Reactive oxygen species and heavy metal stress in plants: Impact on the cell wall and secondary metabolism

The production of reactive oxygen species (ROS) in plants is part of the normal metabolism of chloroplasts, mitochondria and peroxisomes; however, the exposure to environmental constraints, like toxic concentrations of heavy metals, can overwhelm the systems protecting the plants and result in oxidative stress. The formation of ROS is also an event accompanying normal physiological processes, namely pollen tube growth, cell wall loosening during cell expansion, plant fibre growth, lignification, organ senescence and fruit ripening. Since reviews have already been published on broad molecular aspects underlying the production of ROS in plants as a consequence of heavy metal stress, we will here adopt a narrower angle by focusing on two specific aspects: 1) the effects of heavy metal-induced ROS on plant cell wall-related processes and 2) the stimulatory/inhibitory effects of ROS on plant secondary metabolism. We will highlight the role of ROS in important physiological processes, namely fruit maturation, bast fibre intrusive growth (where a model is proposed), pollen tube growth and as regulators of senescence. We will end our survey with an outlook on the importance of deciphering the signaling cascade underlying ROS production in response to heavy metal stress through specific comparisons. In particular, we will hint at comparative studies on 1) ancient local varieties (which often display enhanced resistance to environmental constraints, as well as high secondary metabolite contents) and commercial counterparts, 2) hyperaccumulators and normal plants from the same species. Such studies will enable a better understanding of the impact of ROS on physiological processes, namely the control of plant cell size and organs, as well as on processes of industrial interest, i.e. the production of secondary metabolites.