1. Biology Department, University of Montreal, Montreal , Quebec H1X 2B2, Canada

Correspondence to: Mario Cappadocia Correspondence and requests for materials should be addressed to M.C. (e-mail: Email: mario.cappadocia@umontreal.ca).

Abstract

Many flowering plants avoid inbreeding through a genetic mechanism termed self-incompatibility. An extremely polymorphic S-locus¹ controls the gametophytic self-incompatibility system that causes pollen rejection (that is, active arrest of pollen tube growth inside the style) when an S-allele carried by haploid pollen matches one of the S-alleles present in the diploid style. The only known product of the S-locus is an S-RNase expressed in the mature style². The pollen component to this cell–cell recognition system is unknown and current models^{3, 4} propose that it either acts as a gatekeeper allowing only its cognate S-RNase to enter the pollen tube, or as an inhibitor of non-cognate S-RNases. In the latter case, all S-RNases are presumed to enter pollen tubes; thus, the two models make diametrically opposed predictions concerning the entry of S-RNases into compatible pollen. Here we use immunocytochemical labelling of pollen tubes growing in styles to show accumulation of an S-RNase in the cytoplasm of all pollen-tube haplotypes, thus providing experimental support for the inhibitor model.