The work, which was detailed in the September issue of Plant Physiology, suggests that alterations of biosynthetic pathways could guide plants to produce more of a desired dietary component. In this case, tobacco produced 10 times its usual amount of tryptophan – an amino acid often in short supply in the human diet and vital for the production of serotonin in the brain.
Scientists inserted the gene, known to produce the control enzyme involved in tryptophan production, into the chloroplast genome. Chloroplasts are the chlorophyll containing plastids where photosynthesis occurs. The approach essentially is a reversal of evolution. “The biosynthesis of tryptophan and other essential amino acids occurs in these plastids,” said Archie R. Portis Jr., a University of Illinois crop scientist and researcher in the USDA-Agricultural Research Service Photosynthesis Research Unit at the UI. “However, the genes encoding these enzymes are located in the nucleus and the proteins are imported into the plastids.”
Plastids in today’s plants are believed to have evolved some 2 billion years ago from a unicellular, photosynthetic cyanobacteria containing its own set of genes that was engulfed by
non-photosynthetic cells. “Most of the genes originally located in these early plastids moved to the nucleus,” Portis said. “It is likely that those required for tryptophan biosynthesis were among these.”
The gene the researchers inserted had been isolated previously from a tobacco suspension culture that had been selected for resistance to an inhibitor of tryptophan biosynthesis. That work was done in the laboratory of co-author Jack M. Widholm, a UI professor of crop sciences.
The genetically transformed tobacco plants appeared normal, but they contained a four-fold increase of anthranilate synthase, the control enzyme of tryptophan biosynthesis. The leaves, in turn, had a 10-fold increase in tryptophan compared to non-altered plants.
“The work demonstrates the feasibility of modifying the biosynthetic pathways of important metabolites through transformation of the DNA located in the plastids and relocating native genes in the nucleus,” Portis said. “Plastid transformation is advantageous over nuclear DNA modifications, because it generally allows higher expression of the desired enzymes and restricts unwanted gene movement via pollen, which in most plants does not contain any plastid DNA.”
The technology provides another tool, besides nuclear transformation, for improving yield and value. It could also be used to engineer plants to produce pharmaceuticals such as edible vaccines.
Other co-authors with Portis and Widholm were UI crop scientists Xing-Hai Zhang and Jeffery E. Brotherton. Zhang also is with the USDA-ARS. The Illinois Soybean Program Operating Board, the Illinois Agricultural Experiment Station and the U.S. Department of Agriculture funded the research.
Journal
PLANT PHYSIOLOGY