Direct ligand-receptor complex interaction controls Brassica self-incompatibility

Many higher plants have evolved self-incompatibility mechanisms to prevent self-fertilization(1). In Brassica self-incompatibility, recognition between pollen and the stigma is controlled by the S locus, which contains three highly polymorphic genes: S-receptor kinase (SRK)(2), S-locus protein 11 (SP11)(3) (also called S-locus cysteine-rich protein; SCR)(4) and S-locus glycoprotein (SLG)(5). SRK encodes a membrane-spanning serine/threonine kinase that determines the S-haplotype specificity of the stigma(6), and SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen(4,7,8). SP11 is localized in the pollen coat(8). It is thought that, during self-pollination, SP11 is secreted from the pollen coat and interacts with its cognate SRK in the papilla cell of the stigma to elicit the self-incompatibility response. SLG is a secreted stigma protein(9) that is highly homologous to the SRK extracellular domain. Although it is not required for S-haplotype specificity of the stigma, SLG enhances the self-incompatibility response(6); however, how this is accomplished remains controversial(10-12). Here we show that a single form of SP11 of the S-8 haplotype (S-8-SP11) stabilized with four intramolecular disulphide bonds specifically binds the stigma membrane of the S-8 haplotype to induce autophosphorylation of SRK8, and that SRK8 and SLG(8) together form a high-affinity receptor complex for S-8-SP11 on the stigma membrane.