Arsenic metabolism in plants: an inside story
This article is corrected by:
- Errata: Corrigendum Volume 170, Issue 1, 195, Article first published online: 17 January 2006
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From murder plots to environmental disasters, arsenic (As) has always been in the headlines. As-contaminated soils, sediments and water supplies are major sources of food chain contamination and thereby endanger human health; this is a global problem, but the situation is nowhere worse than in India and Bangladesh, where more than 400 million people are affected by As poisoning in drinking water (Chakraborti et al., 2003). Inorganic species of As, arsenate (AsO4−3, referred to as AsV) and arsenite (AsO4−3, referred to as AsIII), are carcinogenic and have been shown to cause cancer of the lung, liver and kidney and to cause skin pigmentation. Plants too are affected by As; it is a nonessential element and, in general, inorganic As species are phytotoxic. However, some plant species such as Pteris vittata– a Chinese brake fern – have been shown to accumulate high levels of As and hence offer a viable opportunity for the remediation of contaminated environments (Ma et al., 2001). To capitalize on these unique remediation capabilities of plants, it is imperative that we understand the mechanisms by which As tolerance and hyperaccumulation is achieved. We are just at the beginnings of unravelling this story, but a study by Raab et al. in this issue (pp. 551–558) provides an important step forward in understanding the mechanistic details of As detoxification in plants.
‘It is believed that plants trap arsenite below ground to prevent access to above-ground reproductive tissues in order to prevent possible mutagenic consequences’
Mechanism of arsenic uptake and detoxification in plants
Although largely unknown in plants, the mechanisms of As detoxification have been well characterized in bacteria and yeast, which commonly achieve tolerance to As by the reduction of AsV to AsIII by arsenate reductase enzymes, and then the exclusion of toxic oxyanions AsIII from the cell by inducible and selective transporters (Mukhopadhyay et al., 2002; Rosen, 2002). To date, no functional orthologs of these microbial arsenate reductases and AsIII transporters have been identified in plants and thus there is currently no evidence to suggest that plants use these same mechanisms. Recently, however, in addition to the natural As-hyperaccumulating Chinese brake fern, several plants with increased As tolerance have been identified (Meharg & Hartley-Whitaker, 2002). Although the molecular mechanisms of As detoxification and tolerance remain to be fully determined, it has been shown that plants detoxify As by reducing AsV to AsIII (Pickering et al., 2000; Dhankher et al., 2002), which is subsequently detoxified via forming complexes with thiol-reactive peptides such as γ-glutamylcysteine (γ-EC), glutathione (GSH) and phytochelatins (PCs) (Pickering et al., 2000; Hartley-Whitaker et al., 2001; Dhankher et al., 2002; Li et al., 2004). These AsIII-thiol complexes are then suggested to be sequestered into vacuoles by glutathione-conjugating pumps (GCPs) (Dhankher et al., 2002; Wang et al., 2002), although direct evidence of this remains to be proven. In this issue, Raab et al. add new insights into the mechanistic details of As detoxification in plants; they report an extensive study of time-dependent formation of various arsenite–PC complexes in the roots, stems and leaves of sunflower (Helianthus annuus) in response to As exposure.
Multiple arsenic species: multiple tolerance mechanisms?
In As-nontolerant sunflower (H. annuus), Raab et al. use a sophisticated technique to show the formation of 14 different As species, including some that form complexes with arsenite (AsIII–PC3, GS–AsIII–PC2, AsIII–(PC2)2) and newly identified monomethylarsonic–PC2 (MAIII–PC2) in response to As exposure. Previously, Hartley-Whitaker et al. (2001) demonstrated that the As-tolerant nonhyperaccumulator Holcus lanatus contained PC2 in an As-tolerant clone and PC3 in nontolerant clone as dominant species. As tolerance in H. lanatus was found to result via suppression of the high-affinity phosphate/arsenate uptake system (Meharg & Macnair, 1992), which decreases the arsenate influx into plant roots (Hartley-Whitaker et al., 2001). Similarly, in the As hyperaccumulator P. vittata, PC2 and PC3 are reported as the major phytochelatin species induced in response to As exposure (Zhao et al., 2003). P. vittata differs from H. lanatus and sunflower, in that a large amount of As is translocated and stored in above-ground tissues and less As is retained in the rhizome (Ma et al., 2001; Wang et al., 2002). Furthermore, most of the As translocated in the frond is in the form of unbound AsIII. Clearly, there is much to unravel here.
Raab et al. reported two important findings. First, the formation of monomethyl As species complexed with PC2 (MAIII–PC2) in sunflower. Metabolism of AsV to organic As species such as dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) has also been observed in phytoplankton, macrophages (Phillips, 1990) and at low concentration in some terrestrial plant species (Koch et al., 2000). These methylated forms of As are then metabolized to organophospholipids and arsenosugars such as arsenobetaine (Phillips, 1990). These findings, taken together with the new results of Raab et al., indicate the existence of an alternative mechanism of As detoxification in plants that warrant further investigation. Furthermore, Raab et al. revealed several As complexes, which are yet to be identified and hence point towards further experimental work in order to elucidate the biological complexity of As detoxification.
Arsenic: from roots to shoots
The second important finding was the presence of unbound AsV and AsIII and the absence of As–PC complexes in the sap harvested from sunflower plants exposed to arsenate. Similar results were reported in Indian mustard (Brassica juncea) by Pickering et al. (2000), in which they identified unbound AsV and AsIII species in xylem sap. Raab et al. found only PC2 and GSH as the main thiol species in sunflower sap, and PC3 was absent. They concluded that PC2 and GSH can undergo long-distance transport in H. annuus, which is in accordance with earlier results from transgenic Arabidopsis thaliana lines overexpressing the wheat phytochelatin synthase gene, TaPCS1 (Gong et al., 2003). This led Raab et al. to postulate that AsV and AsIII are the main species of As that are translocated from roots to shoot tissues via the xylem and not the AsIII–PC complexes. However, it should be considered that the results by Gong et al. (2003) indicate that, in transgenic Arabidopsis, long-distance Cd2+ transport is PC-dependent, and therefore further work is required to substantiate these findings.
Previous studies in B. juncea and Arabidopsis (Pickering et al., 2000; Dhankher et al., 2002), together with the study of Raab et al. in sunflower, showed that a major fraction of the AsV taken up by plants was retained in roots and that AsV was further reduced to AsIII by endogenous arsenate reductase. Furthermore, most of the AsV in roots was in the form of arsenite–thiol complexes. This suggest that plants have an adaptive mechanism, and it is believed that plants trap arsenite below ground in order to prevent access to above-ground reproductive tissues to prevent possible mutagenic consequences. Although several studies suggested the reduction of AsV to AsIII by endogenous arsenate reductases inside plant cells (Pickering et al., 2000; Dhankher et al., 2002), until very recently no enzymes had been identified from higher plants. We have now identified a gene encoding a putative endogenous arsenate reductase from A. thaliana that reduces AsV to AsIII in plants (O. P. Dhankher & R. B. Meagher, unpublished). The inactivation of this putative arsenate reductase by RNA interference (RNAi) in Arabidopsis enhanced the long distance translocation of As from roots to shoot tissues and thus caused a 10- to 15-fold increase in accumulation of As in above-ground tissues. Duan et al. (2005) also recently reported the presence of an arsenate reductase activity from a root extract of P. vittata that reduces AsV to AsIII in in vitro assays.
In plants, the mechanism of AsIII uptake and further translocation of AsIII from roots to shoots also remains to be elucidated. There is strong evidence that AsO4−3 and phosphate (PO4−3) are taken up by the same transporters in plant roots (Meharg & Macnair, 1992; Wang et al., 2002), but it is not known how and what form of As is translocated from roots to shoots. Compared to AsV and PO4−3, whose chemical properties are very similar, AsIII is quite different. Only the translocation of AsV would therefore be expected to occur via the PO4−3 translocation pathway. Both species of AsV and AsIII were found in xylem sap from stems of B. juncea (Pickering et al., 2000) and sunflower (Raab et al.); however, it is not known whether both species were actually loaded in the xylem sap or occurred as a result of the reduction and oxidation of As species during translocation in the xylem sap. In another study, Quaghebeur & Rengel (2004) suggested that AsIII is the main As species translocated from roots to shoots in A. thaliana. These preliminary studies suggest that plants may have AsIII transporters in roots that translocate AsIII from roots to shoot tissues.
Wider perspectives: phytoremediation
The need to develop efficient strategies for cleaning As-polluted soil and water and also to reduce uptake of As in food crops to minimize the risk of As contamination through the food chain is clear. Physical remediation methods are highly expensive and not practical at the scales required. Phytoremediation, a plant-based technology, however, holds great promise for the purification of contaminated soil and water (Meagher, 2000). For example, the natural As-hyperaccumulating fern P. vittata can be used to clean contaminated soils. However, the mechanism of As hyperaccumulation is not understood and this fern is restricted in growth to the tropics of the southern hemisphere and may not be highly effective in temperate climatic conditions. The uptake and hyperaccumulation capacity of plants can be significantly enhanced by genetic engineering; however, the progress towards developing such genetics-based strategies has been hindered by a lack of thorough understanding of the basic molecular and biochemical mechanisms of As uptake and detoxification in plants. Previously, we developed transgenic plants by overexpressing two bacterial genes, arsenate reductase (ArsC) and γ-glutamylecysteine synthetase (γ-ECS), in Arabidopsis (Dhankher et al., 2002). These plants were super-resistant to arsenate and accumulated a substantially high amount of As in above-ground tissues. Although the potentials of natural as well as genetically modified As hyperaccumulators have raised hopes of reducing As toxicity of water and soil, attention should be focused on engineering high-biomass, fast-growing nonfood plants for soil remediation and aquatic plants for water remediation.