Biology Reference
In-Depth Information
The longitudinal microfilaments are associated with secretory vesicles that contain mater-
ial for the building of new cell walls, and it is possible that their main role in tip extension is to
ensure delivery of these vesicles to the right place. In root hairs growing normally, cyto-
plasmic vesicles and organelles are transported in an 'inverse fountain' pattern, moving
towards the tip in the cortical zone of the cytoplasm and back towards the cell body along
the middle of the cell (some material remains at the tip to be incorporated into the new
wall).
32
This movement of material depends on the presence of an intact microfilament
system and it ceases if root hairs of Hydrocharis are treated with cytochalasin B.
35
This obser-
vation suggests that an important mechanism of focal growth is the accurate targeting of ves-
icles to the growing region and nowhere else. As well as microfilament bundles being needed
for the transport of vesicles, it is possible that the cortical microfilament mesh helps to
prevent inappropriate fusion of vesicles to other parts of the membrane by physically
standing in the way.
Another example of focal growth in plant cells is provided by the pollen tubes of angio-
sperms. The male haploid stage of the life cycle of angiosperms is represented by the pollen
grain, which consists in most species of just two cells. Its development begins when it lands
on the stigma of a flower, the stigma being a 'landing pad' that is attached, via a column
called the 'style', to the ovule that contains the female gametes (
Figure 6.5
). The largest of
these cells, the tube cell, undergoes a dramatic focal growth to produce a long tube that
invades the stigma and grows along the style towards the ovary. It carries with it the smaller
cell, the sperm cell, which divides into two at this time to produce two sperm cells, each of
which is used to fertilize a cell of the female gametophyte (flowering plants have a double-
fertilization: one sperm fuses with the egg cell to produce the zygote proper, the other fuses
with 'polar nuclei' in the female gametophyte to produce a triploid nutritive tissue, the
endosperm).
The growing pollen tube is structurally similar to a growing root hair (
Figure 6.6
).
Microtubules and microfilaments are aligned longitudinally, again sometimes with a hint
FIGURE 6.5
Growth of the pollen tube from the pollen grain, on the stigma, through the style towards the ovary
of a flowering plant.
