existing
ones would be expected to pull the new microtubules to the old orientation, the opposite
effect from the one required. An alternative possibility is that a motor system is aligned
with respect to the axis of the cell and pulls any free microtubules, whether new or old,
into the new desired orientation. The problem, in that case, is to find that motor system
and to understand how it comes to be aligned.
Microtubules are often found, especially when they can be imaged clearly, to be arranged
obliquely in resting plant cells so that they form a helix.
27
It may be that cortical microtu-
bules (and the microfibrils above them) are in fact always arranged helically in interphase
plant cells, but that the pitch of the helix can vary from very small, making the microtubules
seem transverse, to very large, making them seem longitudinal. In this case, the mechanism
for changing between 'transverse' to 'oblique' or 'longitudinal' orientation would be to alter
the natural pitch of the spiral, presumably through altering microtubule associated
proteins. Such a mechanism would have the aesthetic advantage that it is relatively simple,
global, and would scale with any size of cell, but it is so far unsupported by biochemical
evidence.
The microtubule-led model for control of cell elongation has gained widespread accep-
tance in textbooks, but there is a growing body of evidence to suggest that the real mech-
anism may not be so simple. The construction of cell walls where there were none before
can be studied using protoplasts, which are cultured plant cells fromwhich existing cell walls
have been removed enzymatically. Protoplasts of Nicotiana tabacum will regenerate their
walls and, having done so, their cells elongate normally. If construction of the cortical micro-
tubule system lies upstream of the alignment of cellulose microfibrils in a hierarchy of spatial
patterning processes, the microtubule system in regenerating protoplasts ought to be normal
whether or not cellulose biosynthesis is possible. Cellulose biosynthesis can be inhibited
using the herbicide Isoxaben. As expected, treatment of cells with Isoxaben causes cells
that ought to elongate in one direction to expand in all directions instead.
28
Protoplasts regen-
erating in the presence of Isoxaben do construct a cortical microtubule system, but it is
ordered randomly and never acquires the ordered orientation it acquires in control cells.
29, 30
This effect does not depend on starting from protoplasts, for even if cells that still have their
cell walls are treated with Isoxaben, microtubule order is lost over the course of a few days.
Unless Isoxaben has an additional, uncharacterized effect beyond that on cellulose synthe-
sis
d
and attempts to find such an effect have so far failed
d
we are forced to conclude that
cellulose microfibrils determine the direction of the microtubules. If the original idea that
