216 A. Rose
arrangement (reviewed by Scherthan 2001). In animal and yeast cells, this
clustering occurs adjacent to the microtubule organizing center, the centro-
some or spindle pole body, respectively. Despite the lack of these organelles
in plant cells, telomere bouquets at the nuclear envelope can also be observed
in some plant species (Bass et al. 1997; Cowan et al. 2001; Martinez-Perez
et al. 1999). However, not all plant species follow this pattern. In Arabidop-
sis, telomeres cluster inside the nucleus and associate with the nucleolus
(Armstrong et al. 2001). How telomeres are attached to the nuclear or nucle-
olar periphery is unknown. In the maizepam1mutant, bouquet formation
is delayed or abolished, suggesting that PAM1 protein is required for meiotic
bouquet formation (Golubovskaya et al. 2002). In Arabidopsis, the Skp1-like
protein ASK1 is essential for the release of chromatin from the nuclear and
nucleolar periphery (Yang et al. 2006). Since ASK1 is thought to be involved in
ubiquitin-mediated proteolysis, this suggests that nuclear chromatin release
in plants might require the degradation of proteins linking chromatin to the
nuclear envelope.
4
Nuclear and Nuclear Envelope Dynamics in Plants
Plant nuclei are highly mobile organelles. Their movement appears to be me-
diated by actin rather than tubulin in Arabidopsis (Chytilova et al. 2000; Kete-
laar et al. 2002). The nuclear envelope itself exhibits a high degree of plasticity
and can contain extensive grooves and invaginations. In onion and tobacco
cells, these structures were observed having cytoplasmic cores with actin
bundles supporting cytoplasmic streaming and vesicle movement (Collings
et al. 2000). Taken together, these findings suggest a role for actin in nuclear
mobility and nuclear envelope architecture at least during interphase in plant
cells, in contrast to microtubule-dynein based nuclear position and migration
mechanisms in other eukaryotes. Mitotic roles of actin and its motor myosin
appear to concentrate on cell plate formation during later stages of the cell
cycle (Hepler et al. 2002; Sano et al. 2005; Yoneda et al. 2004).
4.1
Markers for the Plant Nuclear Envelope
Marker proteins for the nuclear envelope are valuable tools for studying the
dynamics of the nuclear envelope during mitosis in more detail. In animal
cells, lamins and lamina-associated inner nuclear membrane proteins are fre-
quently used as markers for the nuclear envelope. Since plant cells do not
possess homologs of these proteins, it is surprising that antibodies directed
against lamins are capable of detecting antigens in plant cells (Li and Roux
1992; McNulty and Saunders 1992; Minguez and Morena Diaz de la Espina