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Abstract In Petunia (Solanaceae family), self-incompatibility (SI) is regulated by the polymorphic S-locus, which contains the pistil-specific S-RNase and multiple pollen-specific S-Locus F-box (SLF) genes. SLFs assemble into E3 ubiquitin ligase complexes known as Skp1–Cullin1–F-box complexes (SCFSLF). In pollen tubes, these complexes collectively mediate ubiquitination and degradation of all nonself S-RNases, but not self S-RNase, resulting in cross-compatible, but self-incompatible, pollination. Using Petunia inflata, we show that two pollen-expressed Cullin1 (CUL1) proteins, PiCUL1-P and PiCUL1-B, function redundantly in SI. This redundancy is lost in Petunia hybrida, not because of the inability of PhCUL1-B to interact with SSK1, but due to a reduction in the PhCUL1-B transcript level. This is possibly caused by the presence of a DNA transposon in the PhCUL1-B promoter region, which was inherited from Petunia axillaris, one of the parental species of Pe. hybrida. Phylogenetic and syntenic analyses of Cullin genes in various eudicots show that three Solanaceae-specific CUL1 genes share a common origin, with CUL1-P dedicated to S-RNase-related reproductive processes. However, CUL1-B is a dispersed duplicate of CUL1-P present only in Petunia, and not in the other species of the Solanaceae family examined. We suggest that the CUL1s involved (or potentially involved) in the SI response in eudicots share a common origin.more » « less
Self-incompatibility (SI), an inbreeding-preventing mechanism, is regulated in Petunia inflata by the polymorphic S-locus, which houses multiple pollen-specific S-locus F-box (SLF) genes and a single pistil-specific S-RNase gene. S2-haplotype and S3-haplotype possess the same 17 polymorphic SLF genes (named SLF1 to SLF17), and each SLF protein produced in pollen is assembled into an SCF (Skp1–Cullin1– F-box) E3 ubiquitin ligase complex. A complete suite of SLF proteins is thought to collectively interact with all non-self S-RNases to mediate their ubiquitination and degradation by the 26S proteasome, allowing cross-compatible pollination. For each SCFSLF complex, the Cullin1 subunit (named PiCUL1-P) and Skp1 subunit (named PiSSK1), like the F-box protein subunits (SLFs), are pollen-specific, raising the possibility that they also evolved specifically to function in SI. Here we used CRISPR/Cas9-meditated genome editing to generate frame-shift indel mutations in PiSSK1, and examined the SI behavior of a T0 plant (S2S3) with biallelic mutations in the pollen genome and two progeny plants (S2S2) each homozygous for one of the indel alleles and not carrying the Cas9-containing T-DNA. Their pollen was completely incompatible with pistils of seven otherwise compatible S-genotypes, but fully compatible with pistils of an S3S3 transgenic plant in which production of S3-RNase was completely suppressed by an antisense S3-RNase gene, and with pistils of immature flower buds, which produce little S-RNase. These results suggest that PiSSK1 specifically functions in SI, and support the hypothesis that SLF-containing SCF complexes are essential for compatible pollination.more » « less
Petuniais controlled by the polymorphic S‐locus, which contains S‐RNaseencoding the pistil determinant and 16–20 S‐locus F‐box( SLF) genes collectively encoding the pollen determinant. Here we sequenced and assembled approximately 3.1 Mb of the S2‐haplotype of the S‐locus in Petunia inflatausing bacterial artificial chromosome clones collectively containing all 17 SLFgenes, SLFLike1, and S‐RNase. Two SLFpseudogenes and 28 potential protein‐coding genes were identified, 20 of which were also found at the S‐loci of both the S6a‐haplotype of P. inflataand the SN‐haplotype of self‐compatible Petunia axillaris, but not in the S‐locus remnants of self‐compatible potato ( Solanum tuberosum) and tomato ( Solanum lycopersicum). Comparative analyses of S‐locus sequences of these three S‐haplotypes revealed potential genetic exchange in the flanking regions of SLFgenes, resulting in highly similar flanking regions between different types of SLFand between alleles of the same type of SLFof different S‐haplotypes. The high degree of sequence similarity in the flanking regions could often be explained by the presence of similar long terminal repeat retroelements, which were enriched at the S‐loci of all three S‐haplotypes and in the flanking regions of all S‐locus genes examined. We also found evidence of the association of transposable elements with SLFpseudogenes. Based on the hypothesis that SLFgenes were derived by retrotransposition, we identified 10 F‐boxgenes as putative SLFparent genes. Our results shed light on the importance of non‐coding sequences in the evolution of the S‐locus, and on possible evolutionary mechanisms of generation, proliferation, and deletion of SLFgenes.