Cartoon of a generalised, basic floral morphology. The male anthers shed pollen grains, which are transferred to the stigma. Each pollen adheres, hydrates and germinates, forming a pollen tube that grows through the pistil. The goal is to fertilise an ovule in the ovary. SI prevents fertilisation at different places, depending on the species. The boxes indicate where the SI interactions and pollen inhibition occur for the three best-characterised SI systems. In the Brassicaceae (B) and Papaveraceae (C), inhibition of incompatible pollen takes place on the stigma surface, whereas theS-RNase system found in members of the Solanaceae, Rosaceae, Plantaginaceae and Rutaceae (D) causes incompatible pollen tubes to be inhibited as they grow through the pistil. (B–D) The self-incompatible (SI) pollination (left) and self-compatible (SC) pollination (right) scenarios for each of these SI systems. The pistil and femaleS-determinant are indicated in green; the pollen and maleS-determinant are indicated in orange. (B) In the Brassicaceae, the femaleS-determinant is a receptor kinase, SRK, and the maleS-determinant is a small, secreted ligand, SCR/SP11. Interaction of cognate SRK and SCR/SP11 occurs at the plasma membrane of stigmatic papilla cells and triggers an SI response in the stigma (left); responses triggered within the stigma result in the rejection of incompatible (self) pollen. In a self-compatible (SC) situation (right), non-cognateS-determinants do not interact, and pollen is not inhibited, so pollen germinates and grows a pollen tube. (C) In the Papaveraceae, the femaleS-determinant, PrsS, is a small, secreted ligand and the maleS-determinant, PrpS, is a transmembrane protein. Interaction of cognate PrsS and PrpS at the pollen plasma membrane (left) triggers intracellular signalling within incompatible (self) pollen, resulting in rejection of self-pollen (SI). In a self-compatible situation (right), non-cognateS-determinants do not interact, so pollen is not inhibited. (D) In theS-RNase system, the femaleS-determinant (green) is anS-RNase that is secreted into the pistil extracellular matrix (ECM), and the maleS-determinants are F-box proteins, SLFs. In contrast to the systems shown in (B) and (C), interaction of theS-determinants occurs within the pollen tube as it grows through the pistil ECM and, generally, a non-self-recognition system operates. TheS-RNases are taken up into the pollen tube in a non-specific manner. No interaction occurs between ‘self’S-determinants (left), so RNase toxicity causes failure of incompatible (self) pollen to grow further. In a non-self situation, SLFs interact with non-selfS-RNases (right), which are detoxified and compatible pollen tubes therefore continue to grow. Note that a self-recognition SI system operates in the Brassicaceae (B) and Papaveraceae (C) and a non-self-recognition system operates in theS-RNase system (D).
