Update on Genetic Engineering The Use of Bacterial Choline Oxidase, a Glycinebetaine- Synthesizing Enzyme, to Create Stress-Resistant Transgenic Plants

GB is a zwitterionic, fully N-methyl-substituted derivative of Gly that is found in a large variety of microorganisms, higher plants, and animals (Rhodes and Hanson, 1993). At high concentrations, GB does not interfere with cytoplasmic functions and it efficiently stabilizes the structure and function of many macromolecules. Thus, it belongs to a group of compounds that are known collectively as compatible solutes. GB appears to be a critical determinant of stress tolerance in plants. It is an extremely efficient compatible solute (Le Rudulier et al., 1984) and its presence is strongly associated with the growth of plants in dry and/or saline environments (Rhodes and Hanson, 1993). The accumulation of GB is induced under stress conditions (Gorham, 1995), and the level of GB is correlated with the degree of enhanced tolerance to stress (Saneoka et al., 1995). Exogenous application of GB improves the growth and survival of a wide variety of plants under various stress conditions (Allard et al., 1998; Hayashi et al., 1998). Furthermore, GB is much more effective than other compatible solutes in the stabilization in vitro of the quaternary structure of enzymes and complex proteins, as well as the highly ordered state of membranes, at high concentrations of salts and extreme temperatures (Gorham, 1995; Papageorgiou and Murata, 1995). These properties of GB were deduced for the most part from studies based on comparative physiology and genetics, as well as from experiments in vitro. However, such studies have in fact provided only circumstantial evidence for the important role in vivo of GB in the stress tolerance of plants. A full understanding of the role of GB requires more than circumstantial evidence, and genetic engineering of unicellular cyanobacteria has provided a way for us to examine the physiological significance and the modes of action in vivo of this compatible/ protective solute in the stress tolerance of photosynthetic organisms (Deshnium et al., 1995; Nomura et al., 1995). Similar transgenic approaches have proved fruitful in higher plants such as Arabidopsis, rice (Oryza sativa), and tobacco (Nicotiana tabacum), none of which normally synthesizes GB (Hayashi and Murata, 1998; Sakamoto and Murata, 2000). The aim of this Update is to summarize recent progress in experiments with transgenic phototrophs that has advanced our understanding on the role in vivo of GB in stress tolerance. Aspects of the physiology, biochemistry and genetics of the synthesis, and properties of GB have been covered elsewhere (Rhodes and Hanson, 1993; Gorham, 1995; McNeil et al., 1999).