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Fig. 3. (A) Projection of five consecutive 2.2 nm tomographic images of cell-plate-associated microtubule plus ends in a root-meristem cell (solid-phragmoplast stage). The black and white arrowheads point to morphologically different plus ends of microtubules. The dashed red line on the right-hand half of the image indicates the interface between the ribosome-rich cytosol and the ribosome-depleted cell-plate assembly matrix region. None of the microtubule plus ends pass across the cell-plate midline. (B-D) Mapping of microtubule plus end types, cell-plate assembly matrix (outlined with small red points) and microtubules (light blue). Color coding of the different microtubule plus-end types: blunt-ended, yellow; extended, green; horned, red; flared, blue. (B) A tomographic model of a solid phragmoplast from a shoot-meristem cell, showing all of the microtubules and the cell-plate assembly matrix outline; only microtubules whose plus ends are found within the cell-plate assembly matrix are color coded. (C) The same solid-phragmoplast tomographic models as shown in B, except that only microtubules whose plus ends are located outside the cell-plate assembly matrix are color coded. (D) A transitional phragmoplast from a root meristem. This tomographic model shows all of the microtubules but notice that this stage of phragmoplast development does not have a cell-plate assembly matrix. Color coding of microtubules' plus ends (see also Fig. 2): blunt, yellow; extended, green; horned, red; flared, blue. Notice the high proportion of blunt-ended microtubules that terminate within the cell-plate assembly matrix and the high proportion of horned microtubules that terminate outside a cell-plate assembly matrix. A quantitative analysis of the microtubule end types is presented in Fig. 4. Scale bars, 1 µm.
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