New views of S-RNase-based self-incompatibility.

S-RNase-based self-incompatibility (SI) is the most widespread form of genetically controlled mate selection in plants. S-RNase controls pollination specificity in the pistil, while the newly discovered SLF/SFB controls pollination specificity in the pollen. A widely discussed model suggests that compatibility is explained by ubiquitylation and degradation of nonself-S-RNase and that, conversely, incompatibility is caused by failure to degrade self-S-RNase. This model is consistent with the long-standing view that S-RNase inhibition is central to SI. Recent results show, however, that S-RNase is compartmentalized in pollen tubes and, significantly, that compatibility might not require SLF/SFB. S-RNase compartmentalization and dislocation into the pollen tube cytoplasm might be similar to the trafficking of other cytotoxins such as ricin.