Arsenic Toxicity and Tolerance Mechanisms in Plants: An Overview - Crop Improvement: New Approaches and Modern Techniques - page 369

Phytochelatins
Synthesis of phytochelatins (PCs) and metallothioniens (MTs) is the response of
plants applied with the high concentration of metals or metalloids. Due to their sim-
ilarity with metallothioniens, phytochelatins have been called as class III metallo-
thioniens (Cobbett and Goldsbrough 2002 ). The distribution task between PCs and
MTs is repeatedly reviewed (Cobbett 2000 ; Cobbett and Goldsbrough 2002 ). At
present, PCs primarily functions in detoxification while as MTs have been given
other roles, e.g. in chaperoning the metallic element translocation. MTs belong to
the gene family and PCs are enzymatically produced (Cobbett and Goldsbrough
2002 ). The small molecular weight phytochelatins are cysteine rich polypeptides
with n = 2-11 (Reddy and Prasad 1990 ) or n = 2-5 (Cobbett and Goldsbrough 2002 ).
They were first detected in cell suspension culture of Rauwalfia serpentina exposed
to 0.2 mM CdSO 4 (Grill et al. 1985 ). Phytochelatins are synthesized in response to
Cd, Au, Cu, Ag, Ni, Pb, Sb, Sn, Hg, Te and Zn (Grill et al. 1987 ) and selenate and
arsenate (Grill et al. 1986 ). The proposed sequence to the intensity of induction in
metal specific is: Hg > Cd > As > Te > Ag > Cu > Ni > Sb > Au > Sn > Se > Bi > Pb
> W > Zn (Ernst 1997b ). Many exceptions to this sequence raised, e.g. Pb is strong
inducer of phytochelatins in some legumes (Piechalak et al. 2002 ). To confirm this
the sequence of the root culture of Rubia tinctorium ; Ag > Cd > Pb > Hg > As (III)
> Cu > As (V) > Zn > Pd > In > Ga > Se > Ni has been suggested (Maitani et al.
1996 ). Phytochelatins production leads to toxicity rather than tolerance (Ebbs et al.
2002 ). Current studies revealed that phtochelatins do not contribute to Zn, Cd (Ebbs
et al. 2002 ; Schat et al. 2002 ) or Cu tolerance (Schat et al. 2002 ). In the root cells
of Silene cucubalus the Cu tolerance is associated with the glutathione level, i.e by
restricting the influx of Cu to these cells and PC synthesis reduction (De Vos et al.
1992 ). Apart from toxic element inactivation, PCs perform other functions as well
like micronutrient homeostasis (Schat et al. 2002 ), sulphur metabolism (Tomasze-
wska et al. 1996 ) maintainance of enzyme activity (Kneer and Zenk 1992 ) translo-
cation of metals (Cobbett and Goldsbrough 2002 ) and transport and storage of As
(Hartley-Whitaker et al. 2001b ). It has been anticipated that PCs primary function
is homeostasis and inactivation is secondary (Steffens 1990 ). In higher plants and
microorganisms, the trafficking of PC-complex metals in energy consuming across
tonoplast, the transport is mediated by ABC-type cassette binding ATPase (Nies and
Silver 1995 ). Avena sativa roots have shown that these ATPases transport Cd-PCs
in tonoplast vesicles in the presence of Mg 2+ (Salt and Rauser 1995 ). Most likely,
higher plants have a gene homologue to the hunt gene that regulates the production
of these transporter proteins in Schizosaccharomyces pombe (Ortiz et al. 1992 ).
Five types of PCs have been known on the basis of C-terminal amino-acid and the
length of the chain (PC0-PC4) (Rauser 1995 ). In addition to PCs, i.e. the polymer
of glutamyl-cysteinyl glycine, homo phytochelatins (h-PCs), polymer of glutamyl-
cysteinyl alanine occurs in legumes (Piechalak et al. 2002 ). Evidences point towards
glutathione and homoglutathione acts as precursor of PCs and h-PCs (Cobbett and
Goldsbrough 2002 ). The polymerization of glutathione and PC synthase the metal