Enhanced nitric oxide generation in root transition zone during the early stage of cadmium stress is required for maintaining root growth in barley

The objective of this study was to analyse the role of nitric oxide (NO) in relation to reactive oxygen species (ROS) production and root growth inhibition in the early response of barley root tip to cadmium. We used cell-permeable NO-binding dye DAF-2DA for the NO localization and cell-impermeable dye DAR-4M for the quantification of NO in barley root apex. A considerable increase in NO production was detected in the transition zone of root tips already 30 min after the immersion of barley roots into 15 mu M Cd. This Cd-induced NO generation further increased during the recovery period and reached its maximum level 2 h after the short-term treatment. At this stage an enhanced Cd-induced NO generation was observed in the meristematic, transition and in the beginning of elongation zone in comparison with control roots. Inhibitor of NO synthase activity markedly reduced root growth in control seedlings and had a synergistic growth inhibitory effect with Cd. On the contrary, this Cd-induced root growth inhibition was markedly alleviated by the NO donor. Exposure of roots to Cd markedly increased the production of ROS in the proximal elongation zone of root. While NO did not affect ROS generation, inhibition of its synthesis slightly decreased ROS generation in both control and Cd-treated roots. Our results revealed that enhanced NO production is a very early response of barley root to Cd stress and it is involved in the regulation of root growth mitigating Cd-induced root growth inhibition.