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

Nutrient Stress
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induced Ca 2+ deficiency. Wheeler and his associates (1992c) screened 34 plants species
for Al tolerance. Some plant species achieve Al tolerance by Al-activated exudation of
organic acids from roots (Yang et al., 2000; Ryan et al., 2001). Aluminium tolerance
genes were isolated from Arabidopsis and wheat (Sivaguru et al., 2003; Sasaki et al.,
2004).
Soil salinity is a worldwide problem that consists of salt marshes of the temperate
regions, adjacent to salt lakes and mangrove swamps. Saline soils are mainly
found in semiarid and arid regions. Salinity is characterized by enrichment of salts,
particularly sodium chloride, sodium sulfate and sometimes magnesium chloride and
sulfate at the soil surface. Salinity usually occurs due to evapotranspiration causing a
rise in ground water consisting of salts. Plant growth is severely affected when plants
are subjected to salinity more than any other stress in the natural environment. Salinity
affects the plant growth and development by impairing certain physiological and biochemical
processes like water nutrient imbalance (Flowers et al., 1986), Photosynthesis
(Yeo et al., 1985), Protein synthesis (Helal and Mengel, 1979) and phytohormone levels
(Kuiper et al., 1990).
17. HEAVY METAL TOXICITY AND TOLERANCE
Heavy metals are defined as metals with a density higher than 5g cm -3 . Heavy metal
pollution is posing several problems to mankind and these elements are increasing in
the environment due to industrialization, mining and urban activity. Metals such as Hg,
Pb, Cd, Al, Co, Mn, Ni, Cu and Zn are the important metals and their phytotoxicity is well
documented (Wong and Bradshaw, 1982; Baker, 1987; Breckle, 1991). Metals like Cu,
Mn, Ni and Zn are known as essential elements but when their concentration exceeds
beyond a particular level they inhibit plant growth rather than promoting the growth
and development. Research on metal toxicity and tolerance in plants has focussed on
the potential and realized effects of a variety of metals on physiological and biochemical
functions (Ernst, 1976; Foy et al., 1978; Taylor, 1988; Cumming and Taylor, 1990;
Clemens, 2001).
Metal chelation is the most important mechanism and metal chelating compounds
were first discovered in equine renal cortex by Margoshas and Vallee in 1957.
Phytochelatins and metallothioneins are cysteine rich, heavy metal binding proteins.
Phytochelatins are enzymatically synthesized polypeptides whereas metallothioneins
are gene encoded polypeptides (Cobbett and Goldsruough, 2002). Grill et al., (1985)
isolated phytochelatins from several plants. Phytochelatins are synthesized from glutathione
(GSH). When plants are exposed to heavy metals, the synthesis of
phytochelatins and simultaneous depletion of GSH was observed (Grill et al., 1985;
Rauser, 1990; Cobbett, 2000). An excellent account of molecular biology of heavy metal
stress in plants is given by Gasic and Korban (Chapter 8).