Open Mitosis: Nuclear Envelope Dynamics 219
envelope (Dixit and Cyr 2002). This suggests a connection between these two
membrane systems in plants cells, which is further strengthened by the find-
ing that MAF1 also exhibits a dual targeting to the nuclear envelope as well as
the Golgi (Patel et al. 2005). Likewise, ER-targeted green fluorescent protein
fusions have been used to visualize the nuclear envelope during cell division
(Gupton et al. 2006).
4.2
Nuclear Envelope Dynamics During Mitosis
Microscopy studies suggest that the plant nuclear envelope joins the ER after
it breaks down during open mitosis and reforms from the ER membrane pool
during telophase. In higher plant mitosis, the nuclear envelope becomes in-
distinguishable from tubular and cisternal ER surrounding and invaginating
the spindle apparatus. Based on observations of green fluorescent protein-
labeled ER through cell division, the nuclear envelope appears to accumulate
ER material shortly after chromosome condensation, but before nuclear enve-
lope breakdown, and subsequently elongates with an increased accumulation
of ER at the spindle poles. It dissipates into a network of tubules aligning in
the orientation of the spindle microtubules (Gupton et al. 2006). These tubu-
lar ER/nuclear envelope strands appear to form channels for the separation of
the daughter chromatids (Hawes et al. 1981; Hepler 1980). During later stages
of mitosis, the tubular ER/nuclear envelope material provides the vesicles for
cell plate formation in the phragmoplast (Gupton et al. 2006).
Nuclear envelope labeled with the more specific marker LBR fused to green
fluorescent protein disperses during metaphase into an ER-like meshwork.
Later, it accumulates in tubular structures at the division plate between the
two daughter nuclei, indicating the prevalence of nuclear envelope material
at the newly forming cell plate. The nuclear envelopes around the daughter
nuclei become visible during phragmoplast assembly, and ER fluorescence
decreases, indicating a migration of material from the ER back to the nuclear
envelope (Irons et al. 2003).
4.2.1
Disassembly of the Nuclear Envelope
At the onset of open mitosis in higher eukaryotes, the nuclear envelope breaks
down, but where does it go? Two theories have been proposed:
- Nuclear envelope vesiculation
- Absorption into the endoplasmic reticulum
Evidence for the first theory stems from in vitro experiments onXenopus
cell-free extracts. When disrupted, nuclear membranes formed vesicles and
reformed around chromatin (Vigers and Lohka 1991). However, the alterna-