Arsenic
Arsenic is a toxic metalloid. It is the 20th most abundant element in the Earth’s crust and universally present in the environment. It has a number of therapeutic applications but is generally toxic to all living organisms and classed as a carcinogen by the WHO. Acute arsenic poisoning is relatively rare but chronic poisoning is widespread with over 200 million humans running the risk of arsenic poisoning. The latter is particularly an issue in South-east Asia where the local geology creates considerable arsenic pollution of ground and surface waters.
Arsenic primarily occurs as the inorganic trivalent arsenite (AsIII) and pentavalent arsenate (AsV). Low levels of organic As species also occur such as methylated or dimethylated arsinic acid. The latter are generally considered to be less toxic and therefore are not further considered in this paper. AsV is chemically similar to phosphate (Pi) and causes toxicity by disturbing oxidative phosphorylation, ATP synthesis and a host of metabolic processes that rely on Pi biochemistry. AsIII toxicity is much greater than that of AsV. AsIII in solution takes the form of neutral arsenous acid, and has a similar size to that of essential nutrients such as boric acid and silicic acid. AsIII has a strong propensity to bind sulfhydryl groups and consequently affects general aspects of protein functioning by interfering with secondary, tertiary and quaternary protein structure and with protein-protein interactions.
Although As contamination often derives from geological sources such as marine sedimentary rocks, it is also widely disseminated via human activity such as combustion of coal, the use of pesticides and herbicides, mining or the application of (phosphate) fertiliser. The often encountered contamination of irrigation water inevitably leads to crop exposure to arsenic which depresses plant growth and crop yields.
Plants readily take up inorganic As and a substantial proportion of it can be translocated to edible parts such as the grain of cereals (Zhao et al., 2009, Meharg and Rahman, 2003). Dietary intake of As is particularly relevant in the case of rice (Tripathi et al., 2007).
Processes in the rhizosphere related to arsenic toxicity As stated above, As in the soil is primarily present in the form of inorganic AsIII (as neutral AsOH3) and AsV (as AsO42-). The AsIII-AsV balance depends on the redox potential; for example, paddy rice is cultivated in flooded conditions which are typically reductive and hence rice is exposed predominantly to AsIII. In contrast, aerated soils tend to be oxidative and show a prevalence of AsV.
Soil redox potential is affected by and greatly impacts on the rhizosphere microbial communities. Considerable research has been carried out to assess the role of mycorrhizas in plant As tolerance. Mycorrhizas have been found to promote plant growth in As rich substrates. Mechanistically this may be indirect, i.e. based on their capability to contribute to plant Pi. Adequate Pi supply typically leads to downregulation of Pi uptake systems and consequently reduced As accumulation since AsV is taken up by the same machinery. For example in soybean, establishment of arbuscular mycorrhizas not only significantly promoted plant growth, it greatly reduced the concentration of tissue As in both shoots and roots. Other work hints at a general reduction by mycorrhization of plant oxidative stress (e.g. (Spagnoletti et al., 2016) but how this would work mechanistically is unclear. Yet other work points to the impact of mycorrhizas on transcript level of Pi transporters and metallothioneins (Li et al., 2018). The latter may help lowering As translocation from root to shoot. However, caution should be applied when interpreting these findings; many of these studies find positive effects of mycorrhization in control conditions, which makes it difficult to assess the exact role of mycorrhizas in plant As tolerance.
In both aerobic and anaerobic conditions, As speciation and mobility are also greatly influenced by the bacterial species that populate the rhizosphere. Both arsenite oxidising and arsenate reducing bacteria influence the chemical form and bioavailability of As. Since AsIII is considerably more toxic to plants than AsV, AsV reduction can greatly increase overall As toxicity whereas on the other hand As oxidising bacteria could play a protective role towards plants. Like plants, many rhizosphere fungi and bacteria produce chelates that remove considerable amounts of soil As from the bioavailable fractions (Nair et al., 2007). Excreted (polymeric) substances such as polysaccharides, glycoproteins, lipopolysaccharides, soluble peptides and organic acids are strong metal(loid) chelators that reduce bioavailability. Furthermore, ligands in the form of metallothioneins and phytochelatins allow microorganism to detoxify and store metal(loid) either as a cellular fraction or, in the case of fungi, via deposition in the vacuoles.
Aquatic plants release oxygen from their roots that catalyses the conversion of the relatively soluble Fe2+ to the insoluble Fe3+. The latter causes precipitation of iron-oxides and iron-hydroxides at the root surface creating a brownish plaque that has high affinity for metals and metalloids such as As. Thus, iron plaques can immobilise the otherwise mobile As fraction in soils and as such lower As bioavailability by ~5-10% (Maisch et al., 2020).
Iron plaque formation is partially controlled by the amount of oxygen release by roots but also by the microbial community; in the presence of Fe3+-reducing bacteria a large proportion of the plaque can be dissolved, remobilising plaque metal(loid)s (Maisch et al., 2020). However, AsIII-oxidising bacteria normally form the majority of iron plaque microbes and they promote precipitation and immobilisation of As (Hu et al., 2015, Yu et al., 2017).
Arsenic uptake and distribution The molecular mechanism of As uptake at the soil:root boundary depends on the chemical form of As. AsV enters plants primarily via (active) Pi transporters (Figure 3) that are energised via coupling to the proton gradient. Consequently, As tolerance is closely associated with Pi uptake capacity and loss of function in Pi transporters typically improves As tolerance (Remy et al., 2012). Uptake of AsIII is passive and primarily mediated by NIPs (nodulin-26-like intrinsic proteins) a class of aquaporins (Lindsay and Maathuis, 2017) (Figure 3). Other members of the aquaporin family (e.g. plasma membrane intrinsic proteins or PIPs and tonoplast intrinsic proteins or TIPs) may also contribute but are likely to play a minor role (e.g. (Mosa et al., 2012). AsIII uptake is sustained by active efflux towards the stele through efflux transporters such as Lsi2 which are anion permeases from the ArsB/NhaD superfamily. In rice, Lsi2 is localised to the proximal side of the exo- and endodermal cells (Ma et al., 2008) in the root. Loss of function in NIPs and/or in Lsi2-type transporters results in markedly reduced uptake of As (Ma et al., 2008). As efflux in roots is considerable (Ali et al., 2012, Duan et al., 2012) but the underlying mechanism is unknown.
Arsenic concentrations in root tissues tend to be 5 to 20-fold higher than those found in shoots, (e.g. (Lindsay and Maathuis, 2016)). The identity of the proteins that are responsible for this long distance movement of As are unknown though specific members of the Pi transporter family and specific NIPs probably play a role. The observation that phloem often contains significant amounts of As suggests that net deposition of As in the shoot could be lowered by recirculation via the phloem. Phloem mediated As distribution is also of importance in the context of As partitioning to edible parts of the plant (Lindsay and Maathuis, 2017).
Cellular detoxification of arsenic Cellular detoxification of As is complex. It generally comprises reduction, chelation and sequestration. In most organisms, including plants grown in aerobic conditions, the initial response is the reduction of AsV to AsIII (Figure 3). Consequently, cellular arsenic is predominantly found as AsIII in plants. A number of arsenate reductases has been reported in the literature (e.g. (Xu et al., 2017). These enzymes use glutathione as reductant and are often encoded by ATQ1/HAC1 . ATQ1 is expressed predominantly in the root hairs, epidermal cells and the stele (Chao et al., 2014) and loss of function mutations in ATQ/HAC genes leads to increased sensitivity to AsV but not AsIII. Surprisingly, ATQ/HAC proteins not only influence the cellular AsIII/AsV ratio but also have a remarkable effect on root As efflux and As translocation to the shoot. Reduction of AsV to AsIII is a precondition for chelation of AsIII by glutathione and phytochelatins. Liganded AsIII complexes are then stored in root vacuoles which further neutralises the arsenic species in terms of potential cellular harm.