The Skp1-like protein SSK1 is required for cross-pollen compatibility in S-RNase-based self-incompatibility

The self-incompatibility (SI) response occurs widely in flowering plants as a means of preventing self-fertilization. In these self/non-self discrimination systems, plant pistils reject self or genetically related pollen. In the Solanaceae, Plantaginaceae and Rosaceae, pistil-secreted S-RNases enter the pollen tube and function as cytotoxins to specifically arrest self-pollen tube growth. Recent studies have revealed that the S-locus F-box (SLF) protein controls the pollen expression of SI in these families. However, the precise role of SLF remains largely unknown. Here we report that PhSSK1 (Petunia hybrida SLF-interacting Skp1-like1), an equivalent of AhSSK1 of Antirrhinum hispanicum, is expressed specifically in pollen and acts as an adaptor in an SCF(Skp1-Cullin1-F-box)(SLF) complex, indicating that this pollen-specific SSK1-SLF interaction occurs in both Petunia and Antirrhinum, two species from the Solanaceae and Plantaginaceae, respectively. Substantial reduction of PhSSK1 in pollen reduced cross-pollen compatibility (CPC) in the S-RNase-based SI response, suggesting that the pollen S determinant contributes to inhibiting rather than protecting the S-RNase activity, at least in solanaceous plants. Furthermore, our results provide an example that a specific Skp1-like protein other than the known conserved ones can be recruited into a canonical SCF complex as an adaptor.