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 (O₂⁻), NO, and MG were investigated in tobacco Bright Yellow-2 cells. The Levels of H₂O₂, O₂⁻, NO and MG were higher in the short-term and long-term NaCl-stressed cells than in the non-stressed cells, whereas the O₂⁻ 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 O₂⁻ 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.

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