L-NAME decreases the amount of nitric oxide and enhances the toxicity of cadmium via superoxide generation in barley root tip

Exposure of barley roots to mM concentrations of L-NAME for 30 min caused a considerable root growth inhibition in a dose-dependent manner. The inhibition of root growth was higher in seedlings co-treated with Cd and L-NAME, compared with roots treated with Cd alone, despite the fact that L-NAME markedly reduced the uptake of Cd by roots. Treatment of roots with L-NAME evoked a decrease in NO level in both control and Cd-treated root tips only after a relatively long lag period, which overlaps with an increase in superoxide and H2O2 levels and peroxynitrite generation. L-NAME-induced root growth inhibition is alleviated not only by the application of the NO donor SNP but also by the ROS and peroxynitrite scavengers. Our results indicate that L-NAME, a NOS inhibitor in the animal kingdom, indeed evokes NO depletion also in the plant tissues; however, it does not occur due to the action of L-NAME as an inhibitor of NOS or NOS-like activity, but as a consequence of L-NAME-induced enhanced superoxide generation, leading to increased peroxynitrite level in the root tips due to the reaction between superoxide and NO.