References

88 S.G. Thomas et al.
Fig.6.4. SignalsandtargetsinvolvedinSIinincompatiblePapaver pollen. S proteins (S1)
bindtoaspecificpollenSreceptoronanS1 pollen tube duringan incompatible SI interaction.
This stimulates influx of Ca 2+ and increases in [Ca 2+ ]i, which triggers an intracellular Ca 2+
signalling cascade in the pollen tube. Depolymerization of F-actin is likely to be mediated
by Ca 2+ -regulated actin-binding proteins, including profilin and PrABP80. SI also tri ggers
PCD in incompatible pollen. Hallmarks of PCD, including cytochrome c leakage from
mitochondria, a caspase-3 like cleavage activity and caspase-mediated DNA fragmentation
have been observed in incompatible pollen. There is preliminary evidence for altered actin
dynamics or polymer levels acting as the trigger for PCD, which commits the pollen to
die. Other SI-triggered signalling events in incompatible pollen include rapid increases
in the Ca 2+ -dependent phosphorylation of a number of proteins, including p26, a soluble
inorganic pyrophosphatase and p56, a mitogen-activated protein kinase (MAPK)
tipgrowthisinhibitedwithinafewminutes.Wehaveshownthat[Ca 2+ ]i
acts as a second messenger and triggers a number of downstream events
(Fig. 6.4). Because the tight spatio-temporal regulation of actin dynamics
inpollentubesiscrucialforgrowth,thecytoskeletonisanidealtargetfor
the Ca 2+ -signallingcascadeduringSI.Changestothecytoskeletonduring
SI are probably mediated through the cooperative action of several
Ca 2+ -regulated ABPs, such as profilin and PrABP80. The sustained actin
depolymerization may function to maintain a block on tip growth, but preliminary
data suggest it may also operate in crosstalk with PCD signalling
cascades. The SI-induced PCD cascade includes leakage of cytochrome c
from mitochondria, activation of a caspase-like activity and nuclear DNA
fragmentation. Thus, the cytoskeleton is not only a target for signalling
cascades,butmayalsobeaninitiatorandtransducerofsignals.Activation
ofthep56MAPkinaseoccursaftertipgrowthinhibitionand,therefore,
may signal to downstream events. Phosphorylation of the soluble inorganic
pyrophosphatase, p26, is thought to act as a fail-safe mechanism ensuring
that tip growth remains inhibited. It is becoming clear, therefore, that there