Unlike S-RNases, the pistil-S gene products in Papaver rhoeas (named S-proteins) are not abundant proteins. They are specifically expressed in the stigma, which is the site of pollen inhibition. They are small (~ 15 kDa) secreted proteins without homology to any proteins with known function (Foote et al. 1994). Sequence comparisons between five stigma S-alleles revealed high polymorphism but no hypervariable "blocks" (just a single hypervariable amino acid in a predicted surface loop). Site-directed mutagenesis revealed several residues in this region that participate in pollen rejection (Kakeda et al. 1998). Because these proteins are small, secreted, and unique, it was proposed that they act as S-allele specific ligands that trigger a signalling cascade in incompatible pollen, and this was subsequently confirmed. The Papaver pollen-S receptors for the ligands have not yet been identified. Unlike S-RNase based GSI, the establishment of an in vitro SI system benefited analysis of Papaver SI at the cellular and biochemical level. Recombinant stigmatic S-proteins elicit an S-specific response in incompatible pollen that reproduces the in vivo response. Thus, additional non-S-linked pistil components are not required.
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