Proline and Glycinebetaine Ameliorated NaCl Stress via Scavenging of Hydrogen Peroxide and Methylglyoxal but Not Superoxide or Nitric Oxide in Tobacco Cultured Cells

Efficient detoxification of the reactive oxygen species, nitric oxide (NO) and methylglyoxal (MG), provides protection against NaCl-induced damage in plants. To elucidate the protective mechanisms of proline and glycinebetaine (betaine) against NaCl stress, intracellular levels of hydrogen peroxide (H2O2), superoxide (O-2(-)), NO, and MG were investigated in tobacco Bright Yellow-2 cells. The Levels of H2O2, O-2(-), NO and MG were higher in the short-term and long-term NaCl-stressed cells than in the non-stressed cells, whereas the O-2(-) level was higher in the long-term stressed cells. Exogenous proline and betaine decreased the H2O2 level in both the short-term and the long-term NaCl-stressed cells and the MG level in the long-term NaCl-stressed cells, but did not change the O-2(-) or NO levels. Under salt stress, both proline and betaine increased the transcription levels of glutathione peroxidase, which can contribute to the reduction of H2O2. In conclusion, proline and betaine mitigated salt stress via reduction of H2O2 accumulation during short-term incubation and via reduction of the accumulation of H2O2 and MG during long-term incubation.