Figure 6.

Models of S-RNase-based self-incompatibility. (A) S-RNase degradation model in Petunia. In CPC response, cross S₃ pollen lands on the stigma, germinates and grows into an S₁S₂ style. Both S₁- and S₂-RNases enter the S₃ pollen tube by endocytosis or an unknown pathway (dotted arrows). Subsequently, the hypothetical binding domains of S₁- and S₂-RNases interact with a hypothetical activity domain of the pollen S (i.e., SLF in Solanaceae and Plantaginaceae) in the cytosol of the pollen tube. Then, one or more SLF (SLFN) in S₃ pollen tube form functional SCF^S3-SLFN complexes to tag S₁- and S₂-RNases with a polyubiquitin chain, which are subsequently degraded by the 26S proteasome. Thus, ribosomal RNAs escape from degradation by S-RNases in cross-pollen tubes, resulting in normal pollen tube growth. In SPI, during which self S₁ pollen lands on the S₁S₂ style, both S₁- and S₂-RNases enter the S₁ pollen tube. Similar to CPC response, non-self S₂-RNases bind to a hypothetical activity domain of the pollen S in the cytosol of the pollen tube. Then, one or more SLF (SLFN) in S₁ pollen tube form SCF^S1-SLFN complexes to tag S₂-RNase with a polyubiquitin chain, resulting in its degradation by the 26S proteasome. In contrast, the recognition domain of self S₁-RNase binds to a hypothetical recognition domain of SLF resulting in the formation of a non-functional SCF^S1-SLFN complex, thus self S-RNase escapes degradation and acts as a cytotoxin to inhibit the pollen tube growth. (B) S-RNase compartmentalization model in Nicotiana. S-RNases, 120K and HT-B all enter the pollen tubes. In CPC response, HT-B degradation possibly occurs by a hypothetical pollen protein (PP), thus S-RNase is sorted to a vacuolar compartment. In SPI, the interaction between S-RNase and SLF has been postulated to stabilize HT-B, perhaps by inhibiting the PP action. Subsequently, the vacuolar compartment breaks down, releasing S-RNase and acts as a cytotoxin to inhibit the pollen tube growth.