Identification of the pollen self incompatibility determinant in Papaver rhoeas
Higher plants produce seed through pollination, using specific interactions between pollen and pistil. Self incompatibility is an important mechanism used in many species to prevent inbreeding; it is controlled by a multi allelic S locus. 'Self' (incompatible) pollen is discriminated from 'non self' (compatible) pollen by interaction of pollen and pistil S locus components, and is subsequently inhibited. In Papaver rhoeas, the pistil S locus product is a small protein that interacts with incompatible pollen, triggering a Ca2+ dependent signalling network, resulting in pollen inhibition and programmed cell death. Here we have cloned three alleles of a highly polymorphic pollen expressed gene, PrpS (Papaver rhoeas pollen S) from Papaver and provide evidence that this encodes the pollen S locus determinant. PrpS is a single copy gene linked to the pistil S gene (currently called S, but referred to hereafter as PrsS for Papaver rhoeas stigma S determinant) Sequence analysis indicates that PrsS and PrpS are equally ancient and probably co evolved. PrpS encodes a novel ~20 kDa protein. Consistent with predictions that it is a transmembrane protein, PrpS is associated with the plasma membrane. We show that a predicted extracellular loop segment of PrpS interacts with PrsS and, using PrpS antisense oligonucleotides, we demonstrate that PrpS is involved in S specific inhibition of incompatible pollen. Identification of PrpS represents a major advance in our understanding of the Papaver self incompatibility system. As a novel cell cell recognition determinant it contributes to the available information concerning the origins and evolution of cell cell recognition systems involved in discrimination between self and non self, which also include histocompatibility systems in primitive chordates and vertebrates.
Nature