Homology-dependent Suppression of Stigma Phenotype by an Antisense S-locus Glycoprotein (SLG) Gene in Brassica rapa L.

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Self-incompatibility (SI) in Brassicaceae is sporophytically controlled by haplotypes of the polymorphic S locus complex. Two tightly linked polymorphic genes at the S locus, S-locus glycoprotein (SLG) and S-receptor kinase (SRK) genes, are specifically expressed in the stigma. S-haplotypes have been classified into class I and class II types based on the sequence similarity 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 introduced an antisense class I SLG⁴³ cDNA into a cultivar Osome in Brassica rapa which was heterozygous for class I S⁵² haplotype and class II S⁶⁰. SLG⁴³ is more similar to the endogenous SLG⁵² (87.8% identity) than to SLG⁶⁰ (74.8% identity), Out of ten primary transformants analyzed, two were completely self-compatible; the SI phenotype of stigma was altered from S⁵²S⁶⁰ to S⁶⁰, but that of pollen was not. In these two plants, the expression levels of mRNA and protein of SLG⁵² were reduced, whereas those of SLG⁶⁰ were not. We suggest that an antisense class I SLG-transgene causes homology-dependent suppression, which leads to breakdown of the class I S-haplotype specificity in stigma but not the class II S-haplotype.