Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 260

to pollen release. Since the cloning of the first SLG cDNA (Nasrallah et al. , 1985),
numerous alleles of SLG have been cloned, and these genes exhibit a high level
of sequence polymorphism, pairwise sequence identities varying from 65 to 97.5%
(Kusaba et al. , 1997). As a result, for a considerable time the SLG gene was believed
to play a major role in the determination of S -haplotype specificity in the pistil.
Importantly, the process of screening cDNA libraries for SLG alleles uncov-
ered additional homologous genes. These include two S -locus-related genes ( SLR1
and SLR2 ) that are homologous to SLG and but not linked to the S -locus, and the
S -receptor kinase ( SRK ) gene that is located at the S -locus (Stein et al. , 1991), all
of these genes show stigma-specific expression.

10.4.1 Brassica S-locus glycoproteins

Initial attempts to demonstrate the function of SLG by gain- and loss-of-function
approaches did not yield conclusive results. An attempt to knock out SLG by intro-
duction of an antisense SLG gene into transgenic plants led to the breakdown of SI,
but the transcript levels of both SLG and SRK ( S -locus receptor kinase) were reduced,
complicating interpretation of this result (Shiba et al. , 1995). Similarly, analysis of a
self-compatible mutant ( scf1 ), whose stigma, but not pollen function was defective,
showed that transcript and protein levels of both SLG and SLR ( S -locus-related)
were reduced (Nasrallah et al. , 1992). The difficulty in obtaining plants that had a
reduction in SLG expression without simultaneously reducing expression of other
related genes made it difficult to draw strong conclusions about the function of SLG.
Similar problems plagued gain-of-function experiments. Introduction of SLG of a
new S -haplotype did not confer the new S -haplotype specificity on the stigma of
transgenic plants, but instead, caused breakdown of SI. This phenotype appeared to
be a result of homology-dependent gene silencing, as the expression of the trans-
gene, endogenous SLG , SLR s and SRK were all suppressed (Conner et al. , 1997).
Several lines of evidence have cast doubt on the requirement of SLG in SSI. One
line of self-incompatible Brassica oleracea has been found to produce lower levels
of SLG than self-compatible immature buds of other self-incompatible lines (Gaude
et al. , 1995). SLG is absent in a self-incompatible line of B. oleracea (Okazaki et al. ,
1999). Introduction of SLG 43 into S 52 S 60 plants of Brassica campestris showed that
transgenic plants producing SLG 43 at a normal level fail to acquire the ability to re-
ject S 43 pollen (Takasaki et al. , 1999). Recently reported transformation experiments
have clearly confirmed that SLG is not required for the S -haplotype specificity in
Brassica stigmas; however, they do suggest that the SLG may still play a significant
role in the operation of SSI (Takasaki et al. , 2000) (see below).

10.4.2 SRK encodes S-haplotype specificity in the stigma
SRK is the second highly polymorphic gene identified at the Brassica S -locus (Stein
et al. , 1991). SRK is a classic receptor kinase and consists of an extracellular domain,
a single transmembrane domain, and a cytoplasmic kinase domain that possesses