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

Salt Stress
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macromolecules, irrespective of species and nature of the stress (Yeo, 1998; Nuccio et
al., 1999; Rathinasabapathi, 2000; Hasegawa et al., 2000; Huang et al., 2000). These
Compatible solutes comprise a wide range of organic compounds, such as: simple
sugars (fructose and glucose), sugar alcohols (glycerol and methylated inositols), complex
sugars (trehalose, raffinose and fructans), polyols, quaternary ammonium compounds
(proline, glycine betaine, â-alanine betaine, proline betaine) and tertiary sulfonium
compounds (Rhodes and Hanson, 1993; Nuccio et al., 1999). As compatible solutes
are hydrophilic, they can replace water at the surface of proteins, complex protein
structures and membranes, which explains their action as osmoprotectants and as lowmolecular-weight
chaperones (Hasegawa et al., 2000).
Within this group of molecules, glycine betaine is a ubiquitous protein-stabilizing
osmolyte occurring in all organisms (Rhodes and Hanson, 1993). Glycine betaine
is an amphoteric compound, electrically neutral over a wide range of pH and extremely
soluble in water, allowing it to interact with both hydrophilic and hydrophobic regions
of macromolecules (Sakamoto and Murata, 2002). Glycine betaine a accumulated in
many halophytic species, such as Suaeda maritima (Clipson et al., 1985), Atriplex
nummularia, Spergularia marina, Salicornia europea (Stumpf, 1984) and Salsola
soda (Manetas, 1990) in order to balance the osmotic potential difference between
vacuole and the cytoplasm (Flowers et al., 1977). The accumulation of glycine betaine in
halophytes (e.g. Atriplex griffithii) is induced by salt stress and increases with a raise
of salinity (Khan et al., 1998; 2000b) in the halophyte. Glycine betaine was the major
organic osmolyte of non-halophytes, such as wheat (Saneoka et al., 1999), red-beet
(Subbarao et al., 2001), and sorghum (Yang et al., 2003). Despite its wide presence in
many speeies, glycine betaine is absent in some crops, (rice) and tobacco (Yeo, 1998).
In higher plants the biosynthesis of glycine betaine is seeds as by two-step
oxidation of choline (via the toxic intermediate betaine aldehyde) catalyzed by choline
monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively
(Sakamoto and Murata, 2002). The activity of these enzymes is localized to the chloroplast
stroma, although some BADH activity was found in the cytoplasm (Weigel et al.,
1986). Two kinds of BADH found in the mangrove halophyte Avicennia marina were
characterized by high efficiency in the oxidation of betaine aldehyde (Hibino et al.,
2001). Installing of genes involved in the synthesis of glycine betaine has had a certain
success in improvement of salt tolerance in plants.
Polyols, such as glycerol, mannitol, sorbitol and sucrose, are osmoprotectants
in algae and certain halophytic plants (Yancey et al., 1982). The biosynthesis and accumulation
of specialized polyols (myo-inositol, D-ononitol and D-pinitol) in the cytosol
of the stress-tolerant common ice plant (Mesembryanthemum crystallinum) was reported
to increase with salinity (Nelson et al., 1999).
The sugar alcohol mannitol may serve as a compatible solute in salinity conditions.
The role of mannitol in salt stress tolerance was evaluated in the non-mannitol
producer Arabidopsis by installing the M6PR gene (for synthesis of mannose-6-phosphatase
reductase) from celery, and in the presence of NaCl, mature transgenic plants