Biology
• Metal accumulating plants
• Mechanisms of metal hyperaccumulation in
plants
• Mechanisms of metal resistance: Phytochelatins
and metallothioneins
• Molecular mechanisms of ion transport in plant
cells
Metal accumulating plants
Bioavailability of metals to hyperaccumulators
• 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.
• 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
South Africa
Pentacalia (10 species)
Cuba
Brassicaceae
Bornmuellera (6 taxa)
Greece
Peltaria emarginata
Greece
Streptanthus polygaloides
USA(CA)
Rubiaceae
Psychotria costivenia
Cuba
P. vanhermanii
Cuba
11,600
16,600
17,600
34,400
14,800
38,530
35,720
Hyperaccumulators of Zn, Cd and Pb
Family species
location
Max. Conc (mg/kg)
Zn
Cd Pb
Brassicaceae
Thlaspi
caerulescens
W&Centr Europe
43,710 2,130
Caryophyllaceae
Minuartia verna
Yugoslavia; UK
11,400
Dichapetalaceae
Dichapetalum
gelonioides
Sumatra; Mindanao;
Sabah
30,000
2,740
20,000
Hyperaccumulators of Cu and Co
(from Democratic Republic of Congo)
Family species
Convolvulaceae Ipomoea alpina
Max. Conc (mg/kg)
Cu
Co
12,300
Lamiaceae
Aeollanthus subacaulis var. linearis
Haumaniastrum katangense
H. robertii
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
33,750
22,500
Proteaceae
Macadamia angustifolia
M. neurophylla
11,590
55,200
Hyperaccumulators of Se (from New Caledonia)
Family species
Location
Asteraceae
Haplopappus
condensata
Midwest USA
Brassicaceae
Stanleya pinnata Midwest USA
S. bipinnata
Midwest USA
Lecythidaceae
Lecythis ollaria Venezuela
Leguminosae
Astragalus
bisulcatus
Midwest USA
A. racemosus
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)
Metal toxicity
• 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:
• Formation of stable nontoxic chelates
• 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.
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.
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
• 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
• 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
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:
• 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:
• 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
• Composed of only three amino acids:
glutamate, cysteine, and glycine
• Not coded directly by genes but likely to be
products of biosynthetic pathways,
presumably using GSH (glutathione) as a
substrate
• Have been identified in a wide variety of
plant species, algae, fungal species and
• 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
• Cd was the most effective inducer.
• Exposure of Cd in the range of 1-100
mM, 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 cysteinrich 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
• A role for PCs in the detoxification
of some heavy metals, particularly
Cd, is clearly established.
• 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.
• Movement of ions via the apoplastic
pathway can occur through walls of
cortex cells until restricted by the
impermeable Casparian strips of
endodermal cells.
• 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“ in 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