Betaines and Related Osmoprotectants. Targets for Metabolic Engineering of Stress Resistance1

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Abstract
Osmoprotectants (also termed compatible solutes) occur in all organisms from archaebacteria to higher plants and animals. They are highly soluble compounds that carry no net charge at physiological pH and are nontoxic at high concentrations. Osmoprotectants serve to raise osmotic pressure in the cytoplasm and can also stabilize proteins and membranes when salt levels or temperatures are un- favorable. Osmoprotectants therefore play important roles in the adaptation of cells to various adverse environmental conditions (Yancey, 1994). Chemically, there are three types: betaines and allied compounds, polyols and sugars (e.g. mannitol and trehalose), and amino acids such as Pro. This Update will focus on betaines and their biosynthetic pathways. Advances in polyol and Pro research are cov- ered elsewhere (Delauney and Verma, 1993; Stoop et al., 1996). Betaines are amino acid derivatives in which the nitro- gen atom is fully methylated, i.e. they are quaternary am- monium compounds. Figure 1 shows the structures of the three best-known betaines from plants, Gly betaine, Pro betaine (stachydrine), and b-Ala betaine, as well as choline- O-sulfate and DMSP (Rhodes and Hanson, 1993). The last two compounds are strictly speaking not betaines because DMSP has a tertiary sulfonium in place of a quaternary ammonium group, and choline-O-sulfate has a sulfate ester instead of a carboxyl group—but they are close structural analogs of betaines and have similar chemical and physi- ological properties. The compounds in Figure 1 differ in their taxonomic distribution (Blunden and Gordon, 1986; Rhodes and Han- son, 1993). For instance, Gly betaine is widespread among both flowering plants and algae, whereas DMSP is rare in higher plants but common in algae. Certain crop plants such as rice, soybeans, and potatoes lack significant amounts of betaines or any other osmoprotectant. This deficiency is the rationale for recent interest in using met- abolic engineering technology to install the synthesis of osmoprotectants in such crops in order to improve their tolerance to drought, salinity, and other stresses. The levels of betaines and other osmoprotectants typi- cally rise during exposure to stresses such as salinity, water deficit, and low temperature because the biosynthetic en- zymes are stress induced. Osmoprotectants are largely con- fined to the cytoplasm (including organelles) and are al- most absent from the vacuole, which generally occupies about 90% of the cell volume. For example, the halophyte Atriplex gmelini was found to have 320 mm Gly betaine in the cytoplasm, but only 0.24 mm in the vacuole (Matoh et al., 1987). Isolated chloroplasts of various species have also been shown to contain high concentrations of Gly betaine or DMSP, particularly when isolated from salt-stressed plants; Figure 1B illustrates this point with data from three species.