Kuo Tung Hung

Paraquat toxicity is reduced by nitric oxide in rice leaves

Summary The role of nitric oxide (NO) in plants has been the object of intensive research. In the present work, we evaluated the protective effect of NO against paraquat (PQ) toxicity of rice (Oryza sativa) leaves. PQ toxicity in rice leaves was determined by the decrease of protein content. PQ toxicity of rice leaves was reduced by free radical scavengers such as thiourea, sodium benzoate, ascorbic acid and sodium azide. NO-releasing compounds [N-tert-butyl-α-phenylnitrone (PBN), sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1), and ascorbic acid + NaNO2] were effective in reducing PQ toxicity in rice leaves. SIN-1 and ascorbic acid + NaNO2 prevented both the PQ-induced increase in content of malondialdehyde and the PQ-induced decrease in activities of active oxygen species-detoxifying enzymes. The protective effect of SIN-1 or ascorbic acid + NaNO2 on PQ toxicity, PQ-induced lipid peroxidation and PQ-decreased antioxidative enzyme activities was reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a NO-specific scavenger, suggesting that the protective effect of NO-releasing compounds is attributed to NO released. Reduction of PQ toxicity by NO in rice leaves is most likely mediated through increase in antioxidative enzyme activities and decrease in lipid peroxidation.

Nitric oxide acts as an antioxidant and delays methyl jasmonate-induced senescence of rice leaves.

In the present study, we evaluate the protective effect of nitric oxide (NO) against senescence of rice leaves promoted by methyl jasmonate (MJ). Senescence of rice leaves was determined by the decrease of protein content. MJ treatment resulted in (1) induction of leaf senescence, (2) increase in H2O2 and malondialdehyde (MDA) contents, (3) decrease in reduced form glutathione (GSH) and ascorbic acid (AsA) contents, and (4) increase in antioxidative enzyme activities (ascorbate peroxidase, glutathione reductase, peroxidase and catalase). All these MJ effects were reduced by free radical scavengers such as sodium benzoate and GSH. NO donors [N-tert-butyl-alpha-phenylnitrone (PBN), sodium nitroprusside, 3-morpholinosydonimine, and AsA+NaNO2] were effective in reducing MJ-induced leaf senescence. PBN prevented MJ-induced increase in the contents of H2O2 and MDA, decrease in the contents of GSH and AsA, and increase in the activities of antioxidative enzymes. The protective effect of PBN on MJ-promoted senescence, MJ-increased H2O2 content and lipid peroxidation, MJ-decreased GSH and AsA, and MJ-increased antioxidative enzyme activities was reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, a NO-specific scavenger, suggesting that the protective effect of PBN is attributable to NO released. Reduction of MJ-induced senescence by NO in rice leaves is most likely mediated through its ability to scavenge active oxygen species including H2O2.

Ammonium accumulation is associated with senescence of rice leaves

The relationship between ammonium accumulation and senescence of detached rice leaves was investigated. Ammonium accumulation in detached rice leaves coincided closely with dark-induced senescence. Exogenous NH4Cl and methionine sulfoximine, which caused an accumulation of ammonium in detached rice leaves, promoted senescence. Treatments such as light and benzyladenine, which retarded senescence, decreased ammonium level in detached rice leaves. Abscisic acid, which promoted senescence, increased ammonium level in detached rice leaves. The current results suggest that ammonium accumulation may be involved in regulating senescence. Evidence was presented to show that ammonium accumulated in detached rice leaves increases tissue sensitivity to ethylene. The accumulation of ammonium in detached rice leaves during dark-induced senescence is attributed to a decrease in glutamine synthetase activity and an increase in reduction of nitrate.

Promotive effect of jasmonates on the senescence of detached maize leaves

Promotion of senescence of detached maize leaves by jasmonates was investigated. Senescence of detached maize leaves was promoted by linolenic acid, the precursor of biosynthesis of jasmonic acid, and retarded by inhibitors of lipoxygenase, the first enzyme in the biosynthetic pathway of jasmonic acid. Results support a role of endogenous jasmonates in the regulation of senescence of detached maize leaves. Silver thiosulfate, an inhibitor of ethylene action, was found to inhibit methyl jasmonate, linolenic acid- and abscisic acid-promoted senescence of detached maize leaves. It seems that jasmonate-promoted senescence is mediated through an increase in ethylene sensitivity in detached maize leaves.

An increase in ethylene sensitivity is associated with jasmonate-promoted senescence of detached rice leaves

The role of ethylene in jasmonate-promoted senescence of detached rice leaves was investigated. Ethylene production in methyl jasmonate-treated leaf segments of rice was lower than in the control leaves. Treatment of leaf segments with silver nitrate or/and silver thiosulfate, inhibitors of ethylene action, inhibited methyl jasmonate-, jasmonic acid-, linolenic acid-, and abscisic acid-promoted senescence of detached leaves. We suggest that an increase in ethylene sensitivity, but not ethylene level, is the initial event triggering the enhanced senescence by jasmonates of detached rice leaves.

Involvement of lipid peroxidation in methyl jasmonate-promoted senescence in detached rice leaves

Lipid peroxidation in relation to MJ-promoted senescence of detached rice leaves was investigated. Lipid peroxidation seems to be involved in the regulation of MJ-promoted senescence of detached rice leaves. This conclusion was based on the observations that (a) methyl jasmonate, which was found to promote senescence, increases the level of lipid peroxidation, (b) linolenic and linoleic acid, precursors of the biosynthesis of jasmonic acid, promoted senescence and increased lipid peroxidation level, (c) benzyladenine, a synthetic cytokinin, counteracted MJ-promoted senescence and reduced the increase in lipid peroxidation level, (d) calcium chloride effectively reduced MJ-promoted senescence and at the same time reduced MJ-promoted lipid peroxidation. Free radical scavengers (reduced glutathione and sodium benzoate) and an iron chelator (2, 2′-bipyridine) prevented MJ-promoted senescence, suggesting that lipid peroxidation induced by MJ is mediated through free radicals.

Lipid peroxidation in relation to senescence of maize leaves

Summary Lipid peroxidation in relation to senescence of detached maize leaves in darkness was investigated. There was no change in malondialdehyde (MDA) level in water-treated detached maize leaves during the first day of incubation. Thereafter, there was a significant increase in the level of MDA (lipid peroxidation). Lipid peroxidation seems to be involved in the regulation of the senescence of detached maize leaves. This conclusion was based on the observations that (a) the increase in the level of lipid peroxidation preceded senescence, (b) benzyladenine, which retarded senescence, reduced the increase in lipid peroxidation level, (c) abscisic acid (ABA), methyl jasmonate (MJ) and linolenic acid, which were found to promote senescence, increase the level of lipid peroxidation, and (d) silver thiosulfate, which effectively inhibited MJ-, linolenic acid- and ABA-promoted senescence, reduced MJ-, linolenoc acid- and ABA-in-creased lipid peroxidation level.

Phosphatidylinositol 3-phosphate is required for abscisic acid-induced hydrogen peroxide production in rice leaves

Rice leaves produce H2O2 in response to abscisic acid (ABA), which results in induction of senescence and accumulation of NH4+. The upstream steps of the ABA-induced H2O2 production pathway in rice leaves remain largely unclear. In animal cells, H2O2 production in neutrophils is activated by phosphatidylinositol 3-phosphate (PI3P), a product of phosphatidylinositol 3-knase (PI3K). In the present study, we examined whether PI3P plays a role in H2O2 production in rice leaves exposed to ABA. We found that PI3K inhibitors LY 294002 (LY) or wortmannin (WM) inhibited ABA-induced H2O2 production, senescence and NH4+ accumulation. Hydrogen peroxide almost completely rescued the inhibitory effect of LY or WM. It appears that PI3P plays a role in ABA-induced H2O2 production, senescence, and NH4+ accumulation in rice leaves.

Hydrogen Peroxide, Calcium, and Leaf Senescence in Rice

The possible involvement of calcium in the regulation of H2O2-, abscisic acid (ABA)- or methyl jasmonate (MJ)-induced H2O2 generation, protein loss, and lipid peroxidation in detached rice (Oryza sativa L.) leaves was investigated. Calcium chloride and calcium ionophore A23187 effectively reduced H2O2-promoted ABA- or MJ-induced H2O2 generation, protein loss, and lipid peroxidation in detached rice leaves. It appears that ABA- or MJ-induced H2O2 generation, protein loss, and lipid peroxidation in rice leaves is mediated through blocking the entrance of calcium ions into the cytosol.

Ammonium, calcium, and leaf senescence in rice

The possible involvement of calcium in the regulation of ammonium-promoted senescence of detached rice leaves was investigated. Calcium effectively reduced ammonium-promoted senescence of detached rice leaves. The effect of ammonium on the senescence was also significantly reduced by the calcium ionophore A23187. Ammonium-promoted senescence of detached rice leaves may be mediated through blocking the entrance of calcium ions into the cytosol.