A
hyperaccumulator is a category of metallophyte that is capable of growing in soil or water with a higher concentration of
metals, absorbing the metals through its roots and storing it in its foliage. 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. 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.
Zinc (Zn), and
Molybdenum (Mo) in 100–1000 times the concentration found in sister species or populations.
Phytoextraction is a subprocess of
phytoremediation in which plants remove metal ions from soil or water. For example,
water hyacinth have been demonstrated to remove arsenic from water. Cadmium accumulation has also received attention as this metal is usually toxic.
Caesium-137 and
strontium-90 were removed from a pond using
sunflowers after the
Chernobyl accident. 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.
Phytomining '' plants are hyperaccumulators of nickel
Phytomining, sometimes called
agromining, is the concept of extracting heavy metals from the soil using hyperaccumulating plants. Once the hyperaccumulation has proceeded to some extent, the metals are collected from the plant matter and then refined for sale or disposed of. Phytomining typically follows three steps: 1) Phytoextraction, where metals are sequestered from soil into plants; 2) Enrichment, where plant biomass is eliminated and heavy metals are enriched as solids; 3) Extraction, where the solid remains are processed into more accessible forms. Phytomining would, in principle, minimize
environmental effects compared to conventional mining. Phytomining could also remove low-grade heavy metals from mine waste. He and Alan Baker, a
University of Melbourne professor, first tested it in 1996. 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. 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.
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. 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. 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.
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. 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. 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.
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. 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 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. 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. It has been observed in hyperaccumulating species, that these genes, specifically ZNT1 and ZNT2 alleles are chronically overexpressed. AhHMHA3 is expressed in hyperaccumulating individuals. AhHMHA3 has been identified to be expressed in response to and aid of Zn detoxification. In another study, using metallophytic and non-metallophytic
Arabidopsis populations, back crosses indicated
pleiotropy between Cd and Zn tolerances. This response suggests that plants are unable to detect specific metals, and that hyperaccumulation is likely a result of an overexpressed Zn transportation system. 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. 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.
Genetic Engineering of Hyperaccumulators Genetic engineering has been used to research potential improvements towards hyperaccumulation efficiency and species resistance to biological side effects of metal uptake. Methods have included engineering overexpression of pollutant degrading enzymes or proteins associated with heavy metal transportation pathways, and transgenesis, where genes from hyperaccumulators are inserted into the genome of other hyperaccumulators to target specific metals or metals previously inaccessible to that species. For example,
Sedum plumbizinicicola is a hyperaccumulator of Cd using the heavy metal transporter genes SpHMA2, SpHMA3, and SpNramp6. In 2023, Yang et al. inserted these genes into
Brassica napus, or
Rapeseed plants, resulting in high uptake efficiency and sequestration of Cd compared to the wild-type rapeseed. Transgenic phytoextractors theoretically function to combine favorable traits like high biomass production with hyperaccumulation, showing the potential to improve the speed of phytoremediation. However, research reports often do not include long term data of artificial phytoextraction by transgenic plants to see if they can actually survive their entire life cycle intaking hyperaccumulator-levels of contaminants. Site implementation of transgenic plants for phytoremediation is also controversial, due to how these plants could negatively impact native biodiversity.
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. For example, ligands secreted by plant - phytosiderophores,
organic acids, or
carboxylates -can selectively binds certain ions. == Metal Excluders ==