Enhanced synthesis of choline and glycine betaine in transgenic tobacco plants that overexpress phosphoethanolamine N-methyltransferase

Choline (Cho) is the precursor of the osmoprotectant glycine betaine and is itself an essential nutrient for humans. Metabolic engineering of Cho bios ynthesis in plants could therefore enhance both their resistance to osmotic stresses (drought and salinity) and their nutritional value. The key enzym e of the plant Cho-synthesis pathway is phosphoethanolamine N-methyltransfe rase, which catalyzes all three of the methylations required to convert pho sphoethanolamine to phosphocholine. We show here that overexpressing this e nzyme in transgenic tobacco increased the levels of phosphocholine by 5-fol d and free Cho by 50-fold without affecting phosphatidylcholine content or growth. Moreover, the expanded Cho pool led to a 30-fold increase in synthe sis of glycine betaine via an engineered glycine betaine pathway. Supplying the transgenics with the Cho precursor ethanolamine (EA) further enhanced Cho levels even though the supplied EA was extensively catabolized. These l atter results establish that there is further scope for improving Cho synth esis by engineering an increased endogenous supply of EA and suggest that t his could be achieved by enhancing EA synthesis and/or by suppressing its d egradation.