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

Water Stress
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trations than other compatible solutes. The resurrection plant Myrothamnus flabellifolius
accumulates trehalose to increase the thermostability of its proteins (Kaushik and Bhat,
2003; Drennan et al., 1993), while resistance to various stresses is conferred in plant
cells at low concentrations (Garg et al., 2002). Drought stress causes cellular membrane
damage and leakage of ions from plant cells. Fructans, another compatible solute, have
the ability to stabilize phosphatidylcholine liposomes during freeze-drying (Steponkus,
1984; Hincha et al., 2000).
Leaves close their stomata to avoid evaporation of water during drought, and
consequently, the in-flow of CO 2
into the leaves stops. As a result, the sun’s energy
cannot be used for CO 2
fixation and instead is used for formation of active oxygen
molecules in the chloroplasts. Superoxide and hydrogen peroxide are decomposed by
enzymes specific to these active oxygen species. However, no enzyme has been shown
to decompose hydroxyl radicals, the most dangerous of all active oxygen species.
Some compatible solutes function as scavengers of hydroxyl radicals (Akashi et al.,
2001; Shen et al., 1997). For example, it has been reported that levels of free radicals are
decreased in tobacco plants transformed to accumulate more proline (Hong et al., 2000).
The reactivity of citrulline and mannitol to hydroxyl radicals is much higher that that of
proline; citrulline can promptly decompose all hydroxyl radical molecules at the formation
site (Table 2) (Akashi et al., 2001).
Table 2 . Second-order rate constants for reactions between
hydroxyl radicals and various compounds
Compound Rate constant Concentration Half-life of hydroxyl
(M -1 s -1 ) in vivo (mM) radicals generated
in vivo (ns)
Citrulline 3.9×10 9 200-300 0.59-0.89
Mannitol 2.1×10 9 100-320 1.0-3.3
Proline 5.4×10 8 120-428 3.0-11
Glycine betaine 8.2×10 7 320-1,000 8.5-26
Ascorbic acid 7.3×10 9 25-50 1.9-3.8
Glutathione 8.6×10 9 1-4.5 18-80
3.2. Biosynthesis of Compatible Solutes
The accumulation of metabolites can be accomplished by either promoted synthesis or
repressed degradation, or both. The substrates of compatible solutes are often metabolites
included in primary metabolic pathways with the flux of metabolites for synthesis
of compatible solutes being highly controlled (Nuccio et al., 1999).