Identification of a Novel SBP1-Containing SCF^SFB Complex in Wild Dwarf Almond (Prunus tenella)

S-RNase-based gametophytic self-incompatibility (SI), in which specificities of pistil and pollen are determined by S-RNase and the S locus F-box protein, respectively, has been discovered in the Solanaceae, Plantaginaceae, and Rosaceae families, but some underlying molecular mechanisms remain elusive and controversial. Previous studies discovered SI in wild dwarf almond (Prunus tenella), and pistil S (S-RNase) and pollen S (SFB) determinant genes have been investigated. However, the SCF (SKP1-Cullin1-F-box-Rbx1) complex, which serves as an E3 ubiquitin ligase on non-self S-RNase, has not been investigated. In the current study, PetSSK1 (SLF-interacting-SKP1-like1), SBP1 (S-RNase binding protein 1), CUL1, and SFB genes (S-haplotype-specific F-box) were identified in an accession (ZB1) of P. tenella. Yeast two-hybrid assays revealed interactions between PetSBP1 and PetCUL1 and between PetSBP1 and PetSFBs (SFB16 and SFB17), and subsequent pull-down assays confirmed these interactions, suggesting a novel SBP1-containing SCF^SFB complex in wild dwarf almond. Moreover, despite a putative interaction between PetSSK1 and PetCUL1, we revealed that PetSSK1 does not interact with PetSFB16 or PetSFB17, and thus the canonical SSK1-containing SCF^SFB complex could not be identified. This suggests a novel molecular mechanism of gametophytic SI in Prunus species.