Proline and its metabolism enzymes in cucumber cell cultures during acclimation to salinity

Proline is an important osmolyte appearing as the result of salt stress response of plants. In the present study, we measured the proline concentration, activities of pyrroline-5-carboxylate synthetase (P5CS), pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (PDH) key regulatory enzymes in the biosynthesis and degradation of proline in the acclimated (AC20) and the non-acclimated (NAC) cucumber cell suspension cultures subjected to moderate (150 mM NaCl; AC20-150, NAC-150, respectively) and severe (200 mM NaCl; AC20-200, NAC-200, respectively) salt stress. The data showed that salt stress brought about a linear increase in proline content in both types of cultures. However, in the acclimated culture proline accumulation was observed earlier, in third hour after stress. Only in the acclimated culture moderate and severe stresses up-regulated P5CS activity throughout the experiment, whereas the activity of P5CR grew in response to both NaCl concentrations only in 24th and 48th hour. The severe salt stress resulted in decrease in P5CR in NAC-200 cultures. In response to salt stress, both types of cell suspension cultures reacted with decline in PDH activity below the spectrophotometrically detected level. Cell cultures vigor correlated with salt concentration and time of exposure to the stress factor. Both NaCl concentrations caused linear decline in vigor of the non-acclimated culture up to 80-90 % at the end of the experiment, whereas in the acclimated culture significant decrease by about 30-40 % was reached in 24th hour after stress. The presented data suggest that acclimation to salt stress up-regulated proline synthesis enzyme activity and caused intensive accumulations of proline by inhibiting its oxidation.