Arsenic (As) is an element widely distributed in air, soil, rocks and water. Its presence is due to both the volcanic character of the soil and the erosion of rocks. The highest concentration of this element is found in groundwater. The presence of As in water is mainly due to either natural release of the minerals from the soil (volcanic rocks and iron minerals) or to geothermal activity. The presence of As is also linked to human activities, such as energy production through coal-fired power plants and other fuels derived from fossil fuels, smelters, waste incineration and use of pesticides in agriculture. In the environment As undergoes oxidation, reduction, methylation and demethylation. The oxidation states are: -3, 0, +3 (arsenite) and +5 (arsenate). Commonly As binds to iron, oxygen and sulfur, which form organic and inorganic compounds in different oxidation states. Arsenic is extremely toxic, but the toxicological effects are closely related to the chemical form: inorganic compounds have been identified as the most toxic, followed by organic and finally by arsine gas. The toxicity varies fact, in descending order to the various forms of speciation. For humans the main source of environmental exposure to As is drinking water, where it is present in inorganic form: both as trivalent arsenic As(III) and pentavalent arsenic As(V), but also through the air and food. Arsenic enters the food chain through plant crops, which absorb it through their roots according to its bioavailable levels in soils. Arsenate is transported by roots via phosphate transporters, while arsenite is taken up by a subclass of aquaporins (NIP), some of them also transporting silicon (Si). Methylated forms of As (MetAs) are also taken up by NIP and Si transporters. Inside plants, these types of transporters are also involved in the distribution of As between organs and tissues. However, different forms of As have different mobility efficiencies. Crops exhibit different tendencies to accumulate As in different plant parts in their order: root > stem > leaf . As(V) is enzymatically reduced into As(III) in plant cells by arsenate reductase (AR), leading to the conversion of glutathione (GSH) to its oxidized form (GSSG). Arsenite can be effluxed to the environment by a root Si transporter or methylated. Another pathway of detoxification occurs by the synthesis of phytochelatins (PCs). PC synthesis and their complexation to As(III) are coordinated to the transport of the PC–As(III) complex to the vacuole. For a proper assessment of risk/toxicity of a polluted As soil and to predict its attenuation, after application of remediation techniques, it is crucial to establish the mobility, phytoavailability and biogeochemistry of the toxic element. In this review we describe the mechanisms of transport, metabolism and tolerance that plants show in response to As. Some strategies to reduce As in soil and its transport in plant crops are also summarized.
Keywords: bioavailability, transport, uptake, toxicity, mitigation