Page 61 - Plant-Based Remediation Processes

3 Metal/Metalloid Phytoremediation: Ideas and Future 49

the enhanced compartmentalization of NA in the vacuole directed the vacuolar
accumulation of Zn (Haydon et al. 2012). The increased expression of A. thaliana
ZIF1 (a vacuolar membrane major facilitator superfamily protein required for
basal Zn tolerance) promotes vacuolar sequestration of Zn. In A. thaliana ZIF1
overexpressors Zn is immobilised in roots, and the concomitant sequestration of
NA impairs the translocation of Fe from leaf vasculature to leaf blade and gives
constitutive symptoms of Fe deficiency, similar to biosynthetically NA-deficient
plants (Takahashi et al. 2003;Haydonetal. 2012). A. halleri and T. caerulescens
share elevated NAS expression compared to non-accumulators. Suppression of
AhNAS2 by RNA interference resulted in reduced root NA accumulation,
decrease in root-to-shoot translocation of Zn, increase in Zn-thiol species and
reduced accumulation of Cd in leaves (Deinlein et al. 2012). Transgenic rice
plants overexpressing OsNAS3 (35S enhancers) accumulate more Fe and Zn in
shoots, and two to threefold more Fe, Zn and Cu in seeds, and they exhibit
increased tolerance to Fe and Zn deficiencies and tolerance to Zn, Cu and Ni
toxicity. OsYSL2 is an iron [Fe(II)]–NA and manganese [Mn(II)]–NA complex
transporter, expressed in phloem companion cells and developing seeds, impor-
tant for Fe translocation, especially in the shoots and endosperm (Ishimaru 2010).
Recently Masuda et al. (2012) have tested a combined transgenic approach in rice
expressing ferritin from an endosperm-specific promoter, overproducing NA and
enhancing the Fe flux through expression of OsYSL2 from the endosperm-
specific promoter and sucrose transporter promoter and obtained transgenic
plants which exhibited fourfold higher iron accumulation in polished grains
(Masuda et al. 2012).

3.4.2 Metal(loid) Uptake, Translocation and Partitioning

A. thaliana genome encodes 15 ZIP transporters. The best characterised, IRT1
(Iron-Regulated Transporter 1), is responsible for root uptake of Fe 2+ into epider-
mal cells of the root hair zone (Colangelo and Guerinot 2006). Transcript levels of
IRT1 are regulated by local root and shoot-derived long-distance signals. IRT1
transcripts accumulate during the day, indicating the circadian regulation of Fe
acquisition (Vert 2003). IRT1, IRT2 and transcripts of other genes involved in Zn
and Cd detoxification increase under Fe deficiency (Wu 2012). Transcript levels of
2+ 2+
plasma membrane IRT3, proposed to transport Zn and Fe , increase under Zn
deficiency and are constitutively overexpressed in roots of the Zn hyperaccu-
mulators A. halleri and T. caerulescens (Becher et al. 2004; Talke et al. 2006).
ZIP1, ZIP2, ZIP3 and ZIP4 mediate Zn uptake in a heterologous system and ZIP4
also Cu.
AtHMA1 and AtHMA6 (members of P1B-type ATPase family) encode a high
affinity Cu(I) transporter of the chloroplast envelope (Catty 2011) while soybean
AtHMA8 homologue localises to thylakoid membranes (Bernal et al. 2007).
AtHMA1 can also be a Zn or Ca transporter (Moreno 2008; Kim 2009). Transcripts