Interactions Between Hydrogen Peroxide And Nitric Oxide Signaling In Adaptive Response To Aluminum Toxicity In Wheat

Aluminum (Al) toxicity is the primary constraint of agricultural production on acid soils (pH< 5.5), which constitute approximately 50% of the world’s potential arable lands. Strong binding affinity of Al to cell components resulted in inhibition of cell wall synthesis, distortion of cytoskeleton, destruction of plasma membrane integrity and disruption of various signaling cascades. There is now compelling evidence that hydrogen peroxide (H2O2) and nitric oxide (NO) are ubiquitous signaling molecules involved in a broad spectrum of physiological and developmental processes, and biotic or abiotic stress responses. Previous studies have mainly focused on the individual role of H2O2 or NO in alleviating Al phytotoxicity. However, little is known about the roles and mechanisms of interaction between H2O2 and NO signaling in adaptive responses to Al toxicity in plants.In the present study, two wheat genotypes differing in Al tolerance are employed to examine H2O2 and NO generation and spatial distribution along the root, the role of NO oscillations in tolerance strategies, possible association with NO and AsA-GSH cycle in regulatingAl tolerance, the effect of NO on cell wall composition as well as its subsequent Al-binding capacity, relationship between H2O2 and NO in Al response and MAPK mediated H2O2 and NO signaling transduction. The main results are followed below:(1) Using two wheat genotypes with different sensitivity to aluminum (Al) stress, we studied key components involved in reactive oxygen species (ROS) and reactive nitrogen species (RNS) metabolism. After 24 h of exposure to Al, wheat genotypes exhibited significant difference in root growth and activity. These were accompanied by significant difference of ROS and RNS metabolisms. Sensitive genotype Yang-5 accumulated more O2-, H2O2, NO, ONOO" than that of Al tolerant genotype, with an increase of lipid peroxidation and Evans blue uptake, indicating that Al induced oxidative stress. In contrast, Al tolerant genotype Jian-864 exhibited low levels of lipid peroxidation. The outcome was achieved by adjusting its antioxidant systems. In this process, antioxidants ascorbate and glutathione, and the activity and expression of catalase were significantly induced. Therefore, our results indicate that ROS and RNS homeostasis is important for plant performance under Al stress.(2) The role of nitric oxide (NO) release in tolerance strategies in roots of wheat under aluminum (Al) stress was investigated by using two genotypes differing in Al resistance. An early NO burst at 3 h was observed in root tips of the Al-tolerant genotype Jian-864, whereas the Al-sensitive genotype Yang-5 showed no NO accumulation at 3 h but an extremely high NO levels after 12 h. Stimulating NO production at 3 h in root tips of Yang-5 by NO donor relieved Al-induced root inhibition and callose production, as well as oxidative damage and ROS accumulation, while elimination of early NO burst by NO scavenger aggravated root inhibition of Jian-864. Synthesis of the early NO in roots of Jian-864 was mediated through nitrate reductase (NR) but not through nitric oxide synthase. Elevated antioxidant enzyme activities were induced by Al stress in both wheat genotypes and enhanced by NO donor, but suppressed by NO scavenger or NR inhibitor. These results suggest that NR-mediated early NO burst plays an important role in Al resistance of wheat through modulating enhanced antioxidant defense to adapt to Al stress.(3) The possible association with nitric oxide (NO) and ascorbate-glutathione (AsA-GSH) cycle in regulating aluminum (Al) tolerance of wheat was investigated using two genotypes with different Al resistance. Exposure to Al inhibited root elongation, and triggered lipid peroxidation and oxidation of AsAto dehydroascorbate and GSH to glutathione disulfide in wheat roots. Exogenous NO significantly increased endogenous NO levels, and subsequently alleviated Al-induced inhibition of root elongation and oxidation of AsA and GSH to maintain the redox molecules in the reduced form in both wheat genotypes. Under Al stress, significantly increased activities and gene transcriptional levels of ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase, were observed in the root tips of the Al-tolerant genotype Jian-864. Nitric oxide application enhanced the activity and gene transcriptional level of these enzymes in both wheat genotypes. y-Glutamylcysteine synthetase was not affected by Al or NO, but NO treatments increased the activity of glutathione peroxidase and glutathione S-transferase to a greater extent than the Al-treated wheat seedlings. Proline was significantly decreased by Al, while was not affected by NO. These results clearly suggest that NO protects wheat root against Al-induced oxidative stress, possibly through its regulation of the AsA-GSH cycle.(4) Nitric oxide (NO) is an important bioactive molecule involved in modification of cell wall, which has been recognized as the major target of aluminum (Al) toxicity. The effects of Al-induced NO on cell wall composition and its subsequent Al-binding capacity were investigated in an Al-sensitive wheat (Triticum aestivumL. cv. Yang-5). We found that Al exposure induced an extremely high NO accumulation in root tips of wheat. Eliminating NO production with NO scavenger (cPTIO) significantly alleviated Al-induced root growth inhibition and thus reduced Al accumulation. Elimination of NO, however, did not significantly affect malic acid efflux or rhizosphere pH changes under Al exposure. Cell wall polysaccharides (pectin, hemicellulosel and hemicelluloses 2) and pectin methylesterase (PME) activity, as well as pectin demethylation in the root apex, were significantly increased under Al treatment. Exogenous cPTIO application significantly decreased PME activity and increased the degree of methylation of pectin in root cell wall, thus resulting in a decreased Al retention in pectin. These results suggest that Al-induced extreme elevation of NO decreased pectin methylation of cell wall and subsequently increased its sequestration of Al, and negatively regulated Al tolerance in wheat.(5) We previously reported that nitric oxide (NO) functions as a signal in wheat aluminum (Al) tolerance. To illustrate its relationship with hydrogen peroxide (H2O2) in the tolerance of wheat to Al stress, we investigated the effect of different donors and inhibitors on NO and H2O2 production using two genotypes with different Al resistance. After 2 h Al treatment, the H2O2 levels in the Al-sensitive genotype Yang-5 were lower than that in Al-tolerant genotype Jian-864. Treatment of the seedlings with H2O2 partially rescued their Al sensitivity and induced early NO production. Nitric oxide scavenger (cPTIO) counteracted H2O2 and NO effect; while H2O2 scavenger (DMTU) only reversed H2O2 effect, but did not block NO effect, suggesting that NO is involved in H2O2 signaling as a downstream factor. H2O2 induced the synthesis of the early NO in roots was mediated through nitrate reductase (NR). Elevated antioxidant enzyme activities were induced by Al stress in both wheat genotypes and significantly enhanced by H2O2 donor, but suppressed by NO scavenger or NR inhibitor. These data indicate that H2O2 acts upstream of NO in Al tolerance, which requires enhanced antioxidant defense.(6) Previous work showed that nitric oxide (NO) and hydrogen peroxide (H2O2) are involved in aluminum (Al)-induced antioxidant defense, and H2O2 acts upstream of NO in Al tolerance. In the present study, the role of mitogen-activated protein kinase (MAPK) and the relationship between H2O2, NO and MAPK in Al-induced antioxidant defense were investigated in roots of wheat. Pretreatment with MAPK inhibitor decreased the antioxidant enzyme activities in the Al-tolerant genotype Jian-864. H2O2 and NO significantly enhanced antioxidant enzyme activities in both wheat genotypes, while MAPK inhibitor only reversed H2O2 effect, but did not block NO effect. Pretreatment with MAPK inhibitor substantially reduced the H2O2-induced production of NO, also partially blocked the activation of NR activity. These results suggest that Al-induced H2O2 production activates MAPK, which in turn mediates NO generation through NR and results in the up-regulation of the activities of antioxidant enzymes under Al stress.