Phytochemical Diversity: the Sounds of Silent Metabolism Plant Science
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Plant Science 176 (2009) 161–169 Contents lists available at ScienceDirect Plant Science journal homepage: www.elsevier.com/locate/plantsci Review Phytochemical diversity: The sounds of silent metabolism Efraim Lewinsohn a,*, Mark Gijzen b a Department of Vegetable Crops, Newe Ya’ar Research Center, Agricultural Research Organization, P.O. Box 1021, Ramat Yishay 30095, Israel b Agriculture and Agri-Food Canada, 1391 Sandford Street, London, Ontario N5 V 4T3, Canada ARTICLE INFO ABSTRACT Article history: Plants produce tens of thousands of different natural products also referred to as secondary metabolites. Received 10 June 2008 These metabolites were once thought to be the result of aberrant metabolism, or a form of transient Received in revised form 1 September 2008 storage of byproducts and intermediates thereof. Although the true role of such metabolites in plants Accepted 27 September 2008 remains mostly unknown, it is evident that plants invest a great deal of resources in synthesizing, Available online 21 October 2008 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 Keywords: growing evidence indicating that many biosynthetic pathways leading to the accumulation of plant Silent metabolism Occult metabolic capacity natural products are not fully active. Thus, occult enzymes exist, sometimes without any apparent Plant secondary metabolism endogenous substrate or function, suggesting that plants have a reservoir of metabolic capabilities that Natural product biosynthesis normally remains hidden or unused. It is often difficult to accurately guess what are the actual biological Metabolic engineering roles of such enzymes solely based on bioinformatics, due to promiscuity towards substrates and the Metabolic networks 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. ß 2008 Elsevier Ireland Ltd. All rights reserved. Contents 1. Introduction . 162 2. Occult terpenoid pathways . 162 2.1. Silent metabolism in carotenoid biosynthesis and degradation reactions. 163 3. Silent metabolism in the biosynthesis of volatile ethers . 165 4. Silent metabolism in the phenylpropanoid pathways . 166 4.1. Flavonoid biosynthetic pathways provide numerous examples of silent metabolism . 166 4.2. Silent flavonoid metabolism in soybean seed coats . 166 4.3. Soybean seed coat anthocyanin pigments are substrates for the seed coat peroxidase enzyme . 167 5. Mechanism of action of silent metabolism. 167 5.1. Silent metabolism and relaxed selection. 167 6. Concluding remarks. 167 Acknowledgements. 168 References...................................................................................................... 168 * Corresponding author. E-mail addresses: [email protected] (E. Lewinsohn), [email protected] (M. Gijzen). 0168-9452/$ – see front matter ß 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2008.09.018 162 E. Lewinsohn, M. Gijzen / Plant Science 176 (2009) 161–169 Hello darkness, my old friend The much greater multitude of metabolites in living organisms as I’ve come to talk with you again compared to their unexpected low calculated number by the small Because a vision softly creeping number of relevant genes identified during genome sequencing projects has puzzled biologists [14]. It has been postulated that Left its seeds while I was sleeping promiscuity of enzymes, including multiple product and substrate And the vision that was planted in my brain enzyme specificity as well of changes in compartmentation patterns Still remains may also contribute to plant metabolome diversity [15]. Moreover, Within the sound of silence the more knowledge we mine on specific biosynthetic pathways and the accumulation of information on the complexity of biosynthetic (P. Simon, 1964) networks have revealed that the transcription patterns as a whole are rarely directly reflected in the proteome of an organism [16] and 1. Introduction in turn, the proteome in many cases only vaguely reflects the metabolome [17]. The more knowledge we extract on the regulation Plants produce and accumulate a vast number of different of plant natural product biosynthesis, the more puzzling it gets. natural products, also called secondary metabolites. Although tens Perhaps the old-fashioned unpopular and forgotten theory of being of thousands of secondary metabolites have been chemically natural products part of an ‘‘aberrant metabolism’’ was not so off- identified [1–3], the biological roles of most of these compounds line. It thus seems that the combination of enzymes and substrates remain obscure. Many natural compounds are of commercial and generated by serendipity may be a platform to generate the much industrial importance, imparting colors and scents to flowers, needed variation to attain effectiveness in plant defenses, and this fruits and vegetables, and are also key ingredients in medicinals process might be favored by evolution [7]. In this scenario, it is and nutraceuticals. Many studies have indicated that natural evolutionary advantageous to keep parts of the enzymatic products accumulated in plants have clear ecological roles such as machinery active through generations, and this in turn is the basis protection against predation, protection against fungal and of what we have defined as ‘‘silent metabolism’’: Occult biosynthetic bacterial diseases or against adverse climatic conditions [1,2,4– capacities that is not easily detected but readily active when 6]. Additionally, many natural products serve as signal molecules challenged. ‘‘Silent metabolism’’ can be either constitutive or to attract pollinators and seed-dispersers, or mediate pathogenic, induced by development, the environment and genetic manipula- parasitic, or symbiotic interactions [1]. Still, the biological roles of tion, and might seem to the naı¨ve spectator as if active enzymes and most specialized compounds in the plants producing them are biosynthetic pathways do not have any apparent role in the tissue unknown [3]. According to the so called ‘‘Screening Hypothesis’’ studied, but are uncovered when a change in metabolism is effected. theory [7], it is a rarity for a natural product to possess biological In this review we will discuss a few of many examples of ‘‘silent activity. Therefore, evolution favors means to increase biodiversity metabolism’’ based on our own results and on other published to provide novel compounds to be challenged by evolution. In this work. They all indicate that plants have practically a network of context and according to the ‘‘screening hypothesis’’ many of the occult biosynthetic capacities that confers them an unlimited compounds manufactured and stored by plants (and retained potential to produce a large array of different compounds that can through evolution) are part of the ‘‘screening’’ arsenal that might be activated when novel substrates become available. become important in due evolutionary time. It is therefore advantageous for occult and partial metabolic pathways to be 2. Occult terpenoid pathways retained through evolution as means to readily provide the much needed biochemical diversity. Many monoterpenes are key constituents of essential oils and Plant secondary metabolites were once thought to be the result impart unique aromas to flowers and fruits. The metabolic of aberrant or accidental metabolism or an odd form of storage of engineering of the monoterpene pathway has been the focus of nutrients. Still, numerous studies have demonstrated that plants many studies [18]. The Clarkia breweri floral gene linalool synthase have developed intricate mechanisms to modulate the production (LIS) encodes an enzyme that catalyzes the formation of (S)-linalool and accumulation of such metabolites. In most cases studied, it has from the monoterpene precursor geranyl diphosphate. Over- been found that natural products are usually formed by elaborate expression of Clarkia’s LIS in tomato fruit caused the accumulation arrays of enzymes, concertedly controlled by the expression of of (S)-linalool [19] as expected, but also the unexpected formation their respective genes. Although the chemical structures of more of 8-hydroxylinalool, a compound absent in control fruits (Fig. 1). than 40,000 different terpenes, 20,000 phenolics and 5000 The formation of 8-hydroxylinalool is thought to be mediated by alkaloids are known [1], plants utilize a relatively small number an endogenous ‘‘occult’’ hydroxylase activity able to act on (S)- of biosynthetic pathways for the production of such an impressive linalool. The actual biological role of this hydroxylase in tomato number of different compounds [3]. Thus, the vast phytochemical fruit metabolism is presently unknown, but the novel availability diversity is paradoxically attained by relatively minor