| Summary: | Using a combination of electron microscopic cytochemistry, stereology and experiments with inhibitors, the structure, chemistry, distribution and function of nuclear invaginations characteristic of gymnosperm microspores have been studied. These structures now emerge as specialized ingrowths of the post-meiotic nuclear envelope, rather than as modified nuclear pores as had previously been suspected. The invaginations possess two principal components, a membranous cylinder which abuts onto the nuclear envelope, and a terminal sac, situated some 0.5 μm deep in the karyoplasm. The cylindrical component is hypothesized to provide the pathway by which nascent mRNA is translocated from DNA which invests the invaginaton, first to the cylinder, and thence either to the cytoplasm or to the terminal sac. It is proposed that the RNA that amasses within the sac is finally discharged into the cytoplasm as the invaginations open up to form conventional nuclear embayments. Experiments using the transcriptional inhibitor actinomycin D also indicate that the material accumulating within the invaginations is a product of transcription. Despite the facts that the invagination system may increase the area of the nuclear envelope six fold, and that the terminal sacs possess nuclear pores, it is suggested that the cylindrical component is specially adapted for the transfer of mRNA which, in these structures, appears to pass directly across the envelope and not through a nuclear pore. © 1987 Annals of Botany Company.
|
|---|