Secondary Metabolites - Plant Phys

SECONDARY METABOLITESderived from the chloroplastic MEP pathway. However,cross talk between these two pathways does occasionallyoccur, leading to terpenes that are “mixed” with regard totheir biosynthetic origin.A4-5(A)CH3Some terpenes have roles in growthand developmentCertain terpenes have well-characterized functions in plantgrowth or development and so can be considered primaryrather than secondary metabolites. For example, the gibberellins (see Chapter 25), an important group of planthormones, are diterpenes. Brassinosteroids (see Chapter 15),another class of plant hormones with growth-regulatingfunctions, originate from triterpenes.Sterols are triterpene derivatives that are essential components of cell membranes, which they stabilize by interacting with phospholipids. The red, orange, and yellowcarotenoids are tetraterpenes that function as accessorypigments in photosynthesis and protect photosynthetictissues from photooxidation (see Chapter 7). The hormoneabscisic acid (see Chapter 15) is a C15 terpene produced bydegradation of a carotenoid precursor.Long-chain polyterpene alcohols known as dolicholsfunction as carriers of sugars in cell wall and glycoproteinsynthesis (see Chapter 14). Terpene-derived side chains,such as the phytol side chain of chlorophyll (see Chapter7), help anchor certain molecules in membranes. The vastmajority of terpenes, however, are secondary metabolitespresumed to be involved in plant defenses.Terpenes defend many plants against herbivoresTerpenes are toxins and feeding deterrents to many herbivorous insects and mammals; thus they appear to playimportant defensive roles in the plant kingdom (Gershenzon and Croteau 1992). For example, monoterpene esterscalled pyrethroids, found in the leaves and flowers ofChrysanthemum species, show striking insecticidal activity. Both natural and synthetic pyrethroids are popularingredients in commercial insecticides because of their lowpersistence in the environment and their negligible toxicityto mammals.In conifers such as pine and fir, monoterpenes accumulate in resin ducts found in the needles, twigs, and trunk.These compounds are toxic to numerous insects, including bark beetles, which are serious pests of conifer species throughout the world. Many conifers respond to barkbeetle infestation by producing additional quantities ofmonoterpenes (Trapp and Croteau 2001).Many plants contain mixtures of volatile monoterpenesand sesquiterpenes, called essential oils, that lend a characteristic odor to their foliage. Peppermint, lemon, basil,and sage are examples of plants that contain essential oils.H3CCH2Limonene(B)CH3OHH3CCH3MentholFIGURE A4.3 Structures of limonene (A) and menthol(B). These two well-known monoterpenes serve as defenses against insects and other organisms that feed onthe plants that produce them. (A, lemon tree [Citrus limon], photo Soren Pilman/istockphoto; B, peppermint [genus Mentha], photo Jose Antonio SantisoFernández/istockphoto.)The chief monoterpene constituent of lemon oil is limonene; that of peppermint oil is menthol (FIGURE A4.3).Essential oils have well-known insect repellent properties. They are frequently found in glandular hairs that project outward from the epidermis and serve to “advertise”the toxicity of the plant, repelling potential herbivores evenbefore they take a trial bite. Within the glandular hairs, theterpenes are stored in a modified extracellular space (FIGURE A4.4). Essential oils can be extracted from plants bysteam distillation and are important commercially in flavorPlant Physiology 5/E Taiz/Zeigeringfoods and making perfumes.Sinauer AssociatesAmongthe nonvolatile terpene antiherbivore comMoralesStudioFigure 13.03Date 11-16-09poundsare the limonoids,a group of triterpenes (C30)well known as bitter substances in citrus fruits. Perhapsthe most powerful deterrent to insect feeding known isazadirachtin (FIGURE A4.5A), a complex limonoid from theneem tree (Azadirachta indica) of Africa and Asia. Azadirachtin is a feeding deterrent to some insects at doses aslow as 50 parts per billion, and it exerts a variety of toxiceffects (Aerts and Mordue 1997; Veitch et al. 2008). It hasconsiderable potential as a commercial insect control agentbecause of its low toxicity to mammals, and several preparations containing azadirachtin are now being marketed inNorth America and India.

A4-6 APPENDIX 4were recently found to have a defensive function againstplant-parasitic nematodes (Soriano et al. 2004).Triterpenes that defend plants against vertebrate herbivores include cardenolides and saponins. Cardenolidesare glycosides (compounds containing an attached sugaror sugars) that taste bitter and are extremely toxic to higheranimals. In humans they have dramatic effects on the heartmuscle through their influence on Na /K -ATPases. Incarefully regulated doses, they slow and strengthen theheartbeat. Cardenolides extracted from foxglove (Digitalis)are prescribed to millions of patients for the treatment ofsome types of heart disease.Saponins are steroid and triterpene glycosides, sonamed because of their soaplike properties. The presence of both lipid-soluble (the steroid or triterpene) andwater-soluble (the sugar) elements in one molecule givessaponins detergent properties, and they form a soapylather when shaken with water. The toxicity of saponinsis thought to be a result of their ability to form complexeswith sterols. Saponins may interfere with sterol uptakefrom the digestive system or disrupt cell membranes afterbeing absorbed intoH3theC bloodstream.70 µmFIGURE A4.4 Monoterpenes and sesquiterpenes are commonly found in glandular hairs on the surface of a plant.This false-colored scanning electron micrograph showsglandular trichomes (microscopic hairs, purple) on thecalyx of a clary sage (Salvia sclarea) plant. The trichomesare secreting globules of essential oils (round, white).( Andrew Syred/Photo Researchers, Inc.)H3CCH — CH2 — CH3Phenolic CompoundsH CThe phytoecdysones, first isolated from the commonfern (Polypodium vulgare), are a group of plant steroids thathave the same basic structure as insect molting hormones(FIGURE A4.5B). Ingestion of phytoecdysones by insectsdisrupts molting and other developmental processes, oftenwith lethal consequences. In addition, phytoecdysones3Plants produce a largevarietycompoundsCH— CHof secondaryCH2Hthat contain a phenol2C group: a hydroxyl functional groupon an aromatic ring:OHC6(A) Azadirachtin, a limonoidOCH3OCOCH3 OCH3COOHCH3HOOCH3OOCH3COOC3C6C1C6C3OHFIGURE A4.5 Structure of two triCH3OCOterpenes, azadirachtin (A) andα-ecdysone (B), that serve as pow(B) α-Ecdysone, an insect molting hormoneerful insecticides. Azadirachtinaffects more than 200 species ofOHinsects and can be considered aCH3H3Cnatural insecticide. α-Ecdysone,OHCH3a plant-derived steroidal prohorCH3mone of the insect molting horCH3mone 20-hydroxyecdysone, canHOcause irregular molting in insectOHherbivores. (A, photo of neemleaves RN Photos/istockphoto;HOB, photo of Polypodium vulgareOleaves, blickwinkel/Alamy.)Plant Physiology 5/E Taiz/ZeigerSinauer AssociatesMorales StudioFigure 13.04Date 11-16-09C6OC6

SECONDARY METABOLITESThese substances are classified as phenolic compounds, orphenolics. Plant phenolics are a chemically heterogeneousgroup of nearly 10,000 individual compounds: Some aresoluble only in organic solvents, some are water-solublecarboxylic acids and glycosides, and others are large, insoluble polymers.In keeping with their chemical diversity, phenolicsplay a variety of roles in the plant. Many serve as defensesagainst herbivores and pathogens. Others function inmechanical support, in attracting pollinators and fruitdispersers, in absorbing harmful ultraviolet radiation, orin reducing the growth of nearby competing plants. Aftergiving a brief account of phenolic biosynthesis, we willdiscuss three principal groups of phenolic compounds andwhat is known about their roles in the plant.phosphate pathway into the three aromatic amino acids:phenylalanine, tyrosine, and tryptophan. One of the pathway intermediates is shikimic acid, which has given itsname to this whole sequence of reactions. The well-knownbroad-spectrum herbicide glyphosate (available commercially as Roundup) kills plants by blocking a step in thispathway (see Appendix 1). The shikimic acid pathwayis present in plants, fungi, and bacteria but is not foundin animals. Animals have no way to synthesize aromaticamino acids—phenylalanine, tyrosine, and tryptophan—which are therefore essential nutrients in animal diets.The most abundant classes of phenolic secondary compounds in plants are derived from phenylalanine via theelimination of an ammonia molecule to form cinnamicacid (FIGURE A4.7). This reaction is catalyzed by phenylalanine ammonia lyase (PAL), perhaps the most studiedenzyme in plant secondary metabolism. PAL is situated ata branch point between primary and secondary metabolism, so the reaction it catalyzes is an important regulatorystep in the formation of many phenolic compounds.The activity of PAL is increased by environmental factors such as low nutrient levels, light (through its effect onphytochromes), and fungal infection. The point of controlappears to be the initiation of transcription. Fungal invasion, for example, triggers the transcription of messengerRNA that codes for PAL, thus increasing the amount ofPAL in the plant, which then stimulates the synthesis ofphenolic compounds. The regulation of PAL activity inmany plant species is made more complex by the existence of multiple PAL-encoding genes, some of which arePhenylalanine is an intermediate in the biosynthesisof most plant phenolicsPlant phenolics are synthesized by several different routesand thus constitute a heterogeneous group from a metabolic point of view. Two basic pathways are involved: theshikimic acid pathway and the malonic acid pathway(FIGURE A4.6). The shikimic acid pathway participatesin the biosynthesis of most plant phenolics. The malonicacid pathway, although an important source of phenolicsecondary products in fungi and bacteria, is of less significance in higher plants.The shikimic acid pathway converts simple carbohydrate precursors derived from glycolysis and the pentosePhosphoenolpyruvicacid (from glycolysis)Erythrose4-phosphate(from pentosephosphate pathway)Shikimic -CoAPhenylalanine[C6C3Cinnamic acid[C6C3[C6][ CC36]C1]Malonic acidpathway][C6Simple phenolics[C6]C3 nLigninFIGURE A4.6 Plant phenolics are synthesized in severaldifferent ways. In higher plants, most phenolics are derived at least in part from phenylalanine, a product of theshikimic acid pathway. Formulas in brackets indicate ondensed tanninsbasic arrangement of carbon skeletons: C6 indicates abenzene ring, and C3 is a three-carbon chain. More detailon the pathway from phenylalanine onward is given inFigure 13.7.

A4-8 APPENDIX 4COOHFIGURE A4.7 Outline of phenolic biosynthesis fromphenylalanine onward. The formation of many plantphenolics, including simple phenylpropanoids,coumarins, benzoic acid derivatives, lignin, anthocyanins, isoflavones, condensed tannins, and otherflavonoids, begins with phenylalanine.NH2PhenylalanineNH3expressed only Hin3Cspecific tissues or only underCH — CH2 — CH3certain environmentalconditions (Logemann etH3Cal. 1995).Reactions subsequent to that catalyzed byH3CCH — CHof moreCH2 hydroxylPAL lead to the additionCH2groups and other substituents. The metabolitestrans-cinnamic acid, p-coumaric acid, and theirderivatives are simple phenolic compoundsOHcalled phenylpropanoids becausethey containa benzene ring:H3CH3CCOOHC6HOp-Coumaric acidCoA-SHCoumarins (Figure A.8B)COSCoAC6Lignin precursorsHOp-Coumaroyl-CoA3 Malonyl-CoA moleculesOHC1CH — CHCaffeic acidand other simplephenylpropanoids(Figure A.8A)COOHChalcone synthase Simple phenylpropanoids,such as transH3CCH3 and theirCH — CH2 —acid,cinnamic acid,p-coumaricH3Cderivatives, such as caffeic acid, which haveCC66C6 skeletonC3carbona basic phenylpropanoidH3C )(FIGURE A4.8ABenzoic acidderivatives (Figure A.8C)trans-Cinnamic acidCH — CH2 — CH3C6and a three-carbonside chain.H3CCH — CH CHSimple phenolic compoundsare widespreadC6C3 2H2Cin vascular plants and appear to function invarious capacities. Their structures include thefollowing:H2CPhenylalanine ammonia lyase (PAL)OHHOOHOHCH2OHC3OHOChalconesOHOH (cyclic esters) Phenylpropanoid lactonescalled coumarins,C whichalso have aC16phenylpropanoid carbon skeleton (FIGUREA4.8B)COHOOHOOFlavones6 Benzoic acid derivatives, which have a carbonC6 phenylpropanoidsC6C3skeleton formed frombythe cleavage of a two-carbon fragment fromC6CA4.8C3the side chain ( FIGURE) (see alsoFigure A4.7):C6C1As with many other secondary products, plantscan elaborate on the basicskeletonsofC6 carbonC6C3these simple phenolic compounds to make morecomplexproducts.5/E Taiz/ZeigerPlant PhysiologySinauerAssociatesNowthatthe biosynthetic pathways leadingMorales Studioto most widespread phenolic compounds haveFigure 13.00 in-textDate 02-04-10been determined, researchers have turned theirattention to studying how those pathways areOHHOOOFlavanonesOHOHHOOOOHIsoflavones anins(Figure A.10A)Condensed tannins(Figure A.12A)OHOFlavonols

SECONDARY METABOLITES(A)HHCOOHCCOOHCCCHHHOHOOCH3OHCaffeic acidFerulic acidSimple phenylpropanoids[C]C36(B)Furan ringHOOOO(C)OPsoralen,a furanocoumarinUmbelliferone,a simple coumarinCoumarinsO[C]The release of phenolics into the soil may limit thegrowth of other plantsCOOHCHHOOHOCH3Salicylic acidBenzoic acid derivatives[C6]C1FIGURE A4.8 Simple phenolic compounds play a varietyof roles in plants. (A) Caffeic acid and ferulic acid maybe released into the soil and inhibit the growth of neighboring plants. (B) Psoralen is a furanocoumarin thatexhibits phototoxicity to insect herbivores. (C) Salicylicacid is a plant hormone that is involved in systemic resistance to plant pathogens.regulated. In some cases, specific enzymes, such as PAL,are important in controlling flux through the pathway.Several transcription factors have been shown to regulatephenolic metabolism by binding to the promoter regionsof certain biosynthetic genes and activating transcription.Someof Physiologythese factorsthe transcription of largePlant5/E activateTaiz/Zeigergroupsof genes(Jin and Martin 1999).SinauerAssociatesMorales StudioFigure 13.08causes some furanocoumarins to become activated to ahigh-energy electron state. Activated furanocoumarinscan insert themselves into the double helix of DNA andbind to the pyrimidine bases cytosine and thymine, thusblocking transcription and repair and leading eventuallyto cell death.Phototoxic furanocoumarins are especially abundant inmembers of the Umbelliferae, including celery, parsnip,and parsley. In celery, the concentration of these compounds can increase about one-hundredfold if the plant isstressed or diseased. Celery pickers, and even some grocery shoppers, have been known to develop skin rashesfrom handling stressed or diseased celery. Some insects areadapted to survive on plants that contain furanocoumarinsand other phototoxic compounds by living in silken websor rolled-up leaves, which screen out the activating wavelengths (Sandberg and Berenbaum 1989).C36OVanillinA4-9From leaves, roots, and decaying litter, plants release a variety of primary and secondary metabolites into the environment. The release of secondary compounds by one plantthat have an effect on neighboring plants is referred to asallelopathy. If a plant can reduce the growth of nearbyplants by releasing chemicals into the soil, it may increaseits access to light, water, and nutrients and thus its evolutionary fitness. Generally speaking, the term allelopathyhas come to be applied to the harmful effects of plants ontheir neighbors, although a precise definition also includesbeneficial effects.Simple phenylpropanoids and benzoic acid derivativesare frequently cited as having allelopathic activity. Compounds such as caffeic acid and ferulic acid (see FigureA4.8A) are found in soil in appreciable amounts and havebeen shown in laboratory experiments to inhibit the germination and growth Hof Cmany plants (Inderjit et al. 1995).3CH3CHof—greatCH2 —interestAllelopathy is currentlybecause ofH3Cits potential agricultural applications (Kruse et al. 2000).Reductions in crop yields caused by weeds or residuesH3Cmay in some cases be a result offrom the previous cropCH — CH CH2allelopathy. An excitingprospect is the developmentCH2futureof crop plants genetically engineered to be allelopathic toweeds (see WEB ESSAY 23.7).OHDate 11-16-09Ultraviolet light activates some simple phenolicsLignin is a highly complex phenolic macromoleculeMany simple phenolic compounds have important rolesin plants as defenses against insect herbivores and fungi.Of special interest is the phototoxicity of certain coumarins called furanocoumarins, which have an attachedfuran ring (see Figure A4.8B). These compounds are nottoxic until they are activated by light. Sunlight in theultraviolet A (UV-A) region of the spectrum (320–400 nm)After cellulose, the most abundantorganic substance inC6plants is lignin, a highly branched polymer of phenylpropanoid groups:C6C3C6C1

A4-10 APPENDIX 4Lignin plays both primary and secondary roles in plants.Its precise structure is not known because it is difficult toextract from plants, in which it is covalently bound to cellulose and other polysaccharides of the cell wall.Lignin is generally formed from three different phenylpropanoid alcohols: coniferyl, coumaryl, and sinapyl, all ofwhich are synthesized from phenylalanine via various cinnamic acid derivatives. The phenylpropanoid alcohols arejoined into a polymer through the action of enzymes thatgenerate free-radical intermediates. The proportions of thethree phenylpropanoid alcohols in lignin vary among species, plant organs, and even layers of a single cell wall. Inthe polymer, there are often multiple C—C and C—O—Cbonds in each phenylpropanoid alcohol unit, resulting in acomplex structure that branches in three dimensions.Unlike the monomeric units of polymers such asstarch, rubber, or cellulose, those of lignin do not appearto be linked in a simple, repeating way. However, recentresearch suggests that a guiding protein may bind the individual units during lignin biosynthesis, giving rise to ascaffold that then directs the formation of a large, repeatingunit (Davin and Lewis 2000; Hatfield and Vermerris 2001).Lignin is found in the cell walls of various cell types thatmake up supporting and conducting tissues, notably thetracheids and vessel elements of the xylem. It is depositedchiefly in the thickened secondary wall, but may also bepresent in the primary wall and middle lamella in closecontact with the celluloses and hemicelluloses alreadypresent. The mechanical rigidity of lignin strengthensstems and vascular tissue, allowing upward growth andH3C— CH2 —toCHpermitting water andCHmineralsbe3 conducted throughH3Cthe xylem under negative pressure without collapse of thetissue. Because lignin is such a key component of watertransport tissue, Hthe3C ability to synthesize lignin must haveCH — CH CH2been one of the Hmostimportant adaptations permitting2Cprimitive plants to colonize dry land.Besides providing mechanical support, lignin has significant protective functions inOHplants. Its physical toughness deters herbivory, and its chemical durability makes itrelatively indigestible. By bonding to cellulose and protein,lignin also reduces the digestibility of those substances.C6Lignification blocks the growthof pathogens and is a common response to infection or wounding.C6C3There are four major groups of flavonoidsThe flavonoids are one of the largest classes of plant phenolics. The basic carbonCskeletonof a flavonoid containsC1615 carbons arranged in two aromatic rings connected by athree-carbon bridge:C6C3C6This structure results from two separate biosynthetic pathways: the shikimic acid pathway and the malonic acidpathway (FIGURE A4.9).From shikimic acidpathway via phenylalanine[C]C36From malonicacid pathway[ ]C62′7681OAC541′23′4′B6′5′3The three-carbon bridgeBasic flavonoid skeletonFIGURE A4.9 Basic flavonoid carbon skeleton. Flavonoids are synthesized from products of the shikimic acidand malonic acid pathways. Flavonoids contain 15 carbons in the basic molecular skeleton provided by twoaromatic rings and one 3-carbon bridge. Positions ofcarbons on the flavonoid ring system are numbered asshown.Flavonoids are classified primarily on the basis of thedegree of oxidation of the three-carbon bridge. We will discuss four of these groups here: the anthocyanins, the flavones, the flavonols, and the isoflavones (see Figure A4.7).The basic flavonoid carbon skeleton may have numerous substituents. Hydroxyl groups are usually present atpositions 3, 5, and 7, but they may also be found at otherpositions. Sugars are very common as we

Secondary metabolism is also relevant to agriculture. The very defensive compounds that increase the repro-ductive fitness of plants by warding off fungi, bacteria, . Plant secondary metabolites can be divided into three chemically distinct groups: t