IV. PHYSIOLOGICAL SIGNIFICANCE
Glycine betaine is widely perceived as a compatible solute accumulated in the cytoplasm to reduce the in-
tracellular water activity and s between the cell and its surroundings [2,5,8,19,24,129–131]. This is re-
quired to control the turgor pressure, which is the driving force for cell expansion, division, and thus
growth [132]. The principal characteristic of compatible solutes is their ability to reduce s of cytoplasm
without negative effects on metabolism. Glycine betaine preferentially excludes inorganic ions (such as
Na) from the hydration sphere of proteins and thus protects enzymes from denaturation [15,16,
79,133,134]. Also, compatible solutes are believed to stabilize freeze-thaw cycles and thus act as cry-
oprotectants [135,136].
In general, inorganic ions in the cytoplasm are maintained at relatively constant levels for normal
metabolic functioning [72,137,138]. High concentrations of inorganic solutes can be harmful in the cyto-
plasm as they cause protein denaturation and thus disruption of metabolic functions [25,43,139–141]. In
contrast, vacuoles can tolerate higher concentrations of inorganic ions where they serve in a role of os-
motic adjustment (OA). Potassium and in some cases Na are used for this function [86,142]. For exam-
ple, K is the major cation contributing to OA in sorghum [143], and nearly 78% of the OA in wheat could
be attributed to K accumulation [144,145].
A number of organic solutes such as betaines, tertiary sulfonium compounds such as dimethyl sul-
foniopropionate (DMSP), amino acids such as proline, and polyols such as sucrose, mannitol, and tre-
halose have similar OA functions in the cytoplasm [10,48,50,145,146]. However, GB is widely be-
lieved to be the most effective among compatible solutes in protecting the cytoplasm from dehydration,
ion toxicity (particularly from Na), and temperature stresses [8]. Also, GB can reverse damage to pro-
teins and membranes from high levels of Na [79]. High concentrations of organic solutes are reported
to stabilize macromolecules or molecular assemblies, thus decreasing the loss of either enzyme activ-
ity or membrane integrity when water is limiting [147,148]. Nevertheless, each of the structurally dis-
tinct osmoprotectants could differentially benefit the osmotically sensitive classes of molecules or
structures within the cell [147,148]. The following section will discuss GB’s role in protecting the
metabolic functions of the cytoplasm and thus its direct physiological significance in improving stress
resistance of plants.
890 SUBBARAO ET AL.
Figure 3 Pathways of choline synthesis in higher plants. Faint arrows show steps for which there is only
radiotracer evidence. The bold arrows show steps for which respective enzyme activity is found (EA,
ethanolamine; p-EA, phosphoryl ethanolamine; CDP-EA, cytidine diphosphate ethanolamine ester; Ptd-EA,
phosphatidyl ethanolamine; MEA, methyl ethanolamine; DMEA, dimethyl ethanolamine). (Adapted from
Ref. 8.)