Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 255

protein blots prior to incubation with S-proteins (Hearn et al. , 1996). Correlative
evidence that SBP might be involved in the SI response has come from the use of
mutant S-proteins in an in vitro bioassay combined with Western ligand blotting. In
general, there is a good correlation between binding of mutant S-proteins to SBP
and their functionality in the SI response (Jordan et al. , 1999). Amino acid changes
in the predicted loops 6 and 2 of S 1 -protein simultaneously reduced the ability of
S 1 -protein to bind SBP and to effect the SI response in vitro (changes in loop 6
having the greatest affect). However, an N-terminal deletion of 16 amino acids of
the S 1 -protein completely abolished the ability of the protein to inhibit self-pollen
yet did not affect the binding activity to SBP. One explanation for these results is
that a second non- S -specific interaction involving the N-terminal amino acids and
an as yet unidentified protein is essential for the SI response. Indeed, SBP has been
proposed to function as an accessory receptor, which might modulate the interac-
tion of S-proteins with the pollen S-receptor (Jordan et al. , 1999). In this scenario,
SBP and S-proteins would interact to form a recognition complex. Credence to this
argument comes from analogy to systems found in the animal kingdom, such as
the fibroblast growth factor (FGF) signalling system. Here a family of membrane
proteins, the heparin sulfate proteoglycans (HSPG), function as accessory receptors
and low-affinity, high-capacity, binding of FGF to glycan side chains of HSPG fa-
cilitates oligomerisation of the ligand and enables interaction with the high-affinity
FGF receptor leading to signal transduction (Spivak-Kroizman et al. , 1994).

10.3.5 Changes in the actin cytoskeleton
The actin cytoskeleton has been amply demonstrated to play a critical role in tip
growth in general (Gibbon et al. , 1999). In the poppy SI response, rapid and dra-
matic changes in the organisation of the F-actin cytoskeleton have been observed in
incompatible pollen tubes. Punctate actin foci are formed and continue to form for
several hours after stimulation (Geitmann et al. , 2000; Snowman et al. , 2002). In
addition, the SI response rapidly elicits a large and sustained depolymerisation of
F-actin. Concentrations of F-actin within pollen tubes decrease by 50% within 5 min
of being challenged by self-S-protein and remain low for at least an hour (Snowman
et al. , 2002). Increases in [Ca 2 + ] i in pollen tubes have been shown stimulate actin
depolymerisation; hence, it is envisaged that the SI-induced [Ca 2 + ] i increase dis-
cussed above is likely to be causing this effect, though the mechanism is unknown
(Snowman et al. , 2002).

10.3.6 PCD in the SI response

As mentioned above, SI in Papaver appears to be a two-step process, the initial
step being a rapid but reversible inhibition of tip growth, followed by a second,
more gradual but irreversible, termination of pollen tube viability. Preliminary
evidence suggests that induction of PCD may be the cause of the irreversible step
in SI. There is now good evidence for the involvement of PCD in a number of