Cortical microtubules

The microtubule network in the cell cortex of plants moves by “treadmilling” (i.e, losing polymers from the minus end, while gaining them at a similar rate and the growing plus end). Movie 1 shows the dynamics of cortical microtubules in epidermal cells in the hypocotyl of Arabidopsis thaliana. Because it is difficult to see both ends of the individual microtubules, an experiment was done to show that the physical translocation of the microtubule was not occurring, but instead polymer s were being added to one end and being subtracted from the other end. In the experiment, fluorescent tubulin was photobleached in a line. If the tubules were moving by translocation, the photobleached area would go away very quickly. What happened instead is that it was maintained over time, proving that treadmilling causes microtubule movement.

 

Movie 1 Microtubules

Because most higher plants do not have centrosomes and the associated microtubule organizing centers of animal cells, they initiate microtubule along membranes—the plasma membrane and the nuclear envelope. Sometimes this happens along the sides of other microtubules. This controlled polymerization and depolymerization occurs with the aid of many microtubule associated proteins or MAPs.

Reorganization of microtubules and MAPs through mitosis

Web Figure 1.10.A shows the organization of microtubules (red) and how their kinesin motors (light green) associated MAPs (blue) are involved in their reorganization throughout the cell cycle.

Web Figure 1.10.A Read from left to right, top to bottom. The MAPs (blue) are labeled with their common acronyms (defined in the text of the figure) and show their position relative to the microtubules, preprophase band, spindle, and phragmoplast. The positions of the kinesins (green) are also shown. The nucleus is shown as the central multicolored spheroid, until the nuclear envelope breaks down, and then the individual chromosomes are shown.

Microtubules and cytoplasmic streaming

Microtubules may also serve as tethering spots for the dynamics of other organelles. It has recently been shown that as organelles such as mitochondria and Golgi move along, they pause where they cross microtubules. Microtubules may interact with these organelles in a way that allows these organelles to make brief or more permanent membrane associations with each other, so-called membrane contact sites.