Chuan Chi Lin

Effect of NaCl stress on H2O2 metabolism in rice leaves

The effect of NaCl stress on H2O2 metabolismin detached rice leaves was studied. NaCl (200 mM)treatment did not cause the accumulation ofH2O2 and resulted in no increase in lipidperoxidation and membrane leakage of leaf tissues. The activities of peroxidase, ascorbate peroxidase,superoxide dismutase, and glutathione reductase wereobserved to be greater in NaCl-stressed rice leavesthan in control leaves. However, glycolate oxidasewas lower in NaCl-treated rice leaves than in thecontrol leaves. There was no difference in catalaseactivity between NaCl and control treatments. Theseresults suggest that some antioxidant enzymes can beactivated in response to oxidative stress induced byNaCl.

Cell wall peroxidase activity, hydrogen peroxide level and NaCl-inhibited root growth of rice seedlings

The changes in cell-wall peroxidase (POD) activity and H2O2 level in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mM progressively reduced root growth and increased ionically bound cell-wall POD activity. NaCl had no effect on covalently bound cell-wall POD activities. The reduction of root growth by NaCl is closely correlated with the increase in H2O2 level. Exogenous H2O2 was found to inhibit root growth of rice seedlings. Since ammonium and proline accumulation are associated with root growth inhibition caused by NaCl, we determined the effects of NH4Cl or proline on root growth, cell-wall POD activity and H2O2level in roots. External application of NH4Cl or proline markedly inhibited root growth, increased cell-wall POD activity and increased H2O2 level in roots of rice seedlings in the absence of NaCl. An increase in cell-wall POD activity and H2O2 level preceded inhibition of root growth caused by NaCl, NH4Cl or proline. NaCl or proline treatment also increased NADH-POD and diamine oxidase (DAO) activities in roots of rice seedlings, suggesting that NADH-POD and DAO contribute to the H2O2 generation in the cell wall of NaCl- or proline-treated roots. NH4Cl treatment increased NADH-POD activity but had no effect on DAO activity, suggesting that NADH-POD but not DAO is responsible for H2O2 generation in cell wall of NH4Cl-treated roots.

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.

Regulation of proline accumulation in detached rice leaves exposed to excess copper.

Accumulation of proline in response to excess Cu was studied in detached leaves of rice (Oryza sativa). CuSO(4) was effective in inducing proline accumulation in detached rice leaves under both light and dark conditions. CuSO(4) and CuCl(2) were equally effective in inducing proline accumulation, indicating that proline accumulation is induced by Cu. Sulfate salts of Mg, Mn, and Fe were ineffective in inducing proline accumulation in detached rice leaves. Excess Cu had no effect on relative water content of detached rice leaves, suggesting that Cu-induced proline accumulation is unlikely due to water deficit. Proline accumulation induced by excess Cu was related to proteolysis and an increase in Delta(1)-pyrroline-5-carboxylate reductase or ornithine-delta-aminotransferase activity and could not be explained by proline utilization or stress-induced modifications in proline dehydrogenase or Delta(1)-pyrroline-5-carboxylate dehydrogenase. The content of glutamic acid decreased by excess Cu. The increase in arginine but not ornithine was found to be associated with the increase in proline content in Cu-stressed detached rice leaves. CuSO(4) treatment resulted in an increase in abscisic acid content in detached rice leaves. The possibility that proline accumulation induced by excess Cu is mediated through abscisic acid is discussed.

Copper Toxicity in Rice Seedlings: Changes in Antioxidative Enzyme Activities, H2O2 Level, and Cell Wall Peroxidase Activity in Roots

The changes in lipid peroxidation, antioxidative enzyme activity, H2O2 level, and cell wall peroxidase activity in Cu-stressed roots of rice seedlings and their relation with root growth inhibition were investigated. CuSO4 was effective in inhibiting root growth but not shoot growth. Treatment with CuSO4 resulted in an increase in lipid peroxidation and modulated antioxidative enzyme activities in rice roots. CuSO4 increased the activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and peroxidase, but had no effect on catalase. CuSO4 also increased H2O2 level and cell wall peroxidase in roots of rice seedlings. Exogenous application of H2O2 resulted in an inhibition of root growth. It appears that growth inhibition of root caused by Cu is associated with H2O2 dependent peroxidase-catalyzed formation of cross-linking among cell wall polymers.

NaCl induced changes in ionically bound peroxidase activity in roots of rice seedlings

The changes in ionically bound peroxidase activity in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mM progressively decreases root growth. The reduction of root growth by NaCl is closely correlated with the increase in ionically bound peroxidase activity. Since proline and ammonium accumulations are associated with root growth inhibition caused by NaCl, we determined the effects of proline or NH4Cl on root growth and ionically bound peroxidase activity in roots. External application of proline or NH4Cl markedly inhibited root growth and increased ionically bound peroxidase activity in roots of rice seedlings in the absence of NaCl. An increase in ionically bound peroxidase activity in roots preceded inhibition of root growth caused by NaCl, NH4Cl or proline. Mannitol inhibited root growth, but decreased rather than increased ionically bound peroxidase activity at the concentration iso-osmotic with NaCl. The inhibition of root growth and the increase in ionically bound peroxidase activity in roots by NaClis reversible and is associated with ionic rather than osmotic component.

Disturbed ammonium assimilation is associated with growth inhibition of roots in rice seedlings caused by NaCl

The effects of NaCl on changes in ammonium level and enzyme activities of ammonium assimilation in roots growth of rice (Oryza sativa L.) seedlings were investigated. NaCl was effective in inhibiting root growth and stimulated the accumulation of ammonium in roots. Accumulation of ammonium in roots preceded inhibition of root growth caused by NaCl. Both effects caused by NaCl are reversible. Exogenous ammonium chloride and methionine sulfoximine (MSO), which caused ammonium accumulation in roots, inhibited root growth of rice seedlings. NaCl decreased glutamine synthetase and glutamate synthase activities in roots, but increased glutamate dehydrogenase activity. The growth inhibition of roots by NaCl or MSO could be reversed by the addition of L-glutamic acid or L-glutamine. The current results suggest that disturbance of ammonium assimilation in roots may be involved in regulating root growth reduction caused by NaCl.

Proline accumulation is associated with inhibition of rice seedling root growth caused by NaCl

Abstract The effects of NaCl on changes in the proline level in roots and root growth of rice seedlings were investigated. NaCl was effective in inhibiting root growth and in increasing proline accumulation in roots. Mannitol was also effective in inhibiting root growth, but proline accumulation was observed only at the concentration iso-osmotic with 150 mM NaCl. Accumulation of proline in roots preceded inhibition of root growth caused by NaCl. The inhibition of root growth and accumulation of proline in roots by NaCl are reversible. Exogenous l -proline and gabaculine, which were found to enhance in an accumulation of proline in roots, inhibited root growth of rice seedlings. The relationship between growth inhibition of roots caused by NaCl and changes in the proline level in roots is discussed.

Iron induction of lipid peroxidation and effects on antioxidative enzyme activities in rice leaves

Lipid peroxidation in relation to toxicity of detached rice leavescaused by excess iron (FeSO4) was investigated. ExcessFeSO4, which was observed to induce toxicity, enhanced the contentoflipid peroxidation but not the content of H2O2.Superoxidedismutase activity was reduced by excess FeSO4. Ascorbate peroxidaseand glutathione reductase activities were increased by excess FeSO4.Free radical scavengers, such as mannitol and reduced glutathione, inhibitedexcess iron-induced toxicity and at the same time inhibited excessiron-enhancedlipid peroxidation, suggesting that lipid peroxidation enhanced by excess ironis mediated through free radicals.

Cadmium toxicity of rice leaves is mediated through lipid peroxidation

Oxidative stress, in relation to toxicity of detached rice leaves,caused by excess cadmium was investigated. Cd content inCdCl2-treated detached rice leaves increased with increasingdurationof incubation in the light. Cd toxicity was followed by measuring the decreasein chlorophyll and protein. CdCl2 was effective in inducing toxicityand increasing lipid peroxidation of detached rice leaves under both light anddark conditions. These effects were also observed in rice leaves treated withCdSO4, indicating that the toxicity was indeed attributed to cadmiumions. Superoxide dismutase (SOD), ascorbate peroxidase (APOD), and glutathionereductase (GR) activities were reduced by excess CdCl2 in the light.The changes in catalase and peroxidase activities were observed inCdCl2-treated rice leaves after the occurrence of toxicity in thelight. Free radical scavengers reduced CdCl2-induced toxicity and atthe same time reduced CdCl2-induced lipid peroxidation and restoredCdCl2-decreased activities of SOD, APOD, and GR in the light. Metalchelators (2,2′-bipyridine and 1,10-phenanthroline) reducedCdCl2 toxicity in rice leaves in the light. The reduction ofCdCl2 toxicity by 2,2′-bipyridine (BP) is closely associatedwith a decrease in lipid peroxidation and an increase in activities ofantioxidative enzymes. Furthermore, BP-reduced toxicity of detached riceleaves,induced by CdCl2, was reversed by adding Fe2+ orCu2+, but not by Mn2+ or Mg2+.Reduction of CdCl2 toxicity by BP is most likely mediated throughchelation of iron. It seems that toxicity induced by CdCl2 mayrequire the participation of iron.