Gravistimulation forces the cortical microtubules to reorientate from transverse to random in the lower flank of actively-growing primary roots of French beans

The gravistimulation of primary roots of seedlings of Phaseolus vulgaris for 0.5 h did not cause any conspicuous difference in the microtubule arrays between the upper and the lower flanks. Exposure to gravistimulation of 1 h resulted in the transversely-orientated microtubules being reduced significantly in the lower flank of the actively elongating region, especially in the epidermis and the outer first and second layers of the cortex. This reduction was compensated for by an increase of randomly-orientated microtubules. In the region where the elongation rate of cells began to decrease, the microtubule arrays did not show a distinct difference between the two opposite flanks. When gravistimulation was prolonged to 2 h, the microtubule arrangement was about the same as in the control. The present results are not compatible with the concept that longitudinal microtubule arrays are induced in the lower flank under gravistimulation and thereby hinder the longitudinal expansion of cells in the lower flanks of roots.