Figure 8.

Schematic diagram showing the model of the self-incompatibility mechanism in S4′ pollen tubes in sweet cherry. When the S4′ pollen tube grows in pistils harboring the S4 allele or other S alleles, S-RNase enters into pollen tubes and may be regulated by some proteins like M-locus-encoded GST (MGST) or M-locus-encoded disulfide bond A-like oxidoreductase (MDO). The SFB4′ protein is located at the tip of the pollen tube, but it does not interact with S-RNase. In contrast, the S₄-SLFL protein interacts with S-RNase and ubiquitinates it, the labeled S-RNase is degraded via the 26S proteasome pathway, and so it cannot inhibit the growth of the pollen tube, resulting in compatibility regardless of self- or cross-pollination. Other F-box proteins may also bind to S-RNase and be involved in self-incompatibility, but the mechanism is unclear. In S4 pollen tubes, SFB4 protein could recognize S₄-RNase when it grows in pistils with the S4 haplotype. S₄-RNase was labeled by ubiquitin, which led to self-incompatibility via an unknown pathway. CC, Cross-compatibility; SC, self-compatibility; SI, self-incompatibility.