Plant Cytokinesis – Insights Gained from Electron Tomography Studies 263
MT stability (Inoué 1964). In particular, it was shown that irradiation of this
proximal phragmoplast region with UV light not only led to MT depolymer-
ization but also prevented reformation of a stable phragmoplast. In contrast,
UV irradiation of the distal zone of the phragmoplast, which also caused
MT breakdown, did not prevent the eventual reformation of the phragmo-
plast MT array (Inoué 1964). Further evidence for MT-cell plate interactions
came from studies with isolated phragmoplasts in which the MTs were shown
to remain connected to cell plate membranes throughout the cell disruption
and the phragmoplast isolation and purification processes (Kakimoto and
Shibaoka 1992).
Most cytoplasmic MTs are highly dynamic structures that can form de
novo, grow and then rapidly break down (Baskin 2000). Based on fluores-
cence redistribution after photobleaching experiments, phragmoplast MTs
have a turnover rate oft 1 / 2 = 60 s (Hush et al. 1994). In vitro studies of grow-
ing and shrinking MTs in cryogenically fixed samples that were examined in
the frozen state in a cryoelectron microscope have led to the correlation of
MT (+)-end morphology with the dynamic state of the MT (Mandelkow et al.
1991; Chretien et al. 1995; Carvalho et al. 2003). In particular, extended MT
ends are typical of growing MTs, ramshorn-like ends are seen on disassem-
bling MTs, and blunt ends are characteristic of MTs in a metastable state.
The demonstration that the same types of MT-end morphologies can be vi-
sualized in intact plant cells preserved by HPF-FS has opened up the possibil-
ity of analyzing phragmoplast MT dynamics in such cells by quantifying the
number of MTs exhibiting a specific type of MT (+)-end geometry. This ap-
proach has been employed to analyze the effects of CPAM association on MT
(+)-end dynamics during cytokinesis of meristem cells ofArabidopsis(Austin
et al. 2005). The highlights of that study were that most of the (+)-ends of
the solid phragmoplast MTs terminate within the CPAM, and that of those
MTs 50 – 70 % exhibit a blunt geometry, which corresponds to a metastable
state. In contrast, only 20 % of the (+)-ends of MTs that were located outside
Fig. 5 Microtubule – cell plate linkers. ∼ 30 nm long linker-like structures (arrow-
heads) physically connecting the microtubule (mt) blunt (+)-ends with cell plate mem-
branes (cp), as revealed in properly oriented tomographic reconstructions. Thescale bar
represent0.1μm