CONSISTENT HANDEDNESS OF MICROTUBULE HELICAL ARRAYS IN MAIZE AND ARABIDOPSIS PRIMARY ROOTS

The orientation of cortical microtubules in plant cells has been exten sively studied, in part because of their influence on the expansion of most plant cell types. Cortical microtubules are often arranged in he lical arrays, which are well known to occur with a specific pitch as a function of development or experimental treatment; however, it is not known if the handedness of helical arrays can also be specified. We h ave studied the handedness of helical arrays by using Vibratome sectio ning of maize primary roots and confocal microscopy of Arabidopsis pri mary roots. In cortical cells of maize roots, the helical array was fo und to have the same handedness at a given position, not only for the cells of a single root, but also for the cells of more than one hundre d roots examined. Quantification of angular distribution of apparent i ndividual microtubules showed that defined regions of the root were co mposed of cells with highly uniform microtubule orientation. In the re gion between transverse and longitudinal microtubules (5-10.5 mm from the tip), the array formed a right-handed helix, and basal of cells wi th longitudinal microtubules (11.5-15 mm from the tip), the array form ed a left-handed helix. Similarly, in epidermal cells of Arabidopsis r oots right-handed helical arrays were found in the region between tran sverse and longitudinal microtubules. These results suggest that, in a ddition to the orientation of microtubules, the handedness of helical microtubule arrays is under cellular control.