Do Microtubules Control Growth in Tropism?
A recently proposed hypothesis [Nick et al. (1990) Planta 181: 162] suggests that, in maize coleoptiles, tropistic curvature might be caused by a stimulus-induced trans-organ gradient over the orientation of cortical microtubules adjacent to the outer cell wall of the outer epidermis. This gradient, in turn, is controlled by a light-induced redistribution of auxin. The hypothesis was tested by following the behaviour of microtubules for various light stimuli using indirect immuno-fiuorescence in epidermal strips as assay. Analysis of gravitropic straightening, nastic curvature on the horizontal clinostat, effects of tonic irradiation with red and/or blue light, and experiments involving opposing light pulses demonstrate that bending direction and microtubule orientation gradients are not as closely linked as predicted: Considerable bending can be produced without detectable gradients of microtubule orientation, and conspicuous gradients of microtubule orientation are not necessarily ex