The S receptor kinase gene determines dominance relationships in stigma expression of self-incompatibility in Brassica

Self-incompatibility (SI) in Brassica is sporophytically controlled by the multi-allelic S locus. SI phenotypes of the stigma and pollen in an S heter ozygote are determined by the two S haplotypes it carries; the two haplotyp es may be co-dominant or exhibit a dominant/recessive relationship. Because the S receptor kinase (SRK) gene of the S locus was recently shown to dete rmine the S haplotype specificity of the stigma, we wished to investigate w hether SRK also plays a role in the dominance relationships between S haplo types. We crossed plants carrying an SRK28 transgene with plants homozygous for one of five S haplotypes that are either co-dominant with, or recessiv e to, S-28 haplotype in the stigma, and analyzed the SI phenotypes of the p rogeny. In all cases, the SI phenotype of the stigma of plants carrying the SRK28 transgene could be predicted by the known dominance relationships be tween the S haplotype(s) and the S-28 haplotype. Moreover, in the S-43 homo zygote carrying the SRK28 transgene where the S-43 phenotype in the stigma was masked by the presence of the SRK28, the transcript level of SRK28 was found to be much lower than that of SRK43. All these results suggest that t he dominance relationships between S haplotypes in the stigma are determine d by SRK, but not by virtue of its relative expression level.