INTERACTION OF AUXIN, LIGHT, AND MECHANICAL-STRESS IN ORIENTING MICROTUBULES IN RELATION TO TROPIC CURVATURE IN THE EPIDERMIS OF MAIZE COLEOPTILES

Changes in the orientation of cortical microtubules (longitudinal vs. transverse with respect to the long cell axis) at the outer epidermal wall of maize coleoptile segments were induced by auxin, red or blue l ight, and mechanical stresses (cell extension or compression produced by bending). Immunofluorescent techniques were used for the quantitati ve determination of frequency distributions of microtubule orientation . Detailed kinetic studies showed that microtubule reorientations are temporally correlated with the simultaneously measured changes in grow th rare elicited by auxin, red light, or blue light. Growth inhibition induced by depletion of endogenous auxin produces a longitudinal micr otubule pattern that can be changed into a transverse pattern in a dos e-dependent manner by applying exogenous auxin. A mid-point pattern wi th equal frequencies of longitudinal and transverse microtubules was a djusted at 2 mu mol/l auxin. Bending stress applied under these condit ions adjusts permanent, maximally longitudinal and transverse microtub ule orientations at the compressed and extended segment sides, respect ively, quantitatively mimicking the responses to differential flank gr owth during phototropic and gravitropic curvature. During tropic curva ture the changes in microtubule pattern reflect the distribution of gr owth rather than the distribution of auxin. The microtubule pattern re sponds to auxin-dependent growth changes and mechanical stress in a sy nergistic manner, confirming the functional equivalence of these facto rs in affecting microtubule orientation. Similar results were obtained when segment growth was altered by blue or red light instead of auxin in the presence or absence of mechanical stress. It is concluded from these results that growth changes, elicited by auxin, light, etc., an d mechanical stress affect microtubule orientation through a common si gnal perception and transduction chain.