Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 250

on the stigmatic papillae and initial arrest of pollen growth is rapid, occurring within
minutes. As in the S-RNase-based system, SI is controlled by a single, multiallelic
S -locus (Lawrence et al. , 1978). A number of alleles of the stigmatic S -gene product
have been cloned (Foote et al. , 1994; Walker et al. , 1996; Kurup et al. , 1998). The
proteins encoded by these S -genes are 14 kDa in size and have no homology to

S-RNases or any other proteins of known function. These S-proteins are develop-
mentally expressed, appearing at maturity in the stigmatic papillae and are secreted
to the stigma surface.
This species has proven to be recalcitrant to transformation, preventing trans-
genic approaches to examine gene function; however, a reliable and efficient in vitro
bioassay for the SI response has been developed (Franklin-Tong et al. , 1988). This
assay has been used very productively to identify and study the stigma proteins
involved in pollen rejection, and to characterise the biochemical events that occur
on self-pollination. Both stigmatic extracts and recombinant S-proteins have been
shown to have S -specific biological activity (Franklin-Tong et al. , 1988; Foote
et al. , 1994). Current data suggest that binding of stigmatic S-proteins to an as
yet unidentified pollen S -receptor activates a signal transduction cascade that ulti-
mately leads to inhibition of pollen tube growth (Rudd & Franklin-Tong, 2003). A
number of components of this signalling cascade have been identified. One of the
earliest detected signalling events is an increase in the cytosolic-free Ca 2 + concen-
tration ([Ca 2 + ] i ), which acts as a second messenger (Franklin-Tong et al. , 1993).
This elevation of [Ca 2 + ] i results in a rapid loss of the apical Ca 2 + gradient asso-
ciated with growing pollen tubes and accompanies the inhibition of pollen tube
growth. Changes in the phosphorylation state of pollen phosphoproteins have been
observed, presumably triggered by this [Ca 2 + ] i elevation (Rudd et al. , 1996) along
with dramatic alterations in the actin cytoskeleton (Geitmann et al. , 2000; Snowman
et al. , 2000, 2002; Staiger & Franklin-Tong, 2003). These initial events are thought
to be associated with the cessation of pollen tube growth and are at least temporarily
reversible. Later events including activation of a putative mitogen-activated protein
kinase (MAPK) (Rudd et al. , 2003) are believed to irreversibly seal the fate of in-
compatible pollen, possibly through the triggering of programmed cell death (PCD)
(Jordan et al. , 2000; Rudd & Franklin-Tong, 2003).

∼

10.3.1 The S-gene controlling stigma function in P. rhoeas

Five alleles of the Papaver stigmatic S -gene have been cloned and sequenced (Foote
et al. , 1994; Walker et al. , 1996; Kurup et al. , 1998). Papaver S-proteins, unlike S-
RNases, are not abundant in pistils (nanograms per pistil as opposed to micrograms
per pistil). Like S-RNases, however, they are highly polymorphic sharing between
51.3 and 63.7% amino acid sequence identity, and can be readily differentiated
by isoelectric focusing. Significant sequence similarity has been found between
S-proteins and a large family of open reading frames of the Arabidopsis genome
(Ride et al. , 1999). There are approximately 100 S-protein homologues (SPHs) in
Arabidopsis ,but they are conspicuously absent in EST databases, suggesting that