I I I

MMMWMMMimMiï

ReceptorsIon channels Ca2+

Proteinase inhibitors

Phospholipase

Linolenic acid

Phospholipase

Cell wall

Plasma membrane

G protein kinases

MAP-kinases ^"^Salicylate ^

Auxine

Phenols

Oxylipins (jasmonic acid)

Proteinase inhibitors

Auxine

Phenols

Oxylipins (jasmonic acid)

Volatiles Ethylene

Gene activation Nucleus

Volatiles Ethylene

Gene activation Nucleus

Figure 13 Scheme of signaling pathways involved in the upregulation of induced defense in plants.

Figure 14 Gas chromatogram of the volatile emission of lima bean (Phaseolus lunatus) after 24 h feeding of Spodoptera frugiperda larvae.

Figure 14 Gas chromatogram of the volatile emission of lima bean (Phaseolus lunatus) after 24 h feeding of Spodoptera frugiperda larvae.

Protease inhibitors

The biosynthesis of PIs is activated as a consequence of induced defense. PIs act against proteases of the feeding herbivore so that the herbivore cannot digest the fed plant proteins which results in a decreased fitness of the herbivore.

Generation of phytoalexins

Many plants respond to elicitors of pathogens with the induced production of low molecular weight secondary metabolites - so-called phytoalexins - that render plants more resistant against pathogen attack and can stop their growth (Figure 17). Phytoalexins are derived from

Figure 15 Parasitized Pieris brassicae larva.
Figure 16 Pair of extrafloral nectaries (arrows) of lima bean (Phaseolus lunatus). Photograph taken by Dr. Christian Kost.

diverse biosynthetic pathways. For example, phenylpro-panoids, polyketides, or terpenoids serve as antimicrobial defensive compounds.

The production of phenolics such as chlorogenic acid 67, glyceollin I 68, or 6-methoxymellein 69 has been observed to be induced after pathogen attack. Also stil-benes, for example, resveratrol 70, serve plants as antimicrobial compounds. In case of resveratrol 70, it has been demonstrated by the use of transgenic plants -expressing the grapevine stilbene synthase - that plants become resistant against the fungus Phytophtora infestans. Likewise, terpenoid phytoalexins such as rishitin 71 that are generated by Solanum tuberosum (potato), Nicotiana tabacum (tobacco), or Lycopercion esculentum (tomato) increase the resistance of plants against microbial pathogens.

A defense compound of rather unusual structure is wyerone 72 - a furanone acetylenic acid - generated in high amounts in the broad bean (Vicia faba) after infection with Botrytis fabae or B. cinera. Wyerone 72 kills Botrytis spores. It turned out to be much more effective than rishitin and other phytoalexins.

Conclusions

A very large variety of defensive compounds is found among different plant species. Nevertheless, some general principles can be deduced. It seems that disturbance of membrane potentials is the initial step to provoke defense; most - if not all - plants rely on oxylipin-based signaling processes. Antifeedants with a bitter or pungent taste are often efficient to deter herbivores from feeding. Many toxins are electrophiles that react with biomolecules and thus destroy their function. Other defensive compounds

Figure 17 Phytoalexins.

interfere with respiration. Another general strategy of plants is to reduce the feeding success of enemies by phenolic compounds, proteinase inhibitors, or by the allocation of valuable nutrients.

With more detailed investigations on many plants, the man-made grouping into different defense classes (constitutive defense, activated defense, and induced defense) turns out to be more and more blurred. Many plants use more than one strategy.

With the upcoming whole genome/proteome expression pattern profiling for the major model plants, it can be expected that many more details of the reception and signal processing to induce appropriate defenses will soon be available. Matching this information to the role oftoxins will give exciting detailed insights on the fine-tuning ofthe coordination of plant defense strategies as a whole.

See also: Animal Defense Strategies; Defense Strategies of Marine and Aquatic Organisms; Fungal Defense Strategies.

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