Identification of SFBB-Containing Canonical and Noncanonical SCF Complexes in Pollen of Apple (Malus × domestica)

Gametophytic self-incompatibility (GSI) of Rosaceae, Solanaceae and Plantaginaceae is controlled by a single polymorphic S locus. The S locus contains at least two genes, S-RNase and F-box protein encoding gene SLF/SFB/SFBB that control pistil and pollen specificity, respectively. Generally, the F-box protein forms an E3 ligase complex, SCF complex with Skp1, Cullin1 (CUL1) and Rbx1, however, in Petunia inflata , SBP1 (S-RNase binding protein1) was reported to play the role of Skp1 and Rbx1, and form an SCF SLF -like complex for ubiquitination of non-self S-RNases. On the other hand, in Petunia hybrida and Petunia inflata of Solanaceae, Prunus avium and Pyrus bretschneideri of Rosaceae, SSK1 (SLF-interacting Skp1-like protein1) is considered to form the SCF SLF/SFB complex. Here, we isolated pollen-expressed apple homologs of SSK1 and CUL1 , and named MdSSK1, MdCUL1A and MdCUL1B . MdSSK1 was preferentially expressed in pollen, but weakly in other organs analyzed, while, MdCUL1A and MdCUL1B were almost equally expressed in all the organs analyzed. MdSSK1 transcript abundance was significantly (>100 times) higher than that of MdSBP1 . In vitro binding assays showed that MdSSK1 and MdSBP1 interacted with MdSFBB1- S 9 and MdCUL1, and MdSFBB1- S 9 interacted more strongly with MdSSK1 than with MdSBP1. The results suggest that both MdSSK1-containing SCF SFBB1 and MdSBP1-containing SCF SFBB1 -like complexes function in pollen of apple, and the former plays a major role.