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 cellwalls and the level of H₂O₂ in rice seedling rootstreatedwith mannitol and their correlation with root growth were investigated.Increasing concentrations of mannitol from 92 to 276 mM, which isiso-osmotic with 50 to 150 mM NaCl, progressively reduced rootgrowth and increased POD activities extracted from the cell walls of riceroots.The reduction of growth was also correlated with an increase inH₂O₂ level. Both diamine oxidase (DAO) and NADHperoxidase(NADH-POD) are known to be responsible for the generation ofH₂O₂. Mannitol treatment increased DAO but not NADH-PODactivities in roots of rice seedlings, suggesting that DAO contributes to thegeneration of H₂O₂ in the cell walls of mannitol-treatedroots. An increase in the level of H₂O₂ and the activityof POD extracted from the cell walls of rice roots preceded root growthreduction caused by mannitol. An increase in DAO activity coincided with anincrease in H₂O₂ in roots caused by mannitol. Since DAOcatalyses the oxidation of putrescine, the demonstration that mannitolincreasesthe activity of DAO in roots is consistent with those that mannitol decreasesthe level of putrescine. In conclusion, cell-wall stiffening catalysed by PODispossibly involved in the regulation of root growth reduction caused bymannitol.