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Microtubule organization in the green kingdom: chaos or self-order?

Plant Cell Biology Group, Research School of Biological Sciences, The Australian National University, GPO Box 475, Canberra ACT 2601, Australia

View larger version (55K): [in a new window]   Fig. 1. These schematic illustrations, rendered in 3D at two aspects, show microtubule arrays through the plant cell cycle. (A) A preprophase band, linked to the nucleus by phragmosome microtubules, marks the future division site. (B) Metaphase spindle with a dispersed polar region. (C) In telophase, the phragmoplast forms as a concentrated cylinder of microtubules between daughter nuclei. (D) The cytokinetic phragmoplast expands centrifugally, leading the cell plate towards attachment sites previously established by the preprophase band. Microtubule plus ends meet at midplane. (E) Once cytokinesis is complete, microtubules extend from the nucleus toward the cell cortex and plasma membrane-associated microtubules appear. (F) Plant cells in interphase and those entering terminal differentiation often expand predominantly in one direction. During cell elongation, cortical microtubules are usually arranged in parallel arrays whose predominant orientation is at right angles to the axis of expansion.

View larger version (70K): [in a new window]   Fig. 2. Cortical microtubule recovery patterns after drug-induced microtubule disassembly. (A) Microtubules appear to diverge from the initial assembly site, forming fractal tree-shaped clusters, with microtubules diverging from each other at acute angles (figure adapted from Wasteneys and Williamson, 1989b). (B) Clusters eventually break up. (C) Later in recovery, parallel microtubule order begins to consolidate but some branching configurations and discordant microtubules persist. Bar, 10 µm.

View larger version (21K): [in a new window]   Fig. 3. Model for microtubule assembly by severing and transport of nucleating templates. In this model, a -tubulin ring complex associates with the minus end of a microtubule, while the microtubule extends by the addition of tubulin subunits at the fast-growing, GTP-tubulin-containing plus end (dark green). Severing of the minus end is achieved by the formation of a hexamer of katanin p60 subunits, whose association with the microtubule wall is coordinated by the larger p80 subunit, which may transiently dimerize with the p60 subunits. Microtubule-mediated ATPase activity results in inward movement of the p60 subunits, an action that cleaves the ring complex from the microtubule minus end. Katanin subunits dissociate but the lock-washer-shaped ring complex is transported along the microtubule by a plus-end-directed kinesin. The extent of transport along the microtubule may be regulated by the relative activities of plus- and minus-end-directed kinesins. The ring complex serves as a template for the assembly of additional microtubules. Repeated generation, severing and transport of nucleating templates at the minus end of the original microtubule may explain how the fractal tree complexes shown in Fig. 2A develop.

View larger version (147K): [in a new window]   Fig. 4. Microtubule patterns in the epidermis of Arabidopsis thaliana cotyledons after 4 hours at 29°C. (A) Cortical microtubules are abundant and transversely oriented in wildtype. (B) In the mor1 mutant, microtubules appear short and disoriented. Bar, 10 µm.

View larger version (30K): [in a new window]   Fig. 5. Possible functions of the MOR1 HEAT repeat-1 (HR1) in microtubule stabilization. (A) HR1 links microtubules to the plasma membrane via a plasma-membrane-associated protein. At restrictive temperature, this loss of binding dissociates microtubules from the plasma membrane, promoting their destabilization. (B) HR1 competes with a destabilizing protein (probably a kin1-like kinesin) for binding. At permissive temperature, the high affinity of MOR1 for this site prevents destabilization. At 29°C, this affinity is lost, leading to kin1-dependent destabilization and microtubule shortening.