Plant Stress Biology || Heavy Metal Stress in Plants

8

Heavy Metal Stress in Plants

Ann Cuypers, Karen Smeets, and Jaco Vangronsveld

8.1

Introduction

Over the past two centuries, anthropogenic and industrial activities have led to

high emissions of toxic metals into the environment at concentrations sig-

nificantly exceeding those originating from natural sources (Nriagu, 1988). Mining

and industrial processing are the main sources of heavy metal contamination in

soil [1, 2]. However, heavy metal pollution of soils resulting from the application of

phosphate fertilizers and sewage sludge, and irrigation with sewage effluents or

wastewater also causes major concern due to the potential risks involved [3].

Metals can be subdivided in essential micronutrients (e.g., iron, copper, zinc,

cobalt, and nickel) that are critical for normal development and growth of organisms

[4, 5] andother elements (e.g., such as cadmium, lead, andmercury) that are generally

considered nonessential. Whereas deficiencies of micronutrients can seriously dis-

turb normal development, excess of metals in general adversely affects biochemical

reactions and physiological processes in organisms, causing a major risk for the

environment and human health. Uptake and accumulation by food and feed crop

plants represents a main entry pathway for potentially health-threatening toxic

metals into food chains. Population-based studies as well as cellular studies inves-

tigating the mechanisms of metal toxicity have shown that elevated metal con-

centrations in the environment pose a tremendous risk for human health [3, 6–9].

Improving our knowledge concerning metal acquisition and homeostasis

together with defense and tolerance mechanisms in plants has numerous (bio-

technological) applications with regard to alleviating micronutrient deficiency as

well as soil metal contamination.

This chapter covers general aspects related to metal toxicity ranging from metal

uptake, distribution, and homeostasis, to cellular stress responses. It is our intent

to focus particularly on the mechanisms of metal-induced oxidative stress-related

responses and to point out the relation with signal transduction under metal

stress. This is becoming a scientific subject area of intense investigation and will

provide essential information to comprehend cellular responses to metal toxicity.

Plant Stress Biology. Edited by H. HirtCopyright r 2009 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32290-9

| 161

8.2

Uptake and Distribution of Metals in Plants

To minimize damage caused by nutrient deficiencies or metal toxicity, plants

possess a complex network of processes controlling metal uptake and transport

as well as metal homeostasis. Over the last decade, multiple excellent reviews

have been published describing our knowledge on metal uptake, transport, and

homeostasis in plants [10–17].

Plant metal uptake occurs mainly through roots; direct plant metal uptake

through the leaves is rather limited. Therefore, soil and water metal contents,

and more specifically metal bioavailability, are relevant for plant metal acquisition.

The bioavailability ofmetals to plants depends uponmetal speciation [18, 19], several

soil characteristics, such as pH, texture, and organicmatter [2, 5, 18], the occurrence

of plant-associated microorganisms (i.e., mycorrhiza and bacteria) and the plant

species. As iron deficiency is one of the most widespread nutrient imbalances

in agriculture, different plant species developed new strategies to solubilize and

acquire Fe3þ . Some plants acidify the soil through exudates and reduce Fe3þ before

transport, while others synthesize and transport Fe3þ -chelating agents named

phytosiderophores [14, 15, 17].

Apart from the bioavailable fraction of metals in the soil solution, uptake activity

and translocation efficiency also determine the plant’s metal uptake [11]. The cell

wall exerts binding places for metals [20], but with low selectivity and low affinity.

Transport of cationic metals across the plasma membrane is forced by the negative

membrane potential, and the presence of intracellular binding and storage sites

[12]. Several cation transporters have been identified, for example, transporters

belonging to the zinc- and iron-regulated protein (ZIP) family (zinc-regulated

transporter/iron-regulated transporter-like protein) are essential for iron and zinc

uptake, transporters of the natural resistance-associated macrophage protein

(Nramp) family are involved in iron acquisition, and the copper transporter is

highly specific for copper uptake [17]. Some of these transporters show a broad

substrate range enabling nonessential metals to enter the root cells [12].

Passage of the plasma membrane by metals is enhanced by intracellular binding

and sequestration. Once metal ions enter the cell, they are bound to chelators and

chaperones. Chelators contribute to metal detoxification by buffering cytosolic free

metal concentrations. Chaperones specifically deliver metal ions to organelles and

metal-requiring proteins [21]. Metal chelators include phytochelatins, metallothio-

neins [22], organic acids, and amino acids [23, 24]. Generally, it is assumed that the

major sites of metal sequestration in the roots are the vacuoles. Extensive research,

performed on vacuolar sequestration, has revealed a range of gene families involved

in intracellular metal transport. These include heavy metal ATPases (HMAs), the

multidrug-resistance-associated protein subfamily belonging to ABC transporters,

Nramps, the cation diffusion facilitator family, the ZIP family, and cation anti-

porters that are excellently described in the multiple reviews mentioned above.

The activities of metal-sequestering pathways in root cells are crucial in deter-

mining the rate of metal translocation to the aerial parts. The latter is an essential

162 | 8 Heavy Metal Stress in Plants

prerequisite when phytoextraction is utilized for the clean-up of metal-polluted

soils. Until recently, little was known about these transport mechanisms, but rapid

progress has been made in the identification and characterization of some of these

transporters, and in the role of metal ion ligands in metal homeostasis. Haydon

and Cobbett [16] recently reviewed the role of metal ion ligands for iron, zinc,

copper, manganese, and nickel in plants. In particular, they described the con-

tribution of mugineic acid, nicotianamine, organic acids, histidine, and phytate to

metal homeostasis and the proteins implicated in their transport. Xylem loading of

metal ions/ligands is a tightly controlled process mediated by membrane transport

proteins that are currently under intense study. Recently, FRD3 (FERRIC

REDUCTASE-DEFECTIVE3), a member of the multidrug and toxin extrusion

family of transporters, was shown to be a citrate transporter involved in iron

loading into the xylem [25]. Current findings in the research focusing on the

membrane transporter HMA4 in cadmium/zinc tolerance indicate a role in

the root-to-shoot translocation of cadmium and zinc [26–28].

In general, metal acquisition and sequestration into the leaf cells are more or

less similar as compared to the roots, with the exception of sequestration of metals

to the trichomes, for which scientific information is rather scarce.

8.3

Metal Stress Affects the Plant’s Physiology

Decreased biomass production has commonly been observed in plants subjected to

elevated metal concentrations. From a general biological as well as from plant

physiological point of view, essential and nonessential metals can be distinguished.

Essential micronutrients play a role as components of metalloproteins, as cofactors

in enzymatic catalysis, and in manifold other cellular processes. At supraoptimal

concentrations, however, micronutrients become phytotoxic and affect plant phy-

siology. Although slight growth stimulation might be observed at low concentra-

tions of some non-essential metals (e.g., cadmium, lead, etc.) they distinctly

interfere at higher concentrations, demonstrating similar effects as phytotoxic

amounts of micronutrients.

Several metals show a high affinity for sulfur and nitrogen donors, and

potentially bind to functional groups of macromolecules, causing metabolic

disruption [29, 30]. In general, almost all metals strongly bind to thiol groups

and thus to cysteine-rich proteins. This also makes up one of the most

important detoxification mechanisms against free metal ions in the cytosol –

chelation of metal ions to phytochelatins and/or metallothioneins. Both these

groups of molecules are cysteine-rich and are involved in metal homeostasis (for

a review, see [22]).

Phytotoxic concentrations of metal ions can also lead to substitution of essential

micronutrients in functional and structural proteins, resulting in a disturbance of

the cellular metabolism [31–33].

8.3 Metal Stress Affects the Plant’s Physiology | 163

Since metal phytotoxicity results in leaf chlorosis and growth inhibition, its

interference with photosynthesis, mineral nutrition, and the water balance has

been intensively studied (Table 8.1).

Table 8.1 Physiological processes affected in plants exposed to elevated metal concentrations.

Physiological effects on Metal Plant References

Photosynthesis chloroplast and thylakoid membrane,

PSI/II activity

Cd Hordeumvulgare

[34]

pigment content and thylakoids Cd Oryza sativa [35]

chlorophyll fluorescence parameters Cd Thlaspicaerulescens

[36]

net photosynthetic rate, calvin cycle

enzymes

Cd/

Cu

Cucumissativus

[37]

chlorophyll content and photosynthetic

parameters

Cu Populus [38]

CO2 fixation Cu/

Mn

H. vulgare [39]

carbohydrate metabolism Cu/

Ni

Pinussylvestris

[40]

chlorophyll content and chloroplast

ultrastructure

Ni Brassicaoleracea

[41]

photosystem II activity, CO2 fixation Zn Loliumperenne

[42]

chlorophyll content and photosystem II

activity

Cr L. perenne [43]

chlorophyll content and photosynthetic

rate

Pb H. vulgare,Avena sativa

[44]

Mineral nutrition Fe, Zn, Mn, Cu, Mg contents Cd O. sativa [45]

Fe translocation Cd Nicotianatabacum

[46]

Na, Mg, P, S, K, Ca, Mn, Fe, Cu, Zn

contents

Cd A. thaliana [47]

Mn, Mg, Cu, Zn, Fe, Ca contents Cd

and

Ni

Zea mays [48]

Ca, Mg, Fe contents Cu Rumexjaponicus

[49]

Mg, Mn contents Zn L. perenne [42]

Fe, Mn contents Zn Brassica rapa [50]

Fe, P, Ca, Mn, Mg contents Cr L. perenne [43]

Ca, Fe, Mn, Zn, Cu contents Pb Phaseolusvulgaris

[51]

K, Ca, Mg, Fe, Cu, Zn contents Mn C. sativus [52]

Water balance stomatal conductance Cd/

Cu

C. sativus [37]

transpiration rate Cr L. perenne [43]

water content Hg Lupinus albu [53]


8.4

Unraveling the Cellular Responses of Metal Stress

The search for primary targets of metal injury and thus the complete under-

standing of the mechanisms underlying metal toxicity has become an important

research area in many studies, but many aspects still remain elusive. Examining

global gene and protein expression under metal stress will help us to explore the

functioning and regulation of cellular metabolism under these circumstances.

The gathered data provide a basis for further research in order to unravel the

dynamics of metal-induced biological responses.

Transcriptional and/or proteomic profiling has recently been performed for

different trace elements: cadmium [32, 54, 55], aluminum [56], selenium [57],

arsenic [58, 59], and cesium [60]. Most of the affected genes and/or proteins can be

categorized into the following six groups.

The first group consists of genes related to metal transport and homeostasis

that can be specifically affected by metal exposure (see Section 8.2). A second

group is composed of genes related to cell wall metabolism that are affected

by metal stress. Jones et al. [61] demonstrated a rigidification of the cell wall

in roots of aluminum-exposed maize seedlings. Also, lignification seems to be

important under copper and zinc stress [62], where in this case more extra-

cellular binding sites for metals are hypothesized. A third group comprises

enzymes and proteins involved in energy and cell metabolism. It is clear that

a large number of enzymes involved in energy metabolism are induced under

metal stress as well as enzymes involved in the biosynthesis of amino acids.

A specific feature is the effect on sulfur metabolism that is strongly influ-

enced by most metal stresses. These data are linked with glutathione meta-

bolism. Glutathione plays a crucial role in the defense against metal stress

either through complexation – glutathione is the substrate for phytochelatin

production – or as a constituent of the antioxidant defense system and as

such is important in the cellular redox state [63]. A fourth group includes

genes and proteins involved in proper protein synthesis, folding, and mod-

ification as well as proteolysis. Interaction of metals with biomolecules,

through binding to functional groups or replacement of essential elements, is

known to affect protein turnover. Plant cells often produce heat shock pro-

teins (HSPs), after being subjected to metal stress [64]. These molecular

chaperones induced during metal stress could prevent irreversible protein

denaturation or help to channel their proteolytic degradation [55]. An

important group of genes or proteins coming to the fore in research on

metal-exposed plants are involved in general defense responses. Oxidative

stress-related genes/proteins and glutathione-S-transferases are categorized in

this group and are strongly affected by metal stress [65]. Finally, it is clear

that metal toxicity also activates components of signal transduction pathways

that make up another group of metal-affected genes/proteins. Both metal-

induced oxidative stress responses and signal transduction will be discussed

in the remainder of this chapter.

8.4 Unraveling the Cellular Responses of Metal Stress | 165

8.4.1

Metal-Induced Oxidative Stress

In general, growth reduction is observed in plants exposed to elevated metal

concentrations. Nevertheless, it is difficult to detect a common (path)way of

action at the cellular level, due to complex interactions between metal ions and

metabolism. Metal-induced oxidative stress has been demonstrated in multiple

studies (Table 8.2). This oxidative stress can lead to disruption of cellular

macromolecules (e.g., degradation of proteins, cross-links in DNA, and mem-

brane fatty acid peroxidation). However, the elevated reactive oxygen species

(ROS) concentrations can also act in signal transduction [80, 81]. Owing to the

toxic effects of ROS, it is key to keep their production and detoxification under

tight control. Mittler et al. [82] described a large gene network consisting of at

least 152 genes in Arabidopsis controlling the delicate balance between ROS

toxicity and ROS signaling. Redox sensing and signaling associated with both

chloroplasts and mitochondria are integrated networks that are highly

important in the regulation of cellular processes in both control and stress

conditions [83].

Multiple studies have indicated that exposure of plants to a diverse array of

metals elicits oxidative stress in plant cells (for a review, see [84]; Table 8.2).

Whereas the majority of published articles have focused on metal-induced anti-

oxidant defense mechanisms, it is clear that the sources of ROS production are

currently under investigation (Table 8.2). Under natural conditions ROS are pro-

duced in organelles with a highly oxidizing metabolic rate or that possessing

electron transport chains, such as chloroplasts, mitochondria, and peroxisomes

[81]. Under metal stress, the chemical behavior of the metal studied is relevant in

terms of ROS production. Free redox-active metals, such as copper and iron,

directly enhance the production of hydroxyl radicals through the Fenton reaction.

Reduction of the oxidized metal ion can be achieved by the Haber–Weiss reaction

with superoxide radicals (O2��) as a substrate [81]. In addition to direct metal-

induced ROS production, plant cell NADPH oxidases come into play. They have

an important role in the cellular responses against both biotic and abiotic stresses,

among which is metal stress [70, 74, 85]. NADPH oxidase transfers electrons from

cytoplasmic NADPH to extracellular O2 to form O2��, followed by its dismutation

to H2O2. As such, they can function as intercellular responders to create local ROS

transients, possibly via the generation of a secondary messenger hydrogen per-

oxide (H2O2).

From the results obtained over the last few years, it is clear that elevated ROS

production is a general response of plants exposed to metal stress that either

leads to cellular damage, but can also act in signal transduction [80]. The con-

tribution of ROS in metal-induced signal transduction will be described in the

final section that discusses the findings on different signaling pathways during

metal stress.


8.5

Signaling Under Metal Stress

Genes induced bymetal stress can be classified into two groups: genes encoding for

proteins (i) giving direct protection against metal stress, such as detoxification

enzymes and other functional proteins, and (ii) regulating gene expression and

Table 8.2 Oxidative stress-related parameters affected in

plants exposed to elevated metal concentrations.

Oxidative stress-related parameters Metal Plant References

ROS

production

H2O2 Cd A. thaliana [66]

NADPH oxidases Cd BY-2 tobacco

cells

[67]

O2��, H2O2, NO

�, NADPH oxidases Cd Pisum sativum [68]

H2O2, NADPH oxidases Cd A. thaliana [69]

NADPH oxidases, mitochondrial electron

transfer

Cd several plants [70]

lipoxygenase Cd A. thaliana [47]

ROS, H2O2 Cd/

Hg

Medicago sativa [71]

O2��, H2O2, NADPH oxidases Cd/

Cu

A. thaliana [72]

H2O2 Mn C. sativus [52]

H2O2 Cr Brassica juncea [73]

O2��, H2O2, NADPH oxidases Ni Triticum durum [74]

Antioxidant

defense

mechanism

antioxidant enzymes Cd A. thaliana [66]

ascorbate, glutathione Cd A. thaliana [69]

antioxidant enzymes Cd A. thaliana [47]

antioxidant enzymes, glutathione Cd/

Hg

Z. mays [75]

ascorbate, glutathione, antioxidant

enzymes

Cd/

Hg

M. sativa [71]

ascorbate, glutathione, a-tocopherol,thiol-based reductases and peroxidases

Cd/

Cu

A. thaliana [76]

antioxidant enzymes Cd/

Cu

A. thaliana [72]

glutathione, redox state Cu Brassica napus [77]


enzymes

Cu P. vulgaris [78]

antioxidant enzymes Cu/

Zn

P. vulgaris [79]


enzymes

Mn C. sativus [52]

antioxidant enzymes Cr B. juncea [73]

antioxidant enzymes Pb P. vulgaris [51]

8.5 Signaling Under Metal Stress | 167

signal transduction, that is, transcription factors and kinases [86]. Despite our

growing knowledge regarding toxic responses towards heavy metals and detox-

ification mechanisms, information on metal sensing, regulation, and signal

transduction is rather limited. Kacperska [87] described possible (path)ways in

signal sensing engaged in plant responses to various abiotic stresses, suggesting a

complex network depending on the intensity of the stressor. As many oxidative

processes are influenced by exposure of plants tometal stress (Table 8.2), formation

of ROS might be the basis for interconnecting different signaling pathways.

It has been proposed that the metal-induced formation of ROS, together with

changes in the cellular redox state, provides signals leading to changes in gene

expression. H2O2 production is an immediate response to increased metal con-

centrations [69] and probably is a keymolecule involved in signal transduction events

after plant metal exposure [88–90]. In fact, ROS, such as H2O2, are ideal signaling

molecules as they are small andable todiffuseover short distances. They can influence

the expression of a number of genes involved in signal transduction, metabolism,

cellular organization, cell rescue, and so on [80, 82, 91, 125].

ROS are able to induce antioxidant defense mechanisms directly, such as via the

‘‘antioxidant-responsive element’’ (ARE) commonly found in the promoter region

of such genes. One of these ARE-induced genes is from CAT1 (CATALASE1),

whose gene expression was significantly upregulated during cadmium toxicity

[47, 92]. Cytosolic ascorbate peroxidases such as APX1 and APX2, and the plastidic

iron superoxide dismutase FSD1 are also known to be activated by H2O2 under

various stress situations [93] and are clearly affected by metal stress [47, 92].

Secondly, ROS can activate scavenging mechanisms via redox-sensitive tran-

scription factors or via the activation of kinase cascades, which in turn activate

transcription factors that trigger target gene transcription [82, 94].

Taking the above data into consideration, the sites at which stress-induced

alterations in the redox status of a cell and the generation of H2O2 molecules take

place are likely sensors of the stressful situation [87]. As such, apoplastic H2O2

formation by NADPH oxidases and intracellular production from chloroplasts and

mitochondria are candidates for sensing and consecutively activating gene reg-

ulation under metal stress [70].

Metals affect the gene transcription, expression, and activation of numerous

signaling proteins including mitogen-activated protein kinase (MAPK) proteins

and nuclear transcription factors, proteins involved in calcium and lipid signaling

as well as hormone signaling pathways [90].

The effects of metal stress on intracellular signal transduction may be direct

through the interaction of metals with proteins or indirect through the formation

of metal-induced ROS.

ROS can be detected by several cellular components such as ROS receptors and

redox-sensitive transcription factors [82]. Detection of ROS by receptors can result

in the release of Ca2þ from intracellular stores or in the activation of phospholi-

pases [89]. The generation of ROS, Ca2þ signals, and the activation of specific

phospholipases are thought to activate Ca2þ -dependent kinases as well as other

signal transduction cascades including the MAPK pathway [82, 95]. Previous


studies already demonstrated a coordinated link between cadmium exposure and

Ca2þ signaling, whether or not via H2O2 as an intermediate signaling molecule or

as a second messenger produced via a Ca2þ -induced oxidative burst [67]. In the

case of cadmium toxicity, Garnier et al. [67] demonstrated the accumulation of

H2O2 was preceded by an increase in cytosolic Ca2þ , essential to activate NADPH

oxidases in BY-2 cells. Yakimova et al. [96] also demonstrated the involvement of

calcium in cadmium-induced programmed cell death in tomato suspension cells.

Furthermore, they showed that lipid signaling takes part in this cadmium-induced

programmed cell death. Whereas information on metal-induced lipid signaling is

limited, increasing evidence indicates that lipids also function as mediators in

signal transduction [97]. When toxic metals are taken up from soil solution into

roots cells, the plasma membranes of these cells can be considered as a primary

target for metal action [98]. Moreover, membrane lipid peroxidation is a very

sensitive response caused by metal stress [78, 99].

Various components involved in the phosphatidic acid signaling, such as

phospholipase D, are important elements in the responses of Arabidopsis thalianaroots to cadmium exposure (Smeets, unpublished results).

MAPKs are one of the largest families of serine-threonine kinases in higher plants

that transduce extracellular signals to regulate cellular processes such as cell divi-

sion, hormone production, and defense mechanisms [100]. MAPK can be activated

in a matter of minutes and de novo translation is not required. The MAPK pathway

has been mentioned as a rapid activation mechanism after exposure to cadmium

[101], copper [85, 102], and iron (Tsai et al., 2006). Transcription factors such asHSF,

ZAT, WRKY, and MYB can be activated via multiple components of the MAPK

cascade [82, 103], and are also affected by metal stress (unpublished results).

Plant stress hormones are involved in signal transduction during metal stress.

The first indications suggesting the influence of stress hormones, such as ethylene

and jasmonates, appeared decades ago and evidence is still being produced [90, 104,

105]. Jasmonates and ethylene are considered as stress-responsive hormones that

can act as long-distance signaling compounds, but also can upregulate a defense

network by regulating the expression of transcription factors [57, 90, 106, 107].

The involvement of ethylene in plant responses to a variety of biotic and abiotic

stresses is well known [90, 104]. Ethylene signaling is negatively regulated by the

ethylene receptors; downstream signaling is only activated when ethylene is pre-

sent [108]. EIN2 (ETHYLENE-INSENSITIVE2) is one of the components involved

in the ethylene response and has strong similarities to members of the Nramp

metal-ion transporter family. Based on this similarity EIN2 might regulate ethy-

lene responses by altering ion concentrations of for instance calcium [108].

Exposure to metal stress is correlated to the formation of ethylene. Copper sti-

mulates the production of this phytohormone in intact bean seedlings [109, 110].

Formation of its immediate precursor 1-aminocyclopropane-1-carboxylic acid

(ACC) in the roots could be a result of copper treatment in this organ. ACC may

quickly migrate to the aerial plant organs and there be converted into ethylene,

as was described by Jackson [111]. Cadmium-induced cell death was accompanied

by a small but significant rise in ethylene production in tomato suspension

8.5 Signaling Under Metal Stress | 169

cultures [96]. It is clear that ethylene production is stimulated by severe heavy

metal stress that in turn may be triggered by intracellular oxidative stress [104].

Oxylipins are biologically active signaling molecules derived from oxygenated

polyunsaturated fatty acids [112]. Many oxylipins, in particular those belonging to

the jasmonate family, are discussed as general inter- and intracellular signaling

compounds involved in multiple defense reactions. Jasmonates are oxylipins ori-

ginating from linolenic acid that is oxygenated by lipoxygenases to hydroperoxide

derivatives [88, 107]. Lipid peroxidation initiated by cytoplasmic lipoxygenases

seems to play a prominent role under both cadmium and copper exposure [92,

113]. According to previous research, jasmonates and their derivatives are

important signaling routes under cadmium and copper stress [32, 72, 90,114–116].

WRKY transcription factors were reported to be involved as a downstream com-

ponent of jasmonate signaling [107] and are also induced at transcriptional level

after metal exposure (Smeets, personnal communication).

Extensive cross-talk between signalization pathways is common within various

types of (a)biotic stress situations [86]. Therefore, it is important to emphasize the

activation of multiple signal transduction pathways and their coordinated regula-

tion duringmetal stress. The strong effect ofmetals on oxidative processes can form

the basis for other connections with signaling responses. Current research is

demonstrating cross-talk between multiple signalization pathways, such as cross-

talk between calcium-dependent protein kinases and MAPKs under cadmium

stress as well as NADPH oxidase involvement in cadmium-induced MAPK activa-

tion was demonstrated [85]. Recent evidence suggests that H2O2 increases calcium

influx in Arabidopsis root cells [117] and similar genes involved in calcium signaling

were upregulated in response to selenate and activated a defense response [57].

Despite observations of extensive interactions between distinct hormonal signaling

pathways, our knowledge on the molecular mechanisms involved in these inter-

actions is still rudimentary and restricted to the case of heavy metal stress.

Metal-induced gene expression occurs primarily at the level of transcription, and

regulation of the temporal and spatial expression patterns of specific stress genes is an

important part of the plantmetal stress response. Numerous studies have shown that

transcription factors are important in regulating theplant responses to environmental

stresses [118, 119]. Although our knowledge concerning post-transcriptional regula-

tion of metal-induced gene expression is still in its infancy, it has been shown that

microRNAs are involved in mRNA degradation or translational repression [120].

Sunkar et al. [121] already showed a role of miRNA398 in the regulation of Cu/Zn-

superoxide dismutases during iron and copper stress, but clearly further research is

needed to elucidate and understand these mechanisms in metal stress responses.

8.6

Conclusions

Anthropogenic metal contamination is a worldwide problem. It is of great

importance to better understand the underlying molecular mechanisms of


metal-induced stress responses in plants. This information is most useful to

develop or adjust strategies for growing nonfood crops on heavy metal-con-

taminated agricultural soils, whether or not aiming at phytoremediation. It should

be noted, however, that heavy metal contaminants are usually heterogeneously

distributed in soils leading to a multipollution context, whereas most studies

perform analyses under homogeneous conditions. This should be taken into

consideration in further investigations.

When exploring the fundamental principles of cellular metal stress, differences

in the formation, order, and specificity of the cellular events can lead to the dis-

covery of metal-specific responses, and may be used as potential biomarkers. The

elucidation of the intricate interactions between intracellular signal transduction

and long-distance signaling molecules under metal stress provide new insights

into plant metal responsiveness and will be the subject for future studies.

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Plant Stress Biology || Heavy Metal Stress in Plants