Molecular characterization of S locus genes, SLG and SRK, in a pollen-recessive self-incompatibility haplotype of Brassica rapa L.

In Brassica species that exhibit self-incompatibility, two genes, SLG and SRK, at the S locus are involved in the recognition reaction with self and non-self pollen. From a pollen-recessive S29 haplotype of Brassica rapa, both cDNA and genomic DNA clones for these two genes were isolated and characterized. The nucleotide sequence for the S domain of SRK29 showed a high degree of similarity with that of SLG29, and they belong to Class II type. RNA gel blot analysis showed that the transcript of SLG29 consisted of the first and second exons, and no other transcript containing any part of the intron sequence was detected. Because no transmembrane domain was encoded by the second exon of SLG29, SLG29 was designated a secreted type glycoprotein. SLGs of two other pollen-recessive haplotypes, S40 and S44, of B. rapa also had a similar structure to that of SLG29. Previously, SLG2 from a pollen-recessive haplotype, S2, of Brassica oleracea was found to produce two different transcripts, one for the secreted type glycoprotein and the other for a putative membrane-anchored form of SLG. Therefore, the nature of these SLGs from pollen-recessive haplotypes of B. rapa is different from that of SLG2 of B. oleracea.