PHYSICAL STRAIN-MEDIATED MICROTUBULE REORIENTATION IN THE EPIDERMIS OF GRAVITROPICALLY OR PHOTOTROPICALLY STIMULATED MAIZE COLEOPTILES

During gravitropic and phototropic curvature of the maize coleoptile, the cortical microtubules (MTs) adjacent to the outer epidermal cell w all assume opposite orientations at the two sides of the organ. Starti ng from a uniformly random pattern during straight growth in darkness, the MTs reorientate perpendicularly to the organ axis at the outer (f aster growing) side and parallel to the organ axis at the inner (slowe r growing) side. As similar reorientations can be induced during strai ght growth by increasing or decreasing the effective auxin concentrati on, it has been proposed that these reorientations may be used as a di agnostic test for assessing the auxin status of the epidermal cells du ring tropic curvature. This idea was tested by determining the MT orie ntations in the coleoptile of intact maize seedlings in which the grav itropic or phototropic curvature was prevented or inversed by an appro priate mechanical counterforce. Forces that just prevented the coleopt ile from curving in a gravity or light field prevented reorientations of the MTs. Forces strong enough to overcompensate the tropic stimuli by enforcing curvature in the opposite direction induced reorientation s of the MTs opposite to those produced by tropic stimulation. These r esults show that the MTs at the outer surface of the coleoptile respon d to changes in mechanical tissue strain rather than to gravitropic or phototropic stimuli and associated changes at the level of auxin or a ny other element in the signal transduction chain between perception o f tropic stimuli and asymmetric growth response. It is proposed that c ortical MTs can act as strain gauges in a positive feed-back regulator y circle utilized for amplification and stabilization of environmental ly induced changes in the direction of elongation growth.