RIBOSOMES IN INCOMPATIBLE POLLEN TUBES IN THE SOLANACEAE

Some members of the Solanaceae have a self-incompatibility mechanism p reventing self-fertilization. Stylar ribonucleases (S-RNases) are resp onsible for growth inhibition of self-pollen tubes. A prevalent model postulates that the S-RNases act as intracellular cytotoxins that degr ade ribosomal RNA, and possibly also messenger RNA, in the incompatibl e pollen tubes. Since ribosomes and polysomes are easily noticed with the electron microscope, it should be possible to confirm disintegrati on of these structures. However, our inspection by electron microscopy revealed the presence of ribosomes and polysomes in pollen tubes form ed after self-pollination of the self-sterile species Blugmansia (Datu ra) suaveolens and Nicotiana alata. There was no decrease over time in the number of bound ribosomes per unit of rough endoplasmic reticulum (RER) membrane. The results indicate that the inhibition of tube grow th is not due to a general degradation of ribosomal and messenger RNA. Therefore, the substrate for S-RNases presumably is very specific.