62
63
depends on three basic hallmarks that distinguish hyperaccumulators
from related non-
hyperaccumulator taxa. These common traits are: a much greater capability of taking up heavy met-
als from the soil; a faster and effective root-to-shoot translocation of metals; and a much greater
ability to detoxify and sequester huge amounts of heavy metals in the leaves. Significant progress in
understanding the mechanisms governing metal hyperaccumulation has been made in the last dec-
ade through comparative physiological, genomic, and proteomic studies of hyperaccumulators and
related non-hyperaccumulator plants. A very interesting feature revealed
by this research is that
most key steps in hyperaccumulation do not rely on novel genes, but depend on genes common to
hyperaccumulators and non-hyperaccumulators, that are differently expressed and regulated in the
two kinds of plants.
Heavy metal uptake
Comparative studies have revealed that the enhanced Zn uptake into roots can be attributed
to the constitutive overexpression of some genes belonging to the ZIP (Zinc-regulated transporter
Iron-regulated transporter Proteins) family, coding for plasma membrane located cation transporters
(). Moreover, the expression of these ZIP genes (ZTN1 and ZTN2 in
T. caerulescens and ZIP6 and
ZIP9 in
A. halleri), shows that in non-hyperaccumulating plants it is Zn-regulated and occurs at de-
tectable levels only under Zn deficiency, while in hyperaccumulators is irrespective of Zn supply –
still persisting at high Zn availability.
The decreasing uptake of Cd by roots supplied with increasing Zn concentration, found in
Cd/Zn hyperaccumulator
A. halleri and in most ecotypes of
T. caerulescens, clearly demonstrates
that Cd influx is largely due to Zn transporters, with a strong preference for Zn over Cd. Surprising-
ly, in plants of the Ganges ecotype of
T. caerulescens, which exhibit an exceptionally high ability to
hyperaccumulate Cd in aerial tissues, Cd uptake is not inhibited by Zn, thus suggesting the presence
in root cells of a specific and efficient independent Cd transport system. The supposed existence of
a transporter specific to
this metal, regarded as unessential, raises the question as to whether Cd
might play some physiological roles in that
T. caerulescens accession. The only physiological func-
tion of this heavy metal had previously been noticed in the marine diatom
Thalassiosira weissglogii
owing to its finding in the active metal-binding site of a peculiar Cd-containing carbonic anhydrase.
Specific transporters for Ni hyperaccumulation have not yet been recognized. However, the
preference of Zn over Ni by some Zn/Ni hyperaccumulators supplied with the same concentration
of both heavy metals strongly suggests that a Zn transport system might also be employed in Ni en-
trance into roots. Considerable evidence exists that As can enter plant roots as arsenate via trans-
porters of the chemical analogue phosphate. In root cells of As hyperaccumulator
Pteris vittata
plasma membranes have a higher density of phosphate/arsenate transporters than non-
hyperaccumulator
P. tremula, plausibly due to constitutive gene overexpression. Furthermore, the
enhanced As uptake by the hyperaccumulating fern depends on the higher affinity for arsenate by
the phosphate/arsenate transport systems as well as on the plant‘s ability to increase As bioavailabil-
ity in the rhizosphere by reducing pH via root exudation of large amounts of dissolved organic car-
bon.
The chemical similarity between sulphate and selenate accounts for the root uptake of Se in
this form through high-affinity sulphate transporters, whose activity is regulated by the S status of
the plant. In Se hyperaccumulators, such as
Astragalus bisulcatus (
Fabaceae) and
Stanleya pinnata
(
Brassicaceae), the Se/S ratios in shoots are much higher than in non-hyperaccumulator sister spe-
cies. This supports the idea of a role in this increased Se uptake of one or more sulphate transport-
ers, which may have acquired a Se-specificity, becoming independent of the plant S status.
Root-to-shoot translocation
Differently from non-hyperaccumulator plants, which retain in root cells most of the heavy
metal taken up from the soil, detoxifying them by chelation in the cytoplasm or storing them into
vacuoles, hyperaccumulators rapidly and efficiently translocate these elements to the shoot via the
xylem. This entails, of course, the heavy metal availability for xylem loading, which derives from a
low sequestration into and a ready efflux out of the vacuoles, plausibly due to specific features of
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63
depends on three basic hallmarks that distinguish hyperaccumulators from related non-
hyperaccumulator taxa. These common traits are: a much greater capability of taking up heavy met-
als from the soil; a faster and effective root-to-shoot translocation of metals; and a much greater
ability to detoxify and sequester huge amounts of heavy metals in the leaves. Significant progress in
understanding the mechanisms governing metal hyperaccumulation has been made in the last dec-
ade through comparative physiological, genomic, and proteomic studies of hyperaccumulators and
related non-hyperaccumulator plants. A very interesting feature revealed by this research is that
most key steps in hyperaccumulation do not rely on novel genes, but depend on genes common to
hyperaccumulators and non-hyperaccumulators, that are differently expressed and regulated in the
two kinds of plants.
Heavy metal uptake
Comparative studies have revealed that the enhanced Zn uptake into roots can be attributed
to the constitutive overexpression of some genes belonging to the ZIP (Zinc-regulated transporter
Iron-regulated transporter Proteins) family, coding for plasma membrane located cation transporters
(). Moreover, the expression of these ZIP genes (ZTN1 and ZTN2 in
T. caerulescens and ZIP6 and
ZIP9 in
A. halleri), shows that in non-hyperaccumulating plants it is Zn-regulated and occurs at de-
tectable levels only under Zn deficiency, while in hyperaccumulators is irrespective of Zn supply –
still persisting at high Zn availability.
The decreasing uptake of Cd by roots supplied with increasing Zn concentration, found in
Cd/Zn hyperaccumulator
A. halleri and in most ecotypes of
T. caerulescens, clearly demonstrates
that Cd influx is largely due to Zn transporters, with a strong preference for Zn over Cd. Surprising-
ly, in plants of the Ganges ecotype of
T. caerulescens, which exhibit an exceptionally high ability to
hyperaccumulate Cd in aerial tissues, Cd uptake is not inhibited by Zn, thus suggesting the presence
in root cells of a specific and efficient independent Cd transport system. The supposed existence of
a transporter specific to this metal, regarded as unessential, raises the question as to whether Cd
might play some physiological roles in that
T. caerulescens accession. The only physiological func-
tion of this heavy metal had previously been noticed in the marine diatom
Thalassiosira weissglogii
owing to its finding in the active metal-binding site of a peculiar Cd-containing carbonic anhydrase.
Specific transporters for Ni hyperaccumulation have not yet been recognized. However, the
preference of Zn over Ni by some Zn/Ni hyperaccumulators supplied with the same concentration
of both heavy metals strongly suggests that a Zn transport system might also be employed in Ni en-
trance into roots. Considerable evidence exists that As can enter plant roots as arsenate via trans-
porters of the chemical analogue phosphate. In root cells of As hyperaccumulator
Pteris vittata
plasma membranes have a higher density of phosphate/arsenate transporters than non-
hyperaccumulator
P. tremula, plausibly due to constitutive gene overexpression. Furthermore, the
enhanced As uptake by the hyperaccumulating fern depends on the higher affinity for arsenate by
the phosphate/arsenate transport systems as well as on the plant‘s ability to increase As bioavailabil-
ity in the rhizosphere by reducing pH via root exudation of large amounts of dissolved organic car-
bon.
The chemical similarity between sulphate and selenate accounts for the root uptake of Se in
this form through high-affinity sulphate transporters, whose activity is regulated by the S status of
the plant. In Se hyperaccumulators, such as
Astragalus bisulcatus (
Fabaceae) and
Stanleya pinnata
(
Brassicaceae), the Se/S ratios in shoots are much higher than in non-hyperaccumulator sister spe-
cies. This supports the idea of a role in this increased Se uptake of one or more sulphate transport-
ers, which may have acquired a Se-specificity, becoming independent of the plant S status.
Root-to-shoot translocation
Differently from non-hyperaccumulator plants, which retain in root cells most of the heavy
metal taken up from the soil, detoxifying them by chelation in the cytoplasm or storing them into
vacuoles, hyperaccumulators rapidly and efficiently translocate these elements to the shoot via the
xylem. This entails, of course, the heavy metal availability for xylem loading, which derives from a
low sequestration into and a ready efflux out of the vacuoles, plausibly due to specific features of
root cell tonoplast. As a matter of fact the amount of Zn sequestered into cell root vacuoles is 2–3-
fold lower and the Zn efflux out of vacuoles almost twice as fast in the hyperaccumulators
T. caer-
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