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towards the growing tip. Intense foci of microfilaments are seen at the two future branch
points, and as soon as elongation begins there is a clear arrangement of longitudinal micro-
filaments leading along the processes. There is also a mesh-like network of cortical actin
(
Figure 6.7
).
Depolymerization of microfilaments using cytochalasin D, latrunculin, phalloidin or jas-
plakinolide inhibits the elongation of trichomes and distorts their shapes, but the number
of branches is still correct.
45
Actin is therefore important to trichome elongation but is not
essential to branching. It is interesting that the cortical actin cytoskeleton, rather than the
central filaments, may be particularly important. Actin meshworks tend to be nucleated by
the Arp2/3 protein complex, which is discussed in detail in Chapter 8. Mutations that block
the activation or function of A. thaliana's Arp2/3 complex cause trichomes to be distorted in
a manner very similar to that seen with cytochalasins and similar drugs.
46,47
Treating trichomes with the microtubule-disrupting drugs colchicine, oryzalin, plopyza-
mide or taxol, at an early stage of their development, causes them to bulge and to expand
isotropically. Treating them later, once the process has already begun to head away from
the plane of the leaf but before branching has taken place, causes them to fail to branch
although elongation happens normally.
45
It therefore seems that microtubules are particu-
larly important in setting up the initial process and also the 'new' processes that become
the branches while microfilaments are most important in elongation, as they are in root hairs
and pollen tubes (
Figure 6.8
). This role for microtubules is supported by the phenotypes of
mutants in the Katanin-P60 gene, and weak alleles of the TFCA and TFCC proteins that assist
the formation of tubulin dimers that are the precursors of microtubules; both types of mutant
are characterized by too few trichome branch points.
45
Although the roles of the two types of cytoskeleton have been separated in the discussion
above, they interact strongly; and mutations that disrupt the microfilaments, particularly
cortical microfilaments, cause a distortion of microtubules.
47
Further work on this interaction
may reveal much about the control of plant cell morphogenesis.
FIGURE 6.7
The arrangement of microfilaments in a developing trichome of A thaliana. The diagram is
a schematic representation of data presented in J. Mathur et al.
45
