Increased bound putrescine accumulation contributes to the maintenance of antioxidant enzymes and higher aluminum tolerance in wheat.

The accumulation of bound and conjugated polyamines (PAs) is an important protective trait in plants under adverse environmental conditions. However, their role in plant responses to aluminum (Al) stress remains largely unknown. In this study, we showed that Al treatment increased bound putrescine (Put) levels in the wheat root tips of Al-tolerant Xi Aimai-1, with little effect on its bound spermidine and conjugated PAs or that of Al-sensitive Yangmai-5. Terminating bound Put increments with a synthesis inhibitor (Phenanthroline, o-phen) exacerbated Al-induced root inhibition and callose production. However, it had no significant effect on Al uptake or distribution under Al stress. Instead, Al-induced reactive oxygen species (ROS) production and thus, oxidative damage, was greatly exacerbated by o-phen in the roots of Xi Aimai-1. Application of o-phen barely affected the two ROS generating enzymes (plasma membrane NADPH oxidase and cell wall-bound polyamine oxidase) in wheat roots. However, exogenous o-phen significantly reduced antioxidant enzyme (superoxide dismutase, ascorbate peroxidase, and catalase) activity, which positively correlated with the level of bound Put in Xi Aimai-1. These results clearly suggest that bound Put accumulation works to protect against Al-induced oxidative damage, possibly by maintaining antioxidant capacity in wheat.