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 todays 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 incr
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Contact: Jim Barlow
b-james3@uiuc.edu
217-333-5802
University of Illinois at Urbana-Champaign
28-Sep-2001