REGULATION BY GIBBERELLINS OF THE ORIENTATION OF CORTICAL MICROTUBULES IN PLANT-CELLS

Gibberellins control the direction of expansion of plant cells. They c hange the orientation of cellulose microfibrils by changing the orient ation of cortical microtubules and, hence, the direction of cell expan sion. When gibberellins change the orientation of cortical microtubule s, they also change their stability. If the way in which gibberellins change the orientation of microtubules is identical to the way in whic h they change microtubule stability, then studies on the mechanism tha t regulates this stability should give us some clues to the mechanism that regulates the orientation of microtubules. With this possibility in mind, we undertook a series of studies on the stability of cortical microtubules. These revealed that the association of cortical microtu bules with the plasma membrane is an important part of the mechanism f or their stabilisation. Gibberellins seem to change the stability of m icrotubules by affecting their association with the plasma membrane. T o study the way in which the gibberellins affect this association, it is necessary to clarify the molecular architecture of the structure th at links cortical microtubules with the plasma membrane.