Signal regulation of proline metabolism in callus of the halophyte Nitraria tangutorum Bobr. grown under salinity stress

Callus of the halophyte Nitraria tangutorum Bobr. was used to investigate proline metabolism and its signal regulation under salinity stress. Enhanced levels of proline and hydrogen peroxide (H2O2) were observed in calli exposed to salinity stress, and elevated levels of calcium (Ca) were detected in early responses to 75 mM NaCl treatment. Additionally, NaCl treatment induced significant elevation of ornithine-δ-aminotransferase (OAT) activity, but notable decreases occurred in the activities of glutamyl kinase (GK) and proline dehydrogenase (PDH). H2O2 scavenger dimethylthiourea and pyruvate inhibited the accumulation of proline and the stimulation of OAT in salinity-stressed calli. Moreover, the utilization of Ca chelator EGTA and Ca channel blocker verapamil abolished the enhancement of proline level induced by 75 mM NaCl treatment for 3 days. These results suggest that the accumulation of proline is correlated to the increase of OAT activity and the decrease of PDH activity in response to salinity, and that elevated Ca signal during the early stage of NaCl treatment and the excitation of OAT activity resulting from the increase of H2O2 generation are essential for proline accumulation in salinity-stressed calli.