Heavy metal-induced reactive oxygen species and cell death in barley root tip

- At metal concentrations evoking mild stress, ROS increased linearly in the root tips.
- Under mild stress, ROS function in adaptive responses without damaging the tissue.
- At high metal doses, causing severe stress, ROS production has a biphasic character.
- Rapid burst of ROS under severe stress caused cell death and root growth arrest.
- After this transient burst, ROS production increased again together with hROS.

Transient exposure of roots to Cd, Pb, Hg or Cu for 30 min reduced root growth in a concentration-dependent manner. While the lower concentrations of metals evoked a marked root growth inhibition accompanied by a visible radial root expansion, the higher concentrations caused root growth arrest without the radial expansion of root cells. At lower metal concentrations, evoking mild stress, the amount of reactive oxygen species (ROS) increased linearly after the treatments. They probably function in both stress signalization and metabolic adaptation processes without damaging the root cells. In turn, at high metal concentrations, causing moderate or severe stress, their production has a biphasic character. The amount of ROS, mainly superoxide, increased rapidly in a metal dose-dependent manner, causing a marked cell death at the site of their generation in the root tips. After this transient burst, ROS generation increased again, together with highly ROS, during the destructive processes associated with cell death in the root tips. Our results show that despite the different origin of ROS induced by these heavy metals, the rate of their generation, depending on the metal concentration, determines the development of typical mild stress-activated morphogenic changes or severe stress-caused cell death within a few minutes after the exposure of roots to toxic level of metals.