Secretory Compartments at the Division Plane
by Steven Backues and Sebastian Bednarek
Department of Biochemistry, University of Wisconsin, Madison. 53706, Wisconsin, USA
Figure 3: The endoplasmic reticulum accumulates at the divison plane
The endoplasmic reticulum (teal, er) associates with the phragmoplast microtubules (magenta and light green, mt) and is oriented perpendicular to the cell plate (yellow tubules, cp) during the tubular-vesicular network phase of cell plate maturation (seen to the left, near the yellow plasma membrane). Toward the center of the cell, where the phragmoplast has already disassembled, the cell plate has matured to the planar fenestrated sheet phase, and the ER lies parallel to the cell plate. Places where the ER passes through the cell plate are sites of future plasmodesmata.Bar = 500 mm.
Image courtesy of Dr. José Seguí-Simmaro.
Image courtesy of Dr. José Seguí-Simmaro.
During cytokinesis the secretory pathway becomes polarized toward the division plane and many organelles take a defined position relative to the phragmoplast and cell plate. A clear example are the Golgi stacks, which form a belt around the cell plate [34, 35]. Each Golgi has an associated trans-Golgi-network (TGN) [34], which has been shown by electron tomography to be the source of the secretory vesicles that form the cell plate [1]. Recently, fluorescence microscopy has shown that a novel TGN-associated membrane compartment defined by the small GTPases Rab-A2 and RabA3 contributes to cell plate formation. Rab-A2/A3-YFP fusions localize prominently to the leading edge of the cell plate as well as to TGN-associated cytosolic puncta [36]. KNOLLE, a cell-plate specific syntaxin whose expression is cell-cycle regulated, localizes to the Rab-A2/A3 compartment when it is first expressed during telophase, before it is incorporated into the newly forming cell plate [22, 36]. This suggests that cell plate formation might be initiated by KNOLLE-mediated membrane fusion of the Rab-A2/A3 compartment [36]. This compartment might in fact be identical to the secretory vesicles seen by electron tomography [1], but this has yet to be demonstrated.
The Rab-A2/A3 compartment also functions as an early endosome [36], consistent with other reports that the secretory and endocytic pathways in plants converge at the TGN [37, 38], Given this convergence, it is not surprising that endocytosed material, including both proteins and cell wall polysaccharides, are also found at the cell plate [30, 39]. In addition to the Rab-A2/A3 compartment, other markers of the TGN/endosomes, including GNOM-myc, Ara6, and Ara7 have been reported to localize to the division plane [39], although the localization of GNOM, which defines a different class of recycling endosome, and Ara7, which labels late endosomes, has been disputed [22, 36]. Likewise, a number of plasma-membrane localized proteins that undergo endocytic recycling, including PIN2, PIP2 and BRI1 [39], are also enriched at the cell plate, although it is not clear that any of these have any direct role in cell plate formation. Even though there is substantial evidence that some endocytosed material does go to the cell plate, the relative contribution of endocytosed and newly synthesized material to cell plate formation is still hotly debated [22, 39].
As cell plate formation progresses, an increasing number of multivesicular bodies (MVBs), which represent prevacuolar compartments, also cluster around the cell plate [34]. These likely serve to receive the large amounts of membrane removed by clathrin-coated vesicles during cell plate maturation. KNOLLE localizes transiently to Ara7-labeled multivesicular bodies during late cytokinesis, on its way to the vacuole for degradation [22].
The endoplasmic reticulum (ER) is also highly enriched at the division plane, and tightly associated with the cell plate, especially during the planar-fenestrated sheet phase of cell plate maturation [1] (Figure 3).
The Rab-A2/A3 compartment also functions as an early endosome [36], consistent with other reports that the secretory and endocytic pathways in plants converge at the TGN [37, 38], Given this convergence, it is not surprising that endocytosed material, including both proteins and cell wall polysaccharides, are also found at the cell plate [30, 39]. In addition to the Rab-A2/A3 compartment, other markers of the TGN/endosomes, including GNOM-myc, Ara6, and Ara7 have been reported to localize to the division plane [39], although the localization of GNOM, which defines a different class of recycling endosome, and Ara7, which labels late endosomes, has been disputed [22, 36]. Likewise, a number of plasma-membrane localized proteins that undergo endocytic recycling, including PIN2, PIP2 and BRI1 [39], are also enriched at the cell plate, although it is not clear that any of these have any direct role in cell plate formation. Even though there is substantial evidence that some endocytosed material does go to the cell plate, the relative contribution of endocytosed and newly synthesized material to cell plate formation is still hotly debated [22, 39].
As cell plate formation progresses, an increasing number of multivesicular bodies (MVBs), which represent prevacuolar compartments, also cluster around the cell plate [34]. These likely serve to receive the large amounts of membrane removed by clathrin-coated vesicles during cell plate maturation. KNOLLE localizes transiently to Ara7-labeled multivesicular bodies during late cytokinesis, on its way to the vacuole for degradation [22].
The endoplasmic reticulum (ER) is also highly enriched at the division plane, and tightly associated with the cell plate, especially during the planar-fenestrated sheet phase of cell plate maturation [1] (Figure 3).
Figure 4: CDC48 at the phragmoplast and division plane in a dividing BY2 cell. CDC48-YFP (yellow) marks the phragmoplast (Ph) and the division plane (DP) in a dividing tobacco suspension culture (BY2) cell. Both the phragmoplast and the division plane are areas of high endoplasmic reticulum density during cell division. A neighboring interphase cell (I) shows perinuclear, transvacuolar and cortical CDC48-YFP labeling, typical of the endoplasmic reticulum in non-dividing cells. Nuclei (blue, N) are stained with DAPI.
Image courtesy of Dr. Sookhee Park.
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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)
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