Annual plant reviews volume 40 biochemistry of plant secondary metabolism second edition

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ANNUAL PLANT REVIEWS VOLUME 40

i

Biochemistry of Plant Secondary Metabolism: Second Edition Edited by Michael Wink

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fm BLBK250-Wink March 3, 2010 11:20 Char Count=

ANNUAL PLANT REVIEWS VOLUME 40

Professor of Pharmaceutical Biology

Institute of Pharmacy and Molecular Biotechnology

Heidelberg University

Germany

A John Wiley & Sons, Ltd., Publication

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This edition first published 2010

c

2010 Blackwell Publishing Ltd.

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Library of Congress Cataloging-in-Publication Data

Biochemistry of plant secondary metabolism / edited by Michael Wink – 2nd ed.

p cm – (Annual plant reviews ; v 40)

Includes bibliographical references and index.

ISBN 978-1-4051-8397-0 (hardback : alk paper) 1 Plants–Metabolism.

2 Metabolism, Secondary 3 Botanical chemistry I Wink, Michael.

QK881.B54 2010

A catalogue record for this book is available from the British Library.

Set in 10/12 pt Palatino by Aptara R Inc., New Delhi, India

Printed in Singapore

1 2010

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Annual Plant Reviews

A series for researchers and postgraduates in the plant sciences Each volume

in this series focuses on a theme of topical importance and emphasis is placed

on rapid publication

Editorial Board:

Biosciences, University of Nottingham, Sutton Bonington Campus,Loughborough, Leicestershire, LE12 5RD, UK;

Brookes University, Headington, Oxford, OX3 0BP;

Japan;

University, Palmerston North, New Zealand;

Ithaca, New York 14853, USA

Titles in the series:

1 Arabidopsis

Edited by M Anderson and J.A Roberts

2 Biochemistry of Plant Secondary Metabolism

Edited by M Wink

3 Functions of Plant Secondary Metabolites and Their Exploitation in Biotechnology

Edited by M Wink

4 Molecular Plant Pathology

Edited by M Dickinson and J Beynon

5 Vacuolar Compartments

Edited by D.G Robinson and J.C Rogers

6 Plant Reproduction

Edited by S.D O’Neill and J.A Roberts

7 Protein–Protein Interactions in Plant Biology

Edited by M.T McManus, W.A Laing and A.C Allan

8 The Plant Cell Wall

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Edited by A.J Fleming

17 Plant Architecture and Its Manipulation

21 Endogenous Plant Rhythms

Edited by A Hall and H McWatters

22 Control of Primary Metabolism in Plants

Edited by W.C Plaxton and M.T McManus

23 Biology of the Plant Cuticle

Edited by M Riederer

24 Plant Hormone Signaling

Edited by P Hadden and S.G Thomas

25 Plant Cell Separation and Adhesion

Edited by J.R Roberts and Z Gonzalez-Carranza

26 Senescence Processes in Plants

Edited by S Gan

27 Seed Development, Dormancy and Germination

Edited by K.J Bradford and H Nonogaki

34 Molecular Aspects of Plant Disease Resistance

Edited by Jane Parker

35 Plant Systems Biology

Edited by G Coruzzi and R Guti’errez

36 The Moss Physcomitrella Patens

Edited by C.D Knight, P.-F Perroud and D.J Cove

37 Root Development

Edited by Tom Beeckman

38 Fruit Development and Seed Dispersal

Edited by Lars Østergaard

39 Function and Biotechnology of Plant Secondary Metabolites

Edited by M Wink

40 Biochemistry of Plant Secondary Metabolism

Edited by M Wink

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CONTENTS

1 Introduction: biochemistry, physiology and ecological

Michael Wink

Margaret F Roberts, Dieter Strack and Michael Wink

3 Biosynthesis of cyanogenic glycosides, glucosinolates and

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4 Biosynthesis of phenylpropanoids and related compounds 182

Maike Petersen, Joachim Hans and Ulrich Matern

6 Biochemistry of sterols, cardiac glycosides, brassinosteroids,

Wolfgang Kreis and Frieder M ¨uller-Uri

7 Chemotaxonomy seen from a phylogenetic perspective and

Michael Wink, Flavia Botschen, Christina Gosmann, Holger Sch ¨afer

and Peter G Waterman

7.2 Establishment of chemotaxonomy as a research discipline 3657.3 Developments in small molecule chemotaxonomy over

7.5 Comparison between patterns of secondary metabolites

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Institute of Plant Biology

Technical University Braunschweig

Braunschweig

Germany

Dieter Strack

Department of Secondary Metabolism

Institute of Plant Biochemistry

Halle

Germany

Frieder M ¨uller-Uri

Institute of Botany and Pharmaceutical Biology

University Erlangen-N ¨urnberg

Erlangen

Germany

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Peter G Waterman

Retired from Centre for Phytochemistry

Southern Cross University

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PREFACE

A characteristic feature of plants is their capacity to synthesize and store a

wide variety of low molecular weight compounds, the so-called secondary

metabolites (SMs) or natural products The number of described structures

exceeds 100 000; the real number in nature is certainly much higher becauseonly 20–30% of plants have been investigated in phytochemistry so far Incontrast to primary metabolites, which are essential for the life of every plant,the individual types of SMs usually occur in a limited number of plants,indicating that they are not essential for primary metabolism, i.e anabolism

or catabolism

Whereas SMs had been considered to be waste products or otherwise less compounds for many years, it has become evident over the past threedecades that SMs have important roles for the plants producing them: theymay function as signal compounds within the plant, or between the plantproducing them and other plants, microbes, herbivores, predators of herbi-vores, pollinating or seed-dispersing animals More often SMs serve as de-fence chemicals against herbivorous animals (insects, molluscs, mammals),microbes (bacteria, fungi), viruses or plants competing for light, water andnutrients Therefore, SMs are ultimately important for the fitness of the plantproducing them Plants usually produce complex mixtures of SMs, oftenrepresenting different classes, such as alkaloids, phenolics or terpenoids It islikely that the individual components of a mixture can exert not only additivebut certainly also synergistic effects by attacking more than a single molec-ular target Because the structures of SMs have been shaped and optimizedduring more than 500 million years of evolution, many of them exert interest-ing biological and pharmacological properties which make them useful formedicine or as biorational pesticides

use-In this volume of Annual Plant Reviews, we have tried to provide an

up-to-date survey of the biochemistry and physiology of plant secondary

metabolism A companion volume – M Wink (ed.) Functions of Plant Secondary

Metabolites and Biotechnology – published simultaneously provides overviews

of the modes of action of bioactive SMs and their use in pharmacology asmolecular probes, in medicine as therapeutic agents and in agriculture asbiorational pesticides

In order to understand the importance of SMs for plants, we need detailedinformation on the biochemistry of secondary metabolism and its integra-tion into the physiology and ecology of plants Important issues include

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characterization of enzymes and genes of corresponding biosynthetic ways, and of transport and storage mechanisms, regulation in space/timeand compartmentation of both biosynthesis and storage The study of sec-ondary metabolism has profited largely from the recent progress in molecular

path-biology and cell path-biology and the diverse genome projects Although

Arabidop-sis thaliana is not an excellent candidate to study secondary metabolism on

the first view, the genomic analyses, EST-libraries, mutants and other tools of

A thaliana have been extremely helpful to elucidate secondary metabolism

in other plants

The present volume is the second edition of a successful first edition whichwas published in 1999 and which has received many positive responses fromits readers To achieve a comprehensive and up-to-date summary, we haveinvited scientists who are specialists in their particular areas to update theirprevious chapters This volume draws together results from a broad area ofplant biochemistry and it cannot be exhaustive on such a large and diversegroup of substances Emphasis was placed on new results and concepts whichhave emerged over the last decades

The volume starts with a bird’s eye view of the biochemistry, physiologyand function of SMs (M Wink), followed by detailed surveys of the ma-

jor groups of SMs: alkaloids and betalains (M.F Roberts et al.); cyanogenic

glucosides, glucosinolates and non-protein amino acids (D Selmar); phenyl

propanoids and related phenolics (M Petersen et al.); terpenoids, such as

mono-, sesqui-, di- and triterpenes, cardiac glycosides and saponins (M

Ashour et al., W Kreis and F M ¨uller-Uri) The final chapter discusses the evolution of secondary metabolism (M Wink et al.) The structural types of

SMs are often specific and restricted in taxonomically related plant groups.This observation was the base for the development of ‘chemotaxonomy’ Acloser look indicates that a number of SMs have a taxonomically restricteddistribution Very often, we find the same SMs also in other plant groupswhich are not related in a phylogenetic context There is evidence that some

of the genes, which encode key enzymes of SM formation, have a much widerdistribution in the plant kingdom than assumed previously It is speculatedthat these genes were introduced into the plant genome by horizontal genetransfer, i.e via bacteria that developed into mitochondria and chloroplasts(endosymbiont hypothesis) Evidence is presented that a patchy distributioncan also be due to the presence of endophytic fungi, which are able to produceSMs

The book is designed for use by advanced students, researchers and sionals in plant biochemistry, physiology, molecular biology, genetics, agricul-ture and pharmacy working in the academic and industrial sectors, includingthe pesticide and pharmaceutical industries

profes-The book brought together contributions from friends and colleagues inmany parts of the world As editor, I would like to thank all those who have

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Preface xv

taken part in writing and preparation of this book I would like to thankTheodor C H Cole for help, especially in preparation of the index Specialthanks go to the project editor Catriona Dixon from Wiley-Blackwell and herteam for their interest, support and encouragement

Michael WinkHeidelberg

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Glucose

Glycosides Oligosaccharides Polysaccharides Cyclitols Polyols

Pyruvate

AcCoA

Oxoglutarate Succinate

Malate

Oxalacetate

Citrate

Glutamate Glutamine

Malonyl-CoA

C10 monoterpenes C15 sesquiterpenes C20 diterpenes C30 triterpenes C27 steroids C40 tetraterpenes

C(n) polyterpenes

Saponins Cucurbitacins Terpenoid alkaloids

Anthraquinones Naphthoquinones Phenols Flavonoids

Conium alkaloids

Aspartate

Alkaloids purines NPAAs

Sedum alkaloids

NPAAs

Ornithine

Tropane alkaloids Coca alkaloids

Waxes Fatty acids

L -tyrosine L -phenylalanine

Indole alkaloids Glucosinolates NPAAs Amines Auxines

Isoquinoline alkaloids Phenylpropanoids Flavonoids, stilbenes, catechins Lignin, lignans

Coumarins, furanocoumarins Cyanogenic glycosides Glucosinolates Quinones, NPAAs

Naphthoquinones Anthraquinones

STS

Photosynthesis

Plate 2 Several pathways of secondary metabolites derive from precursors in the shikimate pathway Abbreviation: NPAAs, non-protein amino acids; PAL, phenylalanine ammonia lyase; TDC, tryptophan decarboxylase; STS, strictosidine synthase; CHS, chalcone synthase (Fig 1.3, p 8)

1

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P1: OJL/OTE/SPH P2: OTE/OTE/SPH

cp BLBK250-Wink March 8, 2010 14:34 Char Count=

mitochondrium; cp, chloroplast; nc, nucleus; 1, passive transport; 2, free diffusion; 3,

H+/SM antiporter; 4, ABC transporter for SM conjugated with glutathione; 5, ABC transporter for free SM; 6, H+-ATPase (Fig 1.4, p 9)

Storage of secondary metabolites

Anthraquinone and naphthodianthrones (hypericin), terpenoids

Plate 4 Storage compartments for hydrophilic and lipophilic compounds.

Abbreviation: NPAAs, non-protein amino acids (Fig 1.5, p 11)

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COSTS of Secondary Metabolism

Plate 5 Costs of chemical defence and signal compounds Abbreviations: ATP,

adenosine triphosphate; NADPH 2 , nicotinamide adenine dinucleotide phosphate (reduced form) (Fig 1.6, p 14)

Lupinus

Epidermis (6% QA) Phloem <5 mg/mL Xylem <0.05 mg/mL

Flower: 4% QA Carpels: 3.3%

Quinolizidine alkaloids

N N

H H

Plate 6 Example of the complicated biochemistry and physiology of alkaloid formation:

quinolizidine alkaloids (QAs) in lupins (genus Lupinus, Fabaceae) QAs are formed in leaf

chloroplasts and exported via the phloem all over the plant QAs predominantly

accumulate in vacuoles of epidermal tissue Organs important for survival and

reproduction, such as flowers and seeds, store especially high amounts of defence alkaloids (Fig 1.7, p 15)

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4

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NH2

H N

NH2

NH 2

H N

NH2

N

O O

HO

H N

OH O

HO

N+

O O

Plate 9 Biosynthesis of the pyrrolizidine alkaloid, senecionine-N-oxide ODC, ornithine

decarboxylase; ADC, arginine decraboxylase; SPDS, spermidine synthase; HHS,

homospermidine synthase (Fig 2.4, p 34)

6

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Plate 10 (a) Families and orders of higher plants, placed in a phylogenetic framework according to APG II Branches leading to families, which accumulate benzylisoquinoline alkaloids are highlighted in colour (Fig 7.8a, p 375)

7

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(b)

Plate 11 (b) Families and orders of higher plants, placed in a phylogenetic framework according to APG ll Branches leading to families, which accumulate glucosinolates, cardiac glycosides are highlighted in colour (Fig 7.8b, p 376)

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Plate 12 (c) Families and orders of higher plants, placed in a phylogenetic framework according to APG ll Branches leading to families, which accumulate pyrrolizidine and quinolizidine alkaloids and are highlighted in colour (Fig 7.8c, p 377)

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Arctia caja

S mogadorensis Syntomis phegea Syntomeida epilais

Utetheisa pulchella Tyria jacobaeae Panaxia quadripuntaria

Atolmis rubricollis Eilema depressa Eilema depressa Eilema lurideola Eilema complana Cybosia mesomella Spodoptera frugiperda

Euploea spec

Danaus plexippus Danaus gilippus Papilio machaon Manduca sexta Oncopeltus fasciatus

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Lupinus OW Genista Teline Cytisophyllum Labu rnum

Spartium Stauracanthus Ulex Adenocarpus Retama Chamaecytisus Cytisus Spartocytisus Calicotome

Crotalaria

Loto nonis Rafnia Baptisia Thermopsis Maackia Anagyris Piptanthus Liparia 1 Podalyria Cyclopia Virgilia Sophora II Hovea Brachysema Eutaxia Burtonia Goodia 2 Daviesia Lens Vicia Pisum Lathyrus Medicago Ononis Cicer Abrus 3 Calophaca Caragana Halimodendron Swainsonia Carmichaelia 4 Clianthus Astragalus Sutherlandia Wisteria 5 Dorynium Securigera Coronilla Sesbania Gliricidia Psoralea 6 Glycine Macroptilium Vigna Centrosema Cajanus

Erythrina

Kennedia Hardenbergia Christia 7 Galactia Tephrosia 8 Glycyrrhiza 9 Adesmia 11 Amorpha 12 Sophora I Myroxylon Andira 13 Styphnolobium Cladrastis Castanospermum Leucaena 14 Desmanthus 14 Parkia 16 Acacia 17 Pithecellobium 15 Anadenanthera 14 Neptunia 14 Mimosa 14 Prosopis 14 Gymnocladus 19 Cassia 18 Delonix 19 Parkinsonia 19 Ceratonia 18 Caesalpinia 19 Brownea 20 Tamarindus 20 Bauhinia 21 Cercis 21 Polygala

Genisteae

Crotalarieae Thermop- sideae Podalyrieae

Sophoreae II Bossiaeeae

Mirbelieae Vicieae Trifolieae

Cicereae Abreae

Galegeae

Loteae Robinieae

Phaseoleae

Sophoreae I

SOIDEAE

MIMO- NIOIDEAE

CAESALPI-Occurrence of alkaloids

Quinolizidines

Pyrrolizidines

Simple indoles Erythrina

Indolizidines

β -Carbolines

3 3

3

3 3 3,4

5

5 5

3 3 3 3,6 3,6

12

56

34

Plate 14 (a) Genera and tribes of the Fabaceae, placed in a phylogenetic framework

reconstructed from nucleotide sequences of the rbcL gene Illustrations (a)–(g) are

presented as cladograms of a strict consensus of the six most parsimonious trees

calculated by a heuristic search Due to space limitations, a few tribal names are not listed

in the figures, but are abbreviated by numbers after the genus name: 1 = Liparieae; 2 = Bossiaeeae; 3 = Abreae; 4 = Carmichaelieae; 5 = Millettieae; 6 = Psoraleae; 7 =

Desmodieae; 8 = Tephrosieae (Millettieae); 9 = Galegeae; 10 = Indigofereae; 11 = Adesmieae; 12 = Amorpheae; 13 = Dalbergieae; 14 = Mimoseae; 15 = Ingeae; 16 = Parkieae; 17 = Acacieae; 18 = Cassieae; 19 = Caesalpinieae; 20 = Detarieae; 21 = Cercideae (a) The occurrence of alkaloids Key to branches leading to families that accumulate: quinolizidines, pyrrolizidines (No 1; see arrows); Erythrina (No 3);

indolizidines (No 4);β-carbolines (No 5); or simple indoles (No 2) are marked The rbcL

sequences used (1400 bp) derived from K¨ass and Wink, 1997a,b; Wink and Mohamed (2003) Trees were reconstructed with maximum parsimony (Fig 7.11a, p 389)

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0.001 substitutions/site

N

HH

N

O

OO

(b)

Plate 15 (b) Occurrence of QAs and PAs in the Papilionoideae, tribe Crotalarieae (reconstructed from ITS sequences) (Fig 7.11b, p 390)

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Lupinus NW Lupinus OW Genista Teline Cytisophyllum Laburnum Spartium Stauracanthus Ulex Adenocarpus Retama Chamaecytisus Cytisus Spartocytisus Calicotome Crotalaria Rafnia Baptisia Thermopsis Maackia Anagyris Piptanthus Liparia 1 Podalyria Cyclopia Virgilia Sophora II Hovea Brachysema Eutaxia Burtonia Goodia 2 Daviesia Lens Pisum Lathyrus Medicago Ononis Cicer Abrus 3 Calophaca Caragana Halimodendron Swainsonia Carmichaelia 4 Clianthus Astragalus Sutherlandia Wisteria 5 Dorynium Securigera Coronilla Sesbania Gliricidia Psoralea 6 Glycine Macroptilium Vigna Centrosema Cajanus Erythrina Kennedia Hardenbergia Christia 7 Galactia Tephrosia 8 Glycyrrhiza 9 Adesmia 11 Amorpha 12 Sophora I Myroxylon Andira 13 Styphnolobium Cladrastis Castanospermum

Leucaena 14

Calliandra 15

Desmanthus 14 Parkia 16 Acacia 17 Albizia 15 Pithecellobium 15 Anadenanthera 14 Neptunia 14 Mimosa 14 Prosopis 14

Gleditsia 19 Gymnocladus 19 Cassia 18 Delonix 19 Parkinsonia 19 Ceratonia 18 Caesalpinia 19 Brownea 20 Tamarindus 20 Bauhinia 21 Cercis 21 Polygala

Genisteae

Crotalarieae

sideae

Thermop-Podalyrieae

Mirbelieae

Vicieae

Trifolieae

Cicereae Abreae

Galegeae

Phaseoleae

Sophoreae I

SOIDEAE

MIMO- NIOIDEAE

CAESALPI-Occurrence of NPAAs

Lens Pipecolic acid + derivatives

Acacia Pipecolic acids + djencolic

acidsCanavanine

Other NPAAs

(c)

Plate 16 (c) Occurrence of non-protein amino acids (NPAAs) Key to branches leading

to families that accumulate: pipecolic acid and derivatives (Lens); pipecolic acid and djenkolic acids (Acacia); canavanine; others NPAAs See also legend (a) (Fig 7.11c,

p 391)

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Lupinus NW Lupinus OW Genista Teline Cytisophyllum Laburnum Spartium Stauracanthus Ulex Adenocarpus Retama Chamaecytisus Cytisus Spartocytisus Calicotome Aspalathus Crotalaria Rafnia Baptisia Thermopsis Maackia Anagyris Piptanthus Liparia 1 Podalyria Cyclopia Virgilia Sophora II Hovea Brachysema Eutaxia Burtonia Goodia 2 Daviesia Lens Pisum Lathyrus Medicago Ononis Cicer Abrus 3 Calophaca Caragana Halimodendron Swainsonia Carmichaelia 4 Clianthus Astragalus Sutherlandia Wisteria 5 Dorynium Securigera Coronilla Sesbania Gliricidia Psoralea 6 Glycine Macroptilium Vigna Centrosema Cajanus Erythrina Kennedia Hardenbergia Christia 7 Galactia Tephrosia 8 Glycyrrhiza 9 Adesmia 11 Amorpha 12 Sophora I Myroxylon Andira 13 Styphnolobium Cladrastis Castanospermum Leucaena 14 Calliandra 15 Desmanthus 14 Parkia 16 Acacia 17 Albizia 15 Pithecellobium 15 Anadenanthera 14 Neptunia 14 Mimosa 14 Prosopis 14 Gymnocladus 19 Cassia 18 Delonix 19 Parkinsonia 19 Ceratonia 18 Caesalpinia 19 Brownea 20 Tamarindus 20 Bauhinia 21 Cercis 21 Polygala

Genisteae

Cicereae Abreae

Galegeae

Phaseoleae

Sophoreae I

SOIDEAE

MIMO- NIOIDEAE

CAESALPI-Occurrence of flavonoids

Isoflavones Isoflavones + pterocarpans Catechins/proanthocyanins

(f)

Plate 17 (f) Occurrence of flavonoids Key to branches leading to families that

accumulate: isoflavones; isoflavones and pterocarpans; catechins/proanthocyanins See also legend (a) (Fig 7.11f, p 394)

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Lupinus NW Genista Teline Cytisophyllum Laburnum Spartium Stauracanthus Ulex Adenocarpus Retama Chamaecytisus Cytisus Spartocytisus Calicotome Crotalaria Rafnia Baptisia Thermopsis Maackia Anagyris Piptanthus Liparia 1 Podalyria Cyclopia Virgilia Sophora II Hovea Brachysema Eutaxia Burtonia Goodia 2 Daviesia Lens Pisum Lathyrus Medicago Ononis Cicer Abrus 3 Calophaca Caragana Halimodendron Swainsonia Carmichaelia 4 Clianthus Astragalus Sutherlandia Wisteria 5 Dorynium Securigera Coronilla Sesbania Gliricidia Psoralea 6 Glycine Macroptilium Vigna Centrosema Cajanus Erythrina Kennedia Hardenbergia Christia 7 Galactia Tephrosia 8 Glycyrrhiza 9 Adesmia 11 Amorpha 12 Sophora I Myroxylon Andira 13 Styphnolobium Cladrastis Castanospermum Leucaena 14 Desmanthus 14 Parkia 16 Acacia 17 Albizia 15 Pithecellobium 15 Anadenanthera 14 Neptunia 14 Mimosa 14 Prosopis 14 Gymnocladus 19 Cassia 18 Delonix 19 Parkinsonia 19 Ceratonia 18 Caesalpinia 19 Brownea 20 Tamarindus 20 Bauhinia 21 Cercis 21 Polygala

Genisteae

Cicereae Abreae

Galegeae

Phaseoleae

Sophoreae I

SOIDEAE

MIMO- NIOIDEAE

CAESALPI-Occurrence of triterpenes &

cardiac glycosides

Triterpenes/triterpene saponins

Cardiac glycosides

(g)

Plate 18 (g) Occurrence of triterpenes and cardiac glycosides Key to branches leading

to families that accumulate: triterpenes/triterpene saponins; cardiac glycosides See also legend to (a) (Fig 7.11g, p 395)

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Nerium oleander Thevetia peruviana

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0.001 Substitutions/site

Occurrence of iridoid glycosides in Lamiaceae

toideae

Nepe- oideae

Lami-O

HOH2C

O HO

Glucose

Plate 20 Distribution of iridoid glycosides in the family Lamiaceae, reconstructed from

a rbcL data set (After Wink and Kaufmann, 1996.) (Fig 7.14, p 405)

17

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Symbiotic fungi

-Endophytes-Ectomycorrhiza

SM SM

mRNAProtein

SM

DNADNA

Protobacteria Cyanobacteria

Genes

DuplicationMutationSelectionSpecialization

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Chapter 1

INTRODUCTION:

BIOCHEMISTRY, PHYSIOLOGY AND ECOLOGICAL FUNCTIONS

OF SECONDARY METABOLITES Michael Wink

Institute of Pharmacy and Molecular Biotechnology, Heidelberg University, Heidelberg, Germany

Abstract:Secondary metabolites (SM) occur in plants in a high structural diversity.The different classes of SM and their biosynthetic pathways are summarized in thisintroduction A typical feature of SM is their storage in relatively high concentra-tions, sometimes in organs which do not produce them A long-distance transportvia the phloem or xylem is then required Whereas hydrophilic substances arestored in the vacuole, lipophilic metabolites can be found in latex, resin ducts, oilcells or cuticle SM are not necessarily end products and some of them, especially

if they contain nitrogen, are metabolically recycled Biosynthesis, transport andstorage are energy-dependent processes which include the costs for the replica-tion and transcription of the corresponding genes and the translation of proteins.The intricate biochemical and physiological features are strongly correlated withthe function of SM: SM are not useless waste products (as assumed earlier), butimportant tools against herbivores and microbes Some of them also function assignal molecules to attract pollinating arthropods or seed-dispersing animals and

as signal compounds in other plant – plant, plant – animal and plant – microberelationships

Keywords: secondary metabolites (SM); biosynthesis; transport; storage;turnover; costs; ecological functions

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2 Biochemistry of Plant Secondary Metabolism

Table 1.1 Number of known secondary metabolites from higher plants

a Approximate number of known structures.

b Total of terpenoids number exceeds 22 000 at present.

low molecular weight compounds, the so-called secondary metabolites (SM).Although only 20–30% of higher plants have been investigated so far, severaltens of thousands of SM have already been isolated and their structures deter-mined by mass spectrometry (electron impact [EI]-MS, chemical ionization[CI]-MS, fast atom bombardment [FAB]-MS, electrospray ionization liquidchromatography [ESI-LC]-MS), nuclear magnetic resonance (1H-NMR,13C-NMR) or X-ray diffraction (Harborne, 1993; DNP, 1996; Eisenreich and Bacher,2007; Marston, 2007) In Table 1.1, an estimate of the numbers of known SM

is given Representative structures are presented in Fig 1.1 Within a singlespecies 5000 to 20 000 individual primary and secondary compounds may

be produced, although most of them as trace amounts which usually areoverlooked in a phytochemical analysis (Trethewey, 2004)

Despite the enormous variety of SM, the number of corresponding basicbiosynthetic pathways is restricted and distinct Precursors usually derivefrom basic metabolic pathways, such as glycolysis, the Krebs cycle or theshikimate pathway A schematic overview is presented in Figs 1.2 and 1.3.Plausible hypotheses for the biosynthesis of most SM have been published

(for overviews see Bell and Charlwood, 1980; Conn, 1981; Mothes et al.,

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N

H H

O

N

O H3C

O

OH H

NH2H

H O

H H2N

NH2H

O OH H

Non-protein amino acids

Hyoscyamine

O C H Glucose

O O

Monoterpenes Sesquiterpenes

O

HOH 2 C O HO

O Glucose

Thymol

Allyl isothiocyanate

Artemisinin HelenalinCatalpol (iridoid)

Figure 1.1 Structures of selected secondary metabolites.

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CH 3

O HO

O

OH OH O

O

H 3 C O

O Sugar chain

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COOH HO

HO OH

COOH

HO OH

OH

Flavonoids

Figure 1.1 (Continued)

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O HO

O O

O O O O HO

OH HO

OH

Figure 1.1 (Continued)

1985; Luckner, 1990; Dey and Harborne, 1997; Seigler, 1998; Dewick, 2002)that are based, at least in part, on tracer experiments In addition, genetictools to knock out genes become important to dissect plant secondary path-ways (Memelink, 2005) For pathways leading to cyanogenic glycosides, glu-cosinolates, some alkaloids and non-protein amino acids (NPAAs), amines,flavonoids and several terpenes, the enzymes which catalyse individual steps,have been identified In pathways leading to isoquinoline, indole, pyrroli-dine, pyrrolizidine and tropane alkaloids, flavonoids, coumarins, NPAAs,mono-, sesqui- and triterpenes, some of the genes, which encode biosyn-

thetic enzymes, have already been isolated and characterized (Kutchan et al.,

1991; Kutchan, 1995; Saito and Murakoshi, 1998; Dewick, 2002; Facchini

et al., 2004; Kutchan, 2005; Petersen, 2007; Zenk and Juenger, 2007; Sch¨afer and

Wink, 2009) Whereas, earlier this century, it was argued that SM arise neously or with the aid of non-specific enzymes, we now have good evidencethat biosynthetic enzymes are highly specific in most instances and most havebeen selected towards this special task (although they often derive from com-mon progenitors with a function in primary metabolism or from prokaryoticgenes imported to plant cells through chloroplasts and mitochondria) As

sponta-a consequence of specific enzymsponta-atic synthesis, finsponta-al products nesponta-arly sponta-alwsponta-ayshave a distinct stereochemistry Only the enzymes that are involved in thedegradation of SM, such as glucosidases, esterases and other hydrolases, are

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Glucose

Glycosides Oligosaccharides Polysaccharides Cyclitols Polyols

Pyruvate

AcCoA

Oxoglutarate Succinate

Malate

Oxalacetate

Citrate

Glutamate Glutamine

Malonyl-CoA

C10 monoterpenes C15 sesquiterpenes C20 diterpenes C30 triterpenes C27 steroids C40 tetraterpenes

C(n) polyterpenes

Saponins Cucurbitacins Terpenoid alkaloids

Anthraquinones Naphthoquinones Phenols Flavonoids

Conium alkaloids

Aspartate

Alkaloids purines NPAAs

Sedum alkaloids

NPAAs

Ornithine

Tropane alkaloids Coca alkaloids

Waxes Fatty acids

Polyketides

Figure 1.2 Main pathways leading to secondary metabolites Abbreviations: IPP, isopentenyl diphosphate; DMAPP, dimethyl allyl diphosphate; GAP,

glyceraldehyde-3-phosphate; NPAAs, non-protein amino acids; AcCoA, acetyl coenzyme

A (See Plate 1 in colour plate section.)

less substrate specific The biosynthesis of SM is a highly coordinated process,which includes metabolon formation and metabolic channelling Channelingcan involve different cell types and cellular compartmentation These pro-cesses guarantee a specific biosynthesis and avoid metabolic interferences

(Winkel, 2004; J ¨orgensen et al., 2005).

Some SM are produced in all tissues, but their formation is generallyorgan-, tissue-, cell- and often development-specific Although, in most in-stances, details have not been elucidated, it can be assumed that the genes ofsecondary metabolism are also regulated in a cell-, tissue- and development-specific fashion (as are most plant genes that have been studied so far) Thismeans that a battery of specific transcription factors needs to cooperate inorder to activate and transcribe genes of secondary metabolism Master regu-lators (transcription factors by nature) are apparently present, which controlthe overall machinery of biosynthetic pathways, transport and storage.Sites of biosynthesis are compartmentalized in the plant cell While mostbiosynthetic pathways proceed (as least partially) in the cytoplasm, there isevidence that some alkaloids (such as coniine, quinolizidines and caffeine),furanocoumarins and some terpenes (such as monoterpenes, diterpenes,

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Isoquinoline alkaloids Phenylpropanoids Flavonoids, stilbenes, catechins Lignin, lignans

Coumarins, furanocoumarins Cyanogenic glycosides Glucosinolates Quinones NPAAs

Naphthoquinones Anthraquinones

STS

Photosynthesis

Figure 1.3 Several pathways of secondary metabolites derive from precursors in the shikimate pathway Abbreviation: NPAAs, non-protein amino acids; PAL, phenylalanine ammonia lyase; TDC, tryptophan decarboxylase; STS, strictosidine synthase; CHS, chalcone synthase (See Plate 2 in colour plate section.)

phytol and carotenoids that are formed in the pyruvate/glyceraldehydephosphate pathway) are synthesized in the chloroplast (Roberts, 1981; Winkand Hartmann, 1982; Kutchan, 2005) Sesquiterpenes, sterols and dolicholsare produced in the endoplasmic reticulum (ER) or cytosolic compartment

A schematic overview is presented in Fig 1.4 Coniine and amine formationhas been localized in mitochondria (Roberts, 1981; Wink and Hartmann,

1981) and steps of protoberberine biosynthesis in vesicles (Amann et al.,

1986; Kutchan, 2005; Zenk and Juenger, 2007) Hydroxylation steps are oftencatalysed by membrane-bound enzymes and the ER is the correspondingcompartment The smooth ER is also probably the site for the synthesis ofother lipophilic compounds The various steps in a biosynthesis can proceed

in a channelled array in one compartment; in other instances differentplant organs, cell types or organelles are involved Extensive intra- andintercellular translocation of SM or intermediates would be a consequence.The biosynthesis of the major groups of SM has been reviewed in moredetail in this volume: alkaloids (including betalains) by M Roberts, D Strack

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