Page 63 - Plant-Based Remediation Processes

3 Metal/Metalloid Phytoremediation: Ideas and Future 51

is a Fe–NA influx transporter at pH ¼ 7.0 and Ni–NA transporter at pH ¼ 5.0
(Gendre et al. 2006).
Two ABC transporters, AtABCC1 and ABCC2, have been shown to contribute
to transport of As–, Cd– and Hg–phytochelatin complexes into the vacuole. Simul-
taneous overexpression of AtABCC1 with AtPCS1 (phytochelatin synthase)
resulted in plants exhibiting an increased arsenic tolerance (Song et al. 2010b).
A. thaliana vacuolar membrane major facilitator superfamily protein ZIF1 gene
was selected by genetic screening for Zn-hypersensitive mutants (Haydon and
Cobbett 2007). ZIF1 overexpression enhances NA and Zn partition into and
accumulation in vacuoles and impaired Fe movement and Fe deficiency symptoms
(Haydon et al. 2012). A. thaliana Zn-efflux transporter PCR2, located in the
epidermis and xylem of young roots, and in the epidermis of fully developed
roots, contributes to root-to-shoot Zn transport. The pcr2 mutants are sensitive to
both limitation and excess of Zn, and pcr2 roots accumulate more Zn than WT, and
in Zn limiting conditions Zn is accumulated in the epidermis in pcr2, while in WT it
is found in the stele (Song et al. 2010a). A. thaliana mutants exhibiting As(V)
tolerance harbour null alleles coding for the high affinity Pi transporters PHT1;1 or
PHT1;4, indicating that these transporters play a major role in As(V) uptake.
Additionally, PHF1 (PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR 1),
which is required for efficient trafficking of Pi transporters to the plasma membrane,
also results in a strong tolerance to As(V) (Gonzalez et al. 2005). A. thaliana pht1;1
displays a slow rate of As(V) uptake that ultimately enables the mutant to accumu-
late double the arsenic found in wild-type plants. In A. thaliana As(V) represses the
activation of genes involved in phosphate uptake, which may reflect a regulatory
mechanism which protects plants from As uptake (Catarecha et al. 2007).
Arsenite As(III) uptake and translocation are mediated by members of the NIP
subfamily of aquaporins (aquaglyceroporins) having a larger pore size; thus it is
permeable for additional substrates, such as neutral metalloids, undissociated acids
and small solutes like glycerol (Ali et al. 2009). Three independent As(III)-tolerant
mutants were isolated from ethyl methanesulfonate-mutagenized seeds of A. thaliana;
all mutations were located in the Nodulin 26-like intrinsic protein 1;1 (NIP1;1)
gene. NIP1;1 is localised to the plasma membrane and is highly expressed in roots.
Disruption of NIP1;1 function confers As(III) tolerance to plants and lowers As
accumulation. NIP1;2 and NIP5;1, closely related homologues of NIP1;1, were
also permeable to As(III). Disruption of these genes also reduced the As content
in plants, but As(III) tolerance was not observed in nip1;2 and nip5;1 mutants
(Kamiya et al. 2009). The fern Pteris vittata hyperaccumulates arsenic up to >1%
of the dry weight of a frond, 25 times more than in the root. Two P. vittata genes,
ACR3 and ACR3;1, encode proteins similar to the ACR3 arsenite effluxer of yeast.
ACR3 localises to the vacuolar membrane and its transcription is induced by
arsenic in tissues that directly contact soil. It has been suggested that ACR3 may
participate in transporting arsenic from the root into the xylem for translocation to
the shoot (Indriolo et al. 2010).