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?-Alanine (??Ala) betaine is an osmoprotective compound accumulated by many plants, bacteria, and marine algae. It is quaternary ammonium compounds (QACs) accumulated in response to abiotic stresses such as drought and salinity (Gorham, 1995). QACs also stabilize enzymes and membranes. The synthetic pathway to glycine betaine, the most common QAC, therefore has been the target of recent metabolic engineering efforts to improve plant stress tolerance (McNeil et al., 1999; Sakamoto and Murata, 2000). However, these efforts have met with only limited success due to metabolic constraints on the availability of the precursor choline (Hayashi et al., 1997; Nuccio et al., 1998; Huang et al., 2000). The beta-alanine betaine biosynthesis pathway in Limonium latifolium requires three rounds of N-methylation, each of which is performed by the same N-methyltransferase encoded by the BANMT1 gene. The enzyme converts each methyl acceptor (beta-alanine, N-methyl-beta-alanine and N,N-dimethyl-beta-alanine) with similar catalytic efficiency (Rathinasabapathi et al., 2001). Most members of the highly stress-tolerant plant family Plumbaginaceae accumulate beta-alanine betaine as osmoprotectant. Its potential role in plant tolerance to salinity and hypoxia makes it an interesting target for metabolic engineering.
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