In vivo and in vitro protective role of proline

Accumulation of proline in response to environmental stresses seems to be widespread among plants. To elucidate the role of proline in plant responses, in vivo and in vitro, we studied the effect of proline on catalase (CAT; EC 1.11.1.6), peroxidase (POD; EC 1.11.1.7) and polyphenol oxidase (PPO; EC 1.14.18.1). In vivo, these three enzymes were activated by proline, while CAT and POD were activated and PPO was inactivated by NaCl. In vitro, CAT and POD were activated and PPO was inactivated by proline. Proline appeared to protect these three enzyme activities. The significance of these findings with regard to environmental stress-induced proline accumulation in vivo is discussed. The ability of proline to activate the enzymes may suggest a limited conformational change. These results are important for characterisation of metabolic responses to environmental stresses and can be used as a stress indicator.