Several plant self-incompatibility systems may be controlled by atypical receptor-ligand interactions.

Self-incompatibility (SI) is the single most important mechanism utilized by flowering plants to avoid self-fertilization, thus preventing inbreeding and promoting outcrossing. Many plant SI systems are genetically controlled by a multi-allelic S-locus, containing two tightly linked genes that encode the female and male S-determinants. When pollen lands on a "self" pistil, interaction between cognate female and male S-determinants induces an SI signaling response, resulting in the failure of self-fertilization. Here, we review currently known SI systems that utilize receptor-ligand interactions to control pollen rejection on the stigma surface. Although detailed molecular and cellular information is only known for the SI systems in the Brassicaceae and Papaveraceae, it is apparent that the S-determinants of other SI systems (e.g., in the Poaceae and the Convolvulaceae) are likely to also utilize receptor-ligand interactions to prevent self-fertilization. Strikingly, although most of these systems all appear to utilize cysteine-rich proteins (CRPs) as ligands to induce an SI response, only one of these receptors is a receptor-like kinase (RLK); the other "receptors" identified to date are proteins of unknown function, which we propose to be atypical receptors (ATRs). Although many of these receptors were identified some time ago, their atypical nature raises many questions, including how they function mechanistically, how they evolved and whether they are found in other plant cell-cell communication systems.