REORIENTATION OF MICROTUBULES AT THE OUTER EPIDERMAL WALL OF MAIZE COLEOPTILES BY PHYTOCHROME, BLUE-LIGHT PHOTORECEPTOR, AND AUXIN

The effects of red and blue light on the orientation of cortical micro tubules (MTs) underneath the outer epidermal wall of maize (Zea mays L .) coleoptiles were investigated with immunofluorescent techniques. Th e epidermal cells of dark-grown coleoptiles demonstrated an irregular pattern of regions of parallel MTs with a random distribution of orien tations. This pattern could be changed into a uniformly transverse MT alignment with respect to the long cell axis by 1 h of irradiation wit h red light. This response was transient as the MTs spontaneously shif ted into a longitudinal orientation after 1-2h of continued irradiatio n. Induction/reversion experiments with short red and far-red light pu lses demonstrated the involvement of phytochrome in this response. In contrast to red light, irradiation with blue light induced a stable lo ngitudinal MT alignment which was established within 10 min. The blue- light response could not be affected by subsequent irradiations with r ed or far-red light indicating the involvement of a separate blue-ligh t photoreceptor which antagonizes the effect of phytochrome. In mixed light treatments with red and blue light, the blue-light photoreceptor always dominated over phytochrome which exhibited an apparently less stable influence on MT orientation. Long-term irradiations with red or blue light up to 6 h did not reveal any rhythmic changes of MT orient ation that could be related to the rhythmicity of helicoidal cell-wall structure. Subapical segments isolated from dark-grown coleoptiles ma intained a longitudinal MT arrangement even in red light indicating th at the responsiveness to phytochrome was lost upon isolation. Converse ly auxin induced a transverse MT arrangement in isolated segments even in blue light, indicating that the responsiveness to blue-light photo receptor was eliminated by the hormone. These complex interactions are discussed in the context of current hypotheses on the functional sign ificance of MT reorientations for cell development.