Inhibition of self-incompatible pollen in Papaver rhoeas involves a complex series of cellular events

Self-incompatibility (SI) in Papaver rhoeas involves allele-specific recogn ition between stigmatic S proteins and pollen which results in the rapid in hibition of incompatible pollen. In this article we discuss current underst anding of the components and mechanisms involved in this reaction. Analysis of cDNA sequences encoding several stigmatic S alleles has allowed us to e stablish, using site-directed mutagenesis, that certain amino acids are ess ential for S-specific inhibitory activity. An S protein binding protein (SB P) identified in pollen plasma membrane extracts has been proposed to act a s a receptor. Study of S, gene mutants defective in SEP binding has confirm ed the important role of SEP in the SI reaction, however the exact nature o f this remains to be defined. Although it was previously thought that the P apaver stigmatic S gene was unique. detailed analysis of Arabidopsis thalia na genomic sequences has revealed the presence of a large number of open re ading frames with homology to the S gene. We have named these potential gen es SPH (S-protein homologues). Studies of the molecular and biochemical bas is of self-incompatibility (SI) in P. rhoeas have revealed much about the e arly signals triggered in pollen in this response. We have established that a Ca2+-dependent signal transduction pathway mediates the inhibition of in compatible pollen. Rapid increases in cytosolic free Ca2+ and the rapid los s of the pollen apical Ca2+ gradient are accompanied by the hyperphosphoryl ation of p26 and p68, two soluble pollen phosphoproteins. Studies reveal th at p26 is a soluble inorganic pyrophosphatase. Recent evidence has been obt ained that suggests irreversible pollen tube growth and cell death involves a programmed cell death (PCD) pathway. (C) 2000 Annals of Botany Company.