Ribosomes in incompatible pollen tubes in the Solanaceae

Some members of the Solanaceae have a self-incompatibility mechanism preventing self-fertilization. Stylar ribonucleases (S-RNases) are responsible for growth inhibition of self-pollen tubes. A prevalent model postulates that the S-RNases act as intracellular cytotoxins that degrade ribosomal RNA, and possibly also messenger RNA, in the incompatible pollen tubes. Since ribosomes and polysomes are easily noticed with the electron microscope, it should be possible to confirm disintegration of these structures. However, our inspection by electron microscopy revealed the presence of ribosomes and polysomes in pollen tubes formed after self-pollination of the self-sterile species Brugmansia (Datura) 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 growth is not due to a general degradation of ribosomal and messenger RNA. Therefore, the substrate for S-RNases presumably is very specific.