Linking chemistry and genomics for the study of secondary metabolism in aromatic and medicinal plants

The study of the biosynthesis of secondary metabolites and the genes involved in these processes has been greatly facilitated by novel genomic approaches developed during the last years. Many of the biosynthetic pathways dedicated to secondary metabolism, and the enzymes involved in these pathways, have apparently evolved from the much better studied primary biosynthetic pathways. Therefore by exploiting similarities between functionally-related genes, it has been possible to isolate novel genes involved in the formation of unique natural products. To implement this novel approach, appropriate tissues in the proper physiological state, where the compounds of interest are produced in significant levels, is identified. Next, sequence information on large numbers (thousands) of different ESTs (expressed sequence tags) originating in these tissues is obtained. The information obtained is en masse examined using bioinformatic computer algorithms. Predictions on the physiological and biochemical role of individual ESTs are then made based on DNA similarities, and the patterns of expression of individual ESTs. Identity and biochemical function of the particular EST in question can then be confirmed by functional expression experiments. A few examples of such genomic projects aimed at isolating and characterizing genes involved in the formation of key metabolites are reviewed. Some of the genes responsible for the formation of the volatile phenylpropenes prominent in the essential oil of sweet basil and in the formation of the many compounds that compose the fragrance of roses have been identified utilizing this approach. The potential of utilizing genes that code for the formation of volatile compounds, for the improvement of the quality properties of aromatic plants and other agricultural produce, are discussed.