64
65
stance, is present as free hydrated cations in the xylem sap of
T. caerulescens and
A. halleri, and
only one-third of Ni is bound to citrate in the xylem of hyperaccumulator
Stackhousia tryoni
(
Celastraceae). Conversely, almost all Ni is complexed with citrate and other organic acids in the
latex of the extreme Ni hyperaccumulator
S. acuminata.
Detoxification/sequestration
Heavy metal induced-oxidative stress is a naturally occurring
phenomenon which plants
have evolved to deal with. Heavy metals' signals are perceived by receptors, and receptors transduce
signals via cAMP, pH, etc. causing alterations in electron transport systems of the cell, which re-
sults into an excess production of reactive oxygen species (ROS). ROS cause damage to macromol-
ecules and thus create oxidative stress inside the cell. Plants are coping with it through both toler-
ance and detoxification mechanisms in the plant cell. These involve the use of ascorbic acid (AsA),
catalase (CAT), cysteine (Cys), c-glutamylcysteinesynthetase (c-ECS), glutamine (Glu), glycine
(Gly), glutathione reductase (GR), glutathione synthetase (GS), reduced glutathione (GSH), oxi-
dized glutathione (GSSG), hydrogen peroxide (H
2
O
2
), monodehydroascorbate (MDHA), oxygen
molecule (O
2
), superoxide radicals (O−2), reactive oxygen species (ROS), superoxide dismutase
(SOD), etc. On the other hand, in the presence of plant hormones, signals received by them initiate
a cascade of signal transduction involving haem oxygenase, transcription factors induced by brassi-
nosteroids (BES1 and BZR1) and a gibberellic acid-mediated GA-GID1-DELLA
signaling path-
way. These factors, in turn, initiate expression of the nuclear genes encoding defense proteins, tran-
scription factors (TFs), heat shock proteins (HSP), and metal transporter proteins (MTs). The main
role of MTs is to protect electron transport chains against heavy metals by regulating their uptake.
Other defense proteins protect plant against ROS under heavy metal stress.
Great efficiency in detoxification and sequestration is a key property of hyperaccumulators
which allows them to concentrate huge amounts of heavy metals in above-ground organs without
suffering any phytotoxic effect. This exceptionally high heavy metal accumulation becomes even
more astonishing bearing in mind that it principally occurs in leaves where photosynthesis, essential
for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most
of these contaminants. The preferential heavy metal detoxification/sequestration do occur in loca-
tions, such as epidermis, trichomes and even cuticle, where they do least damage to the photosyn-
thetic machinery. In many cases heavy metals are also excluded from both subsidiary and guard
cells of stomata.
The detoxifying/sequestering mechanisms in aerial organs of hyperaccumulators consist
mainly in heavy metal complexation with ligands and/or in their removal from metabolically active
cytoplasm by moving them into inactive compartments, mainly vacuoles and cell walls. Compara-
tive transcriptome analyses between hyperaccumulator and related non-hyperaccumulator
species
have demonstrated that also the sequestration trait relies, at least in part, on constitutive overexpres-
sion of genes that, in this case, encode proteins operating in heavy metal transfer across the tono-
plast and/or plasma membrane and involved in excluding them from cytoplasm. CDF (Cation Dif-
fusion Facilitator) family members, also named MTPs (Metal Transporter Proteins), which mediate
bivalent cation efflux from the cytosol, are important candidates. The Zn transport into the vacuole,
in fact, may initiate a systemic Zn deficiency response that includes the enhancement of the heavy
metal uptake and translocation via the increased expression of ZIP transporters in hyperaccumulator
plants.
The overexpression of HMA3, coding for a vacuolar P1B-ATPase, plausibly involved in Zn
compartmentation, and that of CAX genes encoding members of a cation exchanger family that
seems to mediate Cd sequestration, have been noticed in
T. caerulescens and
A. halleri and sup-
posed to be involved in heavy metal hyperaccumulation.
Small ligands, such as organic acids, have a major role as detoxifying factors. Such ligands
may be instrumental in preventing the persistence of heavy metals as free ions in the cytoplasm and
even more in enabling their entrapment in vacuoles where the metal–organic acid chelates are pri-
marily located. Citrate, for instance, is the main ligand of Ni in leaves of
T. goesingense, while cit-
64
65
stance, is present as free hydrated cations in the xylem sap of
T. caerulescens and
A. halleri, and
only one-third of Ni is bound to citrate in the xylem of hyperaccumulator
Stackhousia tryoni
(
Celastraceae). Conversely, almost all Ni is complexed with citrate and other organic acids in the
latex of the extreme Ni hyperaccumulator
S. acuminata.
Detoxification/sequestration
Heavy metal induced-oxidative stress is a naturally occurring phenomenon which plants
have evolved to deal with. Heavy metals' signals are perceived by receptors, and receptors transduce
signals via cAMP, pH, etc. causing alterations in electron transport systems of the cell, which re-
sults into an excess production of reactive oxygen species (ROS). ROS cause damage to macromol-
ecules and thus create oxidative stress inside the cell. Plants are coping with it through both toler-
ance and detoxification mechanisms in the plant cell. These involve the use of ascorbic acid (AsA),
catalase (CAT), cysteine (Cys), c-glutamylcysteinesynthetase (c-ECS), glutamine (Glu), glycine
(Gly), glutathione reductase (GR), glutathione synthetase (GS), reduced glutathione (GSH), oxi-
dized glutathione (GSSG), hydrogen peroxide (H
2
O
2
), monodehydroascorbate (MDHA), oxygen
molecule (O
2
), superoxide radicals (O−2), reactive oxygen species (ROS), superoxide dismutase
(SOD), etc. On the other hand, in the presence of plant hormones, signals received by them initiate
a cascade of signal transduction involving haem oxygenase, transcription factors induced by brassi-
nosteroids (BES1 and BZR1) and a gibberellic acid-mediated GA-GID1-DELLA signaling path-
way. These factors, in turn, initiate expression of the nuclear genes encoding defense proteins, tran-
scription factors (TFs), heat shock proteins (HSP), and metal transporter proteins (MTs). The main
role of MTs is to protect electron transport chains against heavy metals by regulating their uptake.
Other defense proteins protect plant against ROS under heavy metal stress.
Great efficiency in detoxification and sequestration is a key property of hyperaccumulators
which allows them to concentrate huge amounts of heavy metals in above-ground organs without
suffering any phytotoxic effect. This exceptionally high heavy metal accumulation becomes even
more astonishing bearing in mind that it principally occurs in leaves where photosynthesis, essential
for plant survival, is accomplished, and that the photosynthetic apparatus is a major target for most
of these contaminants. The preferential heavy metal detoxification/sequestration do occur in loca-
tions, such as epidermis, trichomes and even cuticle, where they do least damage to the photosyn-
thetic machinery. In many cases heavy metals are also excluded from both subsidiary and guard
cells of stomata.
The detoxifying/sequestering mechanisms in aerial organs of hyperaccumulators consist
mainly in heavy metal complexation with ligands and/or in their removal from metabolically active
cytoplasm by moving them into inactive compartments, mainly vacuoles and cell walls. Compara-
tive transcriptome analyses between hyperaccumulator and related non-hyperaccumulator species
have demonstrated that also the sequestration trait relies, at least in part, on constitutive overexpres-
sion of genes that, in this case, encode proteins operating in heavy metal transfer across the tono-
plast and/or plasma membrane and involved in excluding them from cytoplasm. CDF (Cation Dif-
fusion Facilitator) family members, also named MTPs (Metal Transporter Proteins), which mediate
bivalent cation efflux from the cytosol, are important candidates. The Zn transport into the vacuole,
in fact, may initiate a systemic Zn deficiency response that includes the enhancement of the heavy
metal uptake and translocation via the increased expression of ZIP transporters in hyperaccumulator
plants.
The overexpression of HMA3, coding for a vacuolar P1B-ATPase, plausibly involved in Zn
compartmentation, and that of CAX genes encoding members of a cation exchanger family that
seems to mediate Cd sequestration, have been noticed in
T. caerulescens and
A. halleri and sup-
posed to be involved in heavy metal hyperaccumulation.
Small ligands, such as organic acids, have a major role as detoxifying factors. Such ligands
may be instrumental in preventing the persistence of heavy metals as free ions in the cytoplasm and
even more in enabling their entrapment in vacuoles where the metal–organic acid chelates are pri-
marily located. Citrate, for instance, is the main ligand of Ni in leaves of
T. goesingense, while cit-
rate and acetate bind Cd in leaves of
S. nigrum. Moreover, most Zn in
A. halleri and Cd in
T. caer-
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