Transcriptome Analysis Reveals the Same 17 S-Locus F-Box Genes in Two Haplotypes of the Self-Incompatibility Locus of Petunia inflata

Petunia possesses self-incompatibility, by which pistils reject self-pollen but accept non-self-pollen for fertilization. Self-/non-self-recognition between pollen and pistil is regulated by the pistil-specific S-RNase gene and by multiple pollen-specific S-locus F-box (SLF) genes. To date, 10 SLF genes have been identified by various methods, and seven have been shown to be involved in pollen specificity. For a given S-haplotype, each SLF interacts with a subset of its non-self S-RNases, and an as yet unknown number of SLFs are thought to collectively mediate ubiquitination and degradation of all non-self S-RNases to allow cross-compatible pollination. To identify a complete suite of SLF genes of P. inflata, we used a de novo RNA-seq approach to analyze the pollen transcriptomes of S-2-haplotype and S-3-haplotype, as well as the leaf transcriptome of the S3S3 genotype. We searched for genes that fit several criteria established from the properties of the known SLF genes and identified the same seven new SLF genes in S-2-haplotype and S-3-haplotype, suggesting that a total of 17 SLF genes constitute pollen specificity in each S-haplotype. This finding lays the foundation for understanding how multiple SLF genes evolved and the biochemical basis for differential interactions between SLF proteins and S-RNases.