Mechanisms of metal toxicity in plants

Abstract

Metal toxicity in plants is still a global problem for the environment, agriculture and ultimately human health. This review initially addresses the current state of the environmental/agricultural problem, and then discusses in detail the occurrence, mechanisms and relevance of toxicity of selected trace metals (Cd, Cu, Fe, Hg, Ni, and Zn). When discussing the mechanisms, special emphasis is laid on a critical review of their environmental/agricultural relevance, because even now many studies in this field of research are performed under highly artificial lab conditions. The main problems outlined in published studies are artificially high metal concentrations (which never occur even in highly polluted sites) combined with too short treatment times, as well as environmentally and agriculturally irrelevant growth conditions (e.g. constant light and submerged cultivation of seedlings). Furthermore, wherever possible an attempt is made to link the mechanisms published to date in terms of discussing which mechanisms are a direct cause of the observed disturbance of plant function and which are rather a consequence of the primary mechanisms, leading to a complicated toxicity phenotype and ultimately to diminished growth or even death of the plants.

Effects of foliar nickel (Ni) application on mineral nutrition status, urease activity and physiological quality of soybean seeds

“…In this study, the highest copper concentrations in soybean leaves was observed between 40 and 60 g ha -1 of foliar Ni application. Some studies suggest that Ni exerts synergistic effects on copper absorption (Küpper and Andresen, 2016). This synergistic effect was also observed in Alyssum inflatum, a hyperaccumulator species of Ni (Ghasemi et al, 2009;Ghasemi et al, 2015).…”

Modulation of Plant and Fungal Gene Expression Upon Cd Exposure and Symbiosis in Ericoid Mycorrhizal Vaccinium myrtillus

“…Cd can be easily taken up by plant roots thus entering the food chain and causing serious threats to animal and human health (Pinto et al, 2004;Peralta-Videa et al, 2009). Cd adversely affects plant growth and functioning even at low concentrations, with evident symptoms of chlorosis and imbalanced water and nutrients uptake (Küpper and Andresen, 2016). Due to its chemical similarity with trace elements -such as zinc (Zn), iron (Fe), and calcium (Ca) -and its high affinity for protein sulfhydryl groups, Cd can displace divalent cations and interfere with protein functioning.…”

“…Due to its chemical similarity with trace elements -such as zinc (Zn), iron (Fe), and calcium (Ca) -and its high affinity for protein sulfhydryl groups, Cd can displace divalent cations and interfere with protein functioning. Although Cd does not participate directly to the generation of reactive oxygen species (ROS), Cd exposure leads to lipid and protein peroxidation and DNA damage likely because it interferes with the scavenging defense system involved in redox homeostasis (Bertin and Averbeck, 2006;Küpper and Andresen, 2016). Moreover, Cd is mutagenic because it interferes with the cellular mechanisms of DNA damage repair (Giaginis et al, 2006).…”

“…In addition, plants have a range of enzymatic and non-enzymatic scavenging mechanisms to avoid the deleterious effects of Cd within the cell, with the production of active metabolites such as glutathione, metallothioneins, and phytochelatins. These thiol-containing ligands can prevent uncontrolled Cd binding to non-target sites and are likely involved in metal sequestration into the vacuole (Clemens, 2006a;Küpper and Andresen, 2016). On the other hand, enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, peroxidases, and antioxidant secondary metabolites (e.g., ascorbic acid, carotenoids, tocopherols) can buffer increased concentrations of intracellular ROS (Das and Roychoudhury, 2014).…”

Deficiency and toxicity of nanomolar copper in low irradiance—A physiological and metalloproteomic study in the aquatic plant Ceratophyllum demersum

“…This could further influence the fluorescence output. However, all metal toxicity studies we are aware of show a higher degree of damage to PSII compared to PSI, and those showing inhibition of PSI were often carried out using unnaturally high metal concentrations (reviewed by Küpper and Andresen, 2016).…”