Mechanisms of S-RNase-based self-incompatibility.

Abstract

Abstract Gametophytic self-incompatibility (GSI), a genetic mechanism that acts to prevent self-fertilization in many angiosperms, is based on the ability of the pistil to selectively inhibit growth of self pollen while allowing the growth of non-self pollen. In the most widespread variant of GSI (studied most thoroughly in the Solanaceae, Rosaceae and Plantaginaceae), a style-expressed ribonuclease (the S-RNase) is imported into both self and non-self pollen tubes. In pollen tubes recognized as self, the S-RNase degrades RNA, slowing protein synthesis and growth of the pollen tube, whereas in non-self pollen tubes the action of the S-RNase is inhibited and growth continues normally. Recognition of self versus non-self in pollen appears to be mediated through the action of the recently identified SLF/SFB protein. Protein interaction studies suggest that SLF/SFB acts via a modified SCFSLF E3 ubiquitin ligase complex. Several questions remain, however, about the precise mechanism by which the S-RNase remains inactive in non-self pollen tubes. Experiments tracking the fate of protein components of self-incompatibility in pollinated styles suggest that in compatible pollinations, the S-RNase remains sequestered in a membrane-bounded vacuolar compartment, while it is released in incompatible pollen tubes. Other experiments suggest that ubiquitination, and possibly degradation, of S-RNases occurs preferentially in incompatible pollen compared with compatible pollen. Finally, the analysis of SFB mutants in the Rosaceae, along with investigations into the breakdown of GSI in tetraploid plants, suggest that details of the GSI response may differ in the Rosaceae compared with the Solanaceae and Plantaginaceae.