RELATIONSHIP BETWEEN FREEZING TOLERANCE OF ROOT-TIP CELLS AND COLD STABILITY OF MICROTUBULES IN RYE (SECALE-CEREALE L CV PUMA)

The response of cortical microtubules to low temperature and freezing was assessed for root tips of cold-acclimated and non-acclimated winter rye (Secale cereale L. cv Puma) seedlings using indirect immunofluorescence microscopy with antitubulin antibodies. Roots cooled to 0 or -3°C were fixed for immunofluorescence microscopy at these temperatures or after an additional hour at 4°C. Typical arrays of cortical microtubules were present in root-tip cells of seedlings exposed to the cold-acclimation treatment of 4°C 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 -3°C; however, the number of microtubules in these cells was lower than in controls. Microtubules remaining during the -3°C freeze were shorter than microtubules in unfrozen control cells. Repolymerization of microtubules after both the 0 and -3°C treatments occurred within 1 h. Root tips of nonacclimated seedlings had an LT-50 of -9°C. Cold acclimation lowered this value to -14°C. 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 -3°C. Treatment with D-secotaxol, 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.