Page 62 - Plant-Based Remediation Processes

50 M. Mleczek et al.

2+
of vacuolar AtHMA3 (similar to prokaryotic Zn /Cd 2+ pumps) are elevated in
shoots of A. halleri and T. caerulescens (Becher et al. 2004; Talke et al. 2006). The
hma3 mutant of A. thaliana is more sensitive to Zn and Cd, while HMA3
overexpressor plants are more tolerant to Zn and Cd and accumulate more Cd.
AtHMA2 and AtHMA4 are localised to plasma membrane pericycle and xylem
parenchyma cells (Hanikenne et al. 2008) and participate in loading of Zn and Cd
into the xylem for root-to-shoot translocation (Wang et al. 2009). A. thaliana hma2
hma4 double mutants are Zn deficient in the shoots. HMA4 is necessary for Zn
hyperaccumulation in A. halleri shoots (Hanikenne et al. 2008). Transfer of an A.
halleri HMA4 gene to A. thaliana confers Zn translocation into xylem vessels and
up regulation of Zn deficiency response genes, but is not sufficient to increase Zn or
Cd tolerance (Hanikenne et al. 2008).
The 12 A. thaliana MTPs belong to different phylogenetic groups and likely
differ in substrate specificity (Delhaize et al. 2007). MTP1 and MTP3 are localised
to the vacuolar membrane and probably transport Zn into the vacuole. MTP1
increases Zn concentration in leaves while MTP3 has an opposite effect
(Desbrosses-Fonrouge et al. 2005). MTP1 homologues are highly expressed in
hyperaccumulator species such as A. halleri and N. caerulescens (Shahzad 2010).
The His-rich cytoplasmic loop of MTP1 may act as a sensor or a buffer of
cytoplasmic Zn, and deletion of this loop makes MTP1 hyperactive (Kawachi
et al. 2009). Four other A. thaliana MTPs are similar to a legume MTP8 of
Stylosanthes hamata which transports Mn 2+ into the vacuole (Delhaize 2003).
NRAMP proteins are transition metal cation/proton co-transporters or
antiporters with broad specificity (Cailliatte et al. 2009). AtNRAMP1 is a
high affinity Mn uptake transporter. NRAMP5 is responsible for Mn and Cd
uptake in rice (Sasaki et al. 2012). AtNRAMP3 and AtNRAMP4 play a key
role in iron nutrition of the germinating plantlet by remobilizing vacuolar iron.
The Zn and Cd hypersensitivity of nramp3 nramp4 double mutants is likely to
be a result of impaired remobilization of Fe from the vacuole. AtNRAMP6,
which is targeted to a vesicular-shaped endomembrane compartment distinct
from the vacuole or mitochondria, increases sensitivity to Cd without affecting
Cd content. The null allele of NRAMP6 was more tolerant to Cd (Cailliatte
et al. 2009).
Eight A. thaliana YSL oligopeptide transporters are expected to import transition
metals complexed with NA into the cytosol (Schaaf 2005). AtYSL2 is expressed in
cells around vascular tissues and translocates Cu(II)–NA and Fe(II)–NA complexes,
which suggests its function in metal export from the vasculature. AtYSL2 transcript
abundance decreases in shoots in response to Fe deficiency and Cu excess. AtYSL1
is expressed in the xylem parenchyma of leaves, where it is induced in response to
Fe excess; ysl1 mutants accumulate more NA in shoots and less Fe and NA in seeds,
suggesting that YSL1 participates in iron delivery to seeds (Le Jean et al. 2005).
AtYSL1 and AtYSL3 show the highest expression in senescing rosette and
cauline leaves. The double mutant ysl1ysl3 exhibited Fe deficiency and elevated
concentrations of Cu, Mn and Zn (Waters et al. 2006). The N. caerulescens YSL3