The evolution of higher plants has been accompanied by the evolution of enzymes that lead to the production of a wide range of chemical constituents. Chemical structures are known for more than 15,000 chemical constituents on plants and, by extrapolation, it has been estimated that higher plants collective produce more than 100,000 chemical constituents. A relatively simple class of compounds that exhibit substantial chemical diversity is fatty acids that accumulate as major constituents of storage oils in many species. Many of the genes encoding the enzymes required for fatty acid desaturation were cloned by molecular genetic techniques in Arabidopsis. These genes were then used to identify variants of the genes from diverse plant species. These studies revealed that essentially all chemical diversity in plant fatty acids was due to the action of structural variants of a single enzyme. Mechanistic studies of this enzyme family showed that minor changes in the amino acid composition of the protein near the active site could change the outcome of the reaction catalyzed by this family of enzymes. In one instance we were able to show that as few as four amino acid changes in an enzyme could convert and enzyme that normally inserted a double bond into fatty acids into an enzyme that inserted a hydroxyl group. These studies provide an explanation for the mechanism by which chemical diversity in this class of compounds has evolved and provide a model for understanding the mechanisms underlying the evolution of other major families of compounds.

Key words: Arabidopsis, chemical diversity, evolution, genomics, proteomics