Proline status in salt-tolerant and salt-sensitive alfalfa cell lines and plants in response to NaCl

We have investigated proline status in relation to salt-tolerance in cell cultures of alfalfa and salt-tolerant plants regenerated from our salt-tolerant cell cultures. Selection for salt-tolerance in culture correlated in some, but not most cases with selection for constitutively high proline concentrations in callus grown on normal medium. Transfer of salt-tolerant lines with constitutively low proline concentrations to NaCl containing medium led to nearly ten fold increases in proline concentrations, with increased proline detectable in as little as two hours. One selected salt-tolerant cell line which showed a constitutively high proline concentration showed no additional salt dependent increases in proline concentration. In our alfalfa plants, proline is accumulated more rapidly in shoots than in roots when plants are exposed to salt, but both roots and shoots increase their proline concentrations about ten fold in three to five days after growth conditions were chan ed to include 171 mM NaCl. Shoots from all plants showed significant increases in proline accumulation within 24 h of exposure to salt. However, only roots from salt-tolerant plants showed a doubling in proline concentration within 24 h after exposure to salt. In contrast, the salt-sensitive plant roots showed no change in proline concentrations during this time. The rapid adjustment of proline concentration in our salt-tolerant callus and plant roots may serve a protective function.