Phragmoplast Microtubules Undergo Changes in Organization During Cell Plate Formation

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Microtubules are the most visible component of the cytoskeletal elements involved in cytokinesis, such as the premitotic pre-prophase band (Fig. 2A) and the phragmoplast. They have been the subject of hundreds of studies of dividing plant cells, both at the light and the electron microscope level of analysis. Here we only review contributions made by electron microscopy to the study of the phragmoplast and cell plate of cryofixed cells.

Figures 2 and 3 illustrate the organization of the MT arrays associated with phragmoplast formation and maturation in Arabidopsis meristem cells (somatic-type cytokinesis). The two opposing sets of MTs in Fig. 3 are shown in different colors so that their spatial organization can be more readily discerned. The comparison between the late anaphase organization of the mitotic spindle MTs and the organization of the MTs at the time of phrag-moplast initial assembly during very late anaphase (Fig. 3A and B) highlights how the phragmoplast initials arise from clusters of opposing sets of spindle MTs. The distinction between the two types of arrays is based on the presence of multiple CPAMs in the equatorial plane of the phragmoplast initials

Phragmoplast Spindle

Fig. 2 The different stages of somatic-type plant cytokinesis (see text for further details). A G2-M stage (premitotic). B Late mitotic anaphase - phragmoplast initials stage of cytokinesis. C Early telophase stage of caryokinesis - solid phragmoplast stage of cytokinesis. D Mid telophase stage of caryokinesis - transitional phragmoplast stage of cytokinesis. E Late telophase stage of caryokinesis - ring phragmoplast stage of cytokinesis. F G1 stage in the newly formed daughter cells. chr: chromosome; cpam: cell plate assembly matrix; cw: cell wall; db: dumbbell-shaped intermediate; gs: golgi stack; mt: microtubule; mvb: multivesicular body; n: nucleus; ne: nuclear envelope; nu: nucleolus; pd: plasmodesmata; pfs: planar fenestrated sheet cell plate; pgz: peripheral cell plate growth zone; pm: plasma membrane; ppb: pre-prophase band; tn: tubular network cell plate; tvn: tubulo-vesicular network cell plate; v: golgi-derived vesicle

Fig. 2 The different stages of somatic-type plant cytokinesis (see text for further details). A G2-M stage (premitotic). B Late mitotic anaphase - phragmoplast initials stage of cytokinesis. C Early telophase stage of caryokinesis - solid phragmoplast stage of cytokinesis. D Mid telophase stage of caryokinesis - transitional phragmoplast stage of cytokinesis. E Late telophase stage of caryokinesis - ring phragmoplast stage of cytokinesis. F G1 stage in the newly formed daughter cells. chr: chromosome; cpam: cell plate assembly matrix; cw: cell wall; db: dumbbell-shaped intermediate; gs: golgi stack; mt: microtubule; mvb: multivesicular body; n: nucleus; ne: nuclear envelope; nu: nucleolus; pd: plasmodesmata; pfs: planar fenestrated sheet cell plate; pgz: peripheral cell plate growth zone; pm: plasma membrane; ppb: pre-prophase band; tn: tubular network cell plate; tvn: tubulo-vesicular network cell plate; v: golgi-derived vesicle

(Fig. 2B; Seguf-Simarro et al. 2004). Formation of the initials starts when the chromosomes reach the pole regions and the nuclear envelope begins to reassemble. The largest MT arrays are associated with the solid phragmoplast stage of cell plate formation (Figs. 2C, 3C). Based on tomographic data, the

Cell Plate Formation

Fig. 3 Large volume tomographic reconstructions of different stages of mitosis and cytokinesis in Arabidopsis meristem cells. Based on Austin et al. (2005). A Late-anaphase (volume = 5.9 x 5.9 x 0.8 mm3), which shows two opposing sets of spindle microtubules (mt), red and green, above and between the migrating sister chomatids (ch), and vesicles (v). B Phragmoplast initials stage (vol = 5.9 x 5.9 x 0.8 mm3), in which MT arrays arrange parallel to the spindle axis, between the decondensing chromatin masses. Individual phragmoplast initials arise in the cell equator surrounded by clouds of cell plate forming vesicles. C Solid phragmoplast stage (vol = 4.3 x 4.3 x 1.2 mm3), which displays two dense sets of opposing MTs (mt) between the re-forming daughter nuclei (n). The growing cell plate (cp) is surrounded by numerous vesicles and is sandwiched between the two sets of MTs. D Transitional phragmoplast stage (vol = 2.8 x 2.8 x 0.8 mm3), which lacks a CPAM and displays many short MTs on both sides of the maturing cell plate. E Ring phragmoplast stage (vol = 5.9 x 2.8 x 0.9 mm3). Two dense sets of opposing MTs flank the growing edge of the cell plate. The more mature central cell plate region has fewer interacting MTs. The scale bars represents 1 |im

Fig. 3 Large volume tomographic reconstructions of different stages of mitosis and cytokinesis in Arabidopsis meristem cells. Based on Austin et al. (2005). A Late-anaphase (volume = 5.9 x 5.9 x 0.8 mm3), which shows two opposing sets of spindle microtubules (mt), red and green, above and between the migrating sister chomatids (ch), and vesicles (v). B Phragmoplast initials stage (vol = 5.9 x 5.9 x 0.8 mm3), in which MT arrays arrange parallel to the spindle axis, between the decondensing chromatin masses. Individual phragmoplast initials arise in the cell equator surrounded by clouds of cell plate forming vesicles. C Solid phragmoplast stage (vol = 4.3 x 4.3 x 1.2 mm3), which displays two dense sets of opposing MTs (mt) between the re-forming daughter nuclei (n). The growing cell plate (cp) is surrounded by numerous vesicles and is sandwiched between the two sets of MTs. D Transitional phragmoplast stage (vol = 2.8 x 2.8 x 0.8 mm3), which lacks a CPAM and displays many short MTs on both sides of the maturing cell plate. E Ring phragmoplast stage (vol = 5.9 x 2.8 x 0.9 mm3). Two dense sets of opposing MTs flank the growing edge of the cell plate. The more mature central cell plate region has fewer interacting MTs. The scale bars represents 1 |im solid phragmoplasts in Arabidopsis meristem cells are comprised of 800 to 1,000 MTs (Austin and Segui-Simarro, unpublished results).

A controversy has arisen recently over the question as to whether the phragmoplast MTs overlap in the equatorial plane or not. The unambiguous answer to this question, based on the analysis of cells preserved by HPS-FS

methods, is that there is no systematic overlap between the MT (+)-ends in somatic-type cytokinesis phragmoplast arrays, but there is overlap between the MTs in the mini-phragmoplasts of endosperm cells (Otegui and Stae-helin 2000b; Austin et al. 2005). The reason for this difference is unknown, but it could be related to the fact that the mini-phragmoplasts formed during syncytial-type cytokinesis contain only ~2 x 10 MTs versus ~2 x 400 to 500 in the solid phragmoplasts formed during somatic-type cytokinesis. In-terdigitation of the MT ends might be required for the stabilization of the mini-phragmoplast MT assemblies, which have to remain operative during the simultaneous assembly of all of the syncytial cell walls, a process that would be expected to take more time than the assembly of a single cell plate and cell wall during somatic-type cytokinesis (Otegui and Staehelin 2000b).

Upon completion of the tubulo-vesicular stage of cell plate formation (see below), the solid phragmoplast MT array breaks down in conjunction with the disassembly of the CPAM. This stage is called the transitional stage (Figs. 2D, 3D), because it signals the transition from a solid phragmoplast to a ring phragmoplast type of MT organization. Formation of the ring phragmoplast MT arrays follows the reformation of the CPAM around the edges of the centrifugally expanding cell plate (Figs. 2E, 3E). The few MTs and secondary CPAMs that are seen to interact with the central cell plate region at this stage of development are focused on the remaining pores in the fenes-trated sheet-type cell plate region (Segui-Simarro et al. 2004), which have to be closed to produce a new cell wall (Fig. 2F).

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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