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18 Vanillin Biosynthesis - Not as Simple as it Seems?

Richard A. Dixon

18.1 INTRODUCTION

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 so controversial? Why do we know most of the steps involved in taxol biosynthesis (Heinig and Jennewein 2009), all of the steps involved in lignin (monolignol) biosynthesis (a pathway that share similarities to the vanillin pathway(s) (Humphreys and Chapple 2002), many of the steps involved in the formation of complex nitrogen-containing alkaloids (Kutchan 2002; Zeigler et al. 2006), but not how plants make 3-methoxy, 4-hydroxy-benzaldehyde? To be fair to the small body of researchers who have investigated vanillin biosynthesis, this question should probably be re-phrased to ask why we are still confused about the biosynthesis of most C₆-C₃ benzenoid derivatives in plants.

Vanillin is made in the “pods” of an orchid, Vanilla planifolia, a species that lacks genetic or genomic resources, and is stored as its 4-O-glucoside, glucovanillin. It is made in specialized cells within the pod (Joel et al. 2003), although there is still some disagreement as to exactly which cell types do or do not produce vanillin (Joel et al. 2003; Odoux and Brillouet 2009). The nature of the plant species and the restricted cellular location of its famous product should not present insurmountable problems for understanding vanillin biosynthesis, however, since many studies have addressed biosynthetic routes to more complex natural products through the application of molecular genetic approaches to specialized tissues in genetically recalcitrant plant species. Some of the best examples concern the biosynthesis of defensive compounds in glandular trichomes (Gang et al. 2002; Weathers et al. 2006; Nagel et al. 2008). My contention is that the simplicity of vanillin itself poses the major problem, because the structure lends itself to multiple theoretical biosynthetic pathways (Figure 18.1) 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. This certainly seems to be the case from a brief overview of the history of studies on the biosynthesis of vanillin and related compounds (Table 18.1), from which it is clear that our “understanding” of vanillin biosynthesis has not proceeded in a sequential manner. Rather, each new “advance” has provided an alternative model without effectively disproving existing models.