The Cell Plate

by Steven Backues and Sebastian Bednarek

Department of Biochemistry, University of Wisconsin, Madison. 53706, Wisconsin, USA

Movie 2: Cell plate formation
An Arabidopsis root expressing Dynamin-Related Protein 1A-GFP under its native promoter.  Multiple cell plates can be seen to initiate and expand, with DRP1A-GFP concentrated at the leading edge of each plate.  As cell plate formation completes, DRP1A-GFP labeling fades to background levels.  Movie 63x magnification and 360x real speed. Movie courtesy of Dr. Byung-Ho Kang.
  

The cell plate initiates in the center of the cell within the phragmoplast midzone, and expands outward until it fuses with the existing plasma membrane at the cortical division zone. The construction of the cell plate proceeds through morphologically defined stages [1, 17]. The first stage in the formation of the cell plate itself is the fusion of secretory vesicles at the phragmoplast midzone [1]. This fusion reaction is mediated by a machinery that includes KNOLLE, a syntaxin [18], and its regulator KUELE [19], as well as the additional SNARES, NSPN11 [20] and AtSNAP33 [21]. The expression and localization of KNOLLE is cell-cycle regulated, so that a native-promoter driven KNOLLE-GFP is expressed only during cytokinesis, when it localizes to the forming cell plate [22].
The initial vesicle fusion events give rise to dumbbell-shaped membrane structures which have been proposed to grow by fusion with additional vesicles and each other into a narrow tubular network [1] (Figure 1).

Tomography cell plate
Figure 1: Electron tomographic view of cell plate formation
A cell plate in the tubular-vesicular network phase of cell plate formation. Secretory vesicles (small blue and green spheres) are trafficked down the phragmoplast microtubules (light green and magenta rods, mt) to fuse with the growing cell plate (yellow, cp). A few clathrin-coated vesicles (large red spheres) can also be seen budding from more mature sections of the plate and traveling along the phragmoplast MTs. The cell plate is enclosed within a ribosome-excluding cell plate-associated matrix (red dots, cpam). The large blue structures are mitochondria (m). For clarity, the endoplasmic reticulum is not shown. Bar = 500 mm.
Image courtesy of Dr. José Seguí-Simmaro.

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The dumbbell-shaped membrane precursors and the narrow tubules of the tubular network are often wrapped with electron-dense rings and spirals [1, 23]. In cellularizing endosperms these rings have been shown to contain the dynamin-related protein DRP1A [23], and various members of the DRP1 family are known to localize to the forming cell plate. DRP1A-FP and DRP1C-FP are very early cell plate markers, and are brightest at the actively growing edges of the forming cell plate [24, 25] (movie 2). The precise role of these dynamin-related protein rings is unclear but one proposal is that they function to mold the membrane into tubules, or help maintain its characteristic tubular shape.
The process of vesicle fusion and cell plate maturation occurs initially at various points within the phragmoplast midzone [1]. These isolated domains subsequently fuse with each other and the entire cell plate grows outwards as it matures until it fuses with the parental plasma membrane, often in an asymmetrical fashion [1, 24, 26] (see discussion of the cortical division zone by Van Damme and Geelen). The growing edges of the cell plate follow the expanding phragmoplast, with microtubules being at all times associated with areas of the cell plate rich in vesicles and tubular network, but not the flat surface of the planar fenestrated sheet [1].


Clathrin CP
Figure 2: Clathrin Light Chain labels the cell plate and mobile TGN
An Arabidopsis root expressing CLC-GFP. Two cell plates are visible (arrows), as are numerous punctate Trans-Golgi Networks (TGN, arrowheads).
Image courtesy of Dr. Catherine Konopka.


As the cell plate matures, its surface area decreases, as large amounts of membrane material are removed via clathrin-mediated endocytosis [23]. Consistent with this, a Clathrin Light Chain-FP fusion strongly labels the cell plate in addition to the plasma membrane and the Trans-Golgi Network (TGN) [25] (Figure 2). This labeling appears at the cell plate later than that of DRP1A, and is brighter at the central, more mature parts of the cell plate (movie 3). A very similar localization has been reported for DRP2A, a dynamin-related protein that more closely resembles mammalian dynamin 1 and that may be involved in the formation of clathrin coated vesicles [27]. DRP1A and DRP1C have also been shown to colocalize with clathrin light chain at the plasma membrane (though not at the TGN), raising the possibility that their presence at the cell plate may also be due to an involvement in membrane removal, even though their primary localization is to the leading edge [25, 28].
The construction of the new cell wall begins within the lumen of the narrow tubules of the young cell plate. The order in which different cell wall components are deposited has been determined largely by immuno-electron microscopy [17]. The first components to arrive are pectic polysaccharides, hemicelluloses, and arabinogalactan proteins carried by the secretory vesicles that fuse to form the cell plate [29]. These vesicles may carry both newly synthesized polysaccharides as well as polysaccharides derived from the mature cell wall and recycled by endocytosis [30]. The next component to be added is callose, a long-chain polysaccharide that is polymerized directly at the cell plate by callose synthases. CalS1, an Arabidopsis callose synthase, and UGT1, a UDP-glucose transferase, both localize to the cell plate when expressed in cultured tobacco cells [31, 32]. Both have been shown to interact with the soybean homologue of DRP1A, phragmoplastin [31, 32], suggesting that the DRP1 protein rings seen during this stage may also function to position callose synthase appropriately within the cell plate membrane. The highest levels of callose at the cell plate are observed as the narrow membrane tubules broaden and fuse laterally into a wide tubular netwok and, subsequently, a planar fenestrated sheet [17]. The force for this broadening may in fact be provided by callose polymerization, implying that the forming cell wall can act as an “internal” scaffold to stabilize the cell plate membrane [17]. As the cell plate continues to mature and fuses with the parental plasma membrane, the callose is slowly replaced with cellulose [17] , the primary component of a mature cell wall (see discussion of the cell wall by Zheng). The endo-1,4-β-glucanase KORRIGAN, an enzyme putatively involved in cellulose synthesis, localizes to the cell plate when expressed in cultured tobacco cells, and plants lacking KORRIGAN show cytokinetic defects [33].


Fluorescent-protein markers of the cell plate and division plane
(Abbreviations: MT = Microtubule. PPB = Pre-Prophase Band. Phrag = Phragmoplast. MZ = Midzone. CP = Cell Plate. TGN = Trans-Golgi Network. MVB = Multi-vesicular body. PM = Plasma Membrane. DP = Division Plane. ER = Endoplasmic Reticulum)


Table Cell plate

Combined References for Phragmoplast & Cell Plate sections

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