Abscisic acid induced changes in cell wall peroxidase activity and hydrogen peroxide level in roots of rice seedlings.

The changes in the activity of peroxidase (POD) extracted from the cell wall and the level of H(2)O(2) of rice seedling roots treated with abscisic acid (ABA) and their correlation with root growth were investigated. Increasing concentrations of ABA from 3 to 18 µM progressively reduce root growth and increase POD activities (using guaiacol or ferulic acid as a substrate) extracted from the cell wall of rice roots. The reduction of root growth by ABA is also correlated with an increase in H(2)O(2) level. Both diamine oxidase (DAO) and NADH peroxidase (NADH-POD) are known to be responsible for the generation of H(2)O(2). ABA treatment increased NADH-POD and DAO activities in roots of rice seedlings, suggesting that NADH-POD and DAO contribute to the generation of H(2)O(2) in the cell wall of ABA-treated roots. An increase in the level of H(2)O(2) and the activity of POD extracted from the cell wall of rice roots preceded root growth reduction caused by ABA. An increase in DAO and NADH-POD activities coincided with an increase in H(2)O(2) in roots caused by ABA. Since DAO catalyzes the oxidation of putrescine, the results that ABA increases the activity of DAO in roots is consistent with those that ABA decreases the level of putrescine. In conclusion, cell wall stiffening catalyzed by POD is possibly involved in the regulation of root growth reduction caused by ABA.