The latent left–right asymmetry (chirality) of vascular plants is best witnessed as a helical elongation of cylindrical organs in climbing plants. Interestingly, helical handedness is usually fixed in given species, suggesting genetic control of chirality. Arabidopsis thaliana
, a small mustard plant, normally does not twist but can be mutated to exhibit helical growth in elongating organs. Genetic, molecular and cell biological analyses of these twisting mutants are providing mechanistic insights into the left–right handedness as well as how potential organ skewing is suppressed in most plants. Growth direction of elongating plant cells is determined by alignment of cellulose microfibrils in cell walls, which is guided by cortical microtubules localized just beneath the plasma membrane. Mutations in tubulins and regulators of microtubule assembly or organization give rise to helical arrangements of cortical microtubule arrays in Arabidopsis cells and cause helical growth of fixed handedness in axial organs such as roots and stems. Whether tubulins are assembled into a microtubule composed of straight or tilted protofilaments might determine straight or twisting growth. Mechanistic understanding of helical plant growth will provide a paradigm for connecting protein filament structure to cellular organization.
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