Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Gasic and S.S. Korban
In Pteris vittata, the first identified arsenate (As) hyperaccumulator, arsenate
is taken up via phosphate transporters, reduced to arsenite, and sequestered in fronds,
primarily as As (III) (Wang et al., 2002). To investigate uptake, translocation, and
speciation of As in roots and shoots of plants grown either in soil or in a nutrient
solution, Quaghebeur and Rengel (2004) used two mutants of A. thaliana, pho, (P
deficient) and pho2 (P accumulator), defective in regulation and translocation of P (V)
from roots to shoots. Shoots of the pho2 mutant contained higher P concentrations,
but either similar or slightly higher As concentrations than wild type. While for the
pho1 mutant, P levels were lower while As levels were higher in shoots than those
found in wild type. Both pho2 and wild type contained primarily As (III) in both roots
and shoots (67-90% of total As). Arsenic was likely translocated to P (V) via different
pathways in pho2 and pho1 mutants. Therefore, it has been suggested that As (III)
was the main As species translocated from roots to shoots in A. thaliana.
The first plant protein that modulates plant tolerance or accumulation to Pb 2+
was isolated and characterized (Arazi et al., 1999). Transgenic tobacco lines
overexpressing a tobacco calmodulin-binding protein, NtCBP4, showed improved tolerance
to Ni 2+ and hypersensitivity to Pb 2+ associated with reduced Ni 2+ accumulation
and enhanced Pb 2+ accumulation, respectively.
Therefore, it is apparent that plants have developed various systems for essential
metal ion uptake, and based on available evidence, it is believed that there are
several mechanisms for uptake for each essential metal.
Once metal ions enter the cell, they are bound by chelators and chaperones. Chelators
contribute to metal detoxification by buffering cytosolic metal concentrations; while,
chaperones specifically deliver metal ions to organelles and metal-requiring proteins.
There are several known metal-chelators in plants. These include phytochelatins,
metallothioneins, organic acids, and amino acids (Clemens, 2001). Among heavy metalbinding
ligands in plant cells, phytochelatins (PCs) and metallothioneins (MTs) are the
best characterized, and these have been recently reviewed by Cobbett and Goldsbrough
(2002).
3.1. Phytochelatins
3. CHELATION
Plants respond to excess heavy metal ions by synthesizing sulf-hydryl (SH)-containing
peptides having the following general structure [(γ- L
-glutamyl- L
-cysteinyl) n
-glycine],
where n = 2 to 11 (Rauser, 1990). Based on their structure and metal binding properties,
these peptides are classified as class III metallothioneins, but the trivial name
phytochelatins (PCs) has been widely adopted (Grill et al., 1985; Kägi, 1993).
PCs are enzymatically-synthesized directly from glutathione (GSH) by the enzyme
PC synthase (EC. 2.3.2.15) (Grill et al., 1989; Cobbett, 2000) (Fig. 2). In plants, GSH