Genetic engineering of glycinebetaine production toward enhancing stress tolerance in plants: Metabolic limitations

Glycinebetaine (betaine) affords osmoprotection in bacteria, plants and ani mals, and protects cell components against harsh conditions in vitro. This and a compelling body of other evidence have encouraged the engineering of betaine production in plants lacking it. We have installed the metabolic st ep for oxidation of choline, a ubiquitous substance, to betaine in three di verse species, Arabidopsis, Brassica napus, and tobacco (Nicotiana tabacum) , by constitutive expression of a bacterial choline oxidase gene. The highe st levels of betaine in independent transgenics were 18.6, 12.8, and 13 mu mol g(-1) dry weight, respectively, values 10- to 20-fold lower than the le vels found in natural betaine producers. However, choline-fed transgenic pl ants synthesized substantially more betaine. increasing the choline supplem entation further enhanced betaine synthesis, up to 613 mu mol g(-1) dry wei ght in Arabidopsis, 250 mu mol g(-1) dry weight in B. napus, and 80 mu mol g(-1) dry weight in tobacco. These studies demonstrate the need to enhance the endogenous choline supply to support accumulation of physiologically re levant amounts of betaine. A moderate stress tolerance was noted in some bu t not all betaine producing transgenic lines based on relative shoot growth . Furthermore, the responses to stresses such as salinity, drought, and fre ezing were variable among the three species.