Phytochemical diversity: The sounds of silent metabolism

Plants produce tens of thousands of different natural products also referred to as secondary metabolites. These metabolites were once thought to be the result of aberrant metabolism, or a form of transient storage of byproducts and intermediates thereof. Although the true role of such metabolites in plants remains mostly unknown, it is evident that plants invest a great deal of resources in synthesizing, accumulating and sorting such metabolites. often produced through complex and highly regulated biosynthetic pathways operating in multiple cellular and sub-cellular compartments. There is also growing evidence indicating that many biosynthetic pathways leading to the accumulation of plant natural products are not fully active. Thus, occult enzymes exist, sometimes without any apparent endogenous substrate or function, suggesting that plants have a reservoir of metabolic capabilities that normally remains hidden or unused. It is often difficult to accurately guess what are the actual biological roles of such enzymes solely based on bioinformatics, due to promiscuity towards substrates and the relatively ease to change substrate or product specificity by introducing minor changes in sequence of the enzymes. It could be that such orphan enzyme activities are relics of a recent past that have not been fully eliminated through selection and evolution. Additionally, it could be that such occult activities possess unknown biochemical roles and coincidentally are able to accept novel substrates. We have coined the term silent metabolism to describe occult metabolic capacities present or induced in plants. A few examples illustrating silent metabolism in the terpenoid and phenylpropanoid pathways, as well as their repercussion in the metabolic engineering of plant secondary metabolism are discussed in this review. (C) 2008 Elsevier Ireland Ltd. All rights reserved.