METAL TOXICITY; SIMILARITIES IN MECHANISMS AND RESPONSES
In general, metal toxicity has two main effects; firstly, it interferes with nutrient homeostasis and secondly it causes secondary stress particularly in the form of oxidative damage. Some metal(loid)s are nutrients in their own right but may be present at toxic levels. Other metal(loid)s share chemico-physical properties with essential nutrients, and thus affect nutrient homeostasis by disrupting their uptake and distribution. Furthermore, metal(loid)s can affect enzyme activity by competing with physiological metal cofactors. This is particularly the case with elements that have identical charge and similar ionic radius, for example Cd2+ and Ca2+(calcium). The propensity to alter the cellular redox potential, through binding with thiol groups and/or through production of reactive oxygen species, means metal(loid)s are prone to cause oxidative stress. Thus, faced with toxic levels of harmful metals and metalloids plants show great similarities in their responses which are summarised in Figure 1 and further discussed below.
The role of root exudates: Roots extrude H+(protons) and carbon compounds such as organic acids, phenols and phytosiderophores to promote solubility of essential nutrients (Bali et al., 2020). H+ lower the rhizosphere pH which increases soil mineral solubility and hence the release of essential nutrients such as Fe, P (phosphorus), Ca, Mg and S (sulfur), but also the release of toxic metals. Organic exudates have negatively charged reactive groups such as sulfhydryls, carboxyls and hydroxyls with a high affinity for metals. For example, root exuded citrate and malate are known to chelate various metals such as Al (aluminium), Cd and Pb. Exudation levels typically increase in response to metal presence but the details are species related. Thus in sorghum exposure to Cd augmented malate extrusion whereas the same heavy metal induced citrate release in maize roots (Pinto et al., 2008). Phenolics are a further compound that is often released by plant roots in response to heavy metal stress. The hydroxyl group of these aromatic molecules can bind metals (Guo et al., 2016). In general, phenolic and organic acid-metal complexes are not taken up by the plant and hence lower soil toxicity by reducing metal(loid) bioavailability.
Metal(loid) chelation: Two major cellular ligands that prevent the occurrence of metals in ionic (reactive) form are glutathione and phytochelatin (Figure 2). Both contain sulfhydryl groups with high affinity for metal binding (Singh et al., 2016). Phytochelatin is a glutathione oligomer with a structure that consists of multiple γ-glutamate-cysteine moieties, followed by a terminal glycine. The formation of phytochelatins is influenced by the presence of heavy metal(loid)s and under control of the phytochelatin synthase enzyme. Metallothionein is another glutathione-based, cysteine-rich protein that binds metal. Its expression is raised in response to heavy metal stress. Lastly, amino acids like histidine can bind metals and appear particularly important for xylem mediated translocation of metals like Ni and Zn (Kozhevnikova et al., 2014a, Kozhevnikova et al., 2014b).
Vacuolar sequestration: The relative lack of biochemical machinery in the vacuole means it is the preferred cellular storage compartment for toxic metal(loid)s. The transport of metal(loid)s complexed to glutathione or phytochelatins across the tonoplast is carried out by tonoplast ATP-Binding-Cassette (ABC) transporters that are members of the ABCC family (Song et al., 2014). Tonoplast ABC transporters for liganded metal(loid)s are expressed in many tissues. In rice this includes roots, leaves, nodes, peduncles and rachis and ABC proteins are instrumental in the vacuolar sequestration of Cd, As and a range of other metals (Clemens and Ma, 2016). Vacuolar deposition of metals in ionic form also occurs and can be mediated by members of the Cation Diffusion Facilitator family known in plants as MTP (Metal Tolerance Protein) family (Clemens and Ma, 2016). Metal(loid) storage in root vacuoles is also an important factor to prevent large scale translocation of metal(loid)s to above ground tissues.
Alteration of root architecture: Root architecture is sensitive to metal toxicity. This includes biosynthesis of suberin and lignin to strengthen the exo- and endodermic barriers that limit apoplastic transfer (Figure 2). Callose deposition also reduces root metal(loid) permeability. Furthermore, root architecture alteration in the presence of heavy metals often involves the root ‘avoidance response’, i.e. the propensity to either stop growth or grow away from polluted substrates. Root avoidance has been observed in many species for multiple metals including Zn, Ni and Cd (Tognacchini et al., 2020). The opposite response can be found in hyperaccumulators where roots actively forage for metals by increased rooting in soil patches with high metal content (Whiting et al., 2000).
Oxidative stress: Increased levels of heavy metal(loid)s lead to oxidative stress by generating reactive oxygen species (ROS) that can cause lipid oxidation and DNA damage (Noctor et al., 2016). This is countered by increased levels of enzymatic antioxidants like superoxide dismutase, catalase and ascorbate peroxidase, and non-enzymatic antioxidants like tocopherol, glutathione, phenolic compounds and amino acids such as proline. Many studies have shown improved metal(loid) tolerance when the genes and pathways that encode these factors are upregulated (e.g. (Bhaduri and Fulekar, 2012). For example, proline accumulation was observed in rice plants exposed after exposure to Hg stress (Hayat et al., 2012).