Reactive oxygen and nitrogen species as key indicators of plant responses to Cd stress

Although cadmium (Cd), an extremely toxic non-essential heavy metal, has no biological function, it is capable of entering plant roots. Cd not only presents a problem for plants, which have developed specific Cd detection, transport and detoxification mechanisms, but also for humans as it can enter the food chain. After entering the root, Cd can be loaded into the xylem and then into the leaves and fruits, thus constituting a major environmental and health hazard worldwide. Understanding the mechanisms involved in plant responses to Cd stress would facilitate the production of crops with a lower Cd uptake and accumulation capacity as well as plants with greater Cd uptake potential for phytoremediation. One of the most common symptoms of Cd toxicity is the induction of oxidative stress in plants, which have developed various strategies to avoid this toxicity, including the early production of reactive oxygen and nitrogen species (ROS and RNS) with signalling functions. This review focuses on the dual role of ROS and RNS in plant responses to Cd stress: in low concentrations, as signalling molecules capable of orchestrating plant responses on the one hand, and at higher concentrations, as oxidative stress inducers on the other.