Anatomical Defense and Mechanical Barriers

As an important first line of defense, many plants make use of mechanical barriers that offer protection against a variety of possible attacks (Figure 1). Among plants, cellulose 1, a polymer of glucose (^-1,4-glucan), provides stable structures and thus prevents access to the meriste-matic tissues. Lignification (2) further precludes attackers to access labile plant cell layers. Likewise, the lipophilic suberin 3 (e.g., the cork layer of the cork oak, Quercus suber) comprises a complex network of long chain a,u-dicarboxylic acids, long-chain !-hydroxy fatty acids such as !-hydroxy-octadecanoic acid, polyglycerol esters, and phenolic esters. It serves as a protective layer of plant roots and stems mainly against microbial attack. The leaves of many plants are covered with epicuticular waxes consisting of nonpolar compounds, for example, long-chain alkanes (C21-C35), long-chain alcohols (C22-C40), and alkanones (C21-C35). The deposition of such waxes leads to a well-defined crystalline microstructure often exhibiting thorn- and scale-like structures which form a barrier against pathogens. Some waxes combine their function as mechanical barriers with antifungal properties, for example, accumulating antimicrobial ^-lactones such as 4 (Cerinthe minor). The nanostructure of leaf surfaces caused by deposition of waxes efficiently protects plants against dirt and the settlement of other (micro-)-organisms. The rough microstructure reduces the contact surface of the leaf to dirt or microbial spores, thus preventing them from attaching easily. The next rain will sweep them away and clean the leaf. The famous example of lotus leaves even inspired scientists to develop material surfaces that mimic the so-called lotus effect - some applications have already been marketed.

Figure 1

Another protective strategy is the use of thorns and spines that are efficient to prevent animals such as cattle from feeding. Trichomes, that is, hair-like epidermal outgrowths, can serve plants for protection, because they are often filled with toxic compounds that are released upon wounding. Specialized cells (idioblasts) of Dieffenbachia plants contain crystalline needles of calcium oxalate that are shot against attackers causing painful irritation.

Chemical Defense

Constitutive Defense

Constitutive plant defensive compounds belong mainly to three compound classes: alkaloids, terpenoids, and phenolic compounds.


Alkaloids (Figure 2) are preferentially produced by Apocynaceae, Asteraceae, Fabaceae, Liliaceae, Papaveraceae, Ranunculaceae, and Solanaceae. The bitter taste of most alkaloids deters feeding mammals and prevents them from destroying a plant population completely within a short time. However, starvation forces animals to consume even plants with a bitter taste. As a result, Central American farmers face high losses of cattle in dry periods when cattle consume leaves and branches of Melochia pyramidata (Sterunculaceae) which usually are left untouched. The death of cattle is caused by the alkaloid melochinine 5.

Important deterrents and toxins are pyrrolizidine alkaloids such as senecionine 6 that are widespread in Fabaceae and Asteraceae. Wild animals usually do not consume these plants or even if they do, only in low amounts. If grazing cattle feed on plants producing pyr-rolizidine alkaloids, they suffer severe liver poisoning because these alkaloids induce lipid peroxidation processes forming toxic radicals.

Fabaceous plants produce quinolizidine alkaloids, for example, cytisine 7 from laburnum (Laburnum anagyr-oides). Quinolizidine alkaloids deter feeding animals and are toxic because they modulate acetylcholine receptors and inhibit Na+/K+ channels of invertebrates and vertebrates. The tropical plant Strychnos nux-vomica protects its seeds relying on the highly toxic strychnine 8 and brucine 9 that block the glycine receptors of the spinal cord. Moreover, strychnine 8 and brucine 9 cause paralysis and lead, even in small doses, to death.

Another strong nervous poison is nicotine 10, preferentially generated by Nicotiana tabacum (Solanaceae). Nicotine 10 that is stored in trichomes (epidermal hairs) of tobacco leaves is absorbed through the cuticle of insects. Therefore, nicotine acts as an effective contact poison being an antagonist of acetylcholine and thus disturbing signal transduction. As a consequence, most herbivores do not feed on plants containing nicotine 10.

Mechanical Barrier

1 Partial structure of lignin

1 Partial structure of lignin

Examples for mechanical defense.

Mechanical Defence Plants

Figure 2 Plant alkaloids.

However, the specialist Manduca sexta feeds on Nicotiana species tolerating nicotine 10 to some extent because it has acquired the ability to detoxify the poison. Nicotine 10 has been used since more than two centuries as an insecticide. Because nicotine 10 blocks acetylcholine receptors, tobacco alkaloids are toxic to mammals too, especially if applied intravenously. Steroidal alkaloids such as veratridin 11 from Liliaceae also act as neurotox-ins by binding to sodium channels and keeping them open. Besides their antifeedant or toxic potential against mammals and insect herbivores, some alkaloids protect plants against microbial attack, for example, liriodenin 12 from Liriodendron tulipifera (Magnoliaceae) that acts against phytopathogenous fungi.


A very large group of constitutive plant defense compounds are terpenoids (Figure 3). They usually do not prevent mammals from feeding, but are often highly active against insects. The amount of terpenes differs very much in the various parts of plants and is highly dependent on the growth state. Volatile monoterpenes, for example, menthol 13 from peppermint, are often stored in trichomes from which they are released upon rupture.

Some plants, for instance spruces, contain mixtures of monoterpenes, such as a-pinene 14, myrcene 15, limo-nene 16, and @-phellandrene 17 that are lethal to feeding beetles, for example, the spruce beetle (Dendroctonus rufipennis). Often the complex blend of terpenoids exhibits higher toxicity and activity against a broad spectrum of herbivores as antifeedant than isolated single components, explaining the production of a large variety of secondary products by plants.

Very efficient natural insecticides are generated in the buds of some African Pyrethrum species such as pyrethrine I 18, an ester composed of two monoterpenes. Pyrethrine I 18 interferes with Na+-ion channel signaling. It served as the lead structure for the development of important synthetic insecticides.

Antifeedant diterpenoids called baccatins are produced by Taxus baccata trees (death tree). Baccatins are potent inhibitors of the cell cycle. Thus, taxol 19, a member of baccatins, became famous because of its use in anticancer therapy.

The triterpenoid quassine 20 is an example of a bitter-tasting and therefore potent antifeedant of the

Using Plants Structural Barriers
Figure 3 Plant-derived terpenoids.

group of quassinoids that inhibit the NADH oxidase. The inhibiting effect seems to be related to the presence of the «^-unsaturated carbonyl structure element that is prone to undergo reactions with nucleophilic groups in proteins. Quassinoids are especially active against aphids.

Many plants produce phytoecdysones, for example, 20-hydroxyecdysone 21. Since ecdysone serves insects as a hormone to coordinate their moulting, plant analogs are highly toxic for insect herbivores because they interfere with the regulation of their development.

Much attention was paid to the insecticidal compounds isolated from the Indian neem tree (Azadirachta indica) containing triterpenoids such as azadirachtine 22. Azadirachtine 22, a highly functionalized complex triter-penoid, is a very powerful and selective agent against feeding insects disrupting their growth and development. Extracts from the neem tree therefore have been used for a long time to fight herbivorous pests.

Sometimes, the release of highly viscous and sticky resin protects trees after mechanical wounding against feeding insects, because they are trapped by the resin.

Latex, the milky sap of tropical trees such as Hevea brasi-liensis consists mainly of polymers of isoprene and serves to seal wounds and to trap attackers or to jam the mouth-parts of herbivores. Moreover, it contains diterpenoids, for example, abietinic acid 23, that act against microorganisms and insects.

Phenolic compounds

Phenolic compounds represent an important line of defense against feeding herbivores (Figure 4). Besides their toxic potential, many phenolics also exhibit a bitter taste that acts as a deterrent. The concentration of phe-nolics and consequently the toxicity varies very much depending on plant parts, age, and season.

The high molecular weight proanthocyanidins 24 (tannins) reaching molecular weights of up to 7000 Da are deposited in bark and wood in order to prevent herbivore feeding and keep pathogens from entering. Tannins bind to proteins and consequently prevent their digestion as well as denature digestive enzymes causing a reduction of the feeding success.

Besides high molecular weight compounds, low molecular weight phenolics can also act as antifeedants and toxins. For example, the 5-hydroxy-1,4-naphthoquinone (juglone 25) produced mainly by black walnut (Juglans nigra) inhibits respiration in sensitive organisms. Juglone occurs in all parts of black walnut with highest concentration in the roots. Since juglone 25 is released into the surrounding soil, usually no other plants grow near black walnut trees. Phloroglucinol phenolics such as jensenone

26 have been characterized as an antifeedant principle of browse-resistant Eucalyptus species against feeding koalas and the opossum. The aldehydic moieties of jensenone 26 have been shown to readily react with free amino groups of proteins leading to the loss of function of proteins as well as causing disturbance of signaling processes in the gastrointestinal tract.

Very common phenolic plant constituents are coni-feryl alcohol 27 and the related phenols guajacyl alcohol 28 and sinapyl alcohol 29. Upon attack by microorganisms or herbivores, the oxidative polymerization of these compounds is induced to produce lignin 2. This lignification process strengthens the cell walls and prevents further attack.

Famous insecticides are rotenones, for example, 30 and related rotenoids that occur in the roots of the fabaceous Derris elliptica. They inhibit mitochondrial respiration by blocking the electron-transfer chain. Rotenoids have been used for a long time against insects in tropical countries. Moreover, since rotenoids paralyze fish, Derris roots were used for fishing.


Saponins (Figure 5) are glycosides containing two to five monosaccharide molecules condensed to a triterpenoid, steroid, or alkaloid steroid aglycon. Saponins are found in many food plants such as tomato, potato, oat, pea, and soybean. Triterpenoid-derived saponins are preferentially generated by dicotyledonous plants, whereas

Plant Population

Figure 4 Plant phenolics.

Phasine Saponin



Saponin Pea

Figure 5 Triterpenoid, steroid, and alkaloid steroid saponins.

Steroid alkaloid saponin

Figure 5 Triterpenoid, steroid, and alkaloid steroid saponins.

steroid saponins occur mainly in monocots. Steroid alkaloids are typical constituents of the family of Solanaceae.

Saponins are strongly amphiphilic molecules and excellent detergents based on the presence of polar sugar moieties combined with a nonpolar steroid or triterpenoid aglycon. As detergents, saponins cause membrane disruption and consequently destroy cells. They induce hemolysis in mammalians, fish, mollusks, and amphibians. Additionally, many saponins exhibit a deterrent bitter taste. Thus, saponins are feeding deterrents for insects. They often exhibit strong antifungal activity whereas only weak antibacterial activity is usually observed. Their amphiphilic character and thus their toxicity is destroyed by saponification. Consequently, resistant fungi usually degrade saponins with glycosidases.

For instance, the antifungal triterpenoid saponin ave-nacin A-1 31 is an abundant constituent of oat roots.

Some steroid saponins serve the producing plant to compete efficiently with other plants for living space as demonstrated by solamargine from the shrub Solanum incanum that inhibits germination of lettuce seeds.

Other representatives of steroid saponins are produced by Digitalis species (e.g., Digitalis purpurea) such as digi-toxin 32. Digitoxin acts on the Na+/K+-ATPase and strengthens the heart muscle when used in small amounts. It has therefore become an important drug for the therapy of chronic heart failure. However, in larger quantities, digitoxin causes death due to arrhythmia.

The alkaloid steroid saponins, tomatidine 33 and sola-nidine (in which nitrogen is incorporated into a steroidal side chain to form a new ring system), are produced by tobacco and potato plants as defense compounds. The hydroxy group of the aglycon, tomatine, is glycosylated with an oligosaccharide consisting of two molecules of D-glucose, one D-galactose, and one D-xylose. a-Tomatine is effective to deter the larvae of the potato beetle from feeding. Saponins also protect plants against feeding termites and snails.

Fluorinated natural products

Some plants in Africa (Dichapetalum cymosum and Dichapetalum toxicarum) and Australia (Acacia georginae,

Figure 6 Fluorinated fatty acids (fluoroacetate, 18-fluorooctadecanoic acid).

Figure 6 Fluorinated fatty acids (fluoroacetate, 18-fluorooctadecanoic acid).

Gastrolobium, and Oxylobium) have been found to produce fluorinated fatty acid derivatives such as fluoroacetate 34 and long-chain !-fluorofatty acids, for example, 35 (Figure 6). Fluoroacetate 34 is toxic because it is converted to fluoroacetyl coenzyme A which enters the citrate cycle and acts as a strong inhibitor of the aconitase. Fluoroacetate has also been used commercially against rodents.

!-Fluorofatty acids such as 35 occurring in the seeds of Dichapetalum toxicarum undergo fl-oxidation and release fluoroacetyl coenzyme A blocking the citrate cycle as well. Their defensive potential has to be realized regularly by farmers who lose cattle feeding on these plants if there is no other food available.

Amino acids and plant proteins

The nonproteinogenous toxic amino acid canavanine 36 (Figure 7) - an analog of L-arginine - is produced, for example, in the seeds of jack bean (Canavalia ensiformis). During protein biosynthesis, the herbivore incorporates canavanine 36 - instead of L-arginine - into new proteins that become misfolded.

Besides small organic molecules, high molecular weight proteins serve some plants for defense especially as protection of their seeds. For example, the seeds of Ricinus communis contain ricin, those of Abrus precatorius the related abrin. Both proteins are highly toxic. Consumption of a few seeds causes death of humans. Ricin consists of an A- and B-chain that are linked by disulfide bridges. The A-chain is a highly active glucosi-dase and the B-chain specifically binds to the cell surface and induces the uptake of ricin into the cell. The glyco-sidase inactivates protein biosynthesis by cleaving adenine from the ribosomal RNA. Thus, protein synthesis is stopped and the cell dies. It is assumed that one ricin molecule is sufficient to kill one cell. Although being less toxic, phasin from bean plants (e.g., Phaseolus vulgaris) can cause severe poisoning if beans are eaten uncooked. Phasin is a so-called lectin, a protein which binds to carbohydrate moieties of glycoproteins. This reaction is



Figure 7 Canavanine, a toxic amino acid.

Canavanine 36

Figure 7 Canavanine, a toxic amino acid.

similar to antigen-antibody binding and causes agglutination of erythrocytes and leucocytes. Cooking destroys the heat-labile phasin.

Furthermore, plants produce, in analogy to animals, so-called defensins. This class of peptides is characterized by eight conserved cysteine residues that are linked via disulfide bridges. Plant defensins show antifungal activity but usually no activity against bacteria (see Fungal Defense Strategies).

Activated Defense

A large number of plant defensive compounds occur preformed as nontoxic storage forms, avoiding self-intoxication of the producing plant. Common storage forms are, for example, esters, acetals, or glycosides. Such pretoxins develop their defensive potential instantly after enzymatic cleavage, for example, glycosides of fla-vones are hydrolyzed to the corresponding aglycons when a plant is wounded by a feeding herbivore or a microorganism. Since such preformed toxins need to be activated, they constitute a link between constitutive defense and induced defense mechanisms.


Benzoxazinoids (Figure 8) such as DIBOA (2,4-dihy-droxy-2#-1,4-benzoxazin-3(4fl)-one) 39 and DIMBOA (2,4-dihydroxy-7-methoxy-2#-1,4-benzoxazin-3(4H)-one) 40 represent an important group of defense compounds in corn, wheat, and rye plants. The arylhydroxamic acids DIBOA/DIMBOA (39, 40) are stored as biologically inactive glycosides (37, 38) that are cleaved by a ^-glucosidase upon attack. Thus, the active aglycons are released. They are active against bacteria, fungi, and herbivores. DIBOA 39 and DIMBOA 40 undergo spontaneous degradation to benzoxazolinones (BOA 41, MBOA 42). Although the toxic principles of benzoxazi-noids and their breakdown products are not completely understood, it seems that the acetylated AMBOA 43 might be involved. AMBOA 43 as a strong electrophile reacts readily with nucleophiles from biomolecules, for example, free basic amino groups of proteins.

Iridoid glucosides

The terpenoid iridoids (Figure 9) such as plumierid 45 are cyclic glucosylated acetals. Iridoid glucosides are cleaved like other glycosides after attack of herbivores

Figure 8 Benzoxazinoids and reaction of AMBOA with nucleophiles (Nu).
Figure 9 Iridoids as plant defensive compounds generating toxic dialdehydes.

by glucosidases to the free terpenoid aglycon 46. The generated half-acetal 47 is in equilibrium with the dia-ldehyde form 48. 1,5-Dialdehydes are prone to react with free amino groups of proteins to form a dihydro-pyridine ring structure. Thus digestive proteins of herbivores are destroyed, resulting in a decreased fitness of the feeding insect. Besides their toxic potential, iridoids have a bitter taste that is repellent. However, there are specialist herbivores that are resistant against the toxic iridoids and even make use of these compounds for their own defense (sequestration; see Animal Defense Strategies).

Cyanogenic glycosides

More than 2500 plant species, in particular Rosaceae, Fabaceae, Graminae, Araceae, and Euphorbiaceae, use hydrogen cyanide (54, HCN) as a fast-acting, powerful toxin to protect their seeds or leaves against attack by mammalians as well as insect herbivores. There are even several cases of death reported after consumption of cyanogenic plants by humans. HCN blocks the cyto-chrome-3 oxidase of the respiratory chain, preventing the uptake of oxygen from hemoglobin into tissue. The high content of cyanogenic glycosides in some important food plants such as cassava causes problems for human food processing. Being stored in form of cyanogenic glycosides, that is, P-glycosylated a-hydroxynitriles such as amygda-lin 49, HCN 54 is generated upon wounding (Figure 10). Tissue disruption brings together both cyanogenic glyco-sides and the HCN-releasing enzymes, P-glucosidase and hydroxynitrile lyase, which are stored in separate compartments in the intact plant cells. As a consequence, amygdalin 49 is saponified stepwise to prunasine 51 and glucose 50; further loss of the second glucose molecule 50 generates mandelonitrile 52 that is cleaved to benzaldehyde 53 and HCN 54. Remarkably, the content of cyanogenic compounds seems to vary in plants considerably, being more concentrated in young plants as compared to aged ones. Moreover, cyanogenic glycosides are enriched in the stem compared to the leaves.

Plumierid Glycoside

50 OH

Figure 10 Cyanogenic glycosides from plants and their breakdown to release HCN.

50 OH

Figure 10 Cyanogenic glycosides from plants and their breakdown to release HCN.

Radish Myrosinase
Figure 11 Reactions of glucosinolates to produce toxic compounds, in particular, isothiocyanates.


Brassicaceae such as cabbage, rapeseed, or radish produce glucosinolates 55 that are efficient antifeedants against mammals and insects (Figure 11). Upon tissue damage, glucosinolates are cleaved by myrosinase, a glucosi-dase. The instable intermediate 56 undergoes further breakdown to form mainly toxic isothiocyanates 59 (mustard oils) via a Lossen-type rearrangement. Depending on the reaction conditions, for example, pH, nitriles 60, thioepoxides 61, or goitrin 58 and sulfuric acid 57 are also formed. Whereas the nitriles 60 do not exhibit toxic properties (the reason for their formation is still unknown), thioepoxides are highly reactive molecules that exert toxicity by reaction with nucleophiles. Goitrin 58, formed from a labile isocyanate, inhibits the 5'-mono-deiodinase which results in reduced formation of thyroxin causing goiter formation.

Interestingly, glucosinolates are enriched in young cotyledons and young leaves so that those newly developing parts of a plant are especially well protected against herbivores. Consequently, even adapted specialist herbivores tend to feed on older leaves due to the lower concentration of glucosinolates.

Extensive farming of genetically altered rape 00 in recent years has been recognized to cause loss of hares and deer that feed on these rape plants. The 'disease' called rape blindness is caused by the consumption of rapeseed that contains high amounts of glucosinolates.

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