From the Ehrhardt Lab, April, 2003

Real-time imaging reveals the dynamic architecture of plant cells

All animal, fungal and plant cells feature guide-wire-like structures called microtubules, which help move chromosomes into two daughter cells, direct the movement of other organelles within the cell, and create a framework for cell shape and movement. Microtubules are semi-rigid, hollow polymers, or long-chain molecules. In animal cells, they are formed into arrays that radiate from the cell's center to its surface by a centralized organelle that both creates new polymers and hangs onto their ends. In plant cells, most microtubules are arranged quite differently. Instead of having their ends being gathered in the center of the cell, the polymers create an organized shell over the inside surface, or cortex, of the cell. "For years, scientists have been trying to figure out how these cortical arrays are created and become organized", remarked researcher David Ehrhardt of the Department of Plant Biology of the Carnegie Institution in Palo Alto, California. "Now by tagging them with the green fluorescent protein (GFP), we have been able to watch this process in action in living plant cells." The work, which was conducted by scientists at Carnegie and Stanford University, determined where many microtubules originate and how some of them move around to become organized in epidermal cells. The results are published in the April 24, 2003, Science Express .

The scientists were able to image individual microtubules in a transgenic plant of the mustard family, Arabidopsis. They took images at 2 to 5-second intervals for between 3 and 6 minutes. "We found that most of the new microtubules are probably born at multiple sites directly at the cortex and are not formed elsewhere and transferred there," said Ehrhardt. "As we watched individual polymers, it became clear that cortical sites of initiation often did not hang onto the ends of the polymers but released them, after which they migrated around by growing at their leading ends and slowly shortening their lagging ends. This movement is caused entirely by polymerization activity--the addition and deletion of molecules in the chain.  The microtubules didn't slide around to get where they were going." The researchers observed migration by polymerization activity of individual microtubules as they moved into bundles with other microtubules. This is a new view of how microtubules in the cortical arrays can be repositioned to contribute to array organization.