Introduction of SLG (S locus glycoprotein) alters the phenotype of endogenous S haplotype, but confers no new S haplotype specificity in Brassica rapa L.

Self-incompatibility (SI) in Brassicaceae is genetically controlled by the S locus complex in which S locus glycoprotein (SLG) and S receptor kinase (SRK) genes have been identified, and these two genes encoding stigma proteins are believed to play important roles in SI recognition reaction. Here we introduced the SLG43 gene of Brassica rapa into a self-incompatible cultivar, Osome, of B. rapa, and examined the effect of this transgene on the SI behavior of the transgenic plants. Preliminary pollination experiments demonstrated that Osome carried S52 and S60, and both were codominant in stigma, but S52 was dominant to S60 in pollen. S43 was found to be recessive to S52 and codominant with S60 in stigma. The nucleotide sequence of SLG43 was more similar to that of SLG52 (87.8% identity) than to that of SLG60 (74.8% identity). Three of the ten primary transformants (designated No. 1 to No. 10) were either completely (No. 9) or partially (No. 6 and No. 7) self-compatible; the SI phenotype of the stigma was changed from S52S60 to S60, but the SI phenotype of the pollen was not altered. In these three plants, the mRNA and protein levels of both SLG43 and SLG52 were reduced, whereas those of SLG60 were not. All the plants in the selfed progeny of No. 9 and No. 6 regained SI and they produced a normal level of SLG52. These results suggest that the alteration of the SI phenotype of the stigma in the transformants Nos. 6, 7, and 9 was the result of specific co-suppression between the SLG43 transgene and the endogenous SLG52 gene. Three of the transformants (Nos. 5, 8 and 10) produced SLG43 protein, but their SI phenotype was not altered. The S60 homozygotes in the selfed progeny of No. 10 which produced the highest level of SLG43 were studied because S43 was codominant with S60 in the stigma. They produced SLG43 at approximately the same level as did S43S60 heterozygotes, but did not show S43 haplotype specificity at the stigma side. We conclude that SLG is necessary for the expression of the S haplotype specificity in the stigma but the introduction of SLG alone is not sufficient for conferring a novel S haplotype specificity to the stigma.