nation of a regulated uptake of ions in the roots, the cell-, tissue-specific compartmentation of ions, and
osmotic adjustment [133,137,255] (also see Chapter 44).
Most information available on the functional role of GB is based on in vitro studies. Glycine betaine
has only a limited capacity to regulate total leaf s, as its concentrations rarely exceed 100 to 200 mol
g^1 dwt or about 5% of total leaf s. Thus, GB can have significant impact in s only if localized in the
cytoplasm. Also, the functional relationship between GB accumulation and stress resistance in the field
has received limited attention. Characterization of genetic stocks that differ in GB accumulation and their
adaptive potential to stressful environments has not been well established. Development of near-isogenic
lines that differ in GB accumulating capability is essential for evaluating GB as an adaptive trait for crop
improvement programs. But surprisingly little progress has been made in this direction.
Glycine betaine concentrations are tightly correlated with leaf osmotic adjustment under saline and
drought conditions, although a mechanism for this response is not known. Also, to have a positive effect
on plant water relations over extended period of time under water deficits, osmotic adjustment should
stimulate root growth to permit better water extraction. However, to our knowledge, mechanisms that
could link GB accumulation or osmotic adjustment to the root growth stimulation have not been estab-
lished.
Since GB is not metabolized when stress is removed, this represents a permanent cost to plants that
synthesize the compound. Remobilization of C and N compounds plays an important role in limiting wa-
ter stress effects on the seed-filling phase of legumes [256] (G.V. Subbarao et al. unpublished). Negative
osmotic adjustment during seed filling of pigeonpea is observed in genetic stocks that have the highest
yield potential under moisture deficits (G.V. Subbarao et al. unpublished data). Thus, osmotic adjustment
as a mechanism of stress resistance may not limit yield if carbon and nitrogen from the organic solutes
can be utilized subsequently for reproductive growth. Because GB cannot be metabolized like other or-
ganic solutes, a potential exists that GB accumulation can reduce yield. Nevertheless, GB may confer
metabolic stability as it can be redistributed when plants are subjected to intermittent and transient
stresses.
898 SUBBARAO ET AL.
TABLE 4 Transgenic Plants Where Osmotically Active Substances Were Introduced to Improve Osmotic
Adjustmenta
Product Increased
Gene product Source accumulated resistance References
Mannitol-1-phosphate Escherichia coli Mannitol Yes [245]
dehydrogenase [246]
p5C synthetase Vigna aconitifolia Proline Yes [247]
Fructosyltransferase Bacillus subtilis Fructan Yes [248]
(levan-sucrase)
Betaine aldehyde Hordeum vulgare Betaine No [118]
dehydrogenase [162]
(BADH)
Sorbitol-6-phosphate Malus domestica Sorbitol NDc [249]
dehydrogenase
Choline dehydrogenase Escherichia coli Glycine betaine? Yes [250]
Trehalose-6-phosphate Saccharomyces cerevisiae Trehalose Yes [244]
synthetase [251]
Trehalose-6-phosphate E. coli Trehalose No [252]
synthetase
myo-Inositol-3-phosphate Spirodela polyrrhiza Inositol No [253]
synthase
myo-Inositol-3-methyl Mesembryanthemum Ononitol Yes [254]
transferase crystallinum
Choline oxidaseb Arthrobacter globiformis Glycine betaine Yes [200]
aAll these studies involved transformation of tobacco (Nicotiana tabacum).
bIntroduced into Arbidopsis thaliana.
cND, stress resistance was not determined.
Source:Adapted from Ref. 9.