Twisted growth and organization of cortical microtubules.

In plants, directional cell expansion greatly contributes to the final shape of mature cells, and thus to organ architecture. A particularly interesting mode of cell expansion is helical growth in which the growth axis is continuously tilted either to the right or to the left as the cell grows. Fixed handedness of helical growth raises fundamental questions on the possible origin of left-right asymmetry. Twisting mutants of Arabidopsis thaliana offer unique opportunities to study the cellular basis of helical growth. Most of the twisting mutants with fixed handedness have been shown to have defects in microtubule functions, whereas mutants that twist in non-fixed directions appear to be defective in auxin response or transport. Good correlations have been found between the tilted growth direction and alignment of cortical microtubule arrays in twisting mutants with compromised microtubule functions. The present challenge is to understand how particular array patterns are organized during progression of the interphase in rapidly expanding cells. Molecular and cell biological studies on twisting mutants will lead to better understanding on how wild-type plant cells utilize the microtubule cytoskeleton to initiate and rigorously maintain straight growth.