Self-incompatibility in the Brassicaceae: Regulation and mechanism of self-recognition.

Self-incompatibility is one of the most common mechanisms used by plants to prevent self-fertilization. In the Brassicaceae, the inhibition of self-pollen is triggered right at the stigma surface by interaction of two highly polymorphic self-recognition proteins that are encoded by tightly linked genes of the S-locus haplotype: a receptor protein kinase displayed at the surface of stigma epidermal cells and its small diffusible ligand that is localized in the outer coat of pollen grains. It is the specific interaction between receptor and ligand encoded in the same S haplotype that determines specificity in the rejection of self-pollen. The chapter reviews recent results that have shed light on the genetic control, cell biology, and regulation of the self-recognition molecules, as well as the structural basis of ligand recognition. Models that aim to explain how diversification of the self-recognition repertoire can occur in this two-gene self-recognition system are discussed.