The plant nucleoskeleton: Ultrastructural organization and identification of NuMa homologues in the nuclear matrix and mitotic spindle of plant cells

In the present work we investigate the structural organization of the nucleoskeleton of Allium cepa meristematic root cells. Resinless sections reveal for the first time a residual filamentous network in plant nuclei. This network is composed of branched knobbed filaments with associated globular structures, connected to the lamina and to the dense aggregates of different sizes. Results of immunoblotting show that many components of this network are homologues of intermediate filament-type proteins. NUMA, a coiled-coil protein related to intermediate filaments, found in animal cells, can also be detected in this plant nuclear matrix system. Immunofluorescence reveals a diffuse distribution of the animal NuMA homologues in plant nuclear core filaments in interphase. Resinless immunoelectron microscopy further reveals a distribution along the extended filaments and the dense aggregates. During mitosis, in contrast to the accumulation at the poles in animal cells, NuMA homologues in plant onion cells show a diffuse pattern, which may correspond to the spindle matrix. Our data are the first report of the conservation in plants of NuMA proteins, which may be involved in both nuclear and mitotic spindle organizations.