Mercury Toxicity In Plants; by Patra M. and Sharma A.. (Bot. Rev. 66:379-422, 2000)

ABSTRACT

Mercury poisoning has become a problem of current interest as a result of environmental

pollution on a global scale. Natural emissions of mercury form two-thirds of the input while

man-made release form about one-thirds. Considerable amounts of mercury may be added to

agricultural land with sludge, fertilisers, lime, and manures. The most important sources

of contaminating agricultural soil have been the use of organic mercurials as a seed coat

dressing to prevent fungal diseases of seeds. In general, effect of treatment on the

germination capacity is favourable when recommended dosages are used. Injury to the seed

is more pronounced at higher rates, increasing in direct proportion to increasing rates of

application. The availability of soil Hg to plants is low, and there is a tendency for Hg

accumulation in the roots, indicating that the roots serve as a barrier to Hg uptake. Hg

concentration in above ground parts of plants appears to largely depend on foliar uptake

of Hg0 volatilised from the soil.

Uptake of Hg has been found to be plant-specific in bryophytes, lichens, wetland plants,

woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic

content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation

used and total metal content. In general, mercury uptake in plants could be related to the

pollution level.

With lower levels of mercury pollution, the amounts in crops are below the permissible

levels. Aquatic plants have shown to be bioaccumulators of Hg. Hg concentrations in the plants

(stems + leaves) were always greater when the metal was introduced in organic form. In

freshwater aquatic vascular plants, differences in uptake rate depended on the species of

plant, the seasonal growth rate changes, and the metal ion being absorbed.

A part of Hg emitted from the source into the atmosphere is absorbed by plant leaves,

and migrates to humus through fallen leaves. Hg vapour uptake by leaves of the C3 species

oats, barley, and wheat was five times greater than that by leaves of the C4 species corn,

sorghum, and crabgrass. Such differential uptake by C3 and C4 species was largely attributable

to internal resistance to Hg vapour binding. Airborne Hg thus seems to contribute significantly

to the Hg content in crops and thereby to its human food intake. Accumulation, toxicity

response, and Hg distribution differ between plants exposed through shoot or through root,

even when internal Hg concentrations in the treated plants were similar. Throughfall and

litterfall play a significant role in the cycling and deposition of Hg.

The possible causal mechanisms of Hg toxicity are (i) Changes in the permeability of

the cell membrane; (ii) reactions of sulphydryl (-SH) groups with cations; (iii) affinity

for reacting with phosphate groups and active groups of ADP or ATP; and (iv) replacement

of essential ions (mainly major cations). In general, inorganic forms are thought to be more

available to plants than organic ones. The exposure of plants to mercurials may be by - (

1) Direct administration as antifungal agents i.e. mainly to crop plants, through seed

treatment or foliar spray. The end points screened are seed germination, seedling growth,

relative growth of root and shoot and in some case studies of leaf area index, internode

development and other anatomical characters.

2) Accidental exposures through soil, water, and air pollution. The level of toxicity

is tested usually under laboratory conditions using a wide range of concentrations and

different periods of exposure. Additional parameters include biochemical assays and

genetical studies.

The absorption of organic and inorganic Hg from soil by plants is low and there is a

barrier to Hg translocation from plant roots to tops. Thus, large increases in soil Hg

levels produce only modest increases in plant Hg levels by direct uptake from soil. The

seed injury caused by organic mercurials to cereals has been characterized by abnormal

germination and characteristic hypertrophy of the roots and coleoptile.

Mercury affects both light and dark reactions of photosynthesis. Substitution of the

central atom of chlorophyll, magnesium, by mercury in vivo prevents photosynthetic light-

harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis.

The reaction varies with light intensity. A concentration and time dependent protective effect

of GSH seems to be mediated by the restricted uptake of the metal involving cytoplasmic

protein synthesis.

Plant cells contain aquaporins, proteins that facilitate the transport of water, in the

vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive,

and in AQP1, a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a

conserved Asn-Pro-Ala motif.

Mercury has a toxic effect at low concentrations on the degrading capabilities of

microorganisms. Sensitivity to the metal was enhanced by a reduction in pH and tolerance

to Hg by microorganisms was found to be in the order : total population > nitrogen fixers >

nitrifiers.

Numerous experiments have been carried out to study the genetic effects of mercury

compounds in experimental test systems using a variety of genetic endpoints. The most

noticeable and consistent effect was the induction of c-mitosis through disturbance of the

spindle activity, resulting in the formation of polyploid and aneuploid cells, and c-tumours.

Organomercurials had been reported to be 200 times more potent than inorganic mercury.

Exposure to inorganic mercury reduced mitotic index in the root tip cells and increase the

frequency of chromosomal aberrations in degrees directly proportional to the concentrations

used and to the duration of exposure. The period of recovery after removal of Hg was

inversely related to the concentration and duration of exposure.

Bacterial plasmids encode resistance systems for toxic metal ions including Hg2+,

functioning by energy-dependent efflux of toxic ions, through ATPases and chemiosmotic

cation/proton antiporters. The inducible mercury resistance (mer) operon encodes both a

mercuric ion uptake and a detoxification enzymes.

In Gram-negative bacteria, a periplasmic protein, MerP, an inner-membrane transport

protein, MerT, and a cytoplasmic enzyme, mercuric reductase (the MerA protein), are

responsible for the transport of mercuric ions into cell and their reduction to elemental

mercury, Hg(II).

In Thiobacillus ferrooxidans, an acidophilic chemoautotrophic bacterium sensitive to

Hg ions, a group of mercury resistant strains, which volatilized mercury, was isolated.

The entire coding sequence of the mercury ion resistance gene was located within a

2.3kb fragment of chromosomal DNA (encoding 56,000 and 16,000 molecular weight proteins)

from strain E-15 of Escherichia coli.

Higher plants and Schizosaccharomyces pombe respond to heavy metal stress of Hg by

synthesizing phytochelatins (PCs) that act as chelators. The strength of Hg(II) binding to

glutathione and phytochelatins followed the order:

gGlu-Cys-Gly<(gGlu-Cys)2Gly<(gGlu-Cys)3Gly<(gGlu-Cys)4Gly.

Suspension cultures of haploid tobacco (Nicotiana tabacum) cells were subjected to

ethyl methane sulfonate to raise Hg-tolerant plantlets. HgCl2-tolerant variants were selected

from nitrosoguanidine (NTG)-treated suspension cell cultures of cow pea (Vigna unguiculata)

initiated from hypocotyl callus, and incubated with 18mg/ml HgCl2.

Experiments have been carried out to develop Hg-tolerant plants of Hordeum vulgare

through previous exposure to low doses of Hg and subsequent planting in next generation in

Hg-contaminated soil.

Phytoremediation involves the use of plants to extract, detoxify and/or sequester

environmental pollutants from soil and water. Transgenic plants cleave mercury ions from

methyl-mercury complexes; reduce mercury ions to the metallic form; take up metallic mercury

through their roots; and evolve less toxic elemental mercury. Genetically engineered plants

contain modified forms of bacterial genes that break down methyl mercury and reduce mercury

ions. The first gene successfully inserted into plants was merA, which codes for a mercuric

ion reductase enzyme, reducing ionic mercury to the less toxic elemental form. MerB codes

for an organomercurial lyase protein that cleaves mercury ions from highly toxic methyl

mercury compounds. Plants with the merB gene have been shown to detoxify methyl mercury in

soil and water. Both genes have been successfully expressed in Arabidopsis thaliana, Brassica

(mustard), Nicotiana tabacum (tobacco) and Liriodendron tulipifera (tulip poplar). Plants

currently being transformed include cattails, wild rice, and Spartina, another wetlands plant.

The problem of mercury contamination can be reduced appreciably by combining the

standard methods of phytoremediation of removal of Hg from polluted areas through scavenger

plants with raising such plants both by routine mutagenesis and by genetic engineering. The

different transgenics raised utilising the two genes merA and merB are very hopeful prospects.