Biology Reference
In-Depth Information
of helical arrangement.
32,36
As in root hairs, transport and even the formation of secretory
vesicles is inhibited in the presence of cytochalasin D and growth of the pollen tube fails.
37
The control of microfilament polymerization is not understood nearly as well in plants as it
is in animals but a combination of genetic and cell biological studies have succeeded in iden-
tifying some of the key components that are involved in both root hairs and pollen tubes. A.
thaliana has homologues of the Rho family of small GTPases that are important in organizing
stress fibres in animal cells (Chapter 8); they are called 'Rop' proteins ('Rho of plants'). Rop1
and Rop5 are enriched in the tips of growing pollen tubes, and Rop2 in the tips of growing
root hairs.
38
e
40
Dominant negativemutants of Rop1 block the growth of pollen tubes.
39
Consti-
tutive activation of Rop1, on the other hand, results in isotropic growth so that the pollen tube
swells up to become a bulbous cell rather than a fine cylinder. Even over-expression of normal
Rop1 causes growth to become delocalized, apparently because too much Rop1 is associated
with the plasmamembrane in regions other than the growth tip.
39
Together, these observations
suggest that Rop1 encourages cell expansion and that localization of Rop1 activity is important
if growth is to be localized.
FIGURE 6.6
The cytoskeleton and cytoplasmic streaming near the tip of a pollen tube. Cortical microtubules
and cytoplasmic microfilmaments run longitudinally, or nearly so. Vesicles move in an 'inverse fountain', coming
forward in the cortical regions of the cell and returning near the axis.
