180 Y.-R.J.Lee·B.Liu
Fig. 5Organization of microtubules in the phragmoplast. The phragmoplast microtubule
array has a mirrored organization of microtubules with their plus ends in the middle
(arrowheads) and minus ends facing daughter nuclei (arrows)
plate. Cell plate assembly initiates in the middle of the phragmoplast, and
continues centrifugally. Upon the completion of vesicle delivery at a given
area, microtubules in this area will be depolymerized from the minus end.
While microtubules are depolymerized in the middle of the phragmoplast,
new microtubules are being polymerized towards the periphery. Once the as-
sembling cell plate reaches the parental plasma membrane, phragmoplast mi-
crotubules will then be completely depolymerized. One of the most intriguing
questions in plant cell biology is that the cortical site where developing cell
plate fuses is previous determined by the PPB before nuclear envelope break-
down in somatic cells (Wick 1991). But the molecular mechanism underlying
this readout of division plane is still unknown.
In the phragmoplast, kinesins would be required for the following activi-
ties: (1) the establishment of the phragmoplast microtubule array; (2) vesicle
transport along phragmoplast microtubules; (3) microtubule turnover (poly-
merization and depolymerization) in the phragmoplast; and (4) spatial regu-
lation of cell plate assembly.
3.1
Kinesins for Organizing the Phragmoplast Microtubule Array
The development from midzone microtubules to the phragmoplast micro-
tubule array is rather a rapid process (Zhang et al. 1993). The transition
involves microtubule polymerization and bundling/coalescence, as well as