Como citar este trabajo Torri S, Cabrera M, Torres- Duggan. 2013. Plants response to high soil Zn availability. Feasibility of biotechnological improvement. En: Biotechnologic Techniques of Stress in Plants, Editor: M. Miransari, Stadium Press LLC USA, ISBN : 1-62699-031-X, 101-118.

1. Plants response to high soil Zn availability. Feasibility of biotechnological improvement
Silvana Irene Torri
1
, Marisol Natalia Cabrera
2
, Martin Torres Duggan
3
1
Facultad de Agronomía, Universidad de Buenos. Aires, Av. San Martín 4453, Ciudad Autónoma de Buenos Aires, C1417DSE Argentina
2
Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Intendente Güiraldes 2160, Ciudad Autónoma de Buenos Aires,
C1428EGA Argentina
3 Tecnoagro, Girardot 133, Ciudad Autónoma de Buenos Aires, C1427AKC Argentina.
torri@agro.uba.ar
Abstract
Zinc (Zn) is a trace element required in small amounts for most biological systems, although it can be toxic when absorbed
in excess. Some plants have evolved the ability to tightly regulate their internal Zn concentrations in spite of high soil
availability, a process called “zinc homeostasis”. A portion of these species can even hyperaccumulate Zn, with a potential
use to restore contaminated soil environments. A range of gene families that are likely to be involved in Zn transport have
been identified. The growing application of genetic technologies may led to increase plant Zn resistance strategies, as well
as increased uptake of this element from polluted environments. These characteristics may be used for phytoremediation
techniques. This Chapter provides a broad overview of different plant strategies to deal with stress originated from high
Zn availability in soils. Current status of biotechnological improvement and its future prospects are reviewed.
1. Introduction
Zinc (Zn) is a trace element required in small amounts for most biological systems, including plants and animals. The term
“trace element” is widely used for those elements that occur at relatively low concentrations in the dry matter of living
organisms, usually under 100 mg kg
−1
. However, this term is imprecise because it is only based on the concentration of the
element in the plant or animal system, regardless its role.
Several trace elements have been reported to be necessary to complete the vegetative or reproductive stage of plants life
cycle, cannot be replaced by other elements, or are directly involved in plant metabolism (Arnon, Stout 1939). These trace
elements are termed micronutrients, and include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn),
molybdenum (Mo), nickel (Ni) and zinc (Zn). Other trace elements such as arsenic (As), cadmium (Cd), mercury (Hg) and
lead (Pb) have no known biological role in plants but can enter plant cell through existing mineral uptake pathways. These
elements are non-essential, and may form unspecific complex compounds in plant cell causing damage to normal
physiological processes (Sytar et al, 2012).
The importance of Zn for plant cell growth and development has been widely reported for the last century. The fact that
Zn does not take part in oxido-reduction reactions, combined with its geometry makes it a suitable cofactor for many
enzymes. Zinc is the only trace element involved in all six classes of enzymes: oxido-reductases, transferases, hydrolases,
lyases, isomerases and ligases (Barak, Helmke 1993). In plants, enzymes either containing or activated by Zn are involved

2. in a number of physiological processes and cellular metabolism, including protein synthesis; enzyme activation;
metabolism of auxins, carbohydrates, lipids and nucleic acids and antioxidative defense (Marschner 1995; Broadley et al.,
2007). Zinc is also required to regulate plant tolerance of environmental stress, such as high light intensity, high or low
temperatures (Cakmak 2000). The essentiality of Zn in plants was first reported for maize, and afterwards for barley and
dwarf sunflower (Mazé, 1915; Sommer, Lipman 1926).
Zinc is probably one of the most ubiquitous elements in nature. Total Zn concentration in pristine soils is largely
dependent on the parent material and the chemical and physical weathering processes in which the soil has developed
(Mason, Moore 1982). In non-contaminated agricultural soils, total Zn concentration usually ranges from 50 to 300 mg
kg
−1
(Adriano, 2001).
Anthropogenic release of Zn to the environment has greatly increased soil Zn concentration of urban and agricultural soils.
Human activities such as industry, mining, metalliferous smelting processes, atmospheric deposition, together with
improper agriculture practices such as overuse of chemical fertilizers and pesticides, industrial or sewage sludge
amendments, or wastewater irrigation have considerably contributed to increase total Zn concentration in soils (Adriano
2001; Garbisu, Alkorta 2003). Friedland (1990) estimated that the ratio of Zn emissions arising from anthropogenic and
natural inputs were greater than 20 : 1. In certain polluted areas, soil Zn concentration has been reported to be more than
1000 mg Zn kg
−1
dry soil (Castro-Larragoitia et al, 1997). Zn contaminated soils cause injury to soil microorganisms and
reduce crop yield (Hassan and Aarts, 2011). Moreover, high Zn concentration in soils may cause Zn accumulation in the
edible parts of crop plants, representing the principal route of entry into the human food-chain. Unlike organic pollutants,
trace elements cannot biodegrade as a result of biological or chemical processes and, consequently, persist in the
environment for longer periods of time than in other compartments of the biosphere (Lasat, 2002). Therefore, trace
elements are a group of soil pollutants of great concern.
2. Bioavailability of Zn in soils
The evaluation of total Zn concentration in soils is a useful index of the degree of Zn pollution. The evaluation of
background data on soils unaffected by human activities can be useful to determine the rate of soil pollution. Worldwide,
total soil concentration ranges and regulatory guidelines for some trace elements have been proposed (NJDEP 1996; Riley
et al. 1992). However, total Zn concentration provides little or no information on the fraction that plants can absorb (Wolt,
1994). It is now widely recognized that Zn bioavailability strongly depends on its specific chemical forms and on its binding
state in the soil. Many researchers have reported that total Zn concentration in soils does not correlate with the Zn
bioavailable pool (McBride et al., 1997; Torri, Lavado 2009).
When Zn reaches the soil surface, it may distribute between different soil fractions (Marschner 1995, Torri et al 2012). The
distribution of Zn is determined by soil-specific precipitation, complexation or adsorption/desorption reactions. In soils, Zn
can be found as free ions [Zn
2+
, Zn (OH)
+
] or in soluble organic complexes in the soil solution, associated with clay particles,
humic compounds or Fe and Mn hydroxides, precipitated with secondary minerals, or within the matrix of soil primary
minerals (Shuman 1991). Bioavailable Zn fractions are usually considered to be soluble and exchangeable Zn and Zn-bound

3. to organic complexes, although some plants may develop mechanisms by which Zn is selectively absorbed (Torri, Lavado
2009).
Soil chemical properties or changes of environmental conditions may also determine Zn distribution among soil
constituents and, therefore, Zn availability to plants. Soil pH is considered to be the main soil factors controlling the
amounts of plant-available Zn in soils. At high soil pH, Zn is readily adsorbed onto cation exchange sites. Conversely, Zn
solubility increases under acidic conditions (Kabata-Pendias 2001). In accordance with these changes in Zn availability,
plant uptake of Zn increases as soil pH decreases. Other soil characteristics that affect Zn bioavailability include soil
organic matter content, clay content, amorphous iron or manganese oxides, calcium carbonate concentration, redox
conditions, microbial activity in the rhizosphere, soil moisture, climate and concentration of macronutrients or other trace
elements (Alloway 2009). Soil texture was also reported to affect the levels of Zn in the soil solution (Meers et al, 2006).
Nonetheless, the distribution of Zn among soil fractions is subjected to a dynamic equilibrium (Norvell, 1991). For
instance, plant roots directly remove Zn from the soil solution, which is rapidly replenished from Zn adsorbed in easily
desorbed forms. Plant roots may also contribute to increase Zn availability through changes in rhizosphere pH or by root
exudation of organic acids or phytosiderophores (Reichman, Parker 2005). In soils with high organic matter content,
soluble Zn was reported to increase at soil pH 7–7.5, as a result of an increase in the concentration of chelating agents in
the soil solution (Wong et al, 2007). Inorganic precipitates were reported to reduce Zn availability in soils at high pH (Torri,
Lavado 2008).
Several experiments have shown that Zn bioavailability and phytotoxicity depends on the concentration of free Zn species
in the soil solution (Weng et al., 2001). These results were probably due to the fact that free Zn is one of the most
frequent phytotoxic elements after free aluminum and free manganese (Chaney, 1993). Apart from the free divalent
cation, the soil solution may contain Zn as inorganic or organic complexes (Holm et al., 1995). Nevertheless, the
concentration of soluble forms of Zn in soils is low, usually between 4 × 10
−10
and 4 × 10
−6
M (Barber, 1995), even in Zn-
polluted soils (Wu et al. 2000). Free Zn typically accounts for up to 50% of the soluble Zn fraction. Although in many
studies considerable amounts of soluble or exchangeable Zn were anthropogenically added to soils (through waste-water
irrigation, biosolids disposal), with the passage of time Zn transforms gradually from more active and available fractions
into less available species (Ma and Uren 2006). In these soils, the largest fraction of Zn was reported to occurr primarily as
inorganic precipitates, whereas the concentration of water-soluble and exchangeable Zn was usually very low (Schalscha
et al. 1999; Torri, Lavado 2008), a phenomenon known as “aging”. Therefore, the observed Zn phytotoxicity in Zn-polluted
soils that exhibit low concentration of water-soluble and exchangeable Zn is soil high buffer capacity: as free Zn
2+
is taken
up by the plant more Zn solubilizes to the soil solution through different chemical reactions to maintain the initial Zn
concentration. In this case, the total solution Zn concentration may not change appreciably whilst Zn is taken up by plants.
3. Zn plant uptake
Plant uptake of essential trace elements, such as Zn, is a complex process that is regulated by metabolic mechanisms at
the soil-root interface. As discussed in a previous section, Zn is generally taken up as a free divalent cation (Zn
2+
) or as

4. monovalent ZnOH
+
at high soil pH (Marschner 1995). Some studies indicated that uptake of Zn by barley and lettuce not
only depended on the free-Zn concentration in the soil solution, but was also influenced by the level of soluble Zn-
complexes (Bell et al. 1991, McLaughlin et al. 1997).
Two hypotheses have been postulated to explain the contribution of aqueous organic complexes to Zn uptake by plants: i)
uptake is limited by the diffusion rate of free Zn to the root surface, so the presence of Zn-labile organic complexes
increase the diffusive supply of Zn to roots, and free Zn is uptaken as a result of complex dissociation in the diffusive
boundary layer of soil-root interface (Degryse et al, 2006) or ii). Zn-complexes are directly absorbed by plant roots (Wang
et al., 2009). Zn- phytosiderophores complexes have been demonstrated to be absorbed by maize (von Wíren et al. 1996).
Although Zn is an essential trace element for plant growth, high Zn concentration in plant tissue may originate toxicity
symptoms. However, plant stress originated by high Zn availability in soils is far less widespread than stress produced by
Zn deficiency. For normal growth and development, plants must keep the concentration and speciation of intracellular Zn
of essential micronutrients like Zn within an optimal range. This physiological mechanism is known as metal homeostasis,
helping plants to survive both at metal deficiency or at higher metal concentrations. Only when this control mechanism
has reached its capacity, Zn cell concentration rises above certain threshold level and toxicity effects appear (Sinclair,
Kramer 2012). The idea of a threshold toxicity level is often used to establish the point at which trace elements cause
significant growth decrease. Threshold values are often defined as element concentration corresponding to a yield
decrease of 10% (Alloway 1995). In most crops, typical leaf Zn concentration required for adequate growth or maximal
yield is in the range 15 -30 mg Zn kg
−1
DW (Marschner, 1995). Toxicity thresholds are highly variable within species:
although toxicity symptoms usually appear when leaf Zn concentrations exceeds 300 mg Zn kg
−1
leaf DW, some crops may
exhibit toxicity symptoms when leaf Zn concentrations is below 100 mg Zn kg
−1
DW (Marschner, 1995). Therefore, plant Zn
concentration should be maintained in various tissues and compartments within a relatively narrow range to avoid
deficiency or toxicity effects.
4. Plant response to high soil Zn availability
The presence of high trace elements availability in soils exerts a metal-induced stress on plant populations. Stress is
generally defined as the reaction of a biological system, in this case plants, to extreme environmental factors that,
depending on their intensity and duration, may cause significant changes in the system (Nilsen, Orcutt 1996). Excessive Zn
accumulation in root or shoot cells of sensitive plants may induce oxidative stress by the inactivation of proteins by
adventitious binding of Zn (Sarret et al. 2006). The latter may interfere with important metabolic processes, like
photosynthesis (Chaney 1993). Depending on the severity of Zn exposure, stress may induce physiological disorders
resulting in reduced biomass, leaf chlorosis, inhibited root growth, and physiological disorders, often leading to plant
death at excessive exposure. Other plant species, due to natural selection, are able to colonize polluted areas, usually
forming a specific local flora adapted to growth and reproduction in adverse conditions. As a result of ecophysiological
adaptation, some plant species have developed strategies to resist high Zn stress exposure which can be grouped into one
of three categories: i) excluder strategy, ii) tolerant strategy and iii) hypertolerant strategy. These strategies are metal-

5. dependent and plant species-dependent, and can operate until biochemical resistance of plant cells exist. Nonetheless,
whichever strategy plants use to limit the negative effects of Zn toxicity, a complex network of Zn transport, chelation, and
intracellular sequestration processes have to operate to maintain Zn homeostasis in plant cells.
4.1 the excluder strategy
Plants that developed this strategy try to keep low Zn concentrations in the roots across a wide range of soil Zn
concentrationl (Baker, 1981). This is achieved by either restricting Zn soil bioavailability or by reducing Zn uptake.
Zinc can enter the plant symplast by diffusion, by passive transport through channel proteins, or by active transport
through carrier proteins, termed metal transporters (Eide, 2006; Claus, Chavarria-Krauser. 2012) . Zn is most likely crossing
the plasma membrane via members of the ZIP transporter family (Zinc-regulated transporter, Iron-regulated transporter
Protein). Members of the ZIP gene family are capable of transporting a variety of cations, including cadmium, iron and
manganese (Guerinot, 2000). Under high Zn availiability soil conditions, plants will try to inhibit excessive Zn influx through
down regulation of Zn transporters
Another way of avoiding the entry of Zn into the plant is by binding Zn to specific root exudates. Stress stimulates roots to
exude an array of compounds, including soluble and non-soluble organic compounds (Bais et al, 2006). Root exudates can
form Zn complexes in the rhizosphere, making them unavailable to the plant or lessening Zn toxicity.
Most plants have mycorrhizal fungi associated, with an increased capacity to absorb water and nutrients from the soil.
Mycorrhizal symbiosis is affected by anthropogenic stressors including metals (Entry et al. 2002). The presence of
arbuscular mycorrhiza in plant roots may also provide a Zn excluder barrier, immobilizing Zn in vacuoles (Weissenhorn et
al., 1995) or preventing its transport to host plants through the formation of organic complexes (Leyval et al., 1997). Plants
may also modify the composition of the root cell wall to prevent Zn from entering the plant, enhancing its Zn-binding
capacity (Neutelings, 2011).
Remarkably, mMechanisms involved in the exclusion processes are much weaker than those developed by roots for Zn
absorption under deficient conditions. So, high Zn availability in soils is a stronger stress to plants than Zn deficiency. In
highly Zn- polluted soils, it is often difficult to exclude Zn from plant roots due to the high Zn soil availability. When the
tolerance limits of excluder species are exceeded, phytotoxic effects appear, resulting in plant death.
4.2 the tolerant strategy
Some plant species have evolved different mechanisms to tolerate high Zn availability in soils. Even within one plant
species, more than one mechanism could be operating.
When exposed to excess Zn, some plant species accumulate Zn in their tissues, reducing the damage by storing Zn
in unavailable sinks. Plants may accumulate moderate to high amounts of Zn in roots, or Zn may be translocated from
roots to above ground tissues via xylem. In either type, Zn that gets past the plasma membrane is detoxified by physical

6. sinks, like chelation in the cytosol, compartmentation in vacuoles, or confinement in the apoplast in order to reduce an
undesired interaction with cellular compounds.
Of all mechanism involved in preventing or lessening the toxicity effects of Zn, compartmentation in vacuoles is regularly
reported as the most probable. Zinc sequestration in the vacuoles of root cells may limit the translocation from root to
shoot (Lasat et al. 1998) whereas xylem unloading in the shoots and Zn sequestration in the vacuoles of leaf cells may
contribute to translocation to above ground tissues (Palmgren et al. 2008).
4.3 the hypertolerant strategy.
Some plant species growing on naturally or anthropogenically trace elements contaminated soils were found to be
extremely tolerant to exposure to high soil availability of trace elements. In these plants, the concentration of some trace
elements accumulated in the aboveground biomass was found to be up to four orders of magnitude higher than in other
plants. These naturally occurring plant species have evolved the ability to take up, tolerate and accumulate trace elements
to exceptionally high concentrations in their aerial tissues and maintain metal homeostasis (Reeves, Baker 2000).
The term ‘hyperaccumulator’ was used to describe plants that accumulate more than 1000 μg Ni g
−1
of dry weight in their
shoots growing in its natural habitat (Brooks et al., 1977). The phrase ‘growing in its natural habitat’ means that
hyperaccumulators must achieve their high metal concentrations while remaining healthy enough to maintain a self-
sustaining population. Subsequently, the term hyperaccumulator has expanded to other trace elements such as Cd, Co, Cr,
Cu, Mn, Pb, Se and Zn, and more than 500 plant taxa have been reported to be hyperaccumulators as reviewed by Reeves
and Baker (2000). Plant trace element hypertolerance and/or hyperaccumulation are clear ecophysiological adaptations to
metalliferous soils.
The first physiological step in the accumulation process is an enhanced Zn uptake into the root cells. Zinc may enter the
plants through the plasma membrane of the root endodermal cells, through the symplast, or by crossing the root apoplast
through intercellular spaces. A reduced vacuolar sequestration in root cells is the next step, together with an enhanced
symplastic transport through the root towards the vasculature for increased xylem loading. The latter process is
accompanied by Zn distribution and detoxification in shoots, where metals are detoxified and stored in vacuoles with a
high storage capacity, as well as a phloem redistribution (Marschner, 1995). Both symplastic and apoplastic fluxes may
contribute to net Zn fluxes to the shoot (Sattelmacher, 2001; White et al., 2002).
Hyperaccumulator plants represent a low cost treatment for remediating metal polluted soils for their capacity to extract
high concentrations of metals and concentrate them in their upper parts. However, for achieving commercially
remediation programs, the bioconcentration factor (BCF, expressed as the ratio of trace element concentration in plant
shoot to trace element concentration in the soil) should be considered. Hence, the high Zn concentration in aereal tissues
and the higher the biomass production, the less the time necessary to reduce the pollutant content in the soil.
Ideal plants for phytoremediation purposes should be fast growing, have high a biomass, deep roots, be easy to harvest
and should tolerate and accumulate a wide range of trace elements in their aerial or harvestable parts. Native plants

7. should be preferred because they are better adapted in terms of survival, growth and reproduction than plants introduced
from other environment. To date, no plant has been described that fulfils all these criteria. Plant adaptation to high soil
trace elements availability usually include slow growth rates, and high root to shoot biomass ratio (Chaves et al., 2003). In
order to increase the chance of survival in adverse environmental conditions, plants priorize sexual reproduction at the
cost of reducing size and shortening the life cycle. This fact is considered a serious constraint to the use these plants for a
commercial-based phytoextraction programs. To overcome the low growth rate and low biomass production of
hyperaccumulator plants, different strategies can be considered. One of the most promising is the application of
biotechnology for adding specific traits from metal-hyperaccumulator to high biomass crops by using genetic engineering
methods.
5. Feasibility of plant biotechnological improvement for phytoremediation strategies
In general terms, there is an agreement that metal hyperaccumulation is an evolutionary adaptation proving protection
against herbivores and pathogens (Bhargava et al. 2012). Less than 0.2% of angiosperms have this capacity. This property
has been reported in over 450 species of vascular plants from 45 angiosperm families, including members of Asteraceae,
Brassicaceae, Caryophyllaceae, Cyperaceae, Cunonisceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae and
Euphorbiaceae (Bhargava et al. 2011). However, the best known angiosperm hyperaccumulator of metals is Thlaspi (now:
Noccaea) caerulescens (pennycress), that can accumulate large amounts of Zn (39,600 mg kg-1
) and Cd (1800 mg kg-1
)
without showing visual toxicity symptoms. Because of the easy growing under laboratory conditions, this plant has been
widely used in experimental system to study the mechanisms of Zn uptake, accumulation and tolerance in relation to Zn
phytoextraction. As most of the research background on this species has been made on a laboratory scale using hydroponic
systems, there is still a knowledge gap regarding Zn hyperaccumulation behavior under field conditions in contaminated
soils.
There was an impressive progress in characterizing soil chemistry management needed for phytoremediation and
physiology of metal-accumulating plants (Shao et al. 2010). Recent advances using novel methodologies (e.g. microarray-
based transcript analysis, Synchrotton X ray fluorescence microscopy, etc.) contributed to achieve a more comprehensive
global picture of plant metal homeostasis (Puig and Peñarrubia, 2009). However, phytoremediation technology is still an
emerging technology, with only few practical or commercial applications.
The biochemical bases of metal accumulation was recently covered by other authors with great detail (Hall and Williams,
2003; Eide, 2003; Puig and Peñarrubia, 2009; Lal, 2010; Hassan and Aarts, 2011; Claus and Chavarria-Krauser, 2012).
Most of the work on hyperaccumultors has focused on the physiological mechanism of metal uptake, transport, and
sequestration, but relatively little is known about its genetic bases (Lal, 2010). Genetic variations in the ability to
accumulate metals are a key point to be addressed for improving metal accumulation traits, either in conventional and
genetic engineering approaches. Following is described the state of the art and recent advances of genetic-based approaches
for enhancing metal accumulation features in plants:
5.1 Conventional breeding
Traditional breeding uses available genetic variability within species to combine the traits useful for phytoextraction

8. (Bhargava et al. 2012) and evolves both selection strategy (e.g. identification of diverse parental combination) and
transferring of metal tolerance/enhanced uptake phenotype traits from small, slow growing hyperaccumulator species to fast
growing and high biomass producing crops. This is achieved by hybridization strategies. Among breeding tools, symmetric
and asymmetric somatic hybridation bas become an increasingly used tool for genetic improvement. Moreover, in vitro
breeding and somaclonal variation have been used to improve the potential of Brassica juncea to extract and accumulate
toxic metals, with successful results. Beside of this case, little research has been carried out on breeding assessment of metal
accumulation trials in other high yielding species. Therefore, a research effort is needed for increasing scientific knowledge
on this matter. Mayor research needs should be focus on field studies, where a huge lack of information can be observed.
5.2 Genetic engineering
Conventional breeding deals with some severe sexual incompatibility constraints between taxa. For overcoming this issue
Biotechnology open new opportunities by direct transferring of specific genes to target plants. Thus, by applying genetic
engineering technology it can be obtained a transgenic plant that combine both enhanced metal accumulation features and
high biomass yield. Although scientific knowledge of molecular basis underlying metal tolerance/accumulation mechanism
has grown impressively over recent times, there limited information on metal perception and signal transduction pathways
in plants (Shao et al. 2010). Gene expression is a complex process evolving frequently different genes. Furthermore, metal
accumulation related genes have a tissue-depending expression, with a gradient of expression intensities depending on
genotype features, environmental conditions and their interaction. In other words, gene expression takes place at tissue
level, usually with a gradual modulation of the expression depending of genotypic to environment interactions. Heavy
metals affect plant gene expression at different scales, affecting DNA directly or via chromatin structure modification. The
activation of heavy metal stress-responsive genes occurs by a complex array of signaling pathways, which is a dimensional
network. Different secondary mediators participate in the activation of regulatory proteins that bind to promoter regions of
target genes and also posttranscriptional regulation occurs (Figure 1).

9. Figure 1. A framework for the gene expression and regulation when plants are exposed to trace elements
The introduction and over expression of the hyperaccumulating genes into a non-hyperaccumulator plant could be a
promising strategy for enhance metal uptake, accumulation, tolerance and detoxification features (Bhargava et al 2012).
Many promising genes are reported to participate in these processes and different genetic engineering are used by use them
in transgenic plants (Table1 and Table 2).
Table 1. Some important genes involved in metal tolerance and accumulation (adapted from Bhargava et al. 2012)
Metal Gene
Al aha2
Fe frd3
Zn mtp1, mtp3
Cu Cbf
Cd atcax2, atcax 4, mt1
Ni reg2, sat-c
Table 2. Transgenic approaches for enhancement of Zn uptake in different plants (Adapted from Bhargava et al. 2012)
Plant Methodology/approach
Arabidopsis
Brassica juncea
Overexpression of Zn Induced Facilitator 1 (ZIF1)
Overproduction of the glutamylcysteine synthetase and glutathione synthetase

10. Nicotiana tabacum Overexpression of glyoxalase pathway enzymes
Genetic engineering is a powerful technology for improving Zn accumulation traits of high biomass plants for
phytoremediation purposes. By applying this technology an “ideal” plant can be obtained combining both phyto-
accumulation traits (e.g. high metal accumulation) and high biomass productivity. A general model for developing a metal
tolerance transgenic plant is shown in Figure 2.
Figure 2. Development of metal tolerant/accumulator plant using genetic engineering (adapted from Karenlampi et
al. 2000, cited by Bhargava et al. 2012).
An “ideal” transgenic plant for phytoextraction purposes should have the following characteristics: tolerate high levels of
the element in roots and shoot cells, ability to traslocate the metal from roots to shoots, rapid metal uptake, high growth rate
and high aboveground biomass
Apart of metal accumulating genes, other kind of traits has been gaining research interest such as organic acid exudation
and siderophores exudation at rhizosphere level. By introducing genes encoding these phenotypic characteristics, an
improvement in metal uptake may be obtained due to the formation of metal chelating mechanisms.
Nowadays, the transgenic approach is an incipient technology only being tested at field scale for Hg, but environmental
issues has been stressed since Hgº is volatilized at the soil surface and can eventually be re-deposited on soil or water
resources (Bhargava et al. 2012). In spite of this context, clonal propagating has opened new possibilities through tree
remediation cultivars (e.g. Populus x canescens), with huge metal accumulator features.
Some of main reasons behind the quite low applicability of phytoremedition technologies are as follows: long period

11. required for clean up the soil, limited number of target metals that can be extracted, limited depth that can be reached by the
root system, lack of knowledge on the agronomic and management practices to be applied under metal-contaminated soils
and scarce understanding of metal hyperaccumulator plants either at tissue or subcellular level
6. Conclusions and future perspectives
Plants have evolved different responses to deal with stress originated from high trace element availability in soils. In
particular, Zn-tolerant plants developed the following strategies to keep Zn homeostasis: Zn exclusion, Zn tolerant or Zn.-
hypertolerant species. These efficient processes include physiological, molecular, genetic and ecological traits which give
certain species the ability to survive in contaminated sites. A great deal of knowledge has been generated on Zn tolerance
and hyperaccumulation in the last years. Adapted plant response mainly depend on cells molecular mechanism and by a
series of signal transductions. Future research has to ensure a better understanding of these molecular mechanisms. On the
other hand, the use of hyperacumulators for phytoremediation technologies require a rapid metal uptake, high growth rate,
high aboveground biomass and a high rate of root-to-shoot metal transfer. The growing application of genetic technologies
may led to increase plant trace elements resistance strategies, as well as increased uptake of trace elements.
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