INTRODUCTION
Metals and metalloids are ubiquitous in the environment and mainly originate from weathering of the earth’s parental rock material. Some metals play essential roles in all forms of life because they are critical for protein structure and functioning (Rosenzweig, 2002, Frausto da Silva JJR, 2001). Indeed, around 30% of all proteins found in a cell require metal cofactors to be able to carry out vital processes such as respiration, photosynthesis, oxygen transport, nucleic acid synthesis and many others. Consequently, deficiencies of essential metals can cause severe deleterious effects in all organisms. The latter include humans many of whom suffer deficiencies for metals such as Fe (iron) and Zn (zinc) (WHO, 2013, Prasad, 2014, FAO, 2015, GNR, 2020). In contrast, non-essential metal(loids) like As (arsenic), Pb (lead), Hg (mercury), and Cd (cadmium) are also ubiquitous in many environments. These are ranked as highly toxic substances for humans according to the Agency for toxic substances and disease (ATDSR, 2019). Uptake of these elements can be via skin contact or by breathing in fumes in the case of Hg or Pb, but ingestion through contaminated diets is the main mechanism. In particular, consumption of plants, shellfish and fish are important routes for human intake of toxic elements such as As, Cd, Hg and Pb. Some of the afflictions associated with metal(loid) poisoning are neurological impairment of the central nervous system (As, Pb, Hg), coronary and vascular disease leading to cardiac arrest (As, Cd, Hg, Pb), osteoporosis (Cd), renal failure (Cd, Hg), respiratory failure and gastrointestinal lesions (Hg), and cancer (As, Cd) (Chaney, 2015; )see (Renu et al., 2021) for review).
In plants, essential micronutrients include the metals Co (cobalt), Cu (copper), Fe (iron), Mn (manganese), Mo (molybdenum), Ni (nickel) and Zn, and the metalloid B (boron). In addition, the metalloid Si (silicon) has growth promoting effects in many species (Thorne et al., 2020). Other elements are frequently encountered which have no biological function, typically become toxic at low soil concentrations, and have a negative impact on plant growth and development. These include metals such as Cd, Cr (chromium), Cs (cesium), Hg, Pb and metalloids like As and Sb (antimony), all of which can occur at toxic levels due to natural processes and anthropogenic activities. Examples of natural activities include weathering of ultramafic rocks that leads to the formation of serpentine soils that contain high concentrations of heavy metals like Cr, Ni, Co and Fe (Chiarucci and Baker, 2007, Tashakor et al., 2013) or the leaching of As from igneous rock that leads to contamination of aquifers and groundwater.
Post industrial revolution human activities have become an important vehicle for the dissemination of metals in the environment through the use of pesticides and herbicides, mining, smelting, waste disposal, application of inorganic fertiliser and, more recently, use of nanoparticles (Bundschuh et al., 2018, Wilson, 2018). These undertakings have led to wide scale contamination of agricultural soils: For example, it is estimated that more than 6% (~137,000 km2) of the agricultural soils in Europe are in need of remediation because of metal(loid) contamination (Tóth et al., 2016). Agriculture itself has greatly contributed to this problem; many herbicides, pesticides and preservatives contain Cu, Hg, Mn, Pb, Zn or As, such as copper sulphate and lead arsenate used in fruit orchards, arsenic compounds used for banana pest control and cotton defoliation (Wuana and Okieimen, 2011), or the application of wood preservative to fence posts (McLaughlin et al., 2000). Repeated application of inorganic phosphate fertiliser can significantly increase As deposition while treating soils with sewage, manure and other biosolids is another mechanism that causes accumulation of Cd, Cr, Cu, Ni and Zn (Basta et al., 2005, Keller et al., 2002).
Metal(loid) pollution is a global and increasing menace that negatively affects plant growth and development. In an agronomic context, this leads to large yield losses (e.g. (Kotecha et al., 2019) and it is therefore imperative to develop crops with better metal(loid) tolerance. In this paper we will summarise some of the generic responses of plants to toxic heavy metal(loids), using arsenic, cadmium, mercury and zinc to respectively represent metalloids, non-volatile heavy metals, volatile heavy metals, and metallic micronutrients that become toxic at high levels. We will also highlight the specific response mechanisms that differentiate these elements. Based on these analyses, we will critically evaluate scenarios that are likely to contribute to crop metal(loid) tolerance with a focus on the role of roots and the rhizosphere in metal(loid) detoxification.