Supramolecular organization of a chromocentric plant nucleus

Cytochemical staining (EDTA procedure for ribonucleoprotein (RNP) and osmium ammine staining), immunocytochemistry (mAbs anti DNA and trimethyl guanosine cap), hypotonic-formaldehyde fixation and in situ hybridization have been used to study the macromolecular architecture of the interphase nucleus of meristematic cells of a dicotyledon plant with chromocentric nuclei, Chirantodendron pentadactylon. The perichromatin region contains extended chromatin loops, perichromatin fibrils as well as typical and immature perichromatin granules. Hypotonic fixation combined with immunolocalization of trimethyl guanosine cap showed that perichromatin fibrils are associated with snURNPs. These results indicate that in the perichromatin region the splicing of newly synthesized pre-mRNA is taking place. Thus, the same functions are localized in the perichromatin region of plant and animal cell nuclei. RNA-containing granules morphologically similar to perichromatin granules, but significantly smaller in size, were found in the interchromatin region. These granules are frequently grouped in clusters. Filaments of extended chromatin pervade the interchromatin region. These observations show that these structures correspond to Lacandonia granules, previously found in some monocotyledons with reticulate nucleus, but not observed in dicotyledons with a chromocentric nucleus. The presence of these granules in both monocotyledons and dicotyledons suggests that they probably constitute an ancestral character of angiosperms. Perichromatin and Lacandonia granules pediculate and irregular in shape corresponding to immature forms are described for the first time in the nucleus of plants. Immunolocalization indicates that the snURNPs are associated with fibrils, interchromatin granules, coiled bodies and nucleolus associated bodies (NABs). Taken as whole these features demonstrate that intense transcription and splicing occur in the ample interchromatin region of meristematic nuclei of Chirantodendron pentadactylon. (C) Elsevier, Paris.