A recently proposed hypothesis Nick et al. (1990)Planta181: 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 expressed as corresponding curvature. Thus, a monocausal relationship between microtubules and tropism is excluded. Furthermore, a comparison of tonic light effects on microtubules to earlier studies into the impact of light upon auxin content indicate that the relationship between auxin and microtubules might be more complex than hitherto assumed. It is concluded that, at least in maize coleoptiles, growth can be regulated by various mechanisms, and that microtubules, although somehow related to tropism, are probably not the cause of the fast tropistic responses.
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