DYNAMIC REORIENTATION OF CORTICAL MICROTUBULES, FROM TRANSVERSE TO LONGITUDINAL, IN LIVING PLANT-CELLS

The direction in which plant tissue cells expand is reflected in the a lignment of microtubules in the cortical array. When microtubules and coaligned wall microfibrils are arranged transversely around the cell, turgor pressure is chaneled into cell elongation. However, various ag ents (such as wounding, ethylene, abscisic acid) can cause the microtu bules to reorientate by 90 degrees so that they become aligned paralle l to the cell's long axis, allowing lateral expansion instead of elong ation. The mechanism by which microtubules undergo rapid shifts or ali gnment is crucial to understanding growth control in plants, but becau se current models are derived from studies on fixed cells, nothing is known about the dynamics of converting one microtubule alignment to an other. Cells tend to have one predominant microtubule alignment-transv erse, oblique, or longitudinal-but it is not established whether each represents a stable independent set that only changes by rounds of com plete de- and repolymerization, or whether reorientation is a more con tinuous process involving movement of stable or dynamic microtubules. By microinjecting pea (Pisum sativum) epidermal cells with rhodamine-c onjugated brain tubulin and optically sectioning them by confocal lase r scanning microscopy, we could follow labeled microtubules for up to 2 hr as they reorientate. Reorientation does not occur by complete dep olymerization of microtubules in one orientation followed by polymeriz ation of a new array in another orientation. Instead, increased number s of discordant microtubules in nontransverse alignment appear in part icular locations. Neighboring microtubules then adopt the new alignmen t, so that there is a stage during which different alignments coexist before the array on the outer tangential cell face finally adopts a un iform steeply oblique/longitudinal configuration. Rapid fluorescence r ecovery after photobleaching confirms that bundles of cortical microtu bules are not stable but exhibit properties consistent with dynamic in stability, Dynamic microtubules offer a mechanism for rapid growth res ponses to a range of physiological stimuli.