Induced Defense

Elicitors, signal reception, and signaling cascades

In contrast to most other organisms, plants can regenerate completely after loss of biomass. Consequently, plants tolerate some damage. Balancing the costs between risking some damage and freeing resources for development, many plants rely on induced defense mechanisms. The plant does not produce toxins or antifeedants constitu-tively but only on demand in case of an attack. However, there is some time needed for initiating defense. During that time, the plant is not well protected. But induced defense offers the advantage of specific reactions to various threats. Plants have to differentiate between mechanical wounding, for example, by a storm, attack by a pathogen, or by herbivore feeding. There is even evidence that plants can distinguish between various lepi-dopteran herbivores and their larval stage. This requires a highly developed recognition and signaling system.

Both chemical compounds, so-called elicitors, and the type of mechanical wounding serve for plants to recognize dangers and to initiate adequate responses. Elicitors are compounds that are either generated by activation of degrading enzymes by the plant itself or they originate from the herbivore/pathogen and are introduced into the wounding site. For example, feeding lepidopteran larvae introduce low amounts of elicitors with their saliva into the attacked leaves.

The structures of some elicitors from both pathogens and herbivorous insects have been identified ranging from low molecular weight molecules to high molecular weight proteins. Elicitors derived from herbivorous insects that have been identified so far include a ^-glucosidase from Pieris brassicae, volicitin ((17S)-17-hydroxylinolenoyl-L-glutamine (see Animal Defense Strategies)) from the oral secretion of beet armyworm (Spodoptera exigua) larvae, and a peptide fragment from the ATPase - interestingly derived from the plant's own ATPase - isolated from the oral secretion of Manduca sexta larvae.

Besides herbivorous attack, plants have to face invasion by pathogens, for example, Pseudomonas syringae. Several elicitors from microorganisms have been identified and studied in detail. For example, cellulysin, a crude mixture of cellulases from the fungus Trichoderma viride, was found to elicit plant volatile biosynthesis in lima bean. Moreover, ion-channel-forming compounds such as the peptaibol alamethicin induce defense reactions in plants.

On the other hand, plant-derived degradation products such as glucans (e.g., a hepta-^-glucoside) from Phytophtera megasperma or from yeast lead, for example, to the induction of glyceollin production (68; Figure 17) in soybean. Also cell wall fragments of foreign organisms can be recognized by plants. In the same line, flagellin, a protein from the flagellum present in many bacteria, has been observed to induce plant defense. Flagellin is recognized by a receptor that induces the signal transduction via a receptor kinase mechanism. This finding suggests that plants have evolved similar defense mechanisms as known from the mammalian immune system. As proven for flagellin, the early events of defense recognition can be receptor-mediated processes.

Although specific elicitors have been identified to induce defense reactions, nonspecific mechanical wounding - that mimics a feeding insect - is sufficient to initiate many of the defense reactions that are induced by herbivore attack, for example, the emission of volatiles. Nevertheless, simple mechanical wounding - such as cutting a leaf several times - does not induce defense reactions. Instead, continuous microdamage - applied by punching a leaf over a period of several hours - is required to induce the production of defense compounds. This observation indicates that damage of the cell membrane resulting in a change of the membrane potential induces signaling pathways and finally causes upregula-tion of defense mechanisms.

Wounding disrupts the outermost layer of cells and disintegrates the plant cell membranes causing depolarization. Depolarization inevitably causes an alteration of the ion fluxes, for example, inducing influx of Ca2+ ions into the cell. Ca2+ ions together with cyclo-AMP activate a great number of degrading enzymes including gly-cosidases, lipases, peptidases, and oxidoreductases that are involved in initiation of defense reactions. Phospholipases release PUFAs such as linolenic acid 62 from phospholipids. The free PUFAs are substrates of lipooxygenases that produce hydroperoxy fatty acids. Further degradation of hydroperoxy fatty acids results in the generation of a large variety of oxidized lipids (oxylipins).

Particularly well studied is the octadecanoid pathway (Figure 12) that generates the plant hormone jasmonic

OOH O

(13S)-13-Hydroperoxy-Z9,E11 ,Z15-octadecatrienoic acid 63

aliéné oxid synthase

OOH O

(13S)-13-Hydroperoxy-Z9,E11 ,Z15-octadecatrienoic acid 63

aliéné oxid synthase

12-Oxophytodienoic acid (PDA) 65

i, 12-oxo-PDA-reductase " ii,/3-oxidation (3x)

Figure 12 Biosynthesis of jasmonic acid 66 (octadecanoid pathway) as signal involved in activation of induced defense mechanisms.

Jasmonic acid 66

Figure 12 Biosynthesis of jasmonic acid 66 (octadecanoid pathway) as signal involved in activation of induced defense mechanisms.

acid 66 from (13^)-13-hydroperoxy-9Z,11£,15Z-octade-catrienoic acid 63 via an allene oxide intermediate 64 and 12-oxophytodienoic acid 65. Jasmonic acid 66 is responsible for the upregulation of defense-related genes. Other oxylipins - generated enzymatically or by nonenzymatic oxidation processes - either act as toxins or have signaling character.

In addition, salicylate levels rise especially as a consequence of pathogen attack and cause the so-called systemic acquired resistance. In a complex interdependence, the different pathways influence each other (cross talk). The activation of degrading enzymes such as phos-pholipases and the induction of changes in membrane potential are very fast processes (seconds/minutes) whereas the synthesis of jasmonic acid 66 or salicylate takes more time (hours). Furthermore, many additional signal molecules such as ethylene, abscisic acid, or systemin are involved in the regulation of plant defense. The processing of the signals finally leads to gene activation and the induction of defense mechanisms (Figure 13).

Induced defense reactions

A variety of defense strategies are induced after recognition of a danger: the de novo synthesis and emission of volatiles and ethylene, the induction of nectar flow of extrafloral nectaries, the production of protease inhibitors (PIs) and of toxins or antifeedants, for example, phenolic compounds used for cell wall reinforcement by lignification. Also the reallocation of valuable nutrients has been observed. Thus plants both lower the food quality and protect their resources.

Often induced defense is associated with a drastic increase in the production of constitutive defense compounds. For example, herbivore feeding leads to a 220% increase of nicotine in Nicotiana sylvestris while in Brassicae napus induced defense is combined with a shift from the production of aliphatic glucosinolates to indole glucosinolates.

Emission of volatiles

An extensively studied induced defense reaction of many plants is the emission of volatile blends consisting mainly of terpenoids and aromatic compounds (Figure 14). Additionally, the ethylene biosynthesis is induced. The wound hormone ethylene is involved in the fine regulation of defense processes. The volatiles serve both to warn neighboring plants of upcoming dangers and to attract help of predators. The released volatiles attract, for example, parasitic wasps that lay their eggs into the herbivore. When the parasitoid larvae hatch from the eggs, the herbivore is consumed by the larvae of the parasitoid and finally killed. Consequently, the feeding pressure on the plant is reduced. This type of plant defense constitutes an indirect defense mechanism and reflects a form of tritrophic interactions (Figure 15).

Extra floral nectaries

Many plants, for example, ant plants such as Macaranga (Macaranga tanarius) and also the lima bean (Phaseolus lunatus), produce so-called extrafloral nectaries (see arrows in Figure 16) that release sugar-containing droplets. These droplets attract beneficial insects such as ants. As a result, ants patrol on the plant and remove herbivores. Their help is honored with the gift of extrafloral nectar. Some plants such as Macaranga, in addition, also provide living space for specialized ants in their hollow stem.

Enzymes, glucans, peptides, ionophores, volicitin, surfactants, flagellin, abiotic factors

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