Page 255 - Plant-Based Remediation Processes

248 P. Kotrba

form metal chelates with organic acids such as malate or citrate. The same apparently
holds true for the role these organic acids play in hyperaccumulators (Verbruggen
et al. 2009).While PC synthesis is ubiquitous response of plants to heavy metal,
especially Cd, exposure, their accumulation was only been found in Cd-sensitive
populations of N. caerulescens and in non-accumulating ecotypes of S. alfredii
(Schat et al. 2002; Sun et al. 2007). The biosynthesis of PCs via transpeptidation
reaction from glutathione (γ-glutamylcysteinylglycine, GSH) or its homologues
(iso-PCs) is catalyzed by the constitutive PC synthase (PCS) in a metal- or metalloid
(e.g. As)-dependent manner (Clemens 2006).
Glutathione (GSH) and its homologues also act as a fundamental antioxidant
molecule. Glutathione directly eliminates reactive oxygen radicals induced by
heavy metals in cells (Schu ¨tzendu ¨bel and Polle 2002) and provide reducing
equivalents in the ascorbate–glutathione antioxidation cycle to maintain redox
homeostasis (Foyer and Noctor 2005). Such function is also attributed to GSH in
Ni- and Cd-hyperaccumulating populations of N. caerulescens (Freeman et al.
2005; van de Mortel et al. 2008). In yeast, Saccharomyces cerevisiae (Li et al.
1997), and in some ectomycorrhizal fungi (Bellion et al. 2006) which do not
produce PCs, cellular detoxification of Cd depends upon exclusion of bis
(glutathionato)cadmium complex the metal into vacuoles. The differential Cd
stress-dependent expression of homologues of the respective vacuolar ABC trans-
porter (YCF1 in S. cerevisiae) has been reported in A. thaliana (Bovet et al. 2005).
In this plant, GSH also appears to play a role in Cd sequestration in the
mitochondria and bis(glutathionato)cadmium conjugates, transported via ABC
transporter ATM3 into cytoplasm, become substrates for PC synthesis (Kim et al.
2006). Two homologues of A. thaliana MRP10 and ATH13 ABC transporters were
found differentially expressed in N. caerulescens populations displaying
contrasting Zn tolerance (Hassinen et al. 2007); however, a direct evidence for
their role in vacuolar sequestration of Zn is missing. Like ABC transporters,
Ca/cation exchangers (CAX) seem to be involved in vacuolar sequestration of
heavy metals, although they seem to transport free ion rather that a metal chelate.
They are known to respond to heavy metal exposure both non-accumulators and
hyperaccumulators (van de Mortel et al. 2006, 2008; Weber et al. 2006), in which
their metal specificity remains to be elucidated.
Transporters of the cation diffusion facilitator (CDF) family play an important
role in heavy metal homeostasis and detoxification (Kra ¨mer et al. 2007; Montanini
et al. 2007; Migeon et al. 2010). In plants, these are called metal transporter proteins
(MTPs) and mediate not only the transport of variety of metals (Zn, Fe, Cd, Co, Mn)
to vacuoles but also to other organelles and to the apoplast. The best characterized
A. thaliana CDF transporter MTP1 and its homologues from A. halleri (Arrivault
et al. 2006) or poplar (Migeon et al. 2010) are apparently Zn-specific transporters
responsible from Zn efflux from cytoplasm to vacuoles. High expression of MTP1
gene observed in both roots and shoots of A. halleri thus seem to be involved in Zn
hypertolerance trait of this species. Unlike MTP1 of A. halleri, MTP1 from metal
accumulator Thlaspi goesingense MTP1 been reported to have a much broader
substrate range, which was verified to mediate tolerance to Ni, Cd, Co, and Zn in