Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 251

they may be expressed at very low levels, in narrow developmental windows, and/or
transiently in response to certain environmental or biotic stimuli. As Arabidopsis is
a self-compatible species and is also a member of the Brassicaceae, the role of these
S-protein homologues is unclear, but the established role of S-proteins as signalling
molecules in Papaver , combined with the number of members of the gene family in
Arabidopsis , suggests that they may play significant roles in a variety of signalling
pathways.
The pollen S -gene product in this system has not been identified, but is believed
to be a plasma membrane receptor. One pollen protein that specifically binds S-
proteins (S-protein binding protein, SBP) has been identified (Hearn et al. , 1996).
SBP is a plasma membrane protein that is variably glycosylated, resulting in a
size distribution from 70 to 120 kDa. In vitro studies suggest that though SBP
specifically binds S-proteins, this interaction is not allele-specific. As a result, it has
been proposed that SBP may be an accessory receptor involved in SI but is not likely
to be the pollen S-specificity component.

10.3.2 Structure/function of S-proteins
Structural predictions suggest that despite extensive polymorphism between S -
haplotypes, S-proteins share a virtually identical secondary structure. They are pre-
dicted to be composed of a series of six b strands followed by two helical regions
at the C-terminus, all linked by seven hydrophilic loops (Walker et al. , 1996; Kurup
et al. , 1998). A number of these S-proteins are glycosylated; the function of the gly-
can side chains is unclear as non-glycosylated S-protein expressed in Escherichia
coli was found to be fully functional in the in vitro bioassay (Jordan et al. , 1999). As
recombinant S-proteins are functional in the in vitro bioassay, it has been possible
to investigate structure/function relationships using E. coli expressed S-proteins.
Screening a number of mutant S-proteins for functionality in the SI response has
led to the identification of amino acid residues in surface loop 6 as being critical to
S-protein function (Kakeda et al. , 1998). Changing the only hypervariable amino
acid residue in this loop resulted in a complete loss of the ability of S 1 -protein to
inhibit S 1 pollen, as did changing any of several highly conserved amino acids adja-
cent to this residue. Point mutations in other surface loops, however, were found not
to affect S-protein function (Kakeda et al. , 1998). These results suggest that loop 6
may be directly involved in recognition events essential for the SI response.

10.3.3 Biochemical responses in pollen following self-recognition

A number of investigations into the biochemical events that occur in pollen during the
SI response have also been carried out, including changes in pollen gene expression
(Franklin-Tong et al. , 1990), protein phosphorylation (Franklin-Tong et al. , 1992)
and cytosolic calcium levels (Franklin-Tong et al. , 1993).
Inhibition of transcription by actinomycin D does not affect pollen germination,
presumably because mature pollen contains all the RNAs required for pollen tube