Biology

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Metal accumulating plants

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Mechanisms of metal hyperaccumulation in plants

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Mechanisms of metal resistance: Phytochelatins and metallothioneins

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Molecular mechanisms of ion transport in plant cells

Metal accumulating plants

Bioavailability of metals to hyperaccumulators

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Observation of Zn accumulation in plants was first recorded in 1865 by F. Risse (German scientist?). The plant, Thlaspi alperstre var. calaminare grown in zinc-rich soil in a area between Germany and Belgium. The leave tissue of the plant contained Zn exceeding 10,000 mg Zn/kg (1% dry matter), or 10% Zn in the ash.

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Observations of unusual accumulation of other metals have been made only during the twentieth century. E.g. Pb, 1920s, Se 1930s, Ni, 1940s, Co and Cu, 1960s, Cd and Mn, 1970s.

Example: In 1930s, Se was found to be responsible for “alkali disease” in range animals in South Dakota.

Plants, in the genus of Astragalus, are capable of accumulating up to 0.6% Se in dry shoot biomass.

At least 45 plant families are known to contain metal accumulating species and 397 metal accumulating taxa have been identified

Hyperaccumulators of Ni Family species location Max. Conc (mg/kg) Asteraceae

Berkheya coddii

Pentacalia (10 species) South Africa Cuba Brassicaceae Bornmuellera (6 taxa)

Peltaria emarginata Streptanthus polygaloides

Rubiaceae

Psychotria costivenia P. vanhermanii

Greece Greece USA(CA) Cuba Cuba 11,600 16,600 17,600 34,400 14,800 38,530 35,720

Hyperaccumulators of Zn, Cd and Pb Family species Brassicaceae

Thlaspi caerulescens

Caryophyllaceae

Minuartia verna

Dichapetalaceae

Dichapetalum gelonioides

location Yugoslavia; UK Max. Conc (mg/kg) Zn 11,400 Sumatra; Mindanao; Sabah 30,000 Cd Pb W&Centr Europe 43,710 2,130 2,740 20,000

Hyperaccumulators of Cu and Co (from Democratic Republic of Congo) Family species Convolvulaceae Ipomoea alpina Lamiaceae

Aeollanthus subacaulis var. linearis Haumaniastrum katangense H. robertii

Max. Conc (mg/kg) Cu Co 12,300 13,700 9,222 2,070 4,300 2,241 10,232

Hyperaccumulators of Mn (from New Caledonia) Family species Max. Conc (mg/kg) Mn Celastracear

Maytenus bureaviana M. sebertiana

Proteaceae

Macadamia angustifolia M. neurophylla

33,750 22,500 11,590 55,200

Hyperaccumulators of Se (from New Caledonia) Family species Asteraceae

Haplopappus

Location

condensata

Brassicaceae Midwest USA Stanleya pinnata Midwest USA

S. bipinnata

Lecythidaceae Midwest USA Lecythis ollaria Venezuela Leguminosae

Astragalus bisulcatus A. racemosus

Midwest USA Midwest USA Max. Conc (mg/kg) 9,120 1,190 2,380 18,200 8,840 14,920

Mechanisms of metal hyperaccumulation in plants

Definition of an essential element 1. If plant cannot complete its life cycle in the absence of the element 2. It forms part of any molecule of constituent of the plant that is itself essential in the plant

16 elements are believed to be essential for plant growth. These are: C, H, O, N, P, K, S, Ca, Mg, B, Cl, Cu, Fe, Mn, Mo, Zn In addition to the 16 elements essential for plants, higher animals require sodium, iodine, cobalt, selenium, nickel, silicon, chromium, tin, vanadium, and fluorine , but not boron. (including boron, total 25 26 for animals)

Salisbury and Ross, 1985

Metal toxicity

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Genetic variation May be absorbed only to a limited extent, avoidance than true tolerance Accumulate in roots with little transport to shoots Both roots and shoots contain much higher amounts of such elements than nontolerant species could live with Mechanism of true tolerance have not been understood Suggested mechanisms:

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Formation of stable nontoxic chelates

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Storage of elements in vacuoles

Mechanisms of metal hyperaccumulation

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Rhizosphere Interactions Root uptake Root-to-Shoot metal translocation Metal sequestration and complexation

Rhizosphere Interactions Hyperaccumulator species are able to accumulate higher metal concentrations in their shoots than surrounding nonaccumulator plants even from soils containing nonphytoxic background levels of metals. Possibly due to enhance ability to solubilize metals within the rhizosphere of the hyperaccumulator.

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By The release of specific metal-chelating compounds into the rhizosphere by plant roots modification of the rhizosphere pH or redox potential by plant roots.

Root uptake Hyperaccumulation does not appear to be driven by the enhance affinity of root uptake systems for the hyperaccumulated metal, but increased rates of toot uptake .

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Possiblly enhanced expression of metal transporter. E.g. Roots of the zinc hyperaccumulator T. caerulescens appear to contain more zinc transporters per gram fresh weight than the nonaccumulator T. arvense.

Some plants demonstrate metal selectivity Metal selectivity could be due to metal transport across the root plasma membrane during either metal uptake into the symplast or metal export into the xylem

Root-to-Shoot metal translocation

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Limited evidence indicated that rates of metal translocation from root to shoot are similar in hyperaccumulators and related nonaccumulator species.

Possibly, hyperaccumulators may lack the ability to restrict metal movement into the shoot.

Shoot:root ratio of metal concentrations are above unity in hyperaccumulators of Ni, Zn, or Co, suggesting an efficient root-to-shoot translocation system for the hyperaccumulated metals.

Metal sequestration and complexation

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Metal hyperaccumulators tend to accumulate metals in epidermal and subepidermal tissues, including leaf trichomes.

Nickle and zinc are predominantly localized in vacuoles Metal toxicity is reduced by complexing with high affinity ligands or organic acids Evidence: many Ni and Zn hyperaccumulators accumulate high concentrations of organic acids in their leaves.

A young dicot root

A typical root

Basis tissue pattern in a mature root

Summary Evidence suggested that several mechanisms of hyperacumulation have been involved for one metal Hyperaccumulation may require several processes: Increased root uptake as well as reduced root accumulation, sequestration at cellular level as well as the tissue level, and, most importantly metal tolerance.

Mechanisms of metal resistance : Phytochelatins and metallothioneins

Wenzel et al., 1999

Plants have adaptive mechanisms to respond to both nutrient deficiencies and toxicity. Metal tolerance is possibly related to:

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Metal binding to cell walls Metal tolerance of the membrane Reduced membrane transport Active efflux of metals form the cells-plants Metal-tolerant enzymes Compartmentation Chelation of the metal by organic or inorganic ligands Precipitation of metal compounds with low solubility

There are two major heavy metal-binding compounds in plant cells: The

phytochelatin

peptides (PCs) and

metallothioneins

(MTs).

MTs:

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Low molecular weight (<10 kDa) Large fraction of cystein residues High metal content with coordination of metal ions in metal–thiolate clusters

MTs and PCs have been classified into three classes:

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Class I: MTs from mammals and other organisms with a highly conserved arrangement of cystein residues.

Class II: all other MT proteins.

Class III: cystein-rich, metal-binding peptides that are not produced by translation of a mRNA on ribosome and therefore includes PCs.

Phytochelatin

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Composed of only three amino acids: glutamate, cysteine, and glycine

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Not coded directly by genes but likely to be products of biosynthetic pathways, presumably using GSH (glutathione) as a substrate

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Have been identified in a wide variety of plant species, algae, fungal species and marine diatoms.

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Synthesis of PCs can be induced by a wide range of metal ions, including Cd, Ni, Cu, Zn, Ag, Sn, Sb, Te, W, Au, Hg, Pb, and Bi

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Cd was the most effective inducer.

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Exposure of Cd in the range of 1-100

m

M, induction can be detected within hours of exposure.

Mechanism seems a lot more complex than simply chelating the metal ion.

1. The metal ion activate PC synthase, be chelated by the PCs 2. Be transported to the vacuole and possibly form a more complex aggregation in the vacuole with , for example, sulfide or organic acids

The only MT proteins that have been purified from plants are the wheat Ec protein and a number of MTs from Arabidopsis.

There are striking similarity in the cystein rich domains which may have been duplicated within a single MT gene.

Presence of heavy metals may induce the expression of MT genes.

Type I, II and III MTs are more expressed in root than in leaves; while type IV MTs are expressed in developing seeds MTs are also expressed in senescing tissues because they are possibly involved in metal ion transport in this process.

Summary

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A role for PCs in the detoxification of some heavy metals, particularly Cd, is clearly established.

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MTs are likely involved in metal metabolism in plants. However, their role in phytoremediation is highly speculative at this time.

Molecular mechanisms of ion transport in plant cells

Apoplastic --The interconnecting walls and the water-filled xylem elements are considered as a single system Symplastic --the rest of the plant, the “living” part, including the cytoplasm of all the cells in the plant. The cytoplasm of adjoining cells is connected through plasmodesmata in the cell walls.

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Movement of ions via the apoplastic pathway can occur through walls of cortex cells until restricted by the impermeable Casparian strips of endodermal cells.

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Regardless of the pathway across the root, ions transported to the shoot must somehow get into dead conducting cells of the xylem.

Transport across the plant plasma membrane is driven by an electrochemical gradient of protons generated by plasma membrane H + ATPase Many genes encoding transporters have been identified and cloned.

These mustard plants carry a gene that helps them soak up heavy metals (Philip Rea, University of Pennsylvania)

An experimental treatment wetland at the Department of Energy's Savannah River Site tests the ability of native aquatic plants to clean up the acidic, metal contaminated runoff from a coal pile (University of Georgia Savannah River Ecology Laboratory)

Hybrid poplar trees are screened for their ability to extract nickel, cadmium and zinc from contaminated soil (University of Georgia Savannah River Ecology Laboratory)

Phytoremediation can be a cost effective way to clean up contaminated soils, as at this Department of Energy test site (Department of Energy Subsurface Contaminants Focus Area)

In situ remediation of contaminated soil by plants "PHYTOREM“ i

n W. Europe • 1,400,000 contaminated sites in Western Europe; ETCS, 1998) • Many with heavy metals, such as zinc, cadmium, lead and copper. • The residence time of metals in soil is of the order of thousands of years.

• The remediation techniques presently in use are mainly ex-situ using physico-chemical methods of extraction, which are very expensive (ca. US$ 3M/ha) and destroy the soil biology and structure. • Phytoremediation--an emerging technique, low cost and environmentally sustainable (McGrath, 1998).

Root development of Thlaspi caerulescens in the presence of Zn hot spots in an agricultural soil (photo C. Schwartz)

Root development of Thlaspi caerulescens and Lupinus albus in the presence of metals hot spots (Cd and Zn) in soils

Thlaspi caerulescens accumulates more than 2% Zn on a dry weight basis and more than 0.1% of Cd) but has a limited biomass

Salix viminalis takes up reasonably high amounts of Cd and Zn but produces also high biomass which could be further recycled for energy production