Figure 1. Proposed mechanisms for the self-incompatibility reaction in the S-RNAase system. The products of the female S gene, the S-RNAases, are secreted into the transmitting tissue of the style. Pollen growing through the style encounters the S-RNAases. If the pollen carries an S haplotype corresponding to either of the haplotypes present in the style, then inhibition occurs. Two models have been proposed for the inhibition mechanism. Compatible (Sx-, left) and incompatible (Sa-, right) pollinations are shown on an SaSb pistil. Symbols for pistil factors (S-RNAase, HT-B and 120 K) and pollen factor (SLF) are shown below: (a) S-RNAase degradation model. S-RNAase enters the pollen tube cytoplasm from the ECM (arrows). A compatible nonself-S-RNAase/SLF interaction (left) results in ubiquitylation and degradation of S-RNAases by the 26S proteasome, so there is no cytotoxic action and pollen tube growth continues. An incompatible self-S-RNAase/SLF interaction (right) does not result in S-RNAase degradation; cytotoxicity results in RNA degradation and so incompatible pollen tube growth is inhibited. (b) S-RNAase compartmentalization model. S-RNAase, 120 K and HT-B are taken up by endocytosis and sorted to a vacuole. In a compatible interaction (left), S-RNAase remains compartmentalized, so although S-RNAase is present it is not cytotoxic because it is sequestered. Degradation of HT-B in compatible pollen tubes is mediated by a hypothetical pollen protein (PP). How S-RNAase gains access to SLF (arrow, question mark) is not known. In an incompatible interaction (right), HT-B is not degraded and the vacuolar compartment containing S-RNAases degrades. S-RNAase is released into the cytoplasm and RNA is degraded by its cytotoxic action, and pollen tube growth is inhibited. Adapted from McClure and Franklin-Tong (2006). With kind permission of Springer Science and Business Media.

Figure 2. A proposed model for the self-incompatibility mechanism in Papaver rhoeas. Incompatible pollen undergoes an S haplotype-specific interaction. Secreted stigmatic S-proteins interact with the pollen S receptor. An haplotype-specific interaction such as binding S₁ protein to S₁ pollen results in triggering an intracellular Ca²⁺ signalling cascade(s), involving large-scale Ca²⁺ influx and increases in [Ca²⁺]_i. A series of events then occur in the incompatible pollen. Within 1 min there is a dissipation of the tip-focused calcium gradient that is required for continued pollen growth and the activation of calcium-dependent protein kinase (CDPK). The CDPK phosphorylates Pr-p26.1, a soluble inorganic pyrophosphatase (sPPase). Both calcium and phosphorylation inhibit sPPase activity, resulting in a reduction in the biosynthetic capability of the pollen, thereby inhibiting growth. Dramatic changes to pollen cytoskeleton organization are apparent within 1 min, with extensive depolymerization of the F-actin accompanying this, also predicted to cause rapid arrest of tip growth. p56-MAPK is activated and may signal to PCD. PCD is triggered, involving key features of PCD including caspase-like activity, cytochrome c leakage and DNA fragmentation. This ensures that incompatible pollen does not start to grow again. Adapted from McClure and Franklin-Tong (2006). With kind permission of Springer Science and Business Media.

Figure 3. A proposed model for the Brassica self-incompatibility reaction. In Brassica the SI response occurs within the stigma. When a pollen grain alights on the papilla surface the pollen coat flows to form an adhesive ‘foot’, thus making a connection with the surface of the stigmatic papilla. The pollen S locus cysteine-rich (SCR/SP11) protein is carried within this coating and when this is allelic with the recipient stigma, an incompatible reaction is induced. SCR binds to the extracellular domain of the S receptor kinase (SRK), which results in the activation of the kinase. The role of the S locus glycoprotein (SLG) in this recognition event is unclear, as evidence suggests it is not essential for the SI reaction. However in some S haplotypes it does appear to enhance the SI response. MLPK (M locus protein kinase) a membrane localized protein is a positive effector of SI and may form a complex with SRK. Following activation SRK interacts with ARC1 in a phosphorylation-dependent manner. This ultimately leads to pollen rejection by some, as yet, unknown mechanism.