Self-Incompatibility in Petunia inflata: The Relationship between a Self-Incompatibility Locus F-Box Protein and Its Non-Self S-RNases

The highly polymorphic S (for self-incompatibility) locus regulates self-incompatibility in Petunia inflata; the S-RNase regulates pistil specificity, and multiple S-locus F-box (SLF) genes regulate pollen specificity. The collaborative non-self recognition model predicts that, for any S-haplotype, an unknown number of SLFs collectively recognize all non-self S-RNases to mediate their ubiquitination and degradation. Using a gain-of-function assay, we examined the relationships between S-2-SLF1 (for S-2-allelic product of Type-1 SLF) and four S-RNases. The results suggest that S-2-SLF1 interacts with S-7- and S-13-RNases, and the previously identified S-1- and S-3-RNases, but not with S-5- or S-11-RNase. An artificial microRNA expressed by the S-2-SLF1 promoter, but not by the vegetative cell-specific promoter, Late Anther Tomato 52, suppressed expression of S-2-SLF1 in S-2 pollen, suggesting that SLF1 is specific to the generative cell. The S-2 pollen with S-2-SLF1 suppressed was compatible with S-3-, S-5-, S-7-, S-11-, and S-13-carrying pistils, confirming that other SLF proteins are responsible for detoxifying S-5- and S-11-RNases and suggesting that S-2-SLF1 is not the only SLF in S-2 pollen that interacts with S-3-, S-7-, and S-13-RNases. Petunia may have evolved at least two types of SLF proteins to detoxify any non-self S-RNase to minimize the deleterious effects of mutation in any SLF.