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The synthesis of the osmoprotectant glycine betaine (GB) in plants is a metabolic trait of major adaptive importance since it confers tolerance to drought, saline soils and cold, but only some plants can synthesize and accumulate GB. Also, the GB content of a plant is valuable for human and animal nutrition. GB is synthesized from choline in two consecutive reactions catalyzed by choline mono-oxygenase (CMO) and betaine aldehyde dehydrogenase (BADH). Nonaccumulators plants have very low levels of BADH activity due to a single change in an amino acid residue of this enzyme. Two CMO isoenzymes also appear to exist, but they have not been biochemically and structurally characterized yet. Another factor that limits the synthesis of GB is the supply of choline to CMO. Most likely, in GB-accumulator plants the catalytic activities and the regulation of the expression of both BADH and CMO proteins coevolved, since the BADH activity requires an active CMO, and an active CMO without an active BADH would be deleterious for the plant because the toxic betaine aldehyde would accumulate. Biotechnological efforts to engineer this important metabolic trait in GB-nonaccumulators crops have had limited success so far, stressing the need for more basic biochemical studies.

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