Microtubules are polar cytoskeletal filaments that are crucial for intracel-lular motility, mitosis, and cellular organization. A key feature of microtubules is their ability to undergo rapid turnover, which results from the addition and loss of subunits at filament ends. Two types of dynamic turnover characterize microtubule arrays: dynamic instability, in which individual "plus" ends switch stochastically between phases of growth and shortening, and tread-milling, in which the rate of subunit addition at the plus end equals the rate of subunit loss at the opposite "minus" end (1-3). Microtubules of the interphase cortical array of plant cells are known to turn over more rapidly (4) than interphase microtubules in animal cells (5). In a research article on page 1715 of this issue, Shaw et al. (6) now provide a molecular explanation for this difference in dynamic behavior. By in vivo imaging of cells from the model plant Arabidopsis that express green fluorescent protein-tagged tubulin, the authors show mat the faster turnover in the plant array results from a hybrid treadmilling mechanism. This mechanism is characterized by loss of subunits at minus ends and dynamic instability, with a bias toward subunit addition, at the plus ends. Microtubules in the cortical array grow and shorten more slowly (7) than microtubules in animal cells, but spend a much greater fraction of time in a dynamic state. Thus, like the fabled tortoise, faster turnover results from slow but persistent activity.
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