Nitric oxide acts downstream of hydrogen peroxide in regulating aluminum-induced antioxidant defense that enhances aluminum resistance in wheat seedlings

In response to aluminum (Al) stress, various plants increase their antioxidant defense system. In wheat, Al tolerance is a complex phenomenon, and whether this Al-induced response eventually enhances the ability of wheat to tolerate Al or not is still unclear and evidence is also scarce about the signals triggering it. After Al exposure, an increase in the intracellular concentration of H2O2 occurred within 2 h in tolerant genotype, but was not observed in sensitive genotype. Stimulating rapid H2O2 accumulation in the root tips of Yang-5 with H2O2 donor relieved Al-induced root inhibition. In addition, pretreatment with H2O2 donor enhanced the activity of nitrate reductase (NR), which, in turn, induced accumulation of endogenous NO in both genotypes; whereas H2O2 scavenger or inhibitor blocked NO burst in roots of tolerant genotype. Conversely, Al-induced transient H2O2 generation was not sensitive to NO scavenger. Scavenging or inhibition of NO burst aggravated H2O2-relieved Al-induced root inhibition as well as oxidative damage. Elevated antioxidant enzyme activities induced by H2O2 donor were also suppressed by NO scavenger or NR inhibitor. These results suggest a novel signaling pathway where Al-induced transient H2O2 production mediates NO burst in regulating Al-induced antioxidant defense that enhances Al resistance in wheat seedlings.