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48 M. Mleczek et al.

and allocation of other nutrients and depends on regulation of genes involved in
cation uptake, allocation, sequestration and biosynthesis of metal(loid) ligands.
Since the ability to hyperaccumulate metal(loids) (Ni, Zn, Cd, Se, Mn, Co, Cu,
Pb, Sb, Tl or As) without toxicity symptoms, shared by about 500 plant taxa
growing on metalliferous soils, is of polyphyletic origin, it seems likely that only
minor changes in the plant genome can convert it into a hyperaccumulator
(Verbruggen et al. 2009). Several species from the Brassicaceae family have
evolved the ability to accumulate Ni, Zn, Cd, Se and As. Comparative analyses
of transcriptome, ionome and metabolome of the model plant Arabidopsis thaliana
and related hyperaccumulator species is a powerful strategy to investigate adaptive
changes in plant genomes leading to metal(loid) tolerance and hyperaccumulation.

3.4.1 Small Ligands

Metal(loid) homeostasis in plants depends on metal binding proteins and
peptides, as well as on biosynthesis and partitioning of small ligands such as
citrate, acetate, malate, oxalate, phosphate, histidine (His), phytate, glutathione
(GSH), phytochelatins and pectates (Kra ¨mer et al. 2000). The concentration of
histidine and nicotianamine, which form more stable complexes with bivalent
cations than organic acids like citrate, is crucial for hyperaccumulation of Ni and
Zn (Callahan et al. 2006; Haydon and Cobbett 2007). In Alyssum lesbiacum Ni
uptake is proportional to His concentration in xylem, and the gene of ATP-
phosphoribosyltransferase (ATP-PRT) catalysing the first step in His biosynthe-
sis is constitutively overexpressed, which distinguishes A. lesbiacum from its
relative non-accumulator Alyssum montanum. Overexpression of an A. lesbiacum
ATP-PRT cDNA in transgenic A. thaliana increased Ni tolerance and the pool of
free His in the shoot but Ni concentration in neither the xylem sap nor in the shoot
was increased, which indicates that additional factors are necessary for Ni
hyperaccumulation (Ingle et al. 2005). The high rate of root-to-shoot transloca-
tion of Ni in T. caerulescens compared to Thlaspi arvense seems to depend on
enhanced root His concentration and on decreased ability to accumulate Ni–His
complexes in root cell vacuoles. Nicotianamine (NA) is a Fe chelator formed
from S-adenosyl-L-methionine by NA synthase (NAS). The exposure of Thlaspi
caerulescens to Ni triggers the accumulation of NA in roots. Since neither
TcNAS expression nor NAS activity were detected in roots, the NA is most
likely translocated from shoots, partially as a stable Ni–NA complex in the xylem
sap. Such circulation of NA and Ni–NA chelates cannot be detected in the non-
accumulator Thlaspi arvense.In A. thaliana, NAS transcript levels are
upregulated under Fe, Zn and Cu deficiency. In both A. thaliana and A. halleri
(Zn and Cd hyperaccumulator), Zn deficiency induces accumulation of NAS
transcript in the shoot (Talke et al. 2006). Under normal growth conditions,
A. halleri shows high expression of NAS in roots and accumulates more NA.
NA is expected to act in the cytoplasm and in the phloem, but in transgenic plants