Abstract
Many flowering plants have evolved self-incompatibility (SI) systems to prevent inbreeding. SI in Brassica is controlled by a large number of haplotypes at the S-locus. Each S-haplotype encodes the pollen-borne ligand S-locus protein 11 (SP11, also named S-locus cystein rich protein, SCR) and its stigmatic receptor S-receptor protein kinase (SRK). The SI phenotype of pollen is determined by the dominance relationships between the two S-haplotypes the plant carries. Based on these relationships, the S-haplotypes in Brassica have been classified into two groups; the pollen-dominant (class I) and the pollen-recessive (class II) S-haplotypes. Pollen-dominant S-haplotypes are generally codominant with each other, and they are always dominant over recessive S-haplotypes in S-heterozygotes. Previously we have shown that the expression of recessive SP11 allele is silenced as a result of tapetum-specific DNA methylation in its promoter region in the dominant/recessive S-heterozygotes. Here we show that this methylation is controlled by trans-acting small RNA encoded in the flanking region of dominant SP11 allele.
