Self-incompatibility: insights through microscopy

The contributions of microscopy to our understanding of gametophytic self-incompatibility in the ornamental tobacco Nicotiana alata are reviewed. Self-incompatibility is a genetically based system which prevents self-fertilization and thereby promotes outbreeding in plants. Pollen bearing an S-allele identical to either of the S-alleles in the female sporophytic tissues is rejected. Several S-allele-specific style glycoproteins have been isolated from Solanaceous species. All have ribonuclease activity and are termed S-RNases. The cDNA clones encoding N. alata S-RNases have been used in in situ hybridization experiments which show that the genes are expressed in the transmitting tract of the style and in the epidermis of the placenta. Immunocyto-chemistry using an antibody to S₂-RNase demonstrates extracellular accumulation of the enzyme in the same tissues. These tissues are the pathway through which pollen tubes must grow to reach the ovules. The microscopy data are congruent with a model in which self-pollenation is susceptible to an allele-specific cytotoxin produced by the sporophyte.