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 SLG
43 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 S
52 and S
60, and both were codominant in stigma, but S
52 was dominant to S
60 in pollen. S
43 was found to be recessive to S
52 and codominant with S
60 in stigma. The nucleotide sequence of SLG
43 was more similar to that of SLG
52 (87.8% identity) than to that of SLG
60 (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 S
52S
60 to S
60, but the SI phenotype of the pollen was not altered. In these three plants, the mRNA and protein levels of both SLG
43 and SLG
52 were reduced, whereas those of SLG
60 were not. All the plants in the selfed progeny of No. 9 and No. 6 regained SI and they produced a normal level of SLG
52. 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 SLG
43 transgene and the endogenous SLG
52 gene. Three of the transformants (Nos. 5, 8 and 10) produced SLG
43 protein, but their SI phenotype was not altered. The S
60 homozygotes in the selfed progeny of No. 10 which produced the highest level of SLG
43 were studied because S
43 was codominant with S
60 in the stigma. They produced SLG
43 at approximately the same level as did S
43S
60 heterozygotes, but did not show S
43 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.
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