Studies on effects of gibberellin on microtubule-stabilization

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1. We studied the effects of gibberellin on the arrangement and the cold stability of cortical microtubules in epidermal cells of dwarf pea internodes and found that transverse arrays of cortical microtubules are predominant in gibberellin-treated cells, while cortical microtubules parallel to the long axis of the cells are predominant in non-treated cells. Longitudinal microtubules are resistant to low-temperature treatment, but transverse microtubules are sensitive to it. These results suggest that gibberellin gives rise to a predominance of transverse microtubules and thereby to the decrease in cold stablity of microtubules.2. We examined the cold stabilty of cortical microtubules of tobacco suspension cells "BY-2". The cortical microtubules in tobacco cells were resistant to low-temperature treatment, although microtubules which had been polymerized in vitro from isolated tobacco tubulin were disrupted by low-temperature treatment. Cortical microtubules in tobacco protoplasts were sensitive to low-temperature treatment, but those in cells which had regenerated cell walls after culture became resistant to the treatment. These results indicate that cell walls play an important role in the stability of microtubules. Next, we examined the effects of various substances supplied to the medium of protoplasts on the stabilization of microtubules, and found that polylysine and spermine stabilize cortical microtubules in protoplasts. This suggests that some polycations in cell walls are involved in the stabilization of cortical microtubules.3. We established a sensitive method to measure tubulin content, methods to measure tubulin synthetic activity and degrading activity in order to study a relationship between microtubule stability and the turn over of tubulin in plant cells. These methods revealed that tubulin degrading activity is associated with microtubule stability during tracheary element differentiation in isolated Zinnia mesophyll cells.