REORGANIZATION OF INTERPHASE MICROTUBULES IN ROOT CELLS OF MEDICAGO SATIVA L. DURING ACCLIMATION TO OSMOTIC AND SALT STRESS CONDITION.

We have investigated the organization of microtubule system in interphase cells of Medicago sativa L. roots during acclimation to salt and osmotic stress at different concentrations of NaCl, Na2SO4 and mannitol. We have identified several morphological changes in tubulin cytoskeleton that appear during the acclimation to salt and osmotic stress in the cells of different root tissues: 1) decreased density of cortical microtubule network, 2) random orientation of cortical microtubule bundles, 3) non-uniform density of the bundles, 4) thickening of the bundles, 5) fragmentation of the bundles, 6) formation of centers of converging microtubule. Reduced density of the microtubule network and thickening of the bundles was detected during osmotic and salt stress, yet random orientation of cortical microtubules was observed under osmotic stress and not found during salt stress. Fragmentation of microtubule bundles was apparent during salt stress and less evident at high concentration of mannitol. Formation of centers of converging microtubule was common under prolonged action of sodium sulfate, less common under sodium chloride and not found after mannitol treatment. Our data show that cortical microtubules in alfalfa root cells rearrange not only in response to different ions, but also to osmotic pressure. Thus, the signaling pathways and molecular mechanisms inducing reorganization of the microtubule system may be triggered not only by sodium cations but also by sulfate and chloride anion at the concentrations that do not cause irreversible cell damage. Our study show that the osmotic and salt stress differently affect the cortical microtubules, and their reorganization in response to stress depends on the salt cations as well as anions might also show additional effect under salt stress.