The Phragmoplast

An Arabidopsis root expressing Microtubule-Binding-Domain GFP under the control of the 35S promoter. Near the top of the image is a mitotic spindle that transforms into a phragmoplast, with a clearly visible midzone. As the phragmoplast matures, the microtubules are disassembled near its center and reassembled at its phragmoplast edge, leading to the formation of a ring phragmoplast, only two edges of which are visible in this focal plane.

In late anaphase, after chromosomal segregation is completed, the phragmoplast is assembled from the remnants of the mitotic spindle. Like the spindle, the phragmoplast is an antiparallel array of microtubules (MTs) oriented with their minus ends towards the poles of the cell and their plus ends towards the cell midline. Unlike the spindle, the MTs of the phragmoplast do not overlap at their plus ends, but instead terminate in the phragmoplast midzone. The phragmoplast serves both as a track for the trafficking of vesicles to the phragmoplast midzone, and a scaffold upon which the cell plate is assembled. These vesicles contain lipids, proteins and carbohydrates needed for the formation of a new cell boundary. In the phragmoplast midzone these vesicles undergo fusion to form a new organelle, the cell plate [1] (Figure 1). The cell plate grows laterally (movie 2), guided by the phragmoplast, until it fuses with the existing plasma membrane (see the discussion of the cortical division zone by Van Damme and Geelen) and matures into a new plasma membrane and cell wall. During this process, the phragmoplast is continually disassembled at its center, where the cell plate is more mature, and reassembled at its periphery, so that it grows outwards, ahead of the leading edge of the cell plate.

Since the phragmoplast is composed of MTs, it can be easily visualized by any MT probe, such as FP-tubulin fusions (TUA6-GFP, [2]; GFP::TUB1A and GFP::TUB3A [3]) or the MAP4 MT-binding domain fluorescent fusions (MBD-GFP and MBD-RFP) [4, 5] (movie 1). Some plant MT binding proteins, such as MAP65-1 [6] and MOR1 [7], also label the entire phragmoplast, while others label only specific regions of the phragmoplast. MAP65-8 [6] and EB1c [6] label the outer ends of the phragmoplast and the nuclear envelope, where the minus ends of the phragmoplast microtubules are. MAP65-3, in contrast, labels the phragmoplast midzone [6, 8], as does EB1a and EB1b, [9, 10] along with the kinesin ATK5 [11] and the kinesin-related proteins PAKRP1 and PAKRP2 [12].

Many of these phragmoplast-associated proteins, including many of those that localize to the phragmoplast midzone, are thought to be important for maintaining the stability of the phragmoplast [6, 8, 12]. However, the phragmoplast is also a dynamic structure, and as the cell plate grows the phragmoplast microtubules need to continually depolymerize in the center, where the cell plate is relatively mature, and repolymerize on the growing edges [13]. This process is thought to be controlled by a MAP kinase cascade consisting of the MAPK NRK1, the MAPKK NQK1, the MAPKKK NPK1, and the kinesin-related kinase activating proteins NACK1 and NACK2 [13]. NACK1 and NACK2 localize to the phragmoplast midzone [14], as does NPK1 [15]. One downstream target of this kinase cascade is MAP65-1, which is inactivated by phosphorylation specifically at the phragmoplast midzone [16].

Fluorescent-Protein Markers that highlight the Phragmoplast

(Abbreviations: MT = Microtubule. PPB = Pre-Prophase Band .Phrag = Phragmoplast .MZ = Midzone)

Phragmoplast : an overview