Pollen S-locus F-box proteins of Petunia involved in S-RNase-based self-incompatibility are themselves subject to ubiquitin-mediated degradation

Summary Many flowering plants show self-incompatibility, an intra-specific reproductive barrier by which pistils reject self-pollen to prevent inbreeding and accept non-self pollen to promote out-crossing. In Petunia, the polymorphic S-locus determines self/non-self recognition. The locus contains a gene encoding an S-RNase, which controls pistil specificity, and multiple S-locus F-box (SLF) genes that collectively control pollen specificity. Each SLF is a component of an SCF (Skp1/Cullin/F-box) complex that is responsible for mediating degradation of non-self S-RNase(s), with which the SLF interacts, via the ubiquitin-26S proteasome pathway. A complete set of SLFs is required to detoxify all non-self S-RNases to allow cross-compatible pollination. Here, we show that SLF1 of Petunia inflata is itself subject to degradation via the ubiquitin-26S proteasome pathway, and identify an 18 amino acid sequence in the C-terminal region of S₂-SLF1 (SLF1 of S₂ haplotype) that contains a degradation motif. Seven of the 18 amino acids are conserved among all 17 SLF proteins of S₂ haplotype and S₃ haplotype involved in pollen specificity, suggesting that all SLF proteins are probably subject to similar degradation. Deleting the 18 amino acid sequence from S₂-SLF1 stabilized the protein but abolished its function in self-incompatibility, suggesting that dynamic cycling of SLF proteins is an integral part of their function in self-incompatibility. Significance Statement S-locus F-box (SLF) proteins mediate degradation of all non-self S-RNases via the ubiquitin-26S proteasome pathway (UPP), and in this work, we report that SLF proteins themselves are subject to degradation via UPP and that deleting the sequence containing a degron, though stabilizing the proteins, abolishes their function in self-incompatibility.