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

Many flowering plants producing bisexual flowers have adopted self-incompatibility (SI), a reproductive strategy which allows pistils to distinguish between self and nonself pollen, and to only permit nonself pollen to effect fertilization. To date, three different SI mechanisms have been identified, and this article focuses on the S-RNase-based mechanism using Petunia (Solanaceae) as a model. The genetic basis of this type of SI was established nearly a century ago; the polymorphic S-locus specifies the genetic identity of pollen and the pistil. Molecular genetic studies carried out since the late 1980s have led to the identification of the polymorphic genes at the S-locus that control self/nonself-recognition between pollen and the pistil. The S-RNase gene, which controls pistil specificity, was identified first, and subsequent sequencing of the S-locus region containing S-RNase led to the identification of the S-locus F-box (SLF) gene (now named SLF1). A transgenic approach was used to show that S₂-SLF1 (SLF1 of S₂-halotype) of Petunia inflata controls pollen specificity. The S-locus contains additional pollen-expressed F-box genes that show sequence similarity with SLF1, and initially they were thought not to be involved in pollen specificity. However, further studies of SLF1 suggested that it is not the only pollen specificity gene. Indeed, it has recently been shown that two previously identified SLF-like genes in P. inflata (now named SLF2 and SLF3) and a yet unknown number of additional SLF-like genes (named SLF4, SLF5, etc.) collaboratively function to control pollen specificity. The significance and implications of this new finding are discussed.