How Does Vanilla planifolia Make Vanillin?
Richard A. Dixon, Fang Chen and Xiaolan Rao
University of North Texas
Vanillin is the world’s most popular flavor, and as such is probably the world’s most popular plant natural product. It is also an extremely simple molecule. Why then, at a time when the biosynthesis of increasingly complex plant secondary metabolites is being elucidated at both the chemical and molecular genetic levels, should vanillin biosynthesis still be controversial? Vanillin is believed to be made in specialized cells within the pods of the vanilla orchid, Vanilla planifolia, and is stored as its 4-O-glucoside, glucovanillin. The structure of vanillin lends itself to multiple theoretical biosynthetic pathways and, because of a general promiscuity of many enzymes of plant phenolic metabolism, it is possible to find evidence to support any of these pathways from in vitro biochemical approaches. Each new “advance” has provided an alternative model without effectively disproving existing models. In this presentation we evaluate the evidence for and against proposed pathways to vanillin. Our data indicate that the recently proposed one step pathway from ferulic acid catalyzed by a specific vanillin synthase is incorrect.
Richard A. Dixon is Distinguished Research Professor in the Department of Biological Sciences at the University of North Texas, Denton. He was previously Distinguished Professor and Samuel Roberts Noble Research Chair, Senior Vice President and Founding Director of the Plant Biology Division at the Samuel Roberts Noble Foundation, Ardmore, Oklahoma, where he worked from 1988-2013. He received his Bachelor’s and Doctoral degrees in Biochemistry and Botany from the University of Oxford, UK, and postdoctoral training in Plant Biochemistry at the University of Cambridge, UK. He was awarded the Doctor of Science degree for his research achievements by the University of Oxford in 2004. His research interests center on the biochemistry, molecular biology and metabolic engineering of plant natural product pathways and their implications for agriculture and human health, and the engineering of lignocellulosic biomass for the improvement of forage and bioenergy feedstocks. He has published over 430 papers and chapters on these and related topics in international journals. Professor Dixon is a Member of the US National Academy of Sciences (Plant and Soil Sciences Section, elected in May 2007), a Fellow of the American Association for the Advancement of Science (elected in 2003), a Fellow of the National Academy of Inventors (elected in December 2014), a member of the Editorial Boards of five international journals, and has been named by the Institute for Scientific Information as one of the 10 most cited authors in the plant and animal sciences.