Plant nuclei can contain extensive grooves and invaginations

Plant cells can exhibit highly complex nuclear organization. Through dye-la beling experiments in untransformed onion epidermal and tobacco culture cel ls and through the expression of green fluorescent protein targeted to eith er the nucleus or the lumen of the endoplasmic reticulum/nuclear envelope i n these cells, we have visualized deep grooves and invaginations into the l arge nuclei of these cells. In onion, these structures, which are similar t o invaginations seen in some animal cells, form tubular or planelike infold ings of the nuclear envelope. Both grooves and invaginations are stable str uctures, and both have cytoplasmic cores containing actin bundles that can support cytoplasmic streaming. In dividing tobacco cells, invaginations see m to form during cell division, possibly from strands of the endoplasmic re ticulum trapped in the reforming nucleus. The substantial increase in nucle ar surface area resulting from these grooves and invaginations, their appar ent preference for association with nucleoli, and the presence in them of a ctin bundles that support vesicle motility suggest that the structures migh t function both in mRNA export from the nucleus and in protein import from the cytoplasm to the nucleus.