Choline monooxygenase (CMO) catalyzes the committed step of glycine betaine (GlyBet) biosynthesis in many flowering plants. To investigate its effect on various stress tolerances in plant metabolic engineering, we isolated and characterized the CMO gene from Atriplex hortensis, a GlyBet natural accumulator, and introduced it into tobacco to examine the effect of GlyBet on plant drought and salt tolerance, respectively. In A. hortensis, the expression of AhCMO was induced 3-fold in the root and stem, as well as in the leaf, when plants were treated with 400 mM of NaCl, indicating that the acceleration of GlyBet biosynthesis under salt stress was achieved through the whole plant, including organs without chloroplasts. AhCMO transcription was also regulated by drought, ABA and circadian rhythm. Over-expression of AhCMO improved drought tolerance in transgenic tobacco when cultured in medium containing PEG-6000. The transgenic plants also have a better performance under salt stress.