Molecular characterization of the S locus in two self-incompatible Brassica napus lines.

Abstract

In Brassica species, self-incompatibility has been mapped genetically to a single chromosomal location. In this region, there are two closely linked genes coding for the S locus glycoprotein (SLG) and S locus receptor kinase (SRK). They appear to comprise the pistil component of the self-incompatibility reaction. SLG and SRK are thought to recognize an unknown pollen component on the incompatible pollen, and the gene encoding this pollen component must also be linked to the SLG and SRK genes. To further our understanding of self-incompatibility, the chromosomal region carrying the SLG and SRK genes has been studied. The physical region between the SLG-910 and the SRK-910 genes in the Brassica napus W1 line was cloned, and a search for genes expressed in the anther revealed two additional S locus genes located downstream of the SLG-910 gene. Because these two genes are novel and are conserved at other S alleles, we designated them as SLL1 and SLL2 (for S locus-linked genes 1 and 2, respectively). The SLL1 gene is S locus specific, whereas the SLL2 gene is not only present at the S locus but is also present in other parts of the genomes in both self-incompatible and self-compatible Brassica ssp lines. Expression of the SLL1 gene is only detectable in anthers of self-incompatible plants and is developmentally regulated during anther development, whereas the SLL2 gene is expressed in anthers and stigmas in both self-incompatible and self-compatible plants, with the highest levels of expression occurring in the stigmas. Although SLL1 and SLL2 are linked to the S locus region, it is not clear whether these genes function in self-incompatibility or serve some other cellular roles in pollen-pistil functions.

Using S -locus deletions to evaluate self-incompatibility candidate genes: an example for a novel anther-expressed gene

“…In particular, the SLL1 and SLL2 genes (Yu et al 1996) which are included in the deletion in ∆S-1668 (data not shown) can be unambiguously excluded from encoding the pollen SI identity factor, as previously suggested by their lack of polymorphism between S haplotypes (Yu et al 1996). This study points to the usefulness of S-locus deletion strains in evaluating potential S haplotype-specific functions for S locuslinked transcriptional units.…”

“…The chromosomal region containing the S locus is a gene-rich segment in which the polymorphic genes known to be required for self-incompatibility (SI) are interspersed with genes whose functions are unknown (Boyes and Nasrallah 1995;Yu et al 1996;Conner et al 1998;Letham and Nasrallah 1998). Determining which of these genes is required for the specificity of SI is difficult.…”

Expression of stigma- and anther-specific genes located in the S locus region of Ipomoea trifida

“…In the S locus region of Brassica, two non-polymorphic genes, SLL1 and SLL2, are located near the highly polymorphic genes, SLG, SRK and SP11/SCR (Yu et al 1996;Cui et al 1999;Suzuki at al. 1999).…”

Mechanisms of self-incompatibility in flowering plants

“…Based on cDNA sequence analysis, SRK was predicted to encode a plasma membrane receptor kinase comprised of three distinct domains: an extracellular domain followed by a single transmembranespanning region and a cytoplasmic serine/threonine kinase domain [111,112]. The extracellular domain of SRK which is implicated in ligand binding shares a high degree of sequence similarity with SLG implying that both genes co-evolved and that SLG perhaps is the result of a duplication of the SRK extracellular domain [113,114]. The extracellular domain of SRK, like SLG, also encodes a signal peptide, potential N-glycosylation sites, three hypervariable regions and 12 conserved cysteine residues [111,112,115] ( fig.…”