Inhibition of Self-incompatible Pollen in Papaver rhoeas Involves a Complex Series of Cellular Events

Self-incompatibility (SI) in Papaver rhoeas involves allele-specific recognition between stigmatic S proteins and pollen which results in the rapid inhibition of incompatible pollen. In this article we discuss current understanding of the components and mechanisms involved in this reaction. Analysis of cDNA sequences encoding several stigmatic S alleles has allowed us to establish, using site-directed mutagenesis, that certain amino acids are essential for S -specific inhibitory activity. An S protein binding protein (SBP) identified in pollen plasma membrane extracts has been proposed to act as a receptor. Study of S1gene mutants defective in SBP binding has confirmed the important role of SBP in the SI reaction, however the exact nature of this remains to be defined. Although it was previously thought that the Papaver stigmatic S gene was unique, detailed analysis of Arabidopsis thaliana genomic sequences has revealed the presence of a large number of open reading frames with homology to the S gene. We have named these potential genes SPH(S-protein homologues). Studies of the molecular and biochemical basis of self-incompatibility (SI) in P. rhoeas have revealed much about the early signals triggered in pollen in this response. We have established that a Ca2+-dependent signal transduction pathway mediates the inhibition of incompatible pollen. Rapid increases in cytosolic free Ca2+and the rapid loss of the pollen apical Ca2+gradient are accompanied by the hyper-phosphorylation of p26 and p68, two soluble pollen phosphoproteins. Studies reveal that p26 is a soluble inorganic pyrophosphatase. Recent evidence has been obtained that suggests irreversible pollen tube growth and cell death involves a programmed cell death (PCD) pathway.