Impact of the auxin signaling inhibitor p-chlorophenoxyisobutyric acid on short-term Cd-induced hydrogen peroxide production and growth response in barley root tip

Short-term treatment (30 min) of barley roots with a low 10 μM Cd concentration induced significant H2O2 production in the elongation and differentiation zone of the root tip 3 h after treatment. This elevated H2O2 production was accompanied by root growth inhibition and probably invoked root swelling in the elongation zone of the root tip. By contrast, a high 60 μM Cd concentration induced robust H2O2 production in the elongation zone of the root tip already 1 h after short-term treatment. This robust H2O2 generation caused extensive cell death 6 h after short-term treatment. Similarly to low Cd concentration, exogenously applied H2O2 caused marked root growth inhibition, which at lower H2O2 concentration was accompanied by root swelling. The auxin signaling inhibitor p-chlorophenoxyisobutyric acid effectively inhibited 10 μM Cd-induced root growth inhibition, H2O2 production and root swelling, but was ineffective in the alleviation of 60 μM Cd-induced root growth inhibition and H2O2 production. Our results demonstrated that Cd-induced mild oxidative stress caused root growth inhibition, likely trough the rapid reorientation of cell growth in which a crucial role was played by IAA signaling in the root tip. Strong oxidative stress induced by high Cd concentration caused extensive cell death in the elongation zone of the root tip, resulting in the cessation of root growth or even in root death.