Biology Reference
In-Depth Information
How might Rop1 act on microfilaments? One possibility might be to encourage actin-
myosin binding as Rho does in animal cells. Another possibility is that it acts by repressing
actin depolymerizing factors (ADFs). ADFs are cytoplasmic proteins that can both nucleate
and sever microfilaments and they can be inactivated by Rop activity.
41
ADFs are associ-
ated with the zone immediately distal to the tip of a pollen tube, and may be responsible
for cutting filaments there into pieces and preventing them from penetrating all the way
forward.
41
It is known that the pH of the cytoplasm in the tip zone is lower than that else-
where and it is also known that ADFs work best in alkaline conditions; it may be, therefore,
that the pH gradient ensures that ADFs can associate with actin and promote depolymer-
ization and severing of filaments mostly in this zone, and that they leave the microfila-
ments in the shaft of the tube alone. Overexpression of ADF1 in the pollen tube of
Nicotiana tabacum results in ADF/microfilament interaction everywhere, rather than just
in the zone at the tip, and this causes a reduction in the number of longitudinal microfila-
ments in the main body of the pollen tube. Significantly, this overexpression of ADF can
counteract the cell-ballooning effect induced by overexpression of Rop1
41
in pollen tubes
of N. tabacum. This suggests that the main target for this Rop is ADF, and that the Rop
controls microfilament dynamics by regulating the destruction of microfilaments in the
zone just behind the tip.
There is a gradient of cytoplasmic Ca
2
þ
in pollen tubes, with the concentration highest at
the tip and decreasing towards the main cell body of the pollen grain. The establishment
and/or maintenance of this gradient requires the action of Rop1: if dominant negative
Rop1 is expressed in the pollen tube, both Ca
2
þ
influx at the tip and subsequent tube growth
are markedly inhibited.
39
Growing pollen tubes in high-Ca
2
þ
media rescues growth, suggest-
ing that the critical activity of Rop1 is to promote Ca
2
þ
influx. How it does so will remain
obscure until the Ca
2
þ
channel used at the tube tip has been identified. The link may or
may not involve actin. The role of the Ca
2
þ
gradient is not fully understood, but it seems
to be linked to the cytoskeletal system via the Ca
2
þ
-sensitive protein calmodulin; experimen-
tally induced release of activated calmodulin in other parts of the pollen tube reorientates the
growth axis towards those parts.
42
There is probably a feedback system, therefore, in which
Rop proteins set up the gradient of Ca
2
þ
and control the cytoskeleton, and the gradient of
Ca
2
þ
, via calmodulin, controls the activation of Rop.
Root hairs and pollen tubes normally grow as single tubes without branching. Some plant
cells, such as the trichomes
)
of A. thaliana, can produce branched processes. Cells that have
made the decision to form trichomes stop dividing but continue to replicate their DNA by
endoreduplication until they reach a DNA content of 32C.
43
As this takes place, the cell
expands away from the surface of the leaf and emits two new processes, each being a branch
from the existing one. Then each process elongates rapidly. The number of branches is highly
controlled, and can be increased or decreased by specific mutations.
44
Both the microfilament
and microtubule cytoskeletons are required for normal branching morphogenesis to take
place.
Microfilaments show a relatively diffuse organization until the trichome process
emerges, and then they form a more organized longitudinal array leading from the nucleus
)
Trichomes are 'leaf hairs' that stick out from the surface of the leaf and modulate airflows across it.
