Homology-dependent suppression of stigma phenotype by an antisense S-locusglycoprotein (SLG) gene in Brassica rapa L.

Self-incompatibility (SI) in Brassicaceae is sporophytically controlled by haplotypes of the polymorphic S locus complex. Two tightly linked polymorph ic genes at the S locus, S-locus glycoprotein (SLG) and S-receptor kinase ( SRK) genes, are specifically expressed in the stigma. S-haplotypes have bee n classified into class I and class II types based on the sequence similari ty of their SLGs, and their SRKs. To investigate the effect of an antisense SLG gene on the class divergency of the endogenous SLG and SRK genes, we i ntroduced an antisense class I SLG(43) cDNA into a cultivar Osome in Brassi ca rapa which was heterozygous for class I S-52 haplotype and class II S-60 . SLG(43) is more similar to the endogenous SLG(52) (87.8 % identity) than to SLG(60) (74.8 % identity). Out of ten primary transformants analyzed, tw o were completely self-compatible; the SI phenotype of stigma was altered f rom (SS60)-S-52 to S60, but that of pollen was not. In these two plants, th e expression levels of mRNA and protein of SLG(52) were reduced, whereas th ose of SLG(60) were not. We suggest that an antisense class I SLG-transgene causes homology-dependent suppression, which leads to breakdown of the cla ss I S-haplotype specificity in stigma but not the class II S-haplotype.