Identification of the pollen determinant of S-RNase-mediated self-incompatibility

Many flowering plants have adopted self-incompatibility mechanisms to prevent inbreeding and promote out-crosses(1). In the Solanaceae, Rosaceae and Scrophulariaceae, two separate genes at the highly polymorphic S-locus control self-incompatibility interactions: the S-RNase gene encodes the pistil determinant and the previously unidentified S-gene encodes the pollen determinant(2-4). S-RNases interact with pollen S-allele products to inhibit the growth of self-pollen tubes in the style. Pollen-expressed F-box genes showing allelic sequence polymorphism have recently been identified near to the S-RNase gene in members of the Rosaceae and Scrophulariaceae(5-8); but until now have not been directly shown to encode the pollen determinant. Here we report the identification and characterization of PiSLF, an S-locus F-box gene of Petunia inflata ( Solanaceae). We show that transformation of S1S1, S1S2 and S2S3 plants with the S-2-allele of PiSLF causes breakdown of their pollen function in self-incompatibility. This breakdown of pollen function is consistent with 'competitive interaction', in which pollen carrying two different pollen S-alleles fails to function in self-incompatibility(1,9,10). We conclude that PiSLF encodes the pollen self-incompatibility determinant.