Direct ligand–receptor complex interaction controls Brassica self-incompatibility

Many higher plants have evolved self-incompatibility mechanisms to prevent self-fertilization1. 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 stigma6, and SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen4,7,8. SP11 is localized in the pollen coat8. 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 protein9 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 response6; however, how this is accomplished remains controversial10,11,12. Here we show that a single form of SP11 of the S8 haplotype (S8-SP11) stabilized with four intramolecular disulphide bonds specifically binds the stigma membrane of the S8 haplotype to induce autophosphorylation of SRK8, and that SRK8 and SLG8 together form a high-affinity receptor complex for S8-SP11 on the stigma membrane.