Choline Import into Chloroplasts Limits Glycine Betaine Synthesis in Tobacco: Analysis of Plants Engineered with a Chloroplastic or a Cytosolic Pathway
✍ Scribed by Michael L. Nuccio; Scott D. McNeil; Michael J. Ziemak; Andrew D. Hanson; Ravinder K. Jain; Gopalan Selvaraj
- Book ID
- 102614531
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 374 KB
- Volume
- 2
- Category
- Article
- ISSN
- 1096-7176
No coin nor oath required. For personal study only.
✦ Synopsis
The biosynthesis of the osmoprotectant glycine betaine (GlyBet) is a target for metabolic engineering to enhance stress resistance in crops. Certain plants synthesize GlyBet in chloroplasts via a twostep oxidation of choline (Cho). In previous work, a chloroplastic GlyBet synthesis pathway was inserted into tobacco (which lacks GlyBet) by expressing spinach choline monooxygenase (CMO). The transformants had low CMO enzyme activity, and produced little GlyBet ( 70 nmol g &1 fresh wt). In this study, transformants with up to 100-fold higher CMO activity showed no further increase in GlyBet. In contrast, tobacco expressing a cytosolic GlyBet synthesis pathway accumulated significantly more GlyBet (430 nmol g &1 fresh wt), suggesting that subcellular localization influences pathway flux. Modeling of the labeling kinetics of Cho metabolites observed when [ 14 C]Cho was supplied to engineered plants demonstrated that Cho import into chloroplasts indeed limits the flux to GlyBet in the chloroplastic pathway. A high-activity Cho transporter in the chloroplast envelope may therefore be an integral part of the GlyBet synthesis pathway in species that accumulate GlyBet naturally, and hence a target for future engineering.