Hyperaccumulator

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Viola lutea subsp. calaminaria, also known as the zinc violet, grows in soils high in zinc. Gelbes Galmeiveilchen (viola lutea ssp. calaminaria).jpg
Viola lutea subsp. calaminaria , also known as the zinc violet, grows in soils high in zinc.

A hyperaccumulator is a plant capable of growing in soil or water with high concentrations of metals, absorbing these metals through their roots, and concentrating extremely high levels of metals in their tissues. [1] [2] 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. [1] [3] 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]

Contents

Hyperaccumulating plants are of interest for their ability to extract metals from the soils of contaminated sites (phytoremediation) to return the ecosystem to a less toxic state. The plants also hold potential to be used to mine metals from soils with very high concentrations (phytomining) by growing the plants, then harvesting them for the metals in their tissues. [1]

The genetic advantage of hyperaccumulation of metals may be that the toxic levels of heavy metals in leaves deter herbivores or increase the toxicity of other anti-herbivory metabolites. [1]

Physiological basis

Metals are predominantly accumulated in the roots causing an unbalanced shoot to root ratio of metal concentrations in most plants. However, in hyperaccumulators, the shoot to root ratio of metal concentrations are abnormally higher in the leaves and much lower in the roots. [4] As this process occurs, metals are efficiently shuttled from the root to the shoot as an enhanced ability in order to protect the roots from metal toxicity. [4]

Delving into tolerance: Throughout the research of hyperaccumulation, there is a conundrum with tolerance. There are several different understandings of tolerance associated with accumulation; however, there are a few similarities. Evidence has conveyed that the traits of tolerance and accumulation are separate to each other and are moderated by genetic and physiological mechanisms. [5] Moreover, the physiological mechanisms, in relation to tolerance, are classified as exclusion: when the movement of metals at the interfaces of soil/root or root/shoot are blocked, or accumulation: when the uptake of metals that have been rendered as non-toxic are allowed into the aerial plant parts. [6]

Characteristics from certain physiological elements:

There are certain characteristics that are specific to certain species. For example, when presented with a low supply of zinc, Thlaspi caerulescens had higher zinc concentrations accumulated compared to other non-accumulator plant species. [7] Further evidence indicated that when T. caerulescens were grown on soil with an adequate amount of contamination, the species accumulated an amount of zinc that was 24-60 times more than Raphanus sativus (radish) had accumulated. [7] Additionally, the capacity to experimentally manipulate soil metal concentrations with soil amendments has allowed researchers to identify the maximum soil concentrations that hyperaccumulation species can tolerate and the minimum soil concentrations in order to reach hyperaccumulation. [4] Furthermore, with these findings, two distinct categories of hyperaccumulation arose, active and passive hyperaccumulation. [4] Active hyperaccumulation is attained by relatively low soil concentrations. Passive hyperaccumulation is induced by exceedingly high soil concentrations. [4]

Genetic basis

Several gene families are involved in the processes of hyperaccumulation including upregulation of absorption and sequestration of heavy metal metals. [8] These hyperaccumulation genes (HA genes) are found in over 450 plant species, including the model organisms Arabidopsis and Brassicaceae. The expression of such genes is used to determine whether a species is capable of hyperaccumulation. Expression of HA genes provides the plant with capacity to uptake and sequester metals such as As, Co, Fe, Cu, Cd, Pb, Hg, Se, Mn, Zn, Mo and Ni in 100–1000x the concentration found in sister species or populations. [9]

The ability to hyperaccumulate is determined by two major factors: environmental exposure and the expression of ZIP gene family. Although experiments have shown that the hyperaccumulation is partially dependent on environmental exposure (i.e. only plants exposed to metal are observed with high concentrations of that metal), hyperaccumulation is ultimately dependent on the presence and upregulation of genes involved with that process. It has been shown that hyperaccumulation capacities can be inherited in Thlaspi caerulescens (Brassicaceae) and others. As there is a wide variety among hyperaccumulating species that span across different plant families, it is likely that HA genes were eco typically selected for. In most hyperaccumulating plants, the main mechanism for metal transport are the proteins coded by genes in the ZIP family, however other families such as the HMA, MATE, YSL and MTP families have also been observed to be involved. The ZIP gene family is a novel, plant-specific gene family that encodes Cd, Mn, Fe and Zn transporters. The ZIP family plays a role in supplying Zn to metalloproteins. [10]

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. [11] 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 transporter4. Both the ZTP and ZNT families, like the ZIP family, are zinc transporters. [12] It has been observed in hyperaccumulating species, that these genes, specifically ZNT1 and ZNT2 alleles are chronically overexpressed. [13]

While the precise mechanism by which these genes facilitate hyperaccumulation is unknown, expression patterns strongly correlate with individual hyperaccumulation capacity and metal exposure, implying that these gene families play a regulatory role. Because the presence and expression of zinc transporter gene families are highly prevalent in hyperaccumulators, the ability to accumulate a diverse range of heavy metals is most likely due to the zinc transporters' inability to discriminate against specific metal ions. The response of the plants to hyperaccumulation of any metal also supports this theory as it has been observed that AhHMHA3 is expressed in hyperaccumulating individuals. AhHMHA3 has been identified to be expressed in response to and aid of Zn detoxification. [9] In another study, using metallophytic and non-metallophytic Arabidopsis populations, back crosses indicated pleiotropy between Cd and Zn tolerances. [14] This response suggests that plants are unable to detect specific metals, and that hyperaccumulation is likely a result of an overexpressed Zn transportation system. [15]

The overall effect of these expression patterns has been hypothesized to assist in plant defense systems. In one hypothesis, "the elemental defense hypothesis", provided by Poschenrieder, it is 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. [10] Another hypothesis, "the joint hypothesis", provided by Boyd, suggests that expression of these genes assists in systemic defense. [16]

Metal transporters

An important trait of hyperaccumulating plant species is enhanced translocation of the absorbed metal to the shoot. Metal toxicity is tolerated by plant species that are native to metalliferous soils. Exclusion, in which plants resist undue metal uptake and transport, and absorption and sequestration, in which plants pick up vast quantities of metal and pass it to the shoot, where it is accumulated, are the two basic methods for metal tolerance. [17] Hyperaccumulators are plants that have both the second technique and the ability to absorb more than 100 times higher metal concentrations than typical organisms. [18]

T. caerulescens is found mostly in Zn/Pb-rich soils, as well as serpentines and non-mineralized soils. It was discovered to be a Zn hyperaccumulator. Because of its ability to extract vast quantities of heavy metals from soils. [19] 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, Thlaspi 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. [20]

A heavy metal transporter, cDNA, mediates high-affinity Zn2+ uptake as well as low-affinity Cd2+ uptake. This transporter is expressed at very high levels in roots and shoots of the hyperaccumulator. According to (Pence et., al. 1999), 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. [21] This increased gene expression has been shown to be the basis for increased Zn21 uptake from the soil in T. caerulescens roots, and it is possible that the same process underpins the enhanced Zn21 uptake into leaf cells.

See also

Related Research Articles

Abiotic stress is the negative impact of non-living factors on the living organisms in a specific environment. The non-living variable must influence the environment beyond its normal range of variation to adversely affect the population performance or individual physiology of the organism in a significant way.

<span class="mw-page-title-main">Phytoremediation</span> Decontamination technique using living plants

Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronomic techniques to either contain, remove or render toxic environmental contaminants harmless". The term is an amalgam of the Greek phyto (plant) and Latin remedium. Although attractive for its cost, phytoremediation has not been demonstrated to redress any significant environmental challenge to the extent that contaminated space has been reclaimed.

<i>Noccaea caerulescens</i> Species of plant

Noccaea caerulescens, the alpine penny-cress or alpine pennygrass, is a flowering plant in the family Brassicaceae. It is found in Scandinavia and Europe.

<i>Thlaspi</i> Genus of herbs

Thlaspi, or pennycress, is a genus of herbs of temperate regions of the Eurasian continent. They occur in Central and South Europe, South-West Asia and two species are endemic to China. The Thlaspi has been proven to be a hyperaccumulator of heavy metals such as zinc and cadmium and therefore may be used in phytoremediation initiatives.

This list covers known nickel hyperaccumulators, accumulators or plant species tolerant to nickel.

This list covers hyperaccumulators, plant species which accumulate, or are tolerant of radionuclides, hydrocarbons and organic solvents, and inorganic solvents.

<span class="mw-page-title-main">Rhizofiltration</span>

Rhizofiltration is a form of phytoremediation that involves filtering contaminated groundwater, surface water and wastewater through a mass of roots to remove toxic substances or excess nutrients.

A metallophyte is a plant that can tolerate high levels of heavy metals such as lead. Such plants range between "obligate metallophytes", and "facultative metallophytes" which can tolerate such conditions but are not confined to them.

Metallotolerants are extremophile organisms that are able to survive in environments with a high concentration of dissolved heavy metals. They can be found in environments containing arsenic, cadmium, copper, and zinc. Known metallotolerants include Ferroplasma sp. and Cupriavidus metallidurans.

Sedum alfredii is a perennial herb in areas of Asia,

Phytoextraction is a subprocess of phytoremediation in which plants remove dangerous elements or compounds from soil or water, most usually heavy metals, metals that have a high density and may be toxic to organisms even at relatively low concentrations. The heavy metals that plants extract are toxic to the plants as well, and the plants used for phytoextraction are known hyperaccumulators that sequester extremely large amounts of heavy metals in their tissues. Phytoextraction can also be performed by plants that uptake lower levels of pollutants, but due to their high growth rate and biomass production, may remove a considerable amount of contaminants from the soil.

Athyrium yokoscense, commonly known as Asian common ladyfern in English and as Hebino-negoza in Japanese, is a species of fern in the family Athyriaceae. These tough plants live primarily in and around mine sites and thrive in soils contaminated with high concentrations of heavy metals, such as zinc, cadmium, lead, and copper. A. yokoscense is indigenous to Japan, Korea, eastern Siberia and northeastern China and has been known for centuries to tolerate phytotoxic mining sites. The predominance and concentration of this fern species at a particular region was used to identify potential mining sites. The primary potential of A. yokoscense is in its phytoremediative ability to accumulate toxic metals from soils contaminated with heavy metals, so it may have some long-term commercial importance. No medicinal or culinary values of this fern species have been studied or confirmed.

<span class="mw-page-title-main">Ectomycorrhiza</span> Non-penetrative symbiotic association between a fungus and the roots of a vascular plant

An ectomycorrhiza is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. The mycobiont is often from the phyla Basidiomycota and Ascomycota, and more rarely from the Zygomycota. Ectomycorrhizas form on the roots of around 2% of plant species, usually woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine and rose families. Research on ectomycorrhizas is increasingly important in areas such as ecosystem management and restoration, forestry and agriculture.

<span class="mw-page-title-main">Zinc transporter ZIP9</span> Protein found in humans

Zinc transporter ZIP9, also known as Zrt- and Irt-like protein 9 (ZIP9) and solute carrier family 39 member 9, is a protein that in humans is encoded by the SLC39A9 gene. This protein is the 9th member out of 14 ZIP family proteins, which is a membrane androgen receptor (mAR) coupled to G proteins, and also classified as a zinc transporter protein. ZIP family proteins transport zinc metal from the extracellular environment into cells through cell membrane.

<span class="mw-page-title-main">High Affinity K+ transporter HAK5</span>

High Affinity K+ transporter HAK5 is a transport protein found on the cell surface membrane of plants under conditions of potassium deprivation. It is believed to act as a symporter for protons and the potassium ion, K+. Firstly discovered in barley, receiving the name of HvHAK1, it was soon after identified in the model plant Arabidopsis thaliana and named HAK5. These transporters belongs to the subgroup I of the KT-HAK-KUP family of plant proteins with obvious homology with both bacterial and fungal transport systems, which experienced a major diversification following land conquest. KT-HAK-KUP transporters are one of four different types of K+ transporter within the cell, but are unique as they do not have a putative pore forming domain like the other three; Shaker channels, KCO channels, HKT transporters. It is activated when the plant is situated in low soil with low potassium concentration, and has been shown to be located in higher concentration in the epidermis and vasculature of K+ deprived plants. By turning on, it increases the plants affinity (uptake) of potassium. Potassium plays a vital role in the plants growth, reproduction, immunity, ion homeostasis, and osmosis, which ensures the plants survival. It is the highest cationic molecule within the plant, accounting for 10% of the plants dry weight, which makes its uptake into the plant important. Each plant species has its own HAK5 transporter that is specific to that species and has different levels of affinity to K+. To operate and activate the HAK5 transporter, the external concentration of K+ must be lower than 10μM and up to 200μM. In Arabidopsis plants, when external potassium concentration is lower than 10μM, it is only HAK5 that is involved with the uptake of K+, then between 10 and 200μM both HAK5 and AKT1 are involved with the uptake of K+. HAK5 is coupled with CBL9/CIPK23 kinase's although the mechanism behind this has not yet been understood.

Mycorrhizal amelioration of heavy metals or pollutants is a process by which mycorrhizal fungi in a mutualistic relationship with plants can sequester toxic compounds from the environment, as a form of bioremediation.

<i>Phyllanthus balgooyi</i> Species of herbaceous plant in the family Phyllanthaceae, found in Palawan and Sabah

Phyllanthus balgooyi is an herbaceous plant in the family Phyllanthaceae, found in Palawan and Sabah. The plant is a hyperaccumulator of nickel, with a concentration of the metal exceeding 16% in the plant's phloem sap.

<i>Agrostis castellana</i> Species of plant in the family Poaceae

Agrostis castellana, the highland bent, dryland bent or dryland browntop, is a species of cool-season grass in the family Poaceae. It is native to Macaronesia and the Mediterranean, has been widely introduced elsewhere, and is considered an invasive species in some locales. It is a hyperaccumulator of zinc and lead.

References

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