Strong Increase of Foliar Inulin Occurs in Transgenic Lettuce Plants (Lactuca sativa L.) Overexpressing the Asparagine Synthetase A Gene from Escherichia coli

Abstract

Transgenic lettuce (Lactuca sativa L. cv. 'Cortina') lines expressing the asparagine synthetase A gene from Escherichia coli were produced to alter the plant nitrogen status and eventually enhance growth. The relative molecular abundance of water-soluble metabolites was measured by 1H NMR in transgenic and conventional plants at early developmental stages and grown under the same conditions. NMR metabolic profiles assessed that a transgenic line and the wild-type counterpart shared the same compounds, but it also revealed side effects on the carbon metabolism following genetic modification. Concerning the nitrogen status, the amino acid content did not vary significantly, except for glutamic acid and gamma-aminobutyric acid, which diminished in the transgenics. As for the carbon metabolism, in transgenic leaves the contents of sucrose, glucose, and fructose decreased, whereas that of inulin increased up to 30 times, accompanied by the alteration of most Krebs's cycle organic acids and the rise of tartaric acid compared to nontransformed controls. Lettuce leaf inulins consisted of short oligomeric chains made of one glucose unit bound to two/four fructose units. Inulins are beneficial for human health, and they are extracted from plants and commercialized as long-chain types, whereas the short forms are synthesized chemically. Hence, lettuce genotypes with high content of foliar short-chain inulin represent useful materials for breeding strategies and a potential source for low molecular weight inulin.

NMR-Metabolic Methodology in the Study of GM Foods

“…These spectra were conservative, as previously observed for both conventional and transgenic "Cortina" [8]. No novel or unusual signals were observed when the spectra of aqueous extracts from PetE:KNAT1 leaves and conventional "Luxor" counterparts were compared.…”

“…can minimize the exogenous effects on plant metabolite variation. Towards that aim, the glasshouse growing system offers a good solution, since it allows similar, if not identical, conditions for the production of modified and the wild type crops (e.g., lettuce) [8]. The metabolic changes due to variation in environment or the cultivation method can be monitored by NMR to reveal the corresponding metabolic patterns and the interactions of environmental and genetic factors at the level of metabolome [16].…”

“…A limited number of cases where NMR was used for the metabolic study of plant development [8,9,17] showed that each developmental program of plant organs (leaves, fruits, etc.) is reflected in unique metabolic changes, processes that are developmentally regulated.…”

“…Fourteen and nine pools of control and transgenic lines were collected. The samples were lyophilized according to the procedure reported previously [8] and stored at -20 °C. …”

“…The NMR-metabolomic methodology has been applied to study a number of genetically engineered (GM) plant species including lettuce (Lactuca sativa cv. "Cortina") expressing the asparagine synthetase A gene [8], tomato (Solanum lycopersicum cv "Ohio 8245") expressing the yeast SAMdc gene [9], maize expressing the Cry1A(b) gene [10], several transgenic wheat lines [11], and six transgenic lines of pea (Pisum sativum) [12].…”