Self-Incompatibility

Complex pollen–pistil (male–female) interactions play a decisive role in determining reproductive success following pollination. Self-incompatibility (SI) is a genetically controlled mechanism to prevent self-fertilisation, ensuring genetic diversity of offspring. Three SI systems have been well characterised at a molecular/cellular level; they utilise highly polymorphic, tightly linked pollen- and pistil-expressed S-determinants. All three characterised SI systems have different S-determinants and employ contrasting mechanisms to reject incompatible pollen. The Brassica and Papaver SI system operate using self-recognition, while the S-RNase SI system employs self-/non-self recognition. In Brassica, a receptor-kinase ligand interaction, together with modifiers involving a ubiquitin degradation pathway which suppresses compatibility factors, results in the rejection of incompatible pollen. The Papaver SI system utilises a receptor–ligand type interaction that triggers a calcium-dependent signalling network culminating in the programmed cell death of incompatible pollen. In the S-RNase SI system, some form of detoxification of the ribonuclease activity takes place in compatible pollen tubes.