The response of cortical microtubules to low temperature and freezing was assessed for root tips of cold-acclimated and nonacclimated winter rye (Secale cerea/e L. cv Puma) seedlings using indirect immunofluorescence microscopy with antitubulin antibodies. Roots cooled to 0 or -30C were fixed for immunofluorescence microscopy at these temperatures or after an additional hour at 40C. Typical arrays of cortical microtubules were present in root-tip cells of seedlings exposed to the cold-acclimation treatment of 40C for 2 days. Microtubules in these cold-acclimated cells were more easily depolymerized by a 0°C treatment than microtubules in root-tip cells of nonacclimated, 22°C-grown seedlings. Microtubules were still present in some cells of both nonacclimated and cold-acclimated roots at 0 and -30C; however, the number of microtubules in these cells was lower than in controls. Microtubules remaining during the -30C freeze were shorter than microtubules in unfrozen control cells. Repolymenzation of microtubules after both the 0 and -30C treatments occurred within 1 h. Root tips of nonacclimated seedlings had an LT-50 of -90C. Cold acclimation lowered this value to -140C. Treatment of 22°C-grown seedlings for 24 h with the microtubulestabilizing drug taxol caused a decrease in the freezing tolerance of root tips, indicated by a LT-50 of -30C. Treatment with Dsecotaxol, an analog of taxol that was less effective in stabilizing microtubules, did not alter the freezing tolerance. We interpret these data to indicate that a degree of depolymerization of microtubules is necessary for realization of maximum freezing tolerance in root-tip cells of rye.
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