Alterations of the gene expression, lipid peroxidation, proline and thiol content along the barley root exposed to cadmium

Barley seedlings grown on filter paper moistened with 1 mM Cd showed 50% root growth inhibition within 24 h of exposure. The amount of cadmium after 24 h Cd treatment was highest in the first 2 mm-long apical root segment, while it was slightly higher in the fourth segment, 6–8 mm behind the root tip, after 48 h. In recovery experiments, when Cd-treated plants were transferred onto filter paper moistened with distilled water, a large amount of Cd was localised in the apoplast and considerable cell death was detected even though root growth was renewed. This indicates that cell death is likely an active physiological process that contributes to the removal of Cd from the root during root growth recovery. Elevated lipid peroxidation and thiol contents were detected in all individual segments of Cd-treated barley root. On the other hand, proline accumulation was disturbed during Cd stress, showing a significant decrease in all of the studied segments except the first. Cd-induced alteration in the expression of genes involved in metal signalling and detoxification and in drought and oxidative stress responses indicates that Cd-induced water and oxidative stress is responsible for the root growth inhibition, probably through an accelerated differentiation of root tissues.