Osmotic stress-induced changes in cell wall peroxidase activity and hydrogen peroxide level in roots of rice seedlings

The changes in activity of peroxidase (POD) extracted from the cell walls and the level of H2O2 in rice seedling roots treated with mannitol and their correlation with root growth were investigated. Increasing concentrations of mannitol from 92 to 276 mM, which is iso-osmotic with 50 to 150 mM NaCl, progressively reduced root growth and increased POD activities extracted from the cell walls of rice roots. The reduction of growth was also correlated with an increase in H2O2 level. Both diamine oxidase (DAO) and NADH peroxidase (NADH-POD) are known to be responsible for the generation of H2O2. Mannitol treatment increased DAO but not NADH-POD activities in roots of rice seedlings, suggesting that DAO contributes to the generation of H2O2 in the cell walls of mannitol-treated roots. An increase in the level of H2O2 and the activity of POD extracted from the cell walls of rice roots preceded root growth reduction caused by mannitol. An increase in DAO activity coincided with an increase in H2O2 in roots caused by mannitol. Since DAO catalyses the oxidation of putrescine, the demonstration that mannitol increases the activity of DAO in roots is consistent with those that mannitol decreases the level of putrescine. In conclusion, cell-wall stiffening catalysed by POD is possibly involved in the regulation of root growth reduction caused by mannitol.