BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

9. ALLELOPHYSIOLOGY

9.4. Herbivory

In nature, a vast number of species (see Table 9.2) and an immense number of individual herbivorous animals are confronted by plants that remain undamaged or suffer only moderate damage from feeding. For the majority of herbivores, all plants are essentially inedible; like pathogens, herbivores must overcome both constitutive and induced plant defense mechanisms in order to use a plant as a food source. Each herbivore succeeds in doing this with only a few species. Herbivory, much like host-pathogen interactions, is rooted in the coevolution (see 6.15.5) of the participating parties, during the course of which plants have evolved a broad spectrum of defensive mechanisms, while herbivores have developed diverse strategies to overcome them (see 13.6.3, 13.8). The parallels with pathogen defense (see 9.3.4) are readily apparent.

9.4.1. Defense Against Herbivores

As in defense against pathogens, one can distinguish here between constitutive and induced processes. Constitutive defense encompasses both structural barriers (thorns, spines, stinging hairs against larger animals, tough Cell walls and cuticle against smaller ones) and chemical barriers. Many plant metabolites (see 6.16) serve to deter feeding, acting as poisons (toxins), repellents, reducing the nutritional quality of plant tissue (bitter compounds), or even interfering with the developmental cycle of herbivores—especially rapidly reproducing Arthropods (phytoecdysteroids, juvenile hormone analogs). Representative Examples of such substances have already been discussed in the section on metabolic physiology (see 6.16).

Alongside low-molecular-weight compounds that contribute to feeding deterrence, high-molecular-weight substances also exist, particularly Proteins, which either exert toxic effects (e.g., ricin in the seeds of Ricinus communis and the related abrin in rosary pea seeds, both of which block animal Protein Synthesis via inactivation of the ribosomal 60S subunit) or diminish nutritional quality (e.g., the widespread pro-

tease inhibitors found primarily in leaves, fruits, and storage Organs; in the digestive tract of animals, these block various proteases such as Trypsin and Chymotrypsin, thus disrupting Digestion and, in large quantities, sometimes even causing the death of the animal).¹

¹ Another Class of defensive compounds is Lectins. These include phytohemagglutinins, which cause fatal erythrocyte agglutination in the Blood of herbivores. — Transl. note

Effective constitutive defense against insects has been thoroughly studied in the insect-resistant wild tomato species Solanum berthaultii (Fig. 9.18), whose leaves bear Two Types of trichomes (see 3.2.5.3): type B unicellular hairs, which secrete a sticky fluid rich in glucose esters that traps insects (these hairs break off easily and release additional fluid in the process), and type A multicellular hairs, which secrete the volatile compound β-farnesene. This repellent is used by aphids themselves as an alarm pheromone. The pheromone-induced frantic movement of the aphids drives them into contact with an even greater number of glandular trichomes. If the type A trichomes are damaged in the process, they release large amounts of Water-soluble ortho-diphenols and Enzymes (polyphenol oxidases and peroxidases) that oxidize the ortho-diphenols into Quinones. These highly reactive quinones readily polymerize and react with nucleophiles (such as —NH₂—, —OH—, and —SH— groups of proteins). The exudate from the type A trichomes polymerizes within a short time into a resinous mass that glues the insect's mouthparts and tarsi, immobilizing it and causing it to starve to death. Within the plant, ortho-diphenols and oxidases are kept separate through compartmentalization.

Fig. 9.18. Constitutive defense of leaves of the wild tomato Solanum berthaultii against insects (scanning electron micrograph of the leaf surface, magnification approx. 90×)

However, plants also possess induced defense against herbivores, which is particularly effective against small herbivores with individually limited

food intake (though they may occur in large numbers) and is therefore directed primarily against insects. Some of the processes involved in herbivore defense have recently been discovered in tomato leaves (Lycopersicon esculentum). Both species-specific and general mechanisms were identified (Fig. 9.19).

Fig. 9.19. Local and systemic defense against herbivores in tomatoes. When leaves are damaged, e.g., by caterpillars, proteolysis releases the octadecapeptide systemin (A) from the precursor prosystemin; as a systemic wound factor, systemin induces The formation of jasmonic acid (B) in the plant. Jasmonic acid (JA), which is also directly released upon tissue wounding, induces The production of various proteins, including several protease inhibitors (PINs), which render the leaf tissue indigestible for insect herbivore larvae (C)

When a leaf is damaged, for example by a chewing insect larva, triply unsaturated α-linolenic acid is released from Membrane Lipids near the wound site (presumably via Hydrolases or lipases) (see 1.5.2, Fig. 1.21). It serves as a precursor for the synthesis of jasmonic acid (see 7.6.6.2, Fig. 7.66), the accumulation of which can be detected within just a few minutes after wounding (even when inflicted mechanically). In Cells adjacent to the wounded leaf area, jasmonic acid influences the activation of defense genes in a manner not yet fully understood—in the tomato, for instance, activating the genes for several protease inhibitors, which can be detected in the wounded leaf a few hours later and subsequently reach high concentrations (>100 mg/kg) within the leaf. As the chewing insect feeds, it ingests increasing amounts of these inhibitors, progressively impairing its ability to digest food.

Simultaneously, through proteolysis in the wounded region of the tomato leaf, the short 18-amino-acid peptide systemin is cleaved from its precursor prosystemin (see Fig. 9.19). Within a few hours, it travels as a systemic wound signal through the phloem throughout the entire SHOOT, inducing the formation of jasmonic acid and, consequently, the synthesis of protease inhibitors there. Approximately 24 hours after the initial herbivore attack, the entire plant is protected by high concentrations of inhibitors: a mere 1 cm² of a leaf from such a tomato plant constitutes a lethal dose for an insect larva!

While systemin has been found to be active only in tomatoes, mounting evidence shows that many other plants also release

systemic wound factors of yet-unknown types upon wounding (induced by the initial attack) and systemically arm themselves against secondary attacks. Wound-induced substances include not only protease inhibitors, but also proteases themselves, polyphenol oxidases, and numerous secondary metabolites (e.g., the insecticidal compound nicotine in tobacco, and toxic glucosinolates in Brassicaceae). Apparently, all plants investigated to date produce jasmonic acid upon wounding, which influences the activation of defense genes. This is further supported by the fact that in cell cultures of over 150 species, Secondary METABOLISM could be activated using jasmonates. The Role of jasmonic acid in herbivore defense has been clearly demonstrated using tomato mutants that fail to accumulate jasmonic acid following attack (Fig. 9.20).

Fig. 9.20. Significance of jasmonic acid for herbivore defense in tomatoes (courtesy of C. A. Ryan). Symptoms on the defenceless mutant, which has lost The ability to accumulate jasmonic acid upon damage (left plant), compared with the wild type (right plant). Eight-week-old plants were each infested with 8 neonate tobacco hornworm larvae (Manduca sexta), left on the plant for 13 days, and subsequently photographed. The mutant no longer produces protease inhibitors and is severely stunted, whereas the larvae grow intensively (bottom left). In contrast, the wild type suffers only minor feeding damage; these caterpillars (bottom right) exhibit retarded growth. For reasons of space, only a few typical individuals are shown here

9.4.2. Tritrophic Interactions

Plants constantly emit small amounts of volatile compounds through their leaves; alongside Ethylene (see 7.6.5) and, in some cases, isoprene (see 6.16.2), these are predominantly products of the oxidative breakdown of Fatty acids. Herbivorous arthropods frequently use these compounds to locate their host plants. When a plant is wounded by sucking or chewing insects, the emission of volatiles typically increases and—in contrast to purely mechanical damage—the qualitative COMPOSITION OF THE emitted compounds changes radically, giving rise to complex mixtures of 20 or more components. This herbivore-specifically induced production of "aroma mixtures" has a characteristic composition depending on the plant species, age, and physiological state, and bears all the hallmarks of chemical "crys for help". In fact, numerous instances are known (some examples are listed in Table 9.3) where parasitic predatory mites and parasitic wasps (which lay their eggs inside herbivorous host larvae) use these plant signals to locate their hosts. Plant signaling compounds (alarms) primarily include open-chain Terpenes and, in some cases, Aromatic Compounds such as indole or methyl salicylate (Fig. 9.21). Sometimes (e.g., in Brassica oleracea, Table 9.3), even an enhanced release of constitutively formed volatile fatty acid derivatives (some structural examples in Fig. 9.21) is sufficient to attract parasites. Such chemically coordinated three-way relationships between the host plant, the herbivore, and its parasites are termed tritrophic interactions.

Fig. 9.21. Examples of volatile substances released by plants following attack by herbivorous arthropods. Substances (1) and (2) are widespread; (1) through (6) are specifically produced only upon herbivory-induced wounding, not upon mechanical wounding. Substances (7) through (9) are examples of compounds constantly formed in small amounts, but released in significantly greater quantities upon mechanical or herbivory-induced wounding. They also contribute to the scent of a freshly mowed meadow

The formation of herbivore-induced plant volatiles (alarmones) is triggered by components present in insect saliva. The Structure of the first such elicitor was recently elucidated: 17-hydroxylinolenic acid conjugated with α-glutamine. This compound, found in Spodoptera exigua and named volicitin (derived from volatile-eliciting), induces the release of a characteristic blend of alarmones even at extremely low concentrations—about 30 — 40 • 10^-12 mol, an amount contained in just 2 µL of saliva, per 14-day-old corn plant (Fig. 9.22). Remarkably, localized feeding by insects leads not only to local but also to systemic emission of herbivore-specific volatiles. The mechanism underlying this systemic activation remains unknown.

Table 9.3. Examples of tritrophic interactions among plants, herbivores, and their parasites

Note. The numbering of the compounds corresponds to that in Fig. 9.21.

Fig. 9.22. Volicitin, a component of the larval saliva of Spodoptera exigua, which enters the wound during feeding and induces the Formation of Plant alarmones. The precursor of volicitin, linolenic acid, is ingested with plant food; further synthesis of volicitin takes place within the insect body

It has been hypothesized that alarmones released by insect-damaged plants might affect neighboring, undamaged plants, thereby preemptively activating their defenses against herbivores, thus acting as signaling pheromones. However, no conclusive experimental evidence has yet been provided for such a function under natural conditions. Nevertheless, laboratory experiments have demonstrated that a plant emitting volatile methyl jasmonate (e.g., Artemisia tridentata) can affect a neighboring tomato plant and induce the synthesis of protease inhibitors. Yet, for this to occur, both individuals must be kept in close proximity under a sealed Glass bell jar.