Agriculture Reference
In-Depth Information
Figure 10.3
Model for the mechanism of gametophytic SI in
Papaver rhoeas
. S-protein secreted by
the stigma binds to a pollen
S
-receptor complex of which SBP is a component. This causes the opening
of ion channels in the shank of the tube, raising cytosollic Ca
2
+
levels. This leads to a loss of the Ca
2
+
gradient required for tip growth, restructuring of the actin cytoskeleton, and activation of CDPK (which
then phosphorylates the pyrophosphatase p26), leading to rapid cessation of growth. In addition, raised
Ca
2
+
levels ultimately lead to the activation of PCD and death of the pollen tube. Arrows with solid
lines represent established steps, and those with dashed lines are speculative. Based on Rudd and
Franklin-Tong (2003).
p26 by a calcium-regulated protein kinase (Rudd
et al.
, 1996). As p26 has inorganic
pyrophosphatase activity, this is likely to contribute to growth arrest by stopping
or reducing biosynthesis of membrane and wall material essential for growth. The
third effect of [Ca
2
+
]
i
elevation is to cause dramatic alteration of the cytoskele-
ton, involving depolymerisation of F-actin (Geitmann
et al.
, 2000; Snowman
et al.
,
2002). Again it is established that tip growth is highly sensitive to the actin de-
polymerisation (Gibbon
et al.
, 1999). The depolymerisation of F-actin in the SI
response is rapid and could also contribute to growth arrest. In addition, the size and
longevity of depolymerisation response (several hours) suggest that it may play a
role in downstream, longer term, inhibition (Rudd & Franklin-Tong, 2003).
A second series of events appears to be activated after the initial cessation of
tip-growth, and it has been proposed that their function is to make growth inhibition
irreversible (Rudd & Franklin-Tong, 2003). By removing S-proteins experiments at
various time points after challenge in the bioassay, inhibition of pollen tube growth
can be reversed up to 9 min after SI induction (Rudd & Franklin-Tong, 2003).
Current data suggest that a PCD-like mechanism is responsible for cementing the
fate of self-pollen and that it is triggered quite early in the response, but after the
9-min time point. One potential component of this cascade is an MAPK (p56),
