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

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Z . Dajic
8.1. Biochemical Determinants of Salt Tolerance – Enzymes, Compatible Solutes and
Protection Factors
The cytotoxicity of sodium lies in the high charge/mass ratio of the sodium ion (compared
with potassium), causing disruption in water structure and a decrease in hydrophobic
interactions and hydrostatic forces within proteins (Pollard and Wyn Jones,
1979). Additionally, Na + affects the activity of enzymes either by direct binding to
inhibitory sites or by displacing K + from activation sites. It has been suggested that
more than 50 enzymes are activated by K + , and Na + can’t be replaced in this function
(Bhandal and Malik, 1988). Additionally, K + is needed for protein synthesis, as binding
of tRNA to ribosomes requires K + (Blaha et al., 2000).
The effects of salts on enzymatic reactions are multiple and complex, although
to a large extent, their influence is related to the change in cytosolic pH which strongly
affects the activity of enzymes. It is generally accepted that enzymes exhibit slightly
increased activity under low concentrations of ions, whereas they start to be inhibited
in the presence of NaCl concentrations higher than 100mM (Munns, 2002). For instance,
the activity of DNAse and RNAse in alfalfa and lentil seedlings was inhibited in
the presence of 100 mM NaCl (Yupsanis et al., 2001).
Enzymes of halophytes are, in general, just as sensitive as enzymes of
glycophytes (Greenway and Osmond, 1972; Flowers et al., 1977), but some salt tolerant
plants exhibut in vitro tolerance of some enzymes to high concentrations of salts in
(Flowers and Dalmond, 1992). However, the relevance of any assay under in vivo conditions
is uncertain. Enzymes of cell wall compartment could be more salt-tolerant than
cytoplasmic enzymes of higher plants (Thiyagarajah et al., 1996).
The salt tolerance of plants, irrespective of the sensitivity of enzymes and
protein synthesis to high salt concentrations, is significantly related to the sequestration
of salts into the vacuoles, which allows the normal activity of metabolic machinery
in the cytoplasm. Salt-induced increases in the activity of enzymes involved in defense
to oxidative stress are related to the reactive oxygen species scavenging pathway
which takes place in the particular cell compartments, such as chloroplasts, peroxisomes,
glyoxysomes and cytosol (Yeo, 1998; Rathinasabapathi; 2000, Xiong and Zhu,
2002), which, in difference to the vacuoles, do not accumulate the salts.
The cytosolic apparatus of both halophytes and glycophytes is very sensitive
to osmotic and ionic effects of salts. Adverse effects of salts on the cell metabolism
may be alleviated through synthesis and accumulation of compatible solutes and protection
factors of macromolecules (mainly LEA proteins and chaperones). Accumulation
of compatible solutes in response to salt stress is a metabolic adaptation, which
primarily serves for osmotic adjustment and osmotic balance between vacuole and the
cytosol. As found in a number of stress-tolerant species, there is a possibility of convergent
evolution for this trait (Yancey et al., 1982; Rhodes and Hanson, 1993).
Compatible solutes are defined as organic osmolytes, which are compatible
with the cell’s metabolism, referring to protein/solute interactions and stabilization of