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.