Effects of exogenous application of GB as a management strategy for improving stress resistance in
drought-prone and saline environments need to be evaluated. Potential impacts of introducing the GB
biosynthetic pathway into crops (that do not accumulate) are not fully understood. There are concerns that
introducing genes for GB synthesis may not be sufficient if precursor compounds such as choline are still
limiting. Introducing genes that promote synthesis of these precursor compounds (such as choline from
serine) would also need to be evaluated. Also, transport mechanisms for GB between cell compartments
and tissues are not well understood and need further study [8].
Successful development of stress-tolerant crops will require large-scale metabolic engineering of not
only genes encoding osmolytes but also those for antioxidants, water channel proteins, etc. because os-
motic adjustment is just part of a complex web of adaptive strategies in plants [215,257]. The complex-
ity and range of physiological mechanisms that have similar adaptive roles for drought and salinity make
it a challenging task to establish the functional role of GB in crop adaptation to stressful environments.
ACKNOWLEDGMENTS
We would like to thank Drs. Wade Berry (University of California, Los Angeles) and M. Shannon
(USDA Salinity, Riverside, CA), who went through the earlier versions of the manuscript and
made several suggestions to improve it. Also, we would like to thank Dr. William Knott, NASA
Biomedical Office, Kennedy Space Center, for his support and encouragement for this project. This re-
search was supported through the National Research Council and NASA’s Life Support Contract
(NAS10-12180).
REFERENCES
- AD Brown, JR Simpson. Water relations of sugar tolerant yeasts: the role of intracellular polyols. J Gen Mi-
crobiol 72:589–591, 1972. - GR Stewart, JA Lee. The role of proline accumulation in halophytes. Planta 120:279–289, 1974.
- R Storey, RG Wyn Jones. Betaine and choline levels in plants and their relationship to NaCl Stress. Plant Sci
Lett 4:161–168, 1975. - RG Wyn Jones, R Storey. Betaines. In: LG Paleg, D Aspinall, eds. The Physiology and Biochemistry of
Drought Resistance in Plants. Sydney: Academic Press, 1981, pp 171–204. - PH Yancey, ME Clark, SC Hand, RD Bowlus, GN Somero. Living with water stress: evolution of osmolyte
systems. Science 217:1214–1222, 1982. - RG Wyn Jones. An assessment of quaternary ammonium and related compounds as omotic effectors in crop
plants (phytochemical aspects of osmotic adaptation). Recent Adv Phytochem 18:55–58, 1984. - BP Naidu, LG Paleg, GP Jones. Nitrogenous compatible solutes in drought stressed Medicagospp. Phyto-
chemistry 31:1195–1197, 1992. - D Rhodes, AD Hanson. Quaternary ammonium and tertiary sulfonium in higher plants. Annu Rev Plant Phys-
iol Plant Mol Biol 44:357–384, 1993. - PD Hare, WA Cress, J Van Staden. Dissecting the roles of osmolytes accumulation during stress. Plant Cell
Environ 21:535–553, 1998. - AJ Delauney, DPS Verma. Proline biosynthesis and osmoregulation in plants. Plant J 4:215–223, 1993.
- N Smirnoff, QJ Cumber. Hydroxyl radical scavenging activity of compatible solutes. Phytochemistry
28:1057–1060, 1989. - B Schobert. Is there an osmotic regulatory mechanism in algae and higher plants? J Theor Biol 68:17–26, 1977.
- SN Timasheff, T Arakawa. Stabilization of protein structure by solvents. In: TE Creighton, ed. Protein Struc-
ture: A Practical Approach. Oxford: IRL Press, 1989, pp 331–345. - J Gorham. Betaines in higher plants—biosynthesis and role in stress metabolism. In: RM Wallsgrove, ed.
Aminoacids and Their Derivatives in Higher Plants. Cambridge: University Press, 1995, pp 172–203. - HJ Bohnert, RG Jensen. Metabolic engineering for increased salt tolerance—the next step. Aust J Plant Phys-
iol 23:661–667, 1996. - HJ Bohnert, RJ Jensen. Strategies for engineering water-stress tolerance in plants. TIBTECH 14:89–97, 1996.
- JH Crowe, FA Hoekstra, LM Crowe. Anhydrobiosis. Annu Rev Physiol 54:579–599, 1992.
- EA Galinski, HG Truper. Betaine, a compatible solute in the extremely halophilic phototrophic bacterium Ec-
tothiorhodospira halochloris. FEMS Microbiol Lett 13:357–360, 1982. - R Storey, RG Wyn Jones. Quaternary ammonium compounds in plants in relation to salt resistance. Phyto-
chemistry 16:447– 4 53, 1977. - M Briens, F Larher. Osmoregulation in halophytic higher plants: a comparative study of soluble carbohydrates,
polyols, betaines and free proline. Plant Cell Environ 5:287–292, 1982.
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