Hyperaccumulator

Plant that can grow amid high concentrations of metals From Wikipedia, the free encyclopedia

A hyperaccumulator is a plant capable of growing in soil or water with high concentrations of metals, absorbing them through their roots.[1][2][3] The metals are concentrated at levels that are toxic to closely related species not adapted to growing on the metalliferous soils. Compared to non-hyperaccumulating species, hyperaccumulator roots extract the metal from the soil at a higher rate, transfer it more quickly to their shoots, and store large amounts in leaves and roots.[4] The ability to hyperaccumulate toxic metals compared to related species has been shown to be due to differential gene expression and regulation of the same genes in both plants.[1] Hyperaccumulators are regularly discussed within the context of phytoremediation, although their commercialization remains aspirational.

Viola lutea subsp. calaminaria, also known as the zinc violet, grows in soils high in zinc.

Applications

Cartoon for phytoremediation of metal-contaminated soil (violet and yellow = heavy metals) by a hyperaccumulator. The metal ions are moved from the soil to the leaves of the plant. In the case of phytomining, the leaves would be harvested to recover the valuable metals.

Phytoremediation

Hyperaccumulating plants are of interest in the context of phytoremediation: to detoxify contaminated soils.[1][5] Phytoextraction is a subprocess of phytoremediation in which plants remove metal ions from soil or water. Phytoextraction could in principle be used to remove contaminants from an ecosystem.[6] For example, water hyacinth have been demonstrated to remove arsenic from water.[7] Cadmium accumulation has also received attention as this metal is usually toxic.[8][9][10] Salt-tolerant (moderately halophytic) barley and/or sugar beets are commonly used for the extraction of sodium chloride (common salt) to reclaim fields that were previously flooded by sea water.[citation needed] Caesium-137 and strontium-90 were removed from a pond using sunflowers after the Chernobyl accident.[11]

The remediation of metal-contaminated soils recognizes that metals cannot be degraded, they must be removed. Organic pollutants can be, and are generally the major targets for phytoremediation. Field trials support the feasibility of using plants for environmental cleanup.[12] [13]

Phytomining

Odontarrhena plants are hyperaccumulators of nickel

Phytomining, sometimes called agromining,[14] is the concept of extracting heavy metals from the soil using hyperaccumulating plants.[15] Once the hyperaccumulation has proceeded to some extent, the metals are collected from the plant matter and then refined for sale or disposed of.[16][17][18] Phytomining would, in principle, minimize environmental effects compared to conventional mining. Phytomining could also remove low-grade heavy metals from mine waste.[16] A 2021 review concluded that the commercial viability of phytomining was "limited"[14] because it is a slow and inefficient process. Its purpose is either: (i) gathering the metals for economic use (ii) gathering toxic metals to improve the soil.

Phytomining was proposed in 1983 by Rufus Chaney, a USDA agronomist.[19] He and Alan Baker, a University of Melbourne professor, first tested it in 1996.[19] They, as well as Jay Scott Angle and Yin-Ming Li, filed a patent on the process in 1995 which expired in 2015.[20]

Several startups are investigating the process for mining surface-available heavy metals. In 2025, Genomines received 45 million dollars of Series A funding to commercialize nickel phytomining from mine tailings.[21] The French company Econick and the Albanian company MetalPlant both have nickel phytomining projects. As of mid-2024, MetalPlant had extracted less than a kilo of usable nickel, using Odontarrhena plants.[22]

Physiological advantage for hyperaccumulation

The biological advantage of hyperaccumulation may be that the toxic levels of heavy metals in leaves deter herbivores or increase the toxicity of other anti-herbivory metabolites.[1][23] The plant defense hypothesis, "the elemental defense hypothesis", provided by Poschenrieder, suggested that the expression of these genes assist in antiherbivory or pathogen defenses by making tissues toxic to organisms attempting to feed on that plant.[24] Another hypothesis, "the joint hypothesis", provided by Boyd, suggests that expression of these genes assists in systemic defense.[25] The benefit for a plant to hyperaccumulate may be that root-to-shoot transport system drives hyper-accumulation by creating a metal deficiency response in roots.[26]

Metallophytes

Further information: List of hyperaccumulators
The flowering plant Bidens pilosa of the daisy family

A metallophyte are a subset of hyperaccummulators. Metallaphytes are plants that can tolerate high levels of heavy metals.

450 plant species, including the model organisms Arabidopsis and Brassicaceae, have demonstrated the capacity to uptake and sequester metals such as As, Co, Fe, Cu, Cd, Pb, Hg, Se, Mn, Ni,[27] Zn, and Mo in 100–1000x the concentration found in sister species or populations.[28]

European examples include alpine pennycress (Thlaspi caerulescens),[29] the zinc violet (Viola calaminaria), spring sandwort (Minuartia verna), sea thrift (Armeria maritima), Cochlearia, common bent (Agrostis capillaris), Dover catchfly (Silene paradoxa)[30] and plantain (Plantago lanceolata).[31] Few metallophytes are known from Latin America.[32]

Such plants range between "obligate metallophytes" and "facultative metallophytes."[33] Obligate metallophytes can only survive in the presence of heavy metals while facultative metallophytes can tolerate such conditions but are not confined to them.[33] Metallophytes are sometimes classified as metal indicators, excluders, and hyperaccumulators.[18]

T. caerulescens

As a hyperaccumulator variously of Cd, Pb, and Zn, T. caerulescens, pennycress, has received particular attention. Its leaves accumulate up to 380 mg/kg Cd.[34] On the other hand, the presence of copper seems to impair its growth. It is found mostly in Zn/Pb-rich soils, as well as serpentines and non-mineralized soils.[35] When grown on mildly polluted soils, a closely related species, Thlaspi ochroleucum, is a heavy metal-tolerant plant, but it accumulates much less Zn in the shoots than T. caerulescens. Thus, T. ochroleucum is a non-hyperaccumulator and of the same family T. caerulescens is a hyperaccumulator. The transfer of Zn from roots to shoots varied significantly between these two species. T. caerulescens had much higher shoot/root Zn concentration levels than T. ochroleucum, which always had higher Zn concentrations in the roots. When Zn was withheld, the amount of Zn previously accumulated in the roots in T. caerulescens decreased even more than in T. ochroleucum, with a concomitantly greater rise in the amount of Zn in the shoots. The decreases in Zn in roots may be mostly due to transport to shoots, since the volume of Zn in shoots increased during the same time span.[36]

Further examples by element

Genetic basis of hyperaccumulation

An overexpression of a Zn transporter gene, ZNT1, in root and shoot tissue is an essential component of the Zn hyperaccumulation trait in T. caerulescens.[44] This increased gene expression has been shown to be the basis for increased Zn2+ uptake from the soil in T. caerulescens roots, and it is possible that the same process underpins the enhanced Zn2+ uptake into leaf cells.The proteins are coded by genes in the ZIP family, however other families such as the HMA (heavy metal ATPase[26]), MATE, YSL and MTP families have also been observed to be involved. The ZIP gene family encodes Cd, Mn, Fe and Zn transporters. The ZIP family plays a role in supplying Zn to metalloproteins.[24][45]

In one study on Arabidopsis, it was found that the metallophyte Arabidopsis halleri expressed a member of the ZIP family that was not expressed in a non-metallophyte sister species. This gene was an iron-regulated transporter (IRT-protein) that encoded several primary transporters involved with cellular uptake of cations above the concentration gradient. When this gene was transformed into yeast, hyperaccumulation was observed.[46] This suggests that overexpression of ZIP family genes that encode cation transporters is a characteristic genetic feature of hyperaccumulation. Another gene family that has been observed ubiquitously in hyperaccumulators are the ZTP and ZNT families. A study on T. caerulescens identified the ZTP family as a plant specific family with high sequence similarity to other zinc transporter. Both the ZTP and ZNT families, like the ZIP family, are zinc transporters.[47] It has been observed in hyperaccumulating species, that these genes, specifically ZNT1 and ZNT2 alleles are chronically overexpressed.[48]

AhHMHA3 is expressed in hyperaccumulating individuals. AhHMHA3 has been identified to be expressed in response to and aid of Zn detoxification.[28] In another study, using metallophytic and non-metallophytic Arabidopsis populations, back crosses indicated pleiotropy between Cd and Zn tolerances.[49] This response suggests that plants are unable to detect specific metals, and that hyperaccumulation is likely a result of an overexpressed Zn transportation system.[50]

One of the most well-documented HMAs is HMA4, which belongs to the Zn/Co/Cd/Pb HMA subclass and is localized at xylem parenchyma plasma membranes.[1] HMA4 is upregulated when plants are exposed to high levels of Cd and Zn, but it is downregulated in its non-hyperaccumulating relatives.[51] Also, when the expression of HMA4 is increased there is a correlated increase in the expression of genes belonging to the ZIP (Zinc regulated transporter Iron regulated transporter Proteins) family.

Molecular pathway

Often hyperaccumulation is the result of promiscuous zinc binding, i.e. protein-based sequestrants, transporters, etc with a high affinity for zinc that will bind other metal ions. Metals ions in solution are susceptible to extraction.[52] For example, ligands secreted by plant - phytosiderophores, organic acids, or carboxylates -can selectively binds certain ions.[53][54]

Further reading

  • K.B. Axelsen and M.G. Palmgren, Inventory of the superfamily of P-Type ion pumps in Arabidopsis. Plant Physiol., 126 (1998), pp. 696–706.


The flowering plant Silene paradoxa, or Dover Catchyfly, of the Caryophyllaceae family

See also

References

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