The candidates for pistil-encoded S-glycoproteins in Nicotiana alata were identified by pistil protein segregation analysis. Anderson et al. (1986) cloned the first pistil-S cDNA that co-segregated with an S-genotype. They were named S-RNases because of their ribonuclease domains and activity (Mc-Clure et al. 1989). S-RNase expression both coincides developmentally with the onset of SI, and is restricted to cells forming the path from the stigma to the ovary. S-RNases contain secretion signals and are deposited in the ECM in high concentrations. Transgenic plant studies expressing cloned S-RNases provided definitive evidence that S-RNases are pistil-S. Expression of transgenic S-RNase allowed rejection of pollen containing the transgene's S-haplotype, demonstrating that S-RNase determines S-specificity (Lee et al. 1994; Murfett et al. 1994).
Sequence analysis of S-RNase genes showed high levels of polymorphism. They contain highly divergent hypervariable (HV) regions (Ioerger et al. 1991). Site-specific mutagenesis experiments in Solanum showed that S-specificities can be changed by altering amino acids in the HV regions of S-RNases (Matton et al. 1997). Nevertheless, analyses of S-RNases in rosaceous and solanaceous species suggest that regions other than HV regions may be involved in recognition (Ishimizu et al. 1998). Structural analyses established that the most HV regions reside on the surface, where they could interact with pollen-S (Ida et al. 2001). However, how S-RNase specificity is determined remains unclear.
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