S-RNase uptake by compatible pollen tubes in gametophytic self-incompatibility

Many flowering plants avoid inbreeding through a genetic mechanism termed s elf-incompatibility. An extremely polymorphic S-locus(1) controls the gamet ophytic self-incompatibility system that causes pollen rejection (that is, active arrest of pollen tube growth inside the style) when an S-allele carr ied by haploid pollen matches one of the S-alleles present in the diploid s tyle. The only known product of the S-locus is an S-RNase expressed in the mature style(2). The pollen component to this cell-cell recognition system is unknown and current models(3,4) propose that it either acts as a gatekee per allowing only its cognate S-RNase to enter the pollen tube, or as an in hibitor of non-cognate S-RNases. In the latter case, all S-RNases are presu med to enter pollen tubes; thus, the two models make diametrically opposed predictions concerning the entry of S-RNases into compatible pollen. Here w e use immunocytochemical labelling of pollen tubes growing in styles to sho w accumulation of an S-RNase in the cytoplasm of all pollen-tube haplotypes , thus providing experimental support for the inhibitor model.