B. Protection Against Na Ions in Cytoplasm
High intracellular concentrations of both Na and Cl can be deleterious to cellular systems. High salt con-
centrations (greater than 400 mM) inhibit most enzymes because they perturb the hydrophobic-electro-
static balance needed to maintain protein structure [139]. However, toxic effects of Na occur at much
lower concentrations (such as 100 mM Na), suggesting specific Na toxicity targets in the cell [141].
Sodium interferes with cationic sites involved in binding of K, Ca, and Mg [139,141]. For example, 20
mM Na in cytoplasm inhibited several nucleotidases and ribonucleases by displacing the essential Mg
from the protein complexes [166]. Chloride may interfere with anionic binding of RNA and anionic
metabolites such as bicarbonate, carboxylates, and sugar phosphates.
Glycine betaine is widely believed to protect the cytoplasm from Na toxicity [167]. It is hypothe-
sized that the dipole character neutralize Na and Cl during salt stress, and the hydrophobic methyl
groups stabilize the hydrophobic domains of proteins [15,16,133,167]. In vitro studies showed that GB
(200 to 500 mM) protected enzyme activity from Na toxicity [147,168–175]. Glycine betaine also pro-
tects PEP-carboxylase against excessive concentrations of Na ions [176,177]. Also, GB can alter the
thermodynamic properties of membranes by indirectly interacting with phosphatidylcholine moieties
[178,179].
Glycine betaine is less inhibitory to enzyme activity [180] and mRNA translation than equivalent
concentrations of other organic solutes in vitro [42,181]. In wheat and sugar beet, protein synthesis (trans-
lation of mRNA) was maintained at GB concentrations up to 500 mM [42], whereas sucrose levels above
100 mM and proline levels above 300 mM were inhibitory to protein synthesis [42]. In sugar beet, GB
protects the root membranes from heat destabilization [178,182]. These properties are of interest because
crop plants transformed with enzymes involved in osmolyte biosynthesis may also exhibit increased tol-
erance to heat stress [15,16].
- Positive Correlation Between Tissue Salt and Glycine Betaine Levels
As mentioned earlier, if GB is involved in the osmotic adjustment of the cytoplasm, then its production
should be regulated at the biosynthetic level with its production proportional to the severity of salinity or
water stress [21,127]. A positive correlation between salt concentration and GB accumulation was re-
ported in Atriplex semibaccata,A. halimus[183],Spartina alterniflora[184],Sporobolus virginicus
[185],Limoniumsp. [48,50], and Suaeda monoica[163]. In red beet, GB accumulation increased linearly
with leaf Na levels (Figure 6).
GLYCINE BETAINE IN STRESS RESISTANCE 893
Figure 6 Relationship between leaf sodium and glycine betaine accumulation in red beet.