How Insects Overcome Two-Component Plant Chemical Defence: Plant Β-Glucosidases As the Main Target for Herbivore Adaptation
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Biol. Rev. (2013), pp. 000–000. 1 doi: 10.1111/brv.12066 How insects overcome two-component plant chemical defence: plant β-glucosidases as the main target for herbivore adaptation Stefan Pentzold, Mika Zagrobelny, Fred Rook and Søren Bak∗ Plant Biochemistry Laboratory, Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, Copenhagen Dk-1871, Denmark ABSTRACT Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counter- adaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter-adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects’ feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β-glucosidases, bioactivating enzymes that are a key element in the plant’s two-component chemical defence. These adaptations include host plant choice, non-disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions. Key words: insect herbivore-plant interactions, two-component plant chemical defence, β-glucosidases, β-glucosides, insect adaptations, feeding guild, gut pH, sequestration, generalists and specialists. CONTENTS I. Introduction ................................................................................................ 2 (1) Overview: insect herbivores and two-component plant chemical defence .............................. 2 (2) Specialist versus generalist insect herbivores ............................................................ 2 (3) The role of plant β-glucosidases and β-glucosides in two-component plant chemical defence ......... 4 II. How non-adapted insect herbivores are affected by two-component plant chemical defence ............... 6 (1) Alkaloid glucosides ..................................................................................... 6 (2) Benzoxazinoid glucosides .............................................................................. 6 (3) Cyanogenic glucosides ................................................................................. 8 (4) Glucosinolates ......................................................................................... 8 (5) Iridoid glucosides ...................................................................................... 8 (6) Salicinoids ............................................................................................. 9 III. From feeding to digestion: targets for insect herbivore adaptations to two-component plant chemcial defence in a temporal context ....................................................................................... 9 (1) Before feeding: recognition, switching and selection of host plants ..................................... 9 (2) During feeding: impact of the feeding guild ............................................................ 10 * Author for correspondence (Tel: +0045 3533 3346; E-mail: [email protected]). Biological Reviews (2013) 000–000 © 2013 The Authors. Biological Reviews © 2013 Cambridge Philosophical Society 2 Stefan Pentzold and others (a) Piercing-sucking .................................................................................... 10 (b) Leaf-snipping versus leaf-chewing ................................................................... 10 (c) Leaf-mining ......................................................................................... 11 (3) During digestion: physiological and metabolic adaptations to counteract two-component defence .... 12 (a) An alkaline gut pH inhibits the plant β-glucosidase ................................................. 12 (b) Reduction of endogenous insect β-glucosidase activity in the gut ................................... 13 (c) Specialized enzyme activity of insects ............................................................... 14 ( i ) Before hydrolysis by plant β-glucosidases ..................................................... 14 ( ii ) After hydrolysis by plant β-glucosidases ...................................................... 14 (d) Sequestration: spatial separation of plant β-glucosidase and β-glucoside in the insect .............. 15 (e) Single amino acids counteract plant β-glucosidase activity ......................................... 16 (4) Do generalists and specialists have different types of adaptations? ..................................... 17 IV. Conclusions ................................................................................................ 17 V. Acknowledgements ......................................................................................... 18 VI. References .................................................................................................. 18 I. INTRODUCTION counter-adaptations to overcome this conditional toxicity (Fig. 1B, Table 1). The permanent presence of a specific class (1) Overview: insect herbivores and two-component of β-glucosidase-activated defence compound in a particular plant chemical defence plant species is a predictable characteristic that may have facilitated herbivorous insects to evolve adaptations. This Insect herbivores account for more than one quarter of raises the questions: (i) have insect adaptations evolved all living species on Earth (Scudder, 2009), and the co- for each specific class of defence compound, or are there evolution of phytophagous insects and their food plants has general applicable mechanisms that allow insect herbivores continued for more than 350 million years (Chaloner et al., to adapt to all classes of two-component defence, (ii)do 1991; Sinclair & Hughes, 2010). Although insect herbivores generalist and specialist herbivores differ in this respect, (iii) potentially have an abundance of plant species available do adaptations of insect herbivores mainly target activity of for feeding, herbivory is often restricted by the physical and the key enzyme, the plant β-glucosidase, to avoid generation chemical defence mechanisms plants have evolved to fend off of toxic aglucones or are there also other ‘targets’, and (iv)do insect attacks. Whereas mechanical structures like cuticular insect herbivores combine several adaptations to overcome waxes, prickles and thorns provide plants with a physical two-component plant chemical defence? defence, toxic chemical compounds provide an additional Several excellent reviews have described how insect effective defensive barrier (Chen, 2008; Mithofer¨ & Boland, herbivores adapt to toxic chemicals in general, including 2012). To fend off insect herbivores, more than 200,000 behavioural, physiological and metabolic adaptations to specialized metabolites, with toxic, growth-reducing or anti- many different classes of natural plant chemical defence nutritive effects, are known to be produced by plant species compounds as well as synthetic insecticides (Brattsten, 1988; (Zhu-Salzman, Luthe & Felton, 2008; Mithofer¨ & Boland, Hoy, Head & Hall, 1998; Despres,´ David & Gallet, 2007; 2012). Chemical defence compounds can be constitutively Schowalter, 2011). Here we focus more specifically on two- present in the plant, i.e. pre-exist in anticipation of an component chemical defence, as accumulating evidence insect attack (phytoanticipins) or their biosynthesis may be suggests that insect herbivores are able to interfere with inducible (phytoalexins) (VanEtten et al., 1994; Chen, 2008; either one or both components (Boeckler, Gershenzon & Mithofer¨ & Boland, 2012). Unsicker, 2011; Dobler et al., 2011; Winde & Wittstock, 2011; Constitutive plant defence compounds are often stored in Zagrobelny & Møller, 2011). In particular species from the the form of non-active and non-toxic glucosides in the plant Lepidoptera (butterflies and moths), Coleoptera (beetles), and are spatially separated from bioactivating β-glucosidases. Hemiptera (e.g. aphids), Hymenoptera (e.g. sawflies), Well-known classes of these compounds are alkaloid, Orthoptera (e.g. locusts and grasshoppers) and Diptera (true benzoxazinoid, cyanogenic and iridoid glucosides as well as flies) have evolved a remarkable diversity of adaptations. glucosinolates and salicinoids (Halkier & Gershenzon, 2006; Morant et al., 2008; Dobler, Petschenka & Pankoke, 2011; (2) Specialist versus generalist insect herbivores Winde & Wittstock, 2011). Upon insect herbivore attack and tissue damage these glucosylated defence compounds The range of host plant species an insect feeds on defines its come into contact with plant β-glucosidases,