Tree Physiology 31, 1319–1334
doi:10.1093/treephys/tpr090
Tree Physiology review: part of a special section on poplars and the environment
Growth, physiological and molecular traits in Salicaceae trees
investigated for phytoremediation of heavy metals and organics
Marta Marmiroli1,3, Fabrizio Pietrini2, Elena Maestri1, Massimo Zacchini2, Nelson Marmiroli1
and Angelo Massacci2,4
1Department
of Environmental Sciences, Section of Genetic and Environmental Biotechnology, Faculty University of Parma, Parco Area delle Scienze 11/A, Parma, Italy;
of Agro-environment and Forest Biology, National Research Council of Italy, Rome Research Area, Via Salaria Km 29.300, 00015 Monterotondo Scalo (RM), Italy;
3Corresponding author ([email protected]); 4Present address: Institute of Water Research, National Research Council of Italy, Rome Research Area, Via Salaria Km
29.300, 00015 Monterotondo Scalo (RM), Italy
2Institute
Received May 19, 2011; accepted August 7, 2011; published online November 2, 2011; handling editor Roberto Tognetti
Worldwide, there are many large areas moderately contaminated with heavy metals and/or organics that have not been
­remediated due to the high cost and technical drawbacks of currently available technologies. Methods with a good potential
for coping with these limitations are emerging from phytoremediation techniques, using, for example, specific amendments
and/or plants selected from various candidates proven in several investigations to be reasonably efficient in extracting heavy
metals from soil or water, or in co-metabolizing organics with bacteria flourishing or inoculated in their rhizospheres. Populus
and Salix spp., two genera belonging to the Salicaceae family, include genotypes that can be considered among the candidates for this phytoremediation approach. This review shows the recent improvements in analytical tools based on the identification of useful genetic diversity associated with classical growth, physiological and biochemical traits, and the importance
of plant genotype selection for enhancing phytoremediation efficiency. Particularly interesting are studies on the application of
the phytoremediation of heavy metals and of chlorinated organics, in which microorganisms selected for their degradation
capabilities were bioaugmented in the rhizosphere of Salicaceae planted at a high density for biomass and bioenergy
production.
Keywords: biomass, chlorinated compounds, clonal selection, contaminated soil and water, genetic engineering, pollutants,
poplar, rhizoremediation, willow.
Introduction
Salicaceae are a very large resource of useful material that
can be proposed for approaching many environmental issues
(http://www.fao.org/forestry/ipc/32608/en/). More than 30
species are included in the genus Populus, all diploids
(2n = 38). These can be crossed to obtain many fertile
hybrids with a number of unique and different characteristics
that contribute to enrich the diversity of the natural germplasm. The genus Salix includes a number of species even
larger than the genus Populus. There are about 450 species
of Salix worldwide, ranging from dwarf species to 40 m high
trees (Argus 1997). For example, the species used for biomass plantations are mostly shrubs from the subgenus
Caprisalix (Vetrix), containing 125 species worldwide. There
are many common characteristics between the two genera:
they are both spread in similar environments, from the
extreme northern regions of Europe and North America to the
warmer latitudes of Mediterranean areas (Bradshaw et al.
2000).
The small size of the Populus haploid genome (~550 Mbp,
only four times that of Arabidopsis, and 400 times smaller than
that of Pinus) has favoured the creation of 25 genetic maps
and the development of various molecular resources in different species (Cervera et al. 2004, Tuskan et al. 2006,
Markussen et al. 2007, Gaudet et al. 2008, Polle and Douglas
2010).
© The Author 2011. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
1320 Marmiroli et al.
The haploid genome of most members of the Salix spp. is
500 Mbp, distributed on 19 chromosomes. There is a considerable synteny between Populus and Salix genomes; thus the
extended knowledge on Populus genetic features is utilized to
gain new insight into the Salix genome resources. For example,
two genetic linkage maps in willows were constructed, showing a high degree of genomic conservation between willow and
poplar (Berlin et al. 2010).
Experimental approaches to phytoremediation
studies in Salicaceae
Various traits useful for phytoremediation have been studied
testing their variation both within and between species in laboratory and in field trials (see Table 1 for willow and for poplar).
Due to the long life cycle of trees, technologies able to reduce
the time and space needed for the selection of plant material
for phytoremediation of various contaminants, such as in vitro
culture and hydroponic growth, have been currently utilized.
Both these systems represent very useful tools for investigating the effect of metals on plants without the complexity of the
interactions between environmental factors that can affect
metal bioavailability in the soil. The potential of these techniques for the genetic improvement and study of metal tolerance in Salicaceae plants has been highlighted by Confalonieri
et al. (2003) and Castiglione et al. (2007), respectively.
Several examples of in vitro studies have been reported in
Table 1B concerning response to metal contamination, highlighting the efficacy of the approach in identifying relevant phenotypes. Most recently, Iori et al. (2011) showed a different
involvement of thiols, phytochelatins (PCs), organic acids and
antioxidant enzymes in the cadmium (Cd) tolerance and accumulation expressed by callus cultures of two parental clones of
Populus nigra.
Many works have compared the responses of willow and
poplar clones to the presence of heavy metals in a hydroponic
system for selecting the best phytoremediation material (see
Table 1: Šottníková et al. 2003, Lunáčková et al. 2003a,
2003b, Dos Santos Utmazian et al. 2007). Hydroponic culture
is a useful method for carrying out rapid selection from a considerable number of individuals. In fact, it allows a reduction in
growing and treatment time of the plants, space needed for the
experiment, variability due to environmental factors and limitations due to changes in bioavailability of the metal between
different soils. In general, data obtained from hydroponic
screening need to be confirmed by field performance trials,
even though Watson et al. (2003) recently pointed out that the
results obtained from hydroponic and field experiments are
comparable. The results from the laboratory experiments at
times cannot be compared with those coming from field trials
due to the extremely controlled conditions of the former in
respect to the latter. Therefore, enthusiastic reports of
Tree Physiology Volume 31, 2011
l­aboratory experimental achievements fail to perform at the
expected level in the uncontrolled and harsh conditions of the
field.
Classic techniques investigating the physiological and
­biochemical traits of tolerance and heavy metal accumulation
in plant tissues have recently been integrated by the analytical
tools of ‘useful genetic diversity’. The simplest method used is
the analysis of polymorphisms. This analysis is known as amplified fragment length polymorphism (AFLP). The polymorphisms do not concern specific target genes; instead, they are
distributed randomly over the whole genome. This methodology benefits from the availability of the complete poplar
genome sequence (http://genome.jgi-psf.org/Poptr1/Poptr1.
home.html) (Tuskan et al. 2006, Sjödin et al. 2009). The
molecular genetic diversity approach can take advantage of
single-nucleotide polymorphisms (SNPs) in candidate genes
(Morin et al. 2004); these markers are co-dominant and highly
abundant, and their analysis provides results that are reproducible across laboratories (Gupta et al. 2001). Candidate genes
for different traits can be explored using SNPs in the search for
diversity with functional consequences (Joseph and Lexer
2008, Chu et al. 2009, Marmiroli et al. 2011).
High biomass production and relation with
phytoremediation traits
The application of trees to phytoremediation is favoured by the
possibility of long-term growth and high biomass of tissues
accumulating contaminants. Among tree species, Salicaceae
species have the capability of manifesting fast growth starting
from whips and poles. This is seen as a positive aspect in
phyto­remediation installations. Among other features, it gives
the possibility of deep rooting, by planting the poles at several
metres of depth (ITRC 2009). In order to increase the biomass
available for phytoremediation, different strategies are applicable: first of all short rotation coppicing (SRC) because it allows
the repeated harvest of biomass, thus subtracting contaminants
from the soil.
Salix spp. can provide the best yield and economic return in
SRC high plant density plantations, ~15,000 ha−1, coppiced on
a 2–4-year cycle (Bullard et al. 2002). The high biomass production, between 10 and 12 oven dry tonnes (ODT) ha−1 year−1,
is due to the ability to rapidly reach a maximum annual increment (Robinson et al. 2004). Other traits favouring biomass
production are tolerance of high planting densities, rapid
growth rate following coppicing, and vegetative propagation
from dormant shoots, all characteristic of plants adapted to
disturbed environments. The initial planting density and the
length of the rotation strongly influence the time span to reach
maximum mean annual increment. For Salicaceae biomass
crops, with planting densities from 14,000 up to 18,000
plants ha−1, the maximum mean annual increment is reached
Salicaceae and phytoremediation 1321
after 3–5 years (Kopp et al. 1997). Fjell (1985) explains in
detail the technical procedures for efficient planting in order to
obtain maximum biomass production.
At physiological level, coppicing removes apical dominance,
allowing the development of secondary bud on the remaining
stool into shoots (Paukkonen et al. 1992). Ethylene production
caused by coppicing promotes secondary bud development
(Taylor et al. 1982), with consequent increased number of
shoots on each stool, changes in leaf size and specific leaf area
associated with juvenile growth, increased net photosynthetic
rates and rapid development of the canopy (Kauppi et al.
1988). Long-term sustainability of coppice stands requires
harvesting after leaf fall (Bollmark et al. 1999). Tharakan et al.
(2005) reported on the yield of a 3-year Salix coppice stand.
Short rotation coppicing contributes to the fixation of carbon
and can provide an energy source substitute for fossil fuels.
These crops are used to produce heat and electricity directly
through combustion or indirectly through conversion to ­biof­uels.
The high net energy ratio for Salicaceae biomass electricity by
direct fire or gasification ranges from 10 to 13 MJ of electricity/
MJ of fossil fuel consumed across the life cycle of the system
(Keoleian and Volk 2005). Therefore, Salicaceae provide a
great opportunity to conserve fossil fuel resources, limiting the
greenhouse gas emissions within the range of 40–50 g CO2
equivalent/kWhelec.
Salicaceae, being obligate phreatophytes, need a permanent water supply for good biomass production. Many of their
genotypes have tap roots that can go very deep into the soil;
when they reach the water table they develop a capillary
fringe just above it. Thus, ecotypes of willow and poplar can
be found acclimated to the environmental conditions of many
different geographical areas and are regarded as local vegetation with discrete successes in both biomass production for
energy and even in many phytoremediation trials for decontaminating soils, as summarized in Table 1. Some Salix genotypes are known to grow well in soils with excess water and
they can also tolerate moderate chilling conditions. On the
contrary, some Populus genotypes can better tolerate conditions of moderately dry soils with some salinity (Kuzminsky
and Sabatti 1999). Regarding salinity, Mirck et al. (2009)
showed the potential of poplar and willow vegetation to recycle high-salinity waste waters.
The peculiar characteristics of SRC plantations for biomass
production, as shown in the previous subsection, can be considered among the ideal characteristics of candidate plants for
contaminated soil phytoremediation. Undoubtedly, an important
aspect of these characteristics is given by the ability of some
clones to form a tangled skein of coarse and fine capillaries
between the roots of plants growing close together. This large
rhizosphere occupies the soil down to 2–3 m and creates a
high capacity for nutrient uptake and eventual interactions
with contaminants. Furthermore, such a large rhizosphere can
diffuse the contaminants more homogeneously through physical root pathways inside the soil, increasing the probability of
organic degradation by exudates and soil enzymes and of
heavy metals becoming complexed with exuded ligands and
then being taken up by the roots (Pietrini et al. 2005). There
are many examples of these phytoremediation activities being
observed in poplars (see specific sections in this review).
However, some poplar and willow genotypes can be expected,
like most plants, to produce and exude stable metal ligands
from the roots, such as citric, malic or oxalic acids, or some
enzymes degrading organic compounds, such as peroxidase,
dehalogenase, chitinase, and proteolytic enzymes. It has been
widely shown that specific genotypes produce and exude more
of these molecules than other genotypes (Qin et al. 2007),
highlighting the high potential of Salicaceae biodiversity for
phytoremediation purposes.
Short rotation coppicing is not the only way to combine soil
remediation with fast-growing plants and high biomass production. Plants from the Salicaceae family can also be utilized
without coppicing for hydraulic control/evapotranspiration,
groundwater remediation, riparian barriers and landfill reclamation (ITRC 2009). The exploitation of poplar and willow for
urban restoration represents a meaningful example (Dickinson
et al. 2009).
A bottom-up approach in genetic traits studies
Studies of genetic properties for phytoremediation start by
observing phenotypes in clones and variants, and then trying
to identify the genes potentially involved in the observed properties. It has been remarked that tree species are not apt to
rapidly produce heavy-metal-tolerant genotypes (Pulford and
Watson 2003). However, they seem to have particular tolerance mechanisms that are possibly linked to their specific
physiology (Riddell-Black 1993).
Concerning heavy metal contamination, several studies have
tried to analyse the genetic variability that exists for tolerance
and accumulation (for a recent review, see Maestri et al. 2010).
Up to now, model plant species have been the preferred target
of these studies, especially Thlaspi caerulescens J. and C. Presl.
and Arabidopsis halleri (L.) O’Kane and Al Shehbaz. Recent findings point to the existence of multiple major genes for metal
tolerance, and to independent genetic determinations of tolerance and accumulation. Genetic mapping, inter- and intra-specific crosses, and genomic studies are the methodologies
currently applied for these types of plants. These have led to
the identification of candidate genes possibly involved in conferring tolerance or in stimulating hyperaccumulation. One such
example is ZTP1 (van der Zaal et al. 1999), involved in Zn
transport across membranes in Thlaspi. These types of studies
are virtually absent in plants of the Salicaceae family. Populus
and Salix are used and propagated as clones, and the genetic
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1322 Marmiroli et al.
Table 1. ​Review of literature concerning (A) Salix studies and (B) Populus.
Species or genera
Main results
(A) Salix
METALS IN FIELD TRIALS
Salix clones
Salix spp.
Salix discolor, S. viminalis
Salix spp.
Salix viminalis
Variability of Cd and Zn allocation in clones from polluted and
unpolluted areas: 1–72% of metal allocated to shoots
Heavy metal accumulation and tolerance, allocation to organs
Variability in transfer coefficients for heavy metals: Cd, Zn
higher than Ni, Cu, Pb
Exclusion of Cu and accumulation of Zn in shoot tissues
Correlation of plant height with heavy metal content in bark
Reference
Landberg and Greger (1994, 1996),
Greger and Landberg (1999)
Riddell-Black (1994), Punshon et al.
(1995), Watson et al. (1999)
Labrecque et al. (1995)
Nissen and Lepp (1997)
Sander and Ericsson (1998)
Salix spp.
Clonal variation in accumulation of heavy metals and
allocation to organs, effects on photosynthesis
Robinson et al. (2000)
Salix viminalis
Breeding for metal phytoremediation and comparison of
commercial varieties
Larsson (2001), Lindegaard et al. (2001)
Salix spp.
Variability in allocation of heavy metals to bark and wood
Pulford et al. (2002)
Salix viminalis
Leaf response to heavy-metal-contaminated soil
Hermle et al. (2007)
Salix spp.
Accumulation of heavy metals
Meers et al. (2007), Unterbrunner et al.
(2007)
Salix alba, S. caprea, S. purpurea
Metal accumulation from contaminated sites
Migeon et al. (2009)
Salix viminalis
Metal remediation for Cu, Cd, Pb, Zn, in combination with
arbuscular mycorrhizae
Bissonnette et al. (2010)
Salix spp.
Hydroponic experiments for response to heavy metals,
effects on physiological parameters and photosynthesis
Šottníková et al. (2003), Lunáčková et al.
(2003a, 2003b), Dos Santos Utmazian
et al. (2007)
Salix viminalis
Lack of phytochelatin induction in roots and leaves when
exposed to Cd
Landberg and Greger (2004)
Salix spp.
Uptake, accumulation and tolerance for Hg
Wang and Greger (2004)
Salix spp.
Cd accumulation and tolerance
Vassilev et al. (2005)
Salix spp.
Variability in tolerance responses of photosynthesis to Cd
contamination
Cosio et al. (2006), Dos Santos Utmazian
et al. (2007), Kieffer et al. (2009a),
Zacchini et al. (2009), Pietrini et al. (2010b)
Salix viminalis
Selective accumulation of metals in roots, remediation of Co
Mleczeka et al. (2009)
Salix spp.
Photosynthetic responses to a mixture of Cd, Pb and Ni
Pajević et al. (2009)
Salix clones
Response to Cd and physiological parameters, effect on
photosynthesis
Zacchini et al. (2009), Pietrini et al.
(2010a, 2010b)
Salix alba, Salix spp.
Differences among clones in accumulation of Cd and
localization in tissues of roots
Cocozza et al. (2011)
Salix alba
Involvement of sulphur compounds, organic acids and amino
compounds in metal trafficking and detoxification in plant cells
Zacchini et al. (2011)
Salix viminalis
Stimulation of microbial activity and PAH (polycyclic aromatic
hydrocarbon) degradation
Vervaeke et al. (2003)
Salix spp.
Variability of petroleum hydrocarbon remediation
Zalesny et al. (2005)
Salix spp.
Tolerance and degradation of soil PCP (pentachlorophenol)
Mills et al. (2006)
METALS IN LABORATORY TRIALS
ORGANICS IN FIELD TRIALS
ORGANICS IN LABORATORY TRIALS
Salix babylonica
Removal of ethanol and benzene from aquifers
Corseuil and Moreno (2001)
Salix babylonica
Uptake and removal of dieldrin
Skaates et al. (2005)
Salix babylonica
Uptake and phytovolatilization of methyl tert-butyl ether
Yu and Gu (2006)
Salix esigua
Removal of pharmaceutical organic compounds from
municipal treated wastewater
Franks (2007)
Continued
Tree Physiology Volume 31, 2011
Salicaceae and phytoremediation 1323
Table 1. ​Continued
Species or genera
Main results
Reference
Clonal variation in accumulation of heavy metals, allocation to
organs, effects on photosynthesis
Effect of Pb, Cd, Ni and Zn on physiological parameters
Leaf response to heavy-metal-contaminated soil
Accumulation of heavy metals
Phytorecurrent selection for phytoremediation of landfill
leachates
Metal accumulation from contaminated sites
Robinson et al. (2000), Laureysens et al.
(2004, 2005)
Baycu et al. (2006)
Hermle et al. (2007)
Unterbrunner et al. (2007)
Zalesny et al. (2007)
(B) Populus
METALS IN FIELD TRIALS
Populus clones
Populus nigra
Populus tremula
Populus tremula
Populus spp.
P.
deltoides × Populus
nigra,
Populus tremula × Populus alba,
P. tremula × Populus tremuloides,
P. trichocarpa × P. deltoides
Populus × generosa
Populus deltoides × Populus nigra
Populus alba
Migeon et al. (2009)
Metal remediation for Cu, Cd, Pb, Zn, in combination with
arbuscular mycorrhizae
Cd accumulation from contaminated soils
Bissonnette et al. (2010)
Wu et al. (2010)
Phytoremediation of tannery sludge
Shukla et al. (2011)
In vitro plantlets study on accumulation of metals, capacity
for lead accumulation without toxic effects
Hydroponic experiments for response to heavy metals,
effects on physiological parameters and photosynthesis
Kališová-Špirochová et al. (2003)
METALS IN LABORATORY TRIALS
Populus tremula × tremuloides
Populus spp.
Populus deltoides × Populus nigra
(Populus × euramericana)
Response to different Zn concentrations
Šottníková et al. (2003), Lunáčková et al.
(2003a, 2003b), Dos Santos Utmazian
et al. (2007)
Di Baccio et al. (2003, 2005)
Populus tremula × P. alba
In vitro screening of shoot cultures for tolerance to Al, Cu, Pb
Bojarczuk (2004)
Populus trichocarpa × deltoides
Cloning of genes for metallothioneins
Kohler et al. (2004)
Populus deltoides × maximowiczii,
P. × euramericana
Differential response to tanneries material containing heavy
metals: physiological parameters and metal accumulation
Sebastiani et al. (2011), Tognetti et al.
(2004)
Populus clones
Phytoextraction of Cd
Pilipović et. al. (2005)
Populus × euramericana
(P. deltoides × P. nigra)
Response to Cu and physiological parameters
Borghi et al. (2007, 2008)
Populus alba
In vitro studies on molecular and biochemical aspects
­regarding Zn and Cu toxicity
Franchin et al. (2007)
Populus nigra × maximowitzii × P. nigra
Response to Cd stress, effect on photosynthesis
Nikolić et al. (2008)
Populus spp. Populus tremula × P. alba
Proteomics of Cd response
Kieffer et al. (2008, 2009a, 2009b),
Durand et al. (2010)
Populus × euramericana clone
Response to Zn and physiological parameters
Di Baccio et al. (2009)
Populus canescens
Accumulation of Zn
Langer et al. (2009)
Populus spp.
Photosynthetic responses to a mixture of Cd, Pb and Ni
Pajević et al. (2009)
Populus clones
Response to Cd and physiological parameters, effect on
photosynthesis, allocation to organs
Zacchini et al. (2009), Pietrini et al.
(2010a, 2010b)
Populus alba
Mycorrhizal symbiosis effect on growth in heavy-metal-­ Cicatelli et al. (2010)
contaminated soil
Populus alba
In vitro screening for differential clonal response in microshoots for metal tolerance and accumulation
Di Lonardo et al. (2010)
Differences among clones in accumulation of Cd and
localization in tissues of roots
Cocozza et al. (2011)
Populus nigra
Metal-binding compounds and antioxidants in callus cultures
related to Cd tolerance
Iori et al. (2011)
Populus nigra
Involvement of sulphur compounds, organic acids and amino
compounds in metal trafficking and detoxification in plant cells
Zacchini et al. (2011)
Populus × canadensis,
nigra
Populus
Continued
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1324 Marmiroli et al.
Table 1. Continued
Species or genera
Main results
Reference
Uptake, hydrolysis and dealkylation of atrazine
Burken and Schnoor (1996, 1997)
ORGANICS IN FIELD TRIALS
Populus deltoides × nigra
Populus trichocarpa × P. deltoides Uptake and transpiration of carbon tetrachloride
Wang et al. (2004)
Populus spp.
Variability of petroleum hydrocarbon remediation
Zalesny et al. (2005)
Populus spp.
Tolerance and degradation of soil PCP
Mills et al. (2006)
Populus deltoides × (Populus tricho- In situ bioaugmentation with engineered endophytic
bacteria decreases trichloroethylene (TCE) evapotranscarpa × Populus deltoides)
piration, due to increased degradation
Weyens et al. (2009)
Populus spp.
Bianconi et al. (2011)
Removal of recalcitrant HCH isomers in the presence of
inoculation with selected Arthrobacter strains
ORGANICS IN LABORATORY TRIALS
Populus spp.
Biotransformation of TCE to trichloroethanol, di- and trichloracetic acid, mineralization to CO2
Newman et al. (1997), Gordon et al. (1998)
Populus deltoides × nigra
Uptake and degradation of explosives, including TNT
Thompson et al. (1998)
Populus deltoides × nigra
Uptake and transpiration of 1,4-dioxane
Aitchison et al. (2000)
Populus deltoides × nigra
Reduction of perchlorate
Van Aken and Schnoor (2002)
Populus deltoides × Populus nigra In vitro screening of cell cultures for absorption and degradation Mezzari et al. (2004)
of explosives RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and
HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)
Populus deltoids × nigra
Uptake and removal of dieldrin
Populus deltoides × nigra
Hydroponic cultures for gene induction during explosives Tanaka et al. (2007)
decontamination
Populus deltoides × nigra
Uptake and translocation of polychlorinated biphenyls (PCBs);
translocation is inversely related to PCB hydrophobicity
Liu and Schnoor (2008), Liu et al. (2009)
Populus deltoides × nigra
Interaction with mycorrhizal fungi increases explosive
decontamination (RDX)
Thompson and Polebitski (2010)
approaches in these plants have been quite different from those
listed above. Crosses between clones differing in phytoremediation capacities have never been described at a molecular level.
The genus Salix offers a wide range of genetic diversity; therefore, as there are only a limited number of domesticated willow
crops, there is substantial potential for increasing yield and
improving metal tolerance and pest resistance through traditional breeding and hybridization. During the last 15 years,
extensive efforts to breed Salix viminalis and other shrub willows for biomass production and phytoremediation have been
undertaken, mostly in Sweden and the UK. Through these longterm breeding programmes, a considerable wealth of information has been acquired that has elicited, for example, an
increase in yield from 12 to 67% in Sweden (Larsson 2001)
and from 8 to 143% in the UK (Lindegaard et al. 2001).
Molecular approaches
Many genes encoding proteins involved in adaptive tolerance
and the response to heavy metals have been identified and isolated from several plant species (Krämer 2010, Maestri et al.
2010). Specifically, they are genes encoding for plasma
Tree Physiology Volume 31, 2011
Skaates et al. (2005)
­ embrane transporters, tonoplast transporters, metallothioneins
m
for chelation, enzymes for the synthesis of nicotianamine, PCs
and glutathione (GSH), enzymes for the production and secretion of chelating substances in the rhizosphere, such as phytosiderophores, enzymes for sulphate assimilation, and enzymes
for lignin biosynthesis. Genes encoding metallothioneins were
also characterized in Populus (Kohler et al. 2004).
Other candidate genes as descriptors of phytoremediation
capabilities encode proteins involved instead in controlling the
phases of xenobiotic metabolism: cytochrome P450, N- and
O-glucosyltransferases, N- and O-malonyltransferases, sulphotransferases, glutathione-S-transferases and carboxylesterase (Sandermann 1994). Experiments with transgenic plants
suggest that an elevated expression of some of these enzymes
may also confer resistance to heavy metals and increase their
accumulation (Table 2).
Proteomics is an innovative approach that can lead to the
identification of new candidate genes (see, for example, Kieffer
et al. 2008, 2009a, 2009b, Durand et al. 2010). Proteins
responsive to stress induced by metal ions have been identified
and listed. In general, proteins involved in photosynthesis and
energy metabolism are repressed under conditions of metal
Salicaceae and phytoremediation 1325
Table 2. ​Review of literature involving poplars for phytoremediation.
Transgenic event
Main results
Reference
Transgenic poplar (Populus tremula × P. alba)
transformed with bacterial gsh1 (γ-glutamyl-cysteine
synthase)
Increased phytochelatin levels, enhanced
Cd accumulation; tolerance to Cd was not
affected
Transgenic poplar as above
Increased ability to metabolize the
chloroacetanilide herbicides acetochlor
and metolachlor
Modification of sulphur flux to thiol
compounds involved in detoxification of
metals and organic contaminants
Increased resistance and reduction of
mercury and organic mercury
Arisi et al. (1997, 2000), Noctor et al.
(1998), Koprivova et al. (2002),
Rennenberg and Will (2000), Bittsanszky
et al. (2005), Ivanova et al. (2009)
Gullner et al. (2001)
Transgenic poplar (P. tremula × P. alba) transformed
with enzymes for sulphur metabolism, sulphite oxidase
(SO) and adenosine 5′-phosphosulphate reductase
Cottonwood (P. deltoides) transformed with bacterial merA (mercuric ion reductase) or merB
(organomercurial lyase)
Populus alba transformed with pea PsMT
(metallothionein)
Transgenic poplar (P. tremula × P. alba) transformed
with poplar genes PtPCS1 (phytochelatin synthase)
Transgenic poplar (P. tremula × P. alba) transformed
with poplar genes PtHMA4 (heavy metal ATPase)
Transgenic poplar (Populus tremula × P. alba) transformed with mammalian CYP2E1 (cytochrome P450)
Hybrid aspen transformed with fungal MnP
(manganese-dependent peroxidase)
Hybrid aspen (P. tremula × tremuloides) transformed
with bacterial pnrA (nitroreductase)
Hybrid poplar (P. tremula × tremuloides) transformed
with bacterial cbnA (chlorocatechol dioxygenase)
Scheerer et al. (2010)
Che et al. (2003, 2006)
Increased tolerance to Cu and to oxidative
stress
Increase in leaf and stem Zn accumulation
Balestrazzi et al. (2009)
Increased Zn tolerance during rooting
Adams et al. (2011)
Increased metabolism of TCE
Doty et al. (2007), Kang et al. (2010)
Removal of bisphenol A
Iimura et al. (2007)
Increased tolerance and uptake of
explosive TNT
Results: efficient metabolism of
3-­chlorocatechol to 2-chloro-cis,
cis-muconate
Van Dillewijn et al. (2008)
treatment. Among the proteins with increased expression, those
involved in antioxidant systems are probably required in metal
homeostasis (Nikolić et al. 2008). Antioxidants have a fundamental role in metal detoxification: transition metals such as Cu
and Fe enhance the production of reactive oxygen species
(ROS), while non-redox reactive metals such as Hg and Cd
hamper the function of biomolecules via the displacement of
essential metal ions with consequent H2O2 accumulation and
oxidative burst. Reactive oxygen species levels are regulated by
antioxidant systems: ROS are useful at certain concentrations
for defence against pathogens, for marking developmental
stages and for lignification, and they also act as intermediate
signalling molecules in gene expression regulation. At high concentrations they are extremely toxic because they trigger
unspecific oxidation; thus a swathe of molecules such as ascorbate, GSH, tocopherol, and enzymatic scavengers such as peroxidases, catalases and superoxide dismutases act to decrease
the ROS excess (Schutzendubel and Polle 2002). The Populus
deltoides × P. nigra clone I-214 was tested for its response to
different Zn concentrations in order to understand how GSH is
involved in Zn detoxification. Interestingly enough, besides
showing that antioxidant responses are triggered by Zn in the
form of a total decrease in GSH and an increase in GSSG
­(oxidized glutathione), glutathione reductase (GR) expression
Adams et al. (2011)
Ohmiya et al. (2009)
a­ nalyses have shown that GR is regulated at transcriptional and
post-transcriptional levels and that Zn has a different impact on
each of these mechanisms (Di Baccio et al. 2005).
As far as organic contaminants are concerned, the issue of
genetic variability has rarely been addressed. The decontamination and metabolism of organic contaminants can be easily connected to the presence of enzymes in xenobiotic metabolism
(the green liver model), and such enzymes have been described
in several species (Sandermann 1994). However, their variability within species has not been described. Recently, hydroponic
cultures of Populus were used to measure gene induction during xenobiotic decontamination (Tanaka et al. 2007). Exposure
to the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)
led to the induction of genes encoding one glutathione-Stransferase, one cytochrome P450, two reductases and one
peroxidase, in accordance with knowledge on the enzymatic
pathways of detoxification. The approach described effectively
led to the identification of candidate genes.
The genomics of Populus, however, is well advanced and the
draft genome sequence of black cottonwood, Populus trichocarpa, was published in 2006 (Tuskan et al. 2006). The
sequence of the nuclear genome contains >45,000 putative
protein-coding genes. Data can be accessed and explored at
the PopGenIE website (http://www.popgenie.org, Sjödin et al.
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1326 Marmiroli et al.
2009). Populus nigra genotypes have been crossed to generate
an F1 mapping pedigree and to construct a highly informative
genetic map of these species, including genes of adaptive and
economic interest (Gaudet et al. 2008). Microsatellite markers
are available from the website of the Poplar Molecular Genetics
Cooperative (Cervera et al. 2001), and during sequencing >1
million SNPs and indels were identified, with an average of
2.6 polymorphisms per kilobase. Poplar microsatellites from
Salix buriatica (Edwards et al. 1996) were successfully used on
willow species (S. viminalis and Salix alba × Salix fragilis)
together with AFLP to build genetic linkage maps for agronomic
traits such as yield and pathogen resistance (Hanley et al.
2002, Barcaccia et al. 2003, Berlin et al. 2010). Growth and
nitrogen use efficiency, resistance to pathogens, water use and
mineral uptake have been found to be quantitative traits under
polygenic control. Phenotypic correlation and quantitative trait
loci (QTLs) for growth traits and traits related to water use were
analysed in a tetraploid hybrid F2 population originating from a
cross between a female diploid S. viminalis and a male hexaploid Salix dasyclados. This research reported that the QTL analysis suggested the existence of several clusters, one of which,
located on chromosome II (male map), appeared to control several traits such as leaf area productivity and leaf area ratio
(Weih et al. 2006). Advances in genomics for Salicaceae are
promising for future improvement in our knowledge of the basic
mechanisms involved in phytoremediation.
Physiological and biochemical trait studies
Studies of various traits of Salicaceae useful for heavy metal phytoremediation, such as heavy metal uptake, accumulation and
allocation in different plant parts, and tolerance at root and leaf
levels, including the physiological and biochemical mechanisms
at the basis of these activities, started in the 1990s as summarized in Table 1. Recent reviews describing the state of the art
have been published (Memon and Schröder 2009, Vangronsveld
et al. 2009, Rascio and Navari-Izzo 2011). Many studies conducted under both controlled and field conditions (Table 1)
showed that both Salix and Populus spp. extracted relatively high
quantities of various heavy metals from the soil, and remarkable
inter- and intra-species differences in this extraction were evidenced. A comparison of the relative quantity of Cd extracted in
situ by Populus clones selected for their survival in a contaminated area (Pietrosanti et al. 2008) can be done with data of
Thlaspi caerulescens, a hyperaccumulator species tested in soil
with a similar contamination level (McGrath et al. 2000). The
amount of Cd extracted by Salicaceae was smaller than the
amount extracted by the hyperaccumulator species per dry mass
unit, whereas it was higher than the amount extracted by hyperaccumulator species per plant and per hectare due to the greater
biomass production of Salicaceae trees. Specifically, Populus
extracted 250 g Cd ha−1 and Thlaspi about 125 g Cd ha−1 on an
Tree Physiology Volume 31, 2011
annual basis in a soil contaminated with 7–8 kg Cd ha−1. The Cd
shoot accumulation was ~11 mg and 40 mg kg−1 dm, the average plant dry matter was ~2.2 kg and 30 g, and the plant density
was ~1 and 80 plants m−2 for Populus and Thlaspi, respectively.
Using these data, it can be estimated that Populus would take at
least 30 years to remove the Cd from a moderately contaminated
soil, assuming that the plants did not change their extraction
capacity during that time. The objective of shortening this time
factor as much as possible is an experimental target that has
engaged many investigators. In this regard, interesting results
were obtained in high-density plantations of Salicaceae (8000–
10,000 plants ha−1) using a short rotation coppice system that
augmented the root density from top soil down to 2–3 m, and
thus increased the potential extraction of metals by the roots. A
greater interaction seemed to be provided by the proliferation of
bacteria (De Paolis et al. 2011) and by the addition of chemicals
able to form metal complexes. It is suggested that both these
situations increase the bioavailability of heavy metals. Besides
Cd, various clones of Salicaceae have been tested in the laboratory and in field studies (Table 1) for the remediation of soils
singly or simultaneously contaminated with other heavy metals
such as Zn, Ni, Cu, Pb, Co and Cr. Results confirmed that the
removal of most metals, at least below the European Union legal
limits, from typical, moderately contaminated soil, requires a
number of years, even as many as those indicated for Cd.
Differently, when Populus spp. were used to remove heavy metals from water (Pietrini et al. 2010a), the mean extraction efficiency was in the order of hundreds of times the extraction
efficiency in soil, according to Pietrosanti et al. (2008). Metal
availability and plant transpiration (the latter an important driving
force of passive extraction) were both enhanced in water and
were likely to be the main cause of the observed high efficiency.
Another interesting trait under investigation is the allocation
of various extracted heavy metals to above-ground organs
(Table 1). According to Zacchini et al. (2009), the allocation of
extracted heavy metals to these organs (preferentially in the
woody biomass) should be considered as one of the ideal traits
of candidate trees for efficient metal phytoremediation. This indication is supported by the following observations: most plants
accumulate metals in their roots, but the rapid turnover (days) of
their fine capillaries, where metals are ­generally bound (Cocozza
et al. 2008, 2011), would release them again into the soil. A
similar fate would follow the allocation of heavy metals into
leaves. On the contrary, heavy metal allocation to wood, especially when trees are managed as an SRC system, would remove
the metal from the soil with the harvested wood biomass every
2 or 3 years. Via the controlled combustion of the biomass,
most of the metal content could remain in a small volume of
recoverable ash, and it could even be easily isolated and reused
for various purposes (Pulford et al. 2002, Rockwood et al.
2004). Little knowledge on mechanisms involved in ­causing a
preferential allocation of metals to wood and bark with respect
Salicaceae and phytoremediation 1327
to leaves or roots has been gained. So far we have learned that
plant metal accumulation is correlated with wood (xylem) and
bark (phloem + periderm) dry mass, stool biomass, number of
shoots per stool and mean shoot diameter (Laureysens et al.
2004), and that Populus clones are significantly different regarding these properties. The authors concluded that selecting
clones for these biomass and growth features could result in the
corresponding selection of metal accumulation capacity in the
above-ground organs. Other studies showed that wood and bark
are important as large sinks for metals, particularly because they
are formed each growing season (Lepp 1996, Dickinson and
Lepp 1997). In the study of Landberg and Greger (1994), Salix
clones showed a shoot accumulation of Cd about five times
higher than the hyperaccumulators T. caerulescens and Alyssum
murale due to the high biomass production and transport of Cd
to shoots. Another interesting aspect was that the increased
application of metals to the trees did not necessarily lead to
increased metal concentrations in the tissues; Cu, Ni and Pb
plant concentrations were almost independent of soil concentrations, whereas Cd and Zn concentrations were more dependent,
probably because the soil solubility of these metals was higher.
Few studies have dealt with the physiological and b
­ iochemical
activities of Salicaceae in response to short- or long-term exposure to metals such as Cu, Cd, Pb, Ni, As and Cr (Table 1).
According to some authors, it is worth paying attention to the
interaction between heavy metals and photosynthesis. This
interaction can be either direct, when the metal arrives at the
leaf and enters the chloroplast, or indirect, when it competes for
absorption with metals essential for photosynthesis, such as Mg,
Fe and Mn. An inappropriate supply of these metals can cause
limitations to the capacity and activity of some enzymes, with
possible implications for the efficiency of photosynthetic activity.
The analyses of chlorophyll fluorescence and gaseous exchange
or other instrumental non-invasive techniques, performed simultaneously on the same leaf surface, are the technical approaches
used for studying the stress effects on photosynthetic components in vivo, and they could also be adopted for studying similar
metal effects (Pietrini et al. 2003). For example, imaging the
emission of leaf chlorophyll fluorescence and measuring X-ray
fluorescence on the same surface (Figure 1) allowed an assessment and confirmation of the direct interaction of photosynthetic
performances with spatial Cd contents (Pietrini et al. 2010b).
Regarding the indirect effects of the presence of toxic metals in
the rhizosphere on photosynthesis, there is evidence to show
the interference of Cd with the adsorption of other metals, such
as Fe, Mg, Zn and Mn, which are essential cofactors of enzymes,
pigments and structural components of photosynthetic apparatus. Studies regarding this aspect have been conducted on
Arabidopsis thaliana and other higher plants, but to the best of
our knowledge they have not been conducted on Salicaceae
(Krupa 1999, Solti et al. 2008). Pajević et al. (2009) studied the
photosynthetic responses of both willow and poplar clones to a
mixture of Cd, Pb and Ni with concentrations between 10−4 and
10−5 M and reported a significant metal- and genotypic-­
dependent reduction in photosynthesis. These authors showed
differences not only in metal absorption and leaf contents but
also in metal tolerance abilities between genera and their
respective clones. A crucial question arises from studies on photosynthesis and environmental stresses (i.e., metal stress)
regarding which of the main components of photosynthesis
(photochemical, metabolic or stomatal or mesophyll diffusion of
CO2) are altered by metal toxicity. Most studies on Cd, Zn and
Cu effects in higher plants reported major alterations in metabolic-related activities but relatively few photochemical activities
(Sanità di Toppi and Gabbrielli 1999). This high tolerance of the
photochemical process and a noticeable decrease in the photosynthetic potential (i.e., maximum activity of photosynthesis)
were found in some studies with Populus clones treated with
either 50 µM Cd or up to 1000 µM Cu (Borghi et al. 2007,
Pietrini et al. 2010a). A further target for studies was to
­determine the threshold concentrations of metal toxicity on photosynthesis. In this regard, Di Baccio et al. (2009) reported that
in a Populus × euramericana clone, this threshold for externally
bioavailable Zn concentrations was between 1 and 5 mM Zn
(concentrations far higher than the levels registered in
Zn-contaminated soils). Above this value, they reported a reduction in carboxylation activity without significant effects on stomatal functions. Furthermore, studies on various trees, targeting
the metal distribution in various organs, showed that the majority of these contaminants accumulated in roots, trapped in cortical cells, and that generally only small percentages moved to
stems, branches and leaves. For example, different Cd concentrations in roots and leaves were evidenced between Salix and
Populus spp., indicating that the metal translocation ability to
above-ground organs and the metal tolerance of various willow
clones were remarkably greater than those of various poplar
clones (Dos Santos Utmazian et al. 2007). This finding was confirmed by Pietrini et al. (2010b) in other clones of both Salix and
Populus spp. In the same work, the lack of PC induction in Salix
was indicated as the cause of Cd diffusion and sensitivity in
leaves compared with poplars. The lack of PCs was first reported
by Landberg and Greger (2004) for different Salix clones. An
involvement of PCs in Cd transportation was also reported by
Rennenberg and Will (2000), Verbruggen et al. (2009) and
Zacchini et al. (2011). Divergent tolerance responses of photosynthesis were found between different clones of the same
genus. These differences were attributed to changes in the activation of some stress-related biochemical molecules and mechanisms involved in Cd accumulation and tolerance (Cosio et al.
2006, Dos Santos Utmazian et al. 2007, Zacchini et al. 2009,
Kieffer et al. 2009a, Wu et al. 2010, Pietrini et al. 2010b).
The most common and visible effect of heavy metal toxicity
in plants is the loss of pigments (chlorophylls and carotenoids).
Krupa et al. (1996) suggested that this could be used as a
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1328 Marmiroli et al.
Figure 1. ​(a) Images of two representative leaves from Cd-treated poplar (A4A) and willow (SS5) clones. The spectra of Cd peaks obtained with
EDXRF on different areas of the same leaves are also reported. (b) Chlorophyll fluorescence images of the same leaves (ΦPSII) showing the
­different photosynthetic activities in the two clones due to a different Cd distribution on the leaf surface. The false colour code depicted at the
bottom of each image ranges from 0.000 (black) to 1.000 (pink, colour figure or white, black and white figure).
simple and reliable indicator of heavy metal toxicity in higher
plants, including trees, such as Salicaceae, green algae and
cyanobacteria. Pietrini et al. (2010a) reported differences in
the loss of chlorophyll in relation to differences in the sensitivity of various poplar clones to Cd.
Exposure of plants to metals, especially Zn, Ni, Al and Cd,
induced changes in the content of organic acids, namely citric
and malic acid (Saber et al. 1999, Bhatia et al. 2005). Organic
acids can bind metals through the formation of complexes. It
has been reported that feeding plants with citric acid resulted
in an enhanced metal root to shoot transport, probably through
xylem fluids (Chen et al. 2003). An involvement of organic
acids in Cd tolerance and accumulation in poplar and willow
clones was reported (Iori et al. 2011, Zacchini et al. 2011).
Zacchini et al. (2011) proposed a role for the participation of
amino compounds in the response and differential adaptation
of poplar and willow plants to Cd.
Organic degradation trait studies
According to Lynch (1987), plants release ~30% of their photosynthates into the soil in addition to enzymes, organic acids and
other metabolites. A similar or even slightly higher (35%)
­carbon percentage can be estimated for trees, including some
Tree Physiology Volume 31, 2011
Populus spp. (Grayston et al. 1997). The amount of released
carbon is one of the more likely reasons that make these plants
good hosts of microorganisms in their rhizospheres, including
microorganisms useful for metabolic degradation. Because of
this feature, they are capable of a higher rhizoquenching rate of
organic xenobiotics like BTEX (benzene, toluene, ethylbenzene
and xylene compounds) compared with other trees (Jordahl
et al. 1997). Indeed, field and greenhouse studies showed that
Populus spp. have an important role in the uptake and degradation of particular chlorinated organics, and that they can become
established in soils heavily contaminated with petroleum hydrocarbons (Table 1B). It is noteworthy that Populus spp. are
among the few tree genera that can associate in a mutualistic
symbiosis with endophytic ­bacteria (Taghavi et al. 2010) and
both types of mycorrhizal fungi (ectomycorrhizae and vesicular
arbuscular mycorrhizae) (Karliński et al. 2010), which have a
fundamental role in increasing the surface area of roots by up to
800-fold. It is not clear whether the fungi can directly degrade
contaminants or whether they contribute to bacterial degradation (Gunderson et al. 2007). Due to the quality and quantity of
knowledge about rhizodegradation mechanisms and plant–bacteria ­symbiosis, Populus spp. currently seem to be more suitable than willows for the phytoremediation of some persistent
organic pollutants (e.g., chlorinated hydrocarbons, see Table 1).
Salicaceae and phytoremediation 1329
A study on the variability of petroleum hydrocarbon remediation was recently carried out (Zalesny et al. 2005) on 20
Populus and two Salix clones. In the experiment, the clones
differed significantly in height and diameter, and a broad
genetic variation was seen for survival under contaminated
conditions. Commercial Populus clones performed better and,
according to the authors, this was attributed to their generalist
growth performance. The choice of performing genotypes
should be made in short-term treatability studies using clonal
selection indices from plant breeding.
Top-down approach
The study of phytoremediation could also involve genetic engineering with candidate genes, where enhanced properties in
the recipient plant could be expected (for a review, see Maestri
and Marmiroli 2011). The feasibility of genetic engineering in
poplar has enabled this approach for both inorganic and
organic contaminants (Table 2).
Transgenic poplar plants have been produced to improve
the phytoextraction of heavy metals by increasing the production of GSH in order to allow an increased synthesis of
PCs. The transgene was from Escherichia coli, gsh1, and it
encoded gamma-glutamyl-cysteine synthase (Arisi et al.
1997). Results showed improvement in Cd accumulation and
additionally increased tolerance to chloroacetanilide herbicides
(see Table 2). It is known from other plant species that GSH
conjugates to soluble intermediates of the metabolism of
these herbicides, and therefore the authors hypothesized that
increased GSH levels confer tolerance to herbicides. Che et al.
(2006) transformed cottonwood with a bacterial merB gene
involved in the protonolysis of a carbon mercury bond to produce a less toxic form of mercury, showing the need to combine more genes to obtain a transgenic tree useful in the
phytoremediation of mercury-contaminated waters. Recent
experiments (Adams et al. 2011) have used endogenous poplar genes under constitutive control to increase the tolerance
and accumulation of Zn.
Other experiments have addressed transformation with
genes involved in metabolism of organic contaminants (Table 2)
conferring new metabolic activities to poplar plants and cells.
Combining bottom-up and top-down approaches
The exploitation of genetic knowledge combines bottom-up
and top-down approaches in an integrated system. Traits
linked to interesting properties can be identified in a mapping
population and QTLs can be identified. Extreme genotypes on
both sides of the distribution can be identified and used for
microarray analysis in order to identify the main differences.
From this set of analyses, a panel of candidate genes can be
selected for mapping in relation to previously identified QTLs.
Genes of interest may also be tested in transgenic plants to
ascertain their role and function. The natural genetic variation
in these same genes can be studied with specific genetic
markers and applied in breeding. Novel approaches such as
ecotilling can be exploited in searching for genetic variation
and SNPs in natural populations. Ecotilling has been applied
to P. trichocarpa, focusing on nine specific genes (Gilchrist
et al. 2006).
Conclusions
The use of Salicaceae for heavy metal and organic phytoremediation associated with biomass production in high-density plantations is receiving a great deal of interest especially
with respect to large and moderately contaminated sites with
low economic interest. For enhancing the efficiency of this
technology, an in situ effective selection of plant material
and possibly of symbiotic microorganisms able to completely
degrade more recalcitrant organic contaminants in the rhizosphere is essential. A large availability of candidate plants for
selection can be obtained by studies of natural variation
within the Salicaceae germplasm in response to contaminants. Studies to unravel genome regions by QTLs, key
genes and alleles usefully involved in physiological and
­biochemical processes such as high levels of metal extraction and translocation to shoots and organic degradation
are key starting blocks to obtain an efficient in situ
phytoremediation.
Funding
N.M. and E.M. received funding from the University of Parma’s
Local Funding for Research. M.M. received funding from the
Italian Ministry for Education, University and Research (MIUR)
under PRIN2008 project No. 2008XS9YBC_002 ‘Genes,
genes products and decontamination functions of Salicaceae
spp. for metals and metal nanoparticles’. F.P., M.Z. and A.M.
received funding from MIUR under PRIN 2008 project No.
2008XS9YBC_003 ‘An investigation on some physiological
and biochemical aspects involved in the accumulation, distribution, tolerance and degradation of contaminants in poplar
clones’.
References
Adams, J.P., A. Adeli, C.-Y. Hsu, R.L. Harkess, G.P. Page, C.W. d
­ ePamphilis,
E.B. Schultz and C. Yuceer. 2011. Poplar maintains zinc homeostasis
with heavy metal genes HMA4 and PCS1. J. Exp. Bot.
62:3737–3752.
Aitchison, E.W., S.L. Kelley, P.J.J. Alvarez and J.L. Schnoor. 2000.
Phytoremediation of 1,4-dioxane by hybrid poplar trees. Water
Environ. Res. 72:313–321.
Argus, G.W. 1997. Infrageneric classification of Salix (Salicaceae) in
the New World. Syst. Bot. Monogr. 52: 1–121.
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1330 Marmiroli et al.
Arisi, A.C.M., G. Noctor, C.H. Foyer, L. Jouanin. 1997. Modification of
thiol contents in poplars (Populus tremula × P. alba) overexpressing
enzymes involved in glutathione synthesis. Planta 203:362–372.
Arisi, A.C.M., B. Mocquot, A. Lagriffoul, M. Mench, C.H. Foyer and L.
Jouanin. 2000. Responses to cadmium in leaves of transformed
poplars overexpressing γ-glutamylcysteine synthetase. Physiol.
Plant. 109:143–149.
Balestrazzi, A., S. Botti, S. Zelasco et al. 2009. Expression of the
PsMTA1 gene in white poplar engineered with the MAT system is
associated with heavy metal tolerance and protection against
8-hydroxy-2’-deoxyguanosine mediated-DNA damage. Plant Cell
Rep. 28:1179–1192.
Barcaccia, G., S. Meneghetti, E. Albertini, L. Triest and M. Lucchin.
2003. Linkage mapping in tetraploid willows: segregation of molecular markers and estimation of linkage phases support an allotetraploid structure for Salix alba × Salix fragilis interspecific hybrids.
Heredity 90:169–180.
Baycu, G., D. Tolunay, H. Özden and S. Günebakan. 2006.
Ecophysiological and seasonal variations in Cd, Pb, Zn, and Ni concentrations in the leaves of urban deciduous trees in Istanbul.
Environ. Pollut. 143:545–554.
Berlin, S., U. Lagercrantz, S. von Arnold, T. Öst and A.C. RönnbergWästljung. 2010. High-density linkage mapping and evolution of
paralogs and orthologs in Salix and Populus. BMC Genomics 11:129.
(14p.).
Bhatia, N.P., K.B. Walsh and A.J.M. Baker. 2005. Detection and quantification of ligands involved in nickel detoxification in a herbaceous Ni
hyperaccumulator Stackhousia tryonii Bailey. J. Exp. Bot. 56:​
1343–1349.
Bianconi, D., M.R. De Paolis, A.C. Agnello et al. 2011. Field-scale rhyzo­
remediation of a contaminated soil with hexachlorocyclohexane (HCH)
isomers: the potential of poplars for environmental restoration and
economical sustainability. In Handbook of Phytoremediation. Eds. I.A.
Golubev. Nova Science Publishers, New York, pp 783–794.
Bissonnette, L., M. St-Arnaud and M. Labrecque. 2010. Phytoextraction
of heavy metals by two Salicaceae clones in symbiosis with arbuscular mycorrhizal fungi during the second year of a field trial. Plant
Soil 332:55–67.
Bittsanszky, A., T. Komives, G. Gullner, G. Gyulai, J. Kiss, L. Heszky, L.
Radimszky and H. Rennenberg. 2005. Ability of transgenic poplars
with elevated glutathione content to tolerate zinc (2+) stress.
Environ. Int. 31:251–254.
Bojarczuk, K. 2004. Effect of toxic metals on the development of poplar (Populus tremula L. × P. alba L.) cultured in vitro. Pol. J. Environ.
St. 13:115–120.
Bollmark, L., L. Sennerby-Forsse and T. Ericsson. 1999. Seasonal
dynamics and effects of nitrogen and carbohydrate reserves in cutting-derived Salix viminalis plants. Can. J. For. Res. 29:85–94.
Borghi, M., R. Tognetti, G. Monteforti and L. Sebastiani. 2007.
Responses of Populus × euramericana (P. deltoides × P. nigra) clone
Adda to increasing copper concentrations. Environ. Exp. Bot.
61:66–73.
Borghi, M., R. Tognetti, G. Monteforti and L. Sebastiani. 2008. Responses
of two poplar species (Populus alba and Populus × canadensis) to
high copper concentrations. Environ. Exp. Bot. 62:290–299.
Bradshaw, H.D. Jr., R. Ceulemans, J. Davis and R.F. Stettler. 2000.
Emerging model systems: Poplar (Populus) as a model forest tree. J.
Plant Growth Regul. 19:306–313.
Bullard, M.J., S.J. Mustill, P. Carver, and P.M.I. Nixon. 2002. Yield improvements through modification of planting density and harvest frequency
in short rotation coppice Salix spp. 1. Yield response in two morphologically diverse varieties. Biomass Bioenergy 22:15–25.
Burken, J.G. and J.L. Schnoor. 1996. Phytoremediation: plant uptake of
atrazine and role of root exudates. J. Envir. Eng. 122:958–963.
Tree Physiology Volume 31, 2011
Burken, J.G. and J.L. Schnoor. 1997. Uptake and metabolism of atrazine
by poplar trees. Environ. Sci. Technol. 31:1399–1406.
Castiglione, S., C. Franchin, T. Fossati, G. Lingua, P. Torrigiani and S.
Biondi. 2007. High zinc concentrations reduce rooting capacity and
alter metallothionein gene expression in white poplar (Populus alba
L. ‘Villafranca’). Chemosphere 67:1117–1126.
Cervera, M.T., V. Storme, B. Ivens, J. Gusmão, B.H. Liu, V. Hostyn, J. van
Slycken, M. van Montagu and W. Boerjan. 2001. Dense genetic linkage maps of three Populus species (Populus deltoides, P. nigra and
P. trichocarpa) based on AFLP and microsatellite markers. Genetics
158:787–809.
Cervera, M.T., M.M. Sewell, P. Faivre-Rampant, V. Storme and W. Boerjan.
2004. Genome mapping in Populus. In Molecular Genetics and
Breeding of Forest Trees. Eds. S. Kumar and M. Fladung. Food Product
Press, New York, pp 387–410.
Che, D., R.B. Meagher, A.C.P. Heaton, A. Lima, C.L. Rugh and S.A.
Merkle. 2003. Expression of mercuric ion reductase in Eastern cottonwood (Populus deltoides) confers mercuric ion reduction and
resistance. Plant Biotechnol. J. 1:311–319.
Che, D.S., R.B. Meagher, R. Kim, A.C.P. Heaton, A. Lima, C.L. Rugh and
S.A. Merkle. 2006. Expression of methylmercury lyase in Eastern
cottonwood (Populus deltoides) confers organic mercury reduction
and resistance. In Vitro Cell Dev-Pl. 42:228–234.
Chen, Y.X., Q. Lin, Y.M. Luo, Y.F. He, S.J. Zhen, Y.L. Yu, G.M. Tian and
M.H. Wong. 2003. The role of citric acid on the phytoremediation of
heavy metal contaminated soil. Chemosphere 50:807–811.
Chu, Y., X. Su, Q. Huang and X. Zhang. 2009. Patterns of DNA sequence
variation at candidate gene loci in black poplar (Populus nigra L.) as
revealed by single nucleotide polymorphisms. Genetica 137:​141–150.
Cicatelli, A., G. Lingua, V. Todeschini, S. Biondi, P. Torrigiani and
S. Castiglione. 2010. Arbuscular mycorrhizal fungi restore normal
growth in a white poplar clone grown on heavy metal-contaminated
soil, and this is associated with upregulation of foliar metallothionein
and polyamine biosynthetic gene expression. Ann. Bot. 106:791–802.
Cocozza, C., A. Minnocci, R. Tognetti, V. Iori, M. Zacchini and G.
Scarascia-Mugnozza. 2008. Distribution and concentration of cadmium in root tissue of Populus alba determined by scanning electron
microscopy and energy-dispersive X-ray microanalysis. iForest
1:96–103.
Cocozza, C., L. Maiuro and R. Tognetti. 2011. Mapping cadmium distribution in roots of Salicaceae through scanning electron microscopy
with x-ray microanalysis. iForest 4:113–120.
Confalonieri, M., A. Balestrazzi, S. Bisoffi and D. Carbonera. 2003. In
vitro culture and genetic engineering of Populus spp.: synergy for
forest tree improvement. Plant Cell Tiss. Org. 73:109–138.
Corseuil, H.X. and F.N. Moreno. 2001. Phytoremediation potential of
willow trees for aquifers contaminated with ethanol-blended gasoline. Water Res. 35:013–3017.
Cosio, C., P. Vollenweider and C. Keller. 2006. Localization and effects
of cadmium in leaves of tolerant willows (Salix viminalis L.). Part I.
Macrolocalization and phytotoxic effects of cadmium. Environ. Exp.
Bot. 58:64–74.
De Paolis, M.R., L. Pietrosanti, G. Capotorti, A. Massacci and D. Lippi.
2011. Salicaceae establishment in a heavy metal-contaminated site
revealed by eco-physiological characterization of the culturable soil
bacterial fraction. Water Air Soil Pollut. 216:505–512.
Di Baccio, D., R. Tognetti, L. Sebastiani and C. Vitagliano. 2003. Responses
of Populus deltoides × Populus nigra (Populus × ­euramericana) clone
I-214 to high zinc concentrations. New Phytol. 159:443–452.
Di Baccio, D., S. Kopriva, L. Sebastiani and H. Rennenberg. 2005.
Does glutathione metabolism have a role in the defence of poplar
against zinc excess? New Phytol. 167:73–80.
Di Baccio, D. R. Tognetti, A. Minnocci and L. Sebastiani. 2009.
Responses of the Populus × euramericana clone I-214 to excess
Salicaceae and phytoremediation 1331
zinc: Carbon assimilation, structural modifications, metal distribution
and cellular localization. Environ. Exp. Bot. 67:153–163.
Dickinson, N.M. and N.W. Lepp. 1997. Metals and trees: impacts,
responses to exposure and exploitation of resistance traits. In
Contaminated soils: the 3rd International Conference on the
Biogeochemistry of Trace Elements. Ed. R. Prost. INRA, Paris,
pp 247–254.
Dickinson, N.M., A.J.M. Baker, A. Doronila, S. Laidlaw and R.D. Reeves.
2009. Phytoremediation of inorganics : realism and synergies. Int. J.
Phytoremediat. 11:97–114.
Di Lonardo, S., M. Capuana, M. Arnetoli, R. Gabbrielli and C. Gonnelli.
2010. Exploring the metal phytoremediation potential of three
Populus alba L. clones using an in vitro screening. Environ. Sci.
Pollut. Res. 18:82–90.
Dos Santos Utmazian, M.N., G. Wieshammer, R. Vega and W.W. Wenzel.
2007. Hydroponic screening for metal resistance and accumulation
of cadmium and zinc in twenty clones of willows and poplars.
Environ. Pollut. 148:155–165.
Doty, S.L., C.A. James, A.L. Moore et al. 2007. Enhanced phytoremediation of volatile environmental pollutants with transgenic trees.
Proc. Natl Acad. Sci. U.S.A. 104:16816–16821.
Durand, T.C., K. Sergeant, S. Planchon, S. Carpin, P. Label, D. Morabito,
J.F. Hausman and J. Renaut. 2010. Acute metal stress in Populus
tremula × P. alba (717–1B4 genotype): leaf and cambial proteome
changes induced by cadmium2+. Proteomics 10:349–368.
Edwards, K.J., J.H.A. Barker, A. Daly, C. Jones and A. Karp. 1996.
Microsatellite libraries enriched for several microsatellite sequences
in plants. Biotechniques 20:758–760.
Fjell, I. 1985. Preformation of root primordia in shoot and root morphogenesis in Salix viminalis. Nord. J. Bot. 5:357–376.
Franchin, C., T. Fossati, E. Pasquini, G. Lingua, S. Castiglione and
P. Torrigiani. 2007. High concentrations of Zn and Cu induce differential polyamine responses in micropropagated white poplar
(Populus alba L. ‘Villafranca’). Physiol. Plant. 130:77–90.
Franks, C.G. 2007. Phytoremediation of pharmaceuticals with
Salix exigua. MSc thesis, University of Lethbridge, 216 p.
Gaudet, M., V. Jorge, I. Paolucci, I. Beritognolo, G. Scarascia Mugnozza
and M. Sabatti. 2008. Genetic linkage maps of Populus nigra L. including AFLPs, SSRs, SNPs, and sex trait. Tree Genet. Genomes 4:25–36.
Gilchrist, E.J., G.W. Haughn, C.C. Ying et al. 2006. Use of ecotilling as
an efficient SNP discovery tool to survey genetic variation in wild
populations of Populus trichocarpa. Mol. Ecol. 15:1367–1378.
Gordon, M., N. Choe, J. Duffy et al. 1998. Phytoremediation of trichloroethylene with hybrid poplars. Environ. Health Persp. 106:1001–1004.
Grayston, S.J., D. Vaughan and D. Jones. 1997. Rhizosphere carbon
flow in trees, in comparison with annual plants: The importance of
root exudation and its impact on microbial activity and nutrient availability. Appl. Soil Ecol. 5:29–56.
Greger, M. and T. Landberg. 1999. Use of willow in phytoextraction.
Int. J. Phytoremediat. 1:115–123.
Gullner, G., T. Kömives and H. Rennenberg. 2001. Enhanced tolerance of
transgenic poplar plants overexpressing γ–glutamylcysteine ­synthetase
towards chloroacetanilide herbicides. J. Exp. Bot. 52:971–979.
Gunderson, J.J., J.D. Knight and K.C.J. Van Rees. 2007. Impact of
­ectomycorrhizal colonization of hybrid poplar on the remediation of
diesel-contaminated soil. J. Environ. Qual. 36:927–934.
Gupta, P.K., J.K. Roy and M. Prasad. 2001. Single nucleotide polymorphisms: a new paradigm for molecular marker technology and DNA
polymorphism detection with emphasis on their use in plants. Curr.
Sci. 80:524–535.
Hanley, S.J., J.H.A. Barker, J.W. Van Ooijen, C. Aldam, S.L. Harris, I.
Ahman, S. Larsson and A. Karp. 2002. A genetic linkage map of willow (Salix viminalis) based on AFLP and microsatellite markers.
Theor. Appl. Genet. 105:1087–1096.
Hermle, S., P. Vollenweider, M.S. Günthardt-Goerg, C.J. McQuattie and
R. Matyssek. 2007. Leaf responsiveness of Populus tremula and
Salix viminalis to soil contaminated with heavy metals and acidic
rainwater. Tree Physiol. 27:1517–1531.
Iimura, Y., M. Yoshizumi, T. Sonoki, M. Uesugi, K. Tatsumi, K. Horiuchi,
S. Kajita and Y. Katayama. 2007. Hybrid aspen with a transgene for
fungal manganese peroxidase is a potential contributor to phyto­
remediation of the environment contaminated with bisphenol A. J.
Wood Sci. 53:541–544.
Iori, V., F. Pietrini, A. Massacci and M. Zacchini. 2011. Induction of
metal binding compounds and antioxidative defence in callus
­cultures of two black poplar (P. nigra) clones with different tolerance to cadmium. Plant Cell Tiss. Organ Cult. doi:10.1007/
s11240–011-0006-8.
ITRC (Interstate Technology & Regulatory Council). 2009.
Phytotechnology Technical and Regulatory Guidance and Decision
Trees, Revised. PHYTO-3. Interstate Technology & Regulatory
Council, Phytotechnologies Team, Washington, DC, USA, Tech Reg
Update [www.itrcweb.org].
Ivanova, L.A., D.A. Ronzhina, L.A. Ivanov, L.V. Stroukova, A.D. Peuke
and H. Rennenberg. 2009. Chloroplast parameters differ in wild
type and transgenic poplars overexpressing gsh1 in the cytosol.
Plant Biol. 11:625–630.
Jordahl, J.L., L. Foster, J.L. Schnoor and P.J.J. Alvarez. 1997. Effect of
hybrid poplar trees on microbial populations important to hazardous
waste bioremediation. Environ. Toxicol. Chem. 16:1318–1321.
Joseph, J.A. and C. Lexer. 2008. A set of novel DNA polymorphisms
within candidate genes potentially involved in ecological divergence
between Populus alba and P. tremula, two hybridizing European
­forest trees. Mol. Ecol. Res. 8:188–192.
Kališová-Špirochová, I., J. Puncochárová, Z. Kafka, M. Kubal, P. Soudek
and T. Vanek. 2003. Accumulation of heavy metals by in vitro cultures of plants. Water Air Soil Pollut. Focus 3:269–276.
Kang, J.W., H.W. Wilkerson, F.M. Farin, T.K. Bammler, R.P. Beyer, S.E.
Strand and S.L. Doty. 2010. Mammalian cytochrome CYP2E1
triggered differential gene regulation in response to trichloroethylene (TCE) in a transgenic poplar. Funct. Integr. Genomics
10:417–424.
Karliński, L., M. Rudawska, B. Kieliszewska-Rokicka and T. Leski. 2010.
Relationship between genotype and soil environment during colonization of poplar roots by mycorrhizal and endophytic fungi.
Mycorrhiza 20:315–324.
Kauppi, A., M. Kiviniitty and A. Ferm. 1988. Growth habits and crown
architecture of Betula pubescens Ehrh. of seed and sprout origin.
Can. J. For. Res. 18:1603–1613.
Keoleian, G.A. and T.A. Volk. 2005. Renewable energy from willow
biomass crops: life cycle energy, environmental, and economic performance. Crit. Rev. Plant Sci. 24:385–406.
Kieffer, P., J. Dommes, L. Hoffmann, J.F. Hausman and J. Renaut. 2008.
Quantitative changes in protein expression of cadmium-exposed
poplar plants. Proteomics 8:2514–2530.
Kieffer, P., S. Planchon, M. Oufir, J. Ziebel, J. Dommes, L. Hoffmann, J.F.
Hausman and J. Renaut. 2009a. Combining proteomics and metabolite analyses to unravel cadmium stress-response in poplar leaves. J
Proteome Res. 8:400–417.
Kieffer, P., P. Schröder, J. Dommes, L. Hoffmann, J. Renaut and J.F.
Hausman. 2009b. Proteomic and enzymatic response of poplar to
cadmium stress. J. Proteomics 72:379–396.
Kohler, A., D. Blaudez, M. Chalot and F. Martin. 2004. Cloning and
expression of multiple metallothioneins from hybrid poplar. New
Phytol. 164:83–93.
Kopp, R.F., L.P. Abrahamson, E.H. White, K.F. Burns and C.A. Nowak.
1997. Cutting cycle and spacing effects on biomass production by a
willow clone in New York. Biomass Bioenerg. 12:313–319.
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1332 Marmiroli et al.
Koprivova, A., S. Kopriva, D. Jaeger, B. Will, L. Jouanin and
H. Rennenberg. 2002. Evaluation of transgenic poplars overexpressing enzymes of glutathione synthesis for phytoremediation
of cadmium. Plant Biol. 4:664–670.
Krämer, U. 2010. Metal hyperaccumulation in plants. Annu. Rev. Plant
Biol. 61:517–534.
Krupa, Z. 1999. Cadmium against higher plant photosynthesis—a variety of effects and where do they possibly come from? Z. Naturforsch.
C 54:723–729.
Krupa, Z., M. Baranowska and D. Orzol. 1996. Can anthocyanins be
considered as heavy metal stress indicator in higher plants? Acta
Physiol. Plant. 18:147–151.
Kuzminsky, E. and M. Sabatti. 1999. L’impiego di specie forestali nella
prevenzione e nel recupero di suoli salini. In Atti del II Congresso
Nazionale di Selvicoltura. Venezia, 24–27 Giugno 1998. Consulta
Nazionale per le Foreste ed il Legno, Ministero delle politiche
Agricole e Forestali–Direzione Generale per le Risorse Forestali,
Montane ed Idriche–e Accademia Italiana di Scienze Forestali.
Stampa EdAs s.r.l. Vol. 4, pp 203–209.
Labrecque, M., T.I. Teodorescu and S. Daigle. 1995. Effect of wastewater sludge on growth and heavy metal bioaccumulation of two Salix
species. Plant Soil 171:303–16.
Landberg, T. and M. Greger. 1994. Can heavy metal tolerant clones of
Salix be used as vegetation filters on heavy metal contaminated
land. In Report from Symposium; Willow Vegetation Filters for
Municipal Wastewaters and Sludges – a Biological Purification
System. Eds. P. Aronsson and K. Perttu. SLU Info/Repro, Uppsala,
Sweden, pp 133–144.
Landberg, T. and M. Greger. 1996. Differences in uptake and tolerance
to heavy metals in Salix from unpolluted and polluted areas. Appl.
Geochem. 11:175–180.
Landberg, T. and M. Greger. 2004. No phytochelatin (PC2 and PC3)
detected in Salix viminalis. Physiol. Plant. 121:481–487.
Langer, I., D. Krpata, W.J. Fitz, W.W. Wenzel and P.F. Schweiger. 2009.
Zinc accumulation potential and toxicity threshold determined for a
metal-accumulating Populus canescens clone in a dose–response
study. Environ. Pollut. 157:2871–2877.
Larsson, S. 2001. Commercial varieties from the Swedish willow
breeding programme. Aspects Appl. Biol. Biomass Energy Crops II
65:193–198.
Laureysens, I., R. Blust, L. de Temmerman, C. Lemmens and
R. Ceulemans. 2004. Clonal variation in heavy metal accumulation
and biomass production in a poplar coppice culture: I. Seasonal
variation in leaf, wood and bark concentrations. Environ. Pollut.
131:485–494.
Laureysens, I., De Temmerman L., Hastir T., Van Gysel M., Ceulemans
R. 2005. Clonal variation in heavy metal accumulation and biomass
production in a poplar coppice culture. II. Vertical distribution and
phytoextraction potential. Environ. Pollut. 133:541–551.
Lepp, N.W. 1996. Uptake, mobility and loci of concentrations of heavy
metals in trees. In Heavy metals and trees. Proceedings of a
Discussion Meeting. Ed. I. Glimmerveen. Institute of Chartered
Foresters, Edinburgh, pp 68–82.
Lindegaard, K.N., R.I. Parfitt, G. Donaldson, T. Hunter, W.M. Dawson,
E.G.A. Forbes, M.M. Carter, C.C. Whinney, J.E. Whinney and
S. Larsson. 2001. Comparative trials of elite Swedish and UK biomass willow varieties. Aspects Appl. Biol. Biomass Energy Crops II
65:183–192.
Liu, J. and J.L. Schnoor. 2008. Uptake and translocation of lesser-­
chlorinated polychlorinated biphenyls (PCBs) in whole hybrid poplar
plants after hydroponic exposure. Chemosphere 73:1608–1616.
Liu, J., D. Hu, G. Jiang and J.L. Schnoor. 2009. In vivo biotransformation
of 3,3′,4,4′-tetrachlorobiphenyl by whole plants—poplars and
switchgrass. Environ. Sci. Technol. 43:7503–7509.
Tree Physiology Volume 31, 2011
Lunáčková, L., E. Masarovičová, K. Králová and V. Streško. 2003a.
Response of fast growing woody plants from family Salicaceae to
cadmium treatment. Bull. Environ. Contam. Tox. 70:576–585.
Lunáčková, L., A. Šottníková, E. Masarovičová, A. Lux and V. Streško.
2003b. Comparison of cadmium effect on willow and poplar in
response to different cultivation conditions. Biol. Plantarum
47:403–411.
Lynch, J.M. 1987. The Rhizosphere. Wiley Interscience, Chichester.
Maestri, E., M. Marmiroli, G. Visioli and N. Marmiroli. 2010. Metal tolerance and hyperaccumulation: Costs and trade-offs between traits
and environment. Environ. Exp. Bot. 68:1–13.
Maestri, E. and N. Marmiroli. 2011. Transgenic plants for phytoremediation. Int. J. Phytoremediat 13(suppl. 1):264–279.
Markussen, T., B. Pakull and M. Fladung. 2007. Positioning of sex-­
correlated markers for Populus in a AFLP- and SSR-marker based
genetic map of Populus tremula × tremuloides. Silvae Genet.
56:180–184.
Marmiroli, M., G. Visioli, E. Maestri and N. Marmiroli. 2011. Correlating
SNP genotype with the phenotypic response to exposure to cadmium in Populus spp. Environ. Sci. Technol. 45:4497–4505.
McGrath, S.P., S.J. Dunham and R.L. Correll. 2000. Potential for phytoextraction of zinc and cadmium from soils using hyperaccumulator plants.
In Phytoremediation of Contaminated Soil and Water. Eds. N. Terry and
G. Bañuelos. Lewis Publishers, Boca Raton, FL, pp 109–128.
Meers, E., B. Vandecasteele, A. Ruttens, J. Vangronsveld and F.M.G.
Tack. 2007. Potential of five willow species (Salix spp.) for phyto­
extraction of heavy metals. Environ. Exp. Bot. 60:57–68.
Memon, A.R. and P. Schröder. 2009. Implications of metal accumulation mechanisms to phytoremediation. Environ. Sci. Pollut. Res. Int.
16:162–75.
Mezzari, M.P., B. Van Aken, J.M. Yoon, C.L. Just and J.L. Schnoor. 2004.
Mathematical modeling of RDX and HMX metabolism in poplar
(Populus deltoides × Populus nigra, DN34) tissue culture. Int. J.
Phytoremediat. 6:323–345.
Migeon, A., P. Richaud, F. Guinet, M. Chalot and D. Blaudez. 2009.
Metal accumulation by woody species on contaminated sites in the
north of France. Water Air Soil Pollut. 204:89–101.
Mills, T., B. Arnold, S. Sivakumaran, G. Northcott, I. Vogeler, B.
Robinson, C. Norling and D. Leonil. 2006. Phytoremediation and
long term site management of soil contaminated with pentachlorophenol (PCP) and heavy metals. J. Environ. Manage. 79:232–241.
Mirck, J., R.S. Jr. Zalesny, I. Dimitriou, J.A. Zalesny, T.A. Volk and W.E.
Mabee. 2009. The use of short rotation willows and poplars for
the recycling of saline waste waters. In Sixth International
Phytotechnologies Conference. St. Louis, MO. International
Phytotechnology Society, p. 68.
Mleczeka, M., M. Łukaszewskia, Z. Kaczmarekb, I. Rissmanna and
P. Golinskia. 2009. Efficiency of selected heavy metals accumulation
by Salix viminalis roots. Environ. Exp. Bot. 65:48–53.
Morin, P.A., G. Luikart, R.K. Wayne and The SNP Workshop Group. 2004.
Applications of single nucleotide polymorphisms (SNPs) in ecology,
evolution and conservation. Trends Ecol. Evol. 19:208–216.
Newman, L.A., S.E. Strand, N. Choe, J. Duffy, G. Ekuan, M. Ruszaj, B.B.
Shurtleff, J. Wilmoth, P. Heilman and M.P. Gordon. 1997. Uptake and
biotransformation of trichloroethylene by hybrid poplars. Environ.
Sci. Technol. 31:1062–1067.
Nikolič, N., D. Kojić, A. Pilipović, S. Pajević, B. Krstić, M. Borišev and
S. Orlović. 2008. Responses of hybrid poplar to cadmium stress:
­photosynthetic characteristics, cadmium and proline accumulation,
and antioxidant enzyme activity. Acta Biol. Cracov. Bot.
50:95–103.
Nissen, L.R. and N.W. Lepp. 1997. Baseline concentrations of copper
and zinc in shoot tissues of a range of Salix species. Biomass
Bioenerg. 12:115–120.
Salicaceae and phytoremediation 1333
Noctor, G., A.C.M. Arisi, L. Jouanin, K.J. Kunert, H. Rennenberg and
C.H. Foyer. 1998. Glutathione: biosynthesis, metabolism and relationship to stress tolerance explored in transformed plants. J. Exp.
Bot. 49:623–647.
Ohmiya, Y., T. Ono, T. Taniguchi, N. Itahana, N. Ogawa, K. Miyashita, K.
Ohmiya, K. Sakka and T. Kimura. 2009. Stable expression of the
chlorocatechol dioxygenase gene from Ralstonia eutropha NH9 in
hybrid poplar cells. Biosci. Biotechnol. Biochem. 73:1425–1428.
Pajević, S., M. Borišev, N. Nikolić, B. Krstić, A. Pilipović and S. Orlović.
2009. Phytoremediation capacity of poplar (Populus spp.) and willow (Salix spp.) clones in relation to photosynthesis. Arch. Biol. Sci.
61:239–247.
Paukkonen, K., A. Kauppi and A. Ferm. 1992. Origin, structure and
shoot formation ability of buds in cutting-origin stools of Salix
‘Aquatica’. Flora 186:53–65.
Pietrini, F., M.A. Iannelli, S. Pasqualini and A. Massacci. 2003.
Interaction of cadmium with glutathione and photosynthesis in
developing leaves and chloroplasts of Phragmites australis (Cav.)
Trin. Ex Steudel. Plant Physiol. 133:829–837.
Pietrini, F., M.A. Iannelli, R. Montanari, D. Bianconi and A. Massacci.
2005. Cadmium interaction with thiols and photosynthesis in higher
plants. In Advances in Plant Physiology. Ed. Hemantaranjan,
Scientific Publisher, Jodhpur, Vol. 8, Chapter 18, pp 313–326.
Pietrini, F., M. Zacchini, V. Iori, L. Pietrosanti, M. Ferretti and A.
Massacci. 2010a. Spatial distribution of cadmium in leaves and its
impact on photosynthesis: examples of different strategies in willow
and poplar clones. Plant Biol. 12:355–363.
Pietrini, F., M. Zacchini, V. Iori, L. Pietrosanti, D. Bianconi and A.
Massacci. 2010b. Screening of poplar clones for cadmium phytoremediation using photosynthesis, biomass and cadmium content
analyses. Int. J. Phytoremediat. 12:105–120.
Pietrosanti, L., G. Matteucci, F. Pietrini et al. 2008. Hydrological control and phytoremediation: by poplar and willow clones in a contaminated industrial site in Venice lagoon. In Proceedings of the
Fourth European Bioremediation Conference. Eds. N. Kalogerakis,
F. Fava and S.A. Banwart. Chania, Crete, Greece, 4–7 July 2011,
Vol. 138, pp 1–5.
Pilipović, A., N. Nikolić, S. Orlović, N. Petrović and B. Krstić. 2005.
Cadmium phytoextraction potential of poplar clones (Populus spp.).
Z. Naturforsch. C 60:247–251.
Polle, A. and C. Douglas. 2010. The molecular physiology of poplars:
paving the way for knowledge-based biomass production. Plant
Biol. 12:239–241.
Pulford, I.D. and C. Watson. 2003. Phytoremediation of heavy metalcontaminated land by trees—a review. Environ. Int. 29:529–540.
Pulford, I.D., D. Riddell-Black and C. Stewart. 2002. Heavy metal
uptake by willow clones from sewage sludge-treated soil: the potential for phytoremediation. Int. J. Phytoremediat. 4:59–72.
Punshon, T., N.W. Lepp and N.M. Dickinson. 1995. Resistance to ­copper
toxicity in some British willows. J. Geochem. Explor. 52:259–266.
Qin, R., H. Hirano and I. Brunner. 2007. Exudation of organic acid
anions from poplar roots after exposure to Al, Cu and Zn. Tree
Physiol. 27:313–320.
Rascio, N. and F. Navari-Izzo. 2011. Heavy metal hyperaccumulating
plants: How and why do they do it? And what makes them so interesting? Plant Sci. 180:169–181.
Rennenberg, H. and B. Will. 2000. Phytochelatin production and cadmium accumulation in transgenic poplar (Populus tremula × P. alba).
In Sulfur Nutrition and Sulfur Assimilation in Higher Plants. Eds.
C. Brunold, H. Rennenberg, L.J. De Kok, I. Stulen and J.C. Davidian.
Haupt, Bern, pp 393–398.
Riddell-Black, D. 1993. A review of the potential for the use of trees in
the rehabilitation of contaminated land. WRc Report CO 3467, Water
Research Centre, Medmenham.
Riddell-Black, D. 1994. Heavy metal uptake by fast growing willow
species. In Willow Vegetation Filters For Municipal Wastewaters and
Sludges. A Biological Purification System. Eds. P. Aronsson, and K.
Perttu. Department of Ecology and Environmental Research, Section
of Short Rotation Forestry, Uppsala, pp 145–151.
Robinson, B.H., T.M. Mills, D. Petit, L. Fung, S. Green and B. Clothier.
2000. Natural and induced cadmium accumulation in poplar and
willow:
Implications
for
phytoremediation.
Plant
Soil
227:301–306.
Robinson, K.M., A. Karp and G. Taylor. 2004. Defining leaf traits linked to
yield in short-rotation coppice Salix. Biomass Bioenerg. 26:417–431.
Rockwood, D.L., C.V. Naidu, D.R. Carter, M. Rahmani, T.A. Spriggs,
C. Lin, G.R. Alker, J.G. Isebrands and S.A. Segrest. 2004. Shortrotation woody crops and phytoremediation: Opportunities for agroforestry? Agroforest. Syst. 61:51–63.
Saber, N.E., A.M. Abdel-Moneim and S.Y. Barakat. 1999. Role of
organic acids in sunflower tolerance to heavy metals. Biol. Plantarum
42:65–73.
Sander, M.L. and T. Ericsson. 1998. Vertical distributions of plant nutrients and heavy metals in Salix viminalis stems and their implications
for sampling. Biomass Bioenerg. 14:57–66.
Sandermann, H. Jr. 1994. Higher plant metabolism of xenobiotics; the
‘green liver’ concept. Pharmacogenetics 4:225–241.
Sanità di Toppi, L. and R. Gabbrielli. 1999. Response to cadmium in
higher plants. Environ. Exp. Bot. 41:105–130.
Scheerer, U., R. Haensch, R.R. Mendel, S. Kopriva, H. Rennenberg and
C. Herschbach. 2010. Sulphur flux through the sulphate assimilation
pathway is differently controlled by adenosine 5’-phosphosulphate
reductase under stress and in transgenic poplar plants overexpressing γ-ECS, SO, or APR. J. Exp. Bot. 61:609–622.
Schutzendubel, A. and A. Polle. 2002. Plant responses to abiotic
stresses: heavy metal-induced oxidative stress and protection by
mycorrhization. J. Exp. Bot. 53:1351–1365.
Sebastiani, L., F. Scebba and R. Tognetti. 2011. Heavy metal accumulation and growth responses in poplar clones Eridano (Populus
­deltoides × maximowiczii) and I-214 (P. × euramericana) exposed to
industrial waste. Environ. Exp. Bot. 52:79–88.
Shukla, O.P., A.A. Juwarkar, S.K. Singh, S. Khan and U.N. Rai. 2011.
Growth responses and metal accumulation capabilities of woody
plants during the phytoremediation of tannery sludge. Waste
Manage. 31:115–123.
Sjödin, A., N.R. Street, G. Sandberg, P. Gustaffson and S. Jansson. 2009.
The Populus Genome Integrative Explorer (PopGenIE): a new resource
for exploring the Populus genome. New Phytol. 182:1013–1025.
Skaates, S.V., A. Ramaswami and L.G. Anderson. 2005. Transport and
fate of dieldrin in poplar and willow trees analyzed by SPME.
Chemosphere 61:85–91.
Solti, A., L. Gáspár, I. Mészáros, Z. Szigeti, L. Lévai and E. Sárvári. 2008.
Impact of iron supply on the kinetics of recovery of photosynthesis in
Cd-stressed poplar (Populus galuca). Ann. Bot. 102:771–782.
Šottníková, A., L. Lunáčková, E. Masarovičová, A. Lux and V. Streško.
2003. Changes in the rooting and growth of willow and poplars
induced by cadmium. Biol. Plantarum 46:129–131.
Taghavi, S., D. van der Lelie, A. Hoffman, Y.-B. Zhang, M.D. Walla,
J. Vangronsveld, L. Newman and S. Monchy. 2010. Genome
sequence of the plant growth promoting endophytic bacterium
enterobacter sp. 638. PLoS Genet. 6(5):e1000943.
Tanaka, S., L.B. Brentner, K.M. Merchie, J.L. Schnoor, J.M. Yoon and B. Van
Aken. 2007. Analysis of gene expression in poplar trees (Populus deltoides × nigra, DN34) exposed to the toxic explosive hexahydro-1,3,
5-trinitro-1,3,5-triazine (RDX). Int. J. Phytoremediat. 9:15–30.
Taylor, J.S., T.J. Blake, and R.P. Pharis. 1982. The role of plant hormones
and carbohydrates in the growth and survival of coppiced Eucalyptus
seedlings. Physiol. Plant. 55:421–430.
Tree Physiology Online at http://www.treephys.oxfordjournals.org
1334 Marmiroli et al.
Tharakan, P.J., T.A. Volk, C.A. Nowak and L.P. Abrahamson. 2005.
Morphological traits of 30 willow clones and their relationship to
biomass production. Can. J. For. Res. 35:421–431.
Thompson, P.L. and A.S. Polebitski. 2010. Effect of mycorrhizal fungi
on the phytoremediation of hexahydro-1,3,5-trinitro-1,3,5-triazine
(RDX). Environ. Sci. Technol. 44:1112–1115.
Thompson, P.L., L.A. Ramer and J.L. Schnoor. 1998. Uptake and transformation of TNT by hybrid poplar trees. Environ. Sci. Technol.
32:975–980.
Tognetti, R., L. Sebastiani and A. Minnocci. 2004. Gas exchange and
foliar characteristics of two poplar clones grown in soil amended
with industrial waste. Tree Physiol. 24:75–82.
Tuskan, G.A., S.P. DiFazio, S. Jansson et al. 2006. The genome of black
cottonwood, Populus trichocarpa (Torr. & Gray). Science
313:1596–1604.
Unterbrunner, R., M. Puschenreiter, P. Sommer, G. Wieshammer,
P. Tlustos, M. Zupan and W.W. Wenzel. 2007. Heavy metal accumulation in tree growing on contaminated sites in Central Europe.
Environ. Pollut. 148:107–114.
van Aken, B. and J.L. Schnoor. 2002. Evidence of perchlorate (ClO4-)
reduction in plant tissues (poplar tree) using radio-labeled 36ClO4-.
Environ. Sci. Technol. 36:2783–2788.
van der Zaal, B.J., L.W. Neuteboom, J.E. Pinas, A.N. Chardonnens,
H. Schat and J.A.C. Verkleij. 1999. Overexpression of a novel
Arabidopsis gene related to putative zinc transporter genes from
animals can lead to enhanced zinc resistance and accumulation.
Plant Physiol. 119:1047–1055.
van Dillewijn, P., J.L. Couselo, E. Corredoira, A. Delgado, R.-M. Wittich,
A. Ballester and J.L. Ramos. 2008. Bioremediation of
2,4,6-­trinitrotoluene by bacterial nitroreductase expressing transgenic aspen. Environ. Sci. Technol. 42:7405–7410.
Vangronsveld, J., R. Herzig, N. Weyens et al. 2009. Phytoremediation
of contaminated soils and groundwater: lessons from the field.
Environ. Sci. Pollut. Res. 16:765–794.
Vassilev, A., A. Perez-Sanz, B. Semane, R. Carleer and J. Vangronsveld.
2005. Cadmium accumulation and tolerance of two Salix genotypes
hydroponically grown in the presence of cadmium. J. Plant Nutr.
28:2159–2177.
Verbruggen, N., C. Hermans and H. Schat. 2009. Molecular mechanisms of metal hyperaccumulation and tolerance in plants. New
Phytol. 181:759–776.
Vervaeke, P., S. Luyssaert, J. Mertens, E. Meers, F.M. Tack and N. Lust.
2003. Phytoremediation prospects of willow stands on contaminated sediment: a field trial. Environ. Pollut. 126:275–282.
Tree Physiology Volume 31, 2011
Wang, X.P., M.P. Dossett, M.P. Gordon and S.E. Strand. 2004. Fate of
carbon tetrachloride during phytoremediation with poplar under
controlled field conditions. Environ. Sci. Technol. 38:5744–5749.
Wang, Y. and M. Greger. 2004. Clonal differences in mercury tolerance, accumulation, and distribution in willow. J. Environ. Qual.
33:1779–1785.
Watson, C., I.D. Pulford and D. Riddell-Black. 1999. Heavy metal toxicity responses of two willow (Salix) varieties grown hydroponically:
development of a tolerance screening test. Environ. Geochem.
Health 21:359–364.
Watson, C., I.D. Pulford and D. Riddell-Black. 2003. Screening of willow species for resistance to heavy metals: comparison of performance in a hydroponics system and field trials. Int. J. Phytoremediat.
5:351–365.
Weih, M., A.C. Rönnberg-Wästljung and C. Glynn. 2006. Growth analysis and QTL patterns coincide in hybrid willow (Salix dasyclados × S. viminalis) grown under two water regimes. New Phytol.
170:467–477.
Weyens, N., D. van der Lelie, T. Artois, K. Smeets, S. Taghavi,
L. Newman, R. Carleer and J. Vangronsveld. 2009. Bioaugmentation
with engineered endophytic bacteria improves contaminant fate in
phytoremediation. Environ. Sci. Technol. 43:9413–9418.
Wu, F., W. Yang, J. Zhang and L. Zhou. 2010. Cadmium accumulation
and growth responses of a poplar (Populus deltoides × Populus
nigra) in cadmium contaminated purple soil and alluvial soil.
J. Hazard. Mater. 177:268–273.
Yu, X.Z. and J.D. Gu. 2006. Uptake, metabolism and toxicity of methyl
tert-butyl ether (MTBE) in weeping willows. J. Hazard. Mater.
137:1417–1423.
Zacchini, M., F. Pietrini, G.S. Mugnozza, V. Iori, L. Pietrosanti and
A. Massacci. 2009. Metal tolerance, accumulation and translocation
in poplar and willow clones treated with cadmium in hydroponics.
Water Air Soil Pollut. 197:23–34.
Zacchini, M., F. Pietrini, G. Scarascia Mugnozza, V. Iori and A. Massacci.
2011. Cadmium accumulation and tolerance in Populus nigra and
Salix alba. Biol. Plantarum 55:383–386.
Zalesny, J.A., R.S. Zalesny, Jr., A.H. Wiese and R.B. Hall. 2007. Choosing
tree genotypes for phytoremediation of landfill leachate using phytorecurrent selection. Int. J. Phytoremediat. 9:513–530.
Zalesny, R.S. Jr., E.O. Bauer, R.B. Hall, J.A. Zalesny, J. Kunzman, C.J. Rog
and D.E. Riemenschneider. 2005. Clonal variation in survival and
growth of hybrid poplar and willow in an in situ trial on soils heavily
contaminated with petroleum hydrocarbons. Int. J. Phytoremediat.
7:177–197.