Spermidine-mediated hydrogen peroxide signaling enhances the antioxidant capacity of salt-stressed cucumber roots

Hydrogen peroxide (H2O2) is a key signaling molecule that mediates a variety of physiological processes and defense responses against abiotic stress in higher plants. In this study, our aims are to clarify the role of H2O2 accumulation induced by the exogenous application of spermidine (Spd) to cucumber (Cucumis sativus) seedlings in regulating the antioxidant capacity of roots under salt stress. The results showed that Spd caused a significant increase in endogenous polyamines and H2O2 levels, and peaked at 2 h after salt stress. Spd-induced H2O2 accumulation was blocked under salt stress by pretreatment with a H2O2 scavenger and respective inhibitors of cell wall peroxidase (CWPOD; EC: 1.11.1.7), polyamine oxidase (PAO; EC: 1.5.3.11) and NADPH oxidase (NOX; EC: 1.6.3.1); among these three inhibitors, the largest decrease was found in response to the addition of the inhibitor of polyamine oxidase. In addition, we observed that exogenous Spd could increase the activities of the enzymes superoxide dismutase (SOD; EC: 1.15.1.1), peroxidase (POD; EC: 1.11.1.7) and catalase (CAT; EC: 1.11.1.6) as well as the expression of their genes in salt-stressed roots, and the effects were inhibited by H2O2 scavengers and polyamine oxidase inhibitors. These results suggested that, by regulating endogenous PAs-mediated H2O2 signaling in roots, Spd could enhance antioxidant enzyme activities and reduce oxidative damage; the main source of H2O2 was polyamine oxidation, which was associated with improved tolerance and root growth recovery of cucumber under salt stress.