Self‐incompatibility in Petunia: a self/nonself‐recognition mechanism employing S‐locus F‐box proteins and S‐RNase to prevent inbreeding

Genetic basis of gametophytic self‐incompatibility (GSI) and of its breakdown in tetraploids. (a) Self‐pollination and cross‐pollination of a diploid self‐incompatible S₁S₂ plant. Both S₁ and S₂ pollen produced by the S₁S₂ plant are rejected during their tube growth in the pistil due to matching of the S‐haplotypes, whereas pollen of any other S‐haplotype is accepted by the S₁S₂ pistil for fertilization. (b) Breakdown of self‐incompatibility (SI) in a tetraploid S₁S₁S₂S₂ plant due to presence of two different S‐alleles in diploid pollen. The tetraploid plant produces three different S‐genotypes of diploid pollen. Upon self‐pollination, S₁S₁ and S₂S₂ pollen are rejected, but S₁S₂ (heteroallelic) pollen is accepted. The tetraploid plant rejects S₁ and S₂ pollen, consistent with the notion that the breakdown of SI lies on the pollen side.

Strategy for studying structure–function relationships of SLF proteins involved in pollen specificity. First, all types of SLF genes encoding pollen specificity will be identified from a particular S‐haplotype (S₂ given as an example). Second, each type of SLF gene will be introduced into transgenic plants of various S‐genotypes to test whether it can cause breakdown of SI in pollen carrying one of the test S‐haplotypes. For example, S₂‐SLF1 is introduced into plants of S_xS_y genotype, and if it causes breakdown of SI in pollen of S_x‐haplotype but not S_y‐haplotype, this would suggest that S₂‐SLF1 interacts with S_x‐RNase. Third, the interaction between an SLF and any S‐RNase identified in Step 2 will be confirmed by protein–protein interaction assays and/or coimmunoprecipitation experiments. Having established a comprehensive relationship between each type of SLF and all nonself‐S‐RNases tested, the potential region(s) responsible for the differential interactions with S‐RNases will be identified from comparing the amino acid sequences of different types of SLFs that interact with different subsets of S‐RNases. Finally, the putative specificity region(s) will be examined by constructing chimeric genes between different types of SLF genes, with the coding sequences for the specificity region(s) swapped, and examining the SI behavior of the chimeric genes using the transgenic approach as shown in Step 2.

Biochemical models for S‐RNase‐based self‐incompatibility. (a) Competitive interaction explained by a protein degradation model based on a single SLF gene controlling pollen specificity. For S₁S₂ heteroallelic pollen, its S₁‐locus contains the S₁‐RNase and S₁‐SLF genes and its S₂‐locus contains the S₂‐RNase and S₂‐SLF genes. S₁‐SLF and S₂‐SLF are produced in the cytoplasm of the S₁S₂ heteroallelic pollen, and upon self‐pollination, S₁‐RNase and S₂‐RNase are taken up during tube growth in the pistil. As indicated by the arrow connecting S₁‐SLF and S₂‐RNase, and by the arrow connecting S₂‐SLF and S₁‐RNase, S₁‐SLF and S₂‐SLF interact with S₂‐RNase and S₁‐RNase, respectively, to mediate their ubiquitination and degradation. S₁‐SLF and S₂‐SLF can also interact with all other nonself‐S‐RNases (S₃‐RNase, S₄‐RNase, etc.) during cross‐pollinations. As a result, the S₁S₂ heteroallelic pollen will be universally accepted by pistils of any S‐genotype. (b) Collaborative nonself‐recognition model. Pollen of each S‐haplotype has a single S‐RNase gene controlling pistil specificity, but an as yet unknown number of SLF genes controlling pollen specificity. Each type of SLF protein recognizes and interacts with a specific subset of nonself‐S‐RNases to mediate their ubiquitination and degradation. As indicated by the arrows connecting a particular type of SLF and S‐RNases, S₇‐SLF1 of P. hybrida interacts with S₉‐RNase and S₁₇‐RNase, and causes breakdown of SI in S₉ and S₁₇ transgenic pollen due to competitive interaction; S₇‐SLF2 interacts with S₉‐RNase, S₁₁‐RNase, and S₁₉‐RNase, and causes breakdown of SI in S₉, S₁₁, and S₁₉ transgenic pollen.40 However, none of the three types of SLF proteins examined interact with S₅‐RNase, suggesting that additional yet unidentified type(s) of SLF genes are also involved in controlling pollen specificity.40