In 1667, after more than a century of bloody battles, the Dutch and the English settled their dispute over the spice trade. Although the conflict centered on cloves and nutmeg, plant researchers have long known that the fuss was really about two closely related organic molecules, eugenol and isoeugenol, which give the respective spices their characteristic aromas. But the researchers did not know exactly how plants make these compounds. Now a team has elucidated the biochemical pathway responsible, as well as identified the key enzymes involved. The findings could have important applications in the food and flower industries.
The research group, led by molecular biologist Eran Pichersky of the University of Michigan, Ann Arbor, studied two model organisms that are easily manipulated in the laboratory: basil, which produces large amounts of eugenol, and the petunia flower, whose scent is caused by three aromatic compounds including isoeugenol. In the wild, such aromatic compounds repel herbivores and bacteria while attracting pollinating insects. When the researchers scanned their database of DNA sequences from basil leaves and petunia flowers, they spotted a sequence that matched a gene implicated by another research group in producing the petunia's scent. Pichersky's team fully sequenced this gene and found that it was very similar to another gene in the database that came from the basil plant, the team reports online this week in the Proceedings of the National Academy of Sciences.
Suspecting that these genes might code for enzymes involved in eugenol and isoeugenol synthesis, the researchers genetically engineered Escherichia coli bacteria to carry either the petunia or basil versions. When the bacteria were cultured with a precursor in the synthetic pathway, coniferyl alcohol, they obligingly produced the two aromatic molecules--which control bacteria did not do. In further experiments with basil plants, the researchers discovered how coniferyl alcohol is metabolized into an intermediate molecule, at least in the lab, then converted to eugenol.
The work is a "major step forward," says Robert Schuurink, a plant molecular biologist at the University of Amsterdam in the Netherlands. Schuurink adds that the identification of the enzymes as well as the intermediate step in the biochemical pathway "now opens the possibility of genetically engineering the aromatic contents of spices." And Pichersky sees other applications as well, such as making plants more resistant to animals and engineering the scents of crop plant flowers to improve their pollination, thus increasing their yields.