Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 261

serine/threonine kinase activity (Goring & Rothstein, 1992). The extracellular do-
main is called the S -domain and is 75-99% identical to SLG at the amino acid
level. In addition to allelic polymorphism, other characteristics of SRK are also
consistent with a key role in stigmatic haplotype specificity. SRK is predominantly
expressed in stigmatic papillae, and localises to the plasma membrane (Delorme
et al. , 1995) right at the site of pollination. Further like SLG ,expression of SRK is
low in immature buds that do not reject self-pollen and increases as SSI becomes
activated.
Attempts to directly confirm SRK function in vivo by transformation were ini-
tially plagued by co-suppression effects that hindered expression of gain-of-function
transgenes as well as endogenous SRK and SLG . This problem has been overcome
by the use of genotypes with a high degree of sequence divergence. In a definitive set
of experiments, SLG 28 and SRK 28 of B. campestris were introduced into S 52 S 60 and
S 60 S 60 plants, respectively (Takasaki et al. , 2000). It was found that expression of
SRK 28 alone, but not of SLG 28 alone, in the stigmas of the transgenic plants conferred
the ability to reject S 28 pollen. These results somewhat surprisingly demonstrate that
SRK is the sole determinant of stigmatic S -haplotype specificity and rule out the
involvement of SLG in this function.
If SLG is not required for stigmatic S -specificity, a question obviously arises
as to what its function(s) might be. Interestingly, though SLG was not absolutely
required for SI, the transgenic experiments did demonstrate that the strength of
incompatibility may involve SLG .Itwas determined that the degree of self-pollen
rejection by SRK 28 varied according to the S -haplotype of the transformant. A
good correlation was found between the efficiency of self-pollen rejection and the
degree of amino acid sequence identity between the S -domain of SRK 28 and the
SLG produced in the same stigma: the higher the identity, the stronger the rejection
(Takasaki et al. , 2000). As a result, it has been hypothesised that SLG interacts with
the S -domain of SRK to facilitate the recognition reaction between SRK and the
pollen S -specificity determinant. The strength of the association between SLG and
SRK (which is akin to dimerisation) might then be affected by the level of sequence
similarity between them. It is important to note, however, that the transgenic plants
expressed SRK 28 at only approximately 30% of the wild-type level and hence this
phenomenon may not be significant in the natural situation. Another potential role
for SLG is in pollen adhesion to the stigma surface. It has been demonstrated that
treatment of stigmas with an anti-SLG antibody prior to pollination reduces the
strength of pollen adhesion to the stigmatic papillae (Luu et al. , 1999).

10.4.3 SCR/SP11 encodes pollen S-haplotype specificity

Because of the genetically sporophytic nature of SSI, the determinant of pollen
S -haplotype specificity has long been suspected to be a component of the pollen
coat, which is derived from the diploid cells of the tapetum and also makes direct
contact with the stigma surface (for a review of pollen coating, see Doughty et al. ,
2000). A cyclohexane-based protocol has been developed for removing the highly