1. Poisonous plant compounds, which are con-stitutively formed and are usually products of secondary metabolism, protect plants from herbivory, however, never completely. Some phytophagous insects take up such compounds while feeding on plants and accumulate them to protect themselves from their enemies. During the course of evolution, however, counteractions developed again. Volatile secondary plant metabolites often act as messengers which either frighten off or entice.
2. In addition to the constitutively expressed defence system of plants, reactions are also induced after wounding or infestation by pathogens. These reactions are triggered by pressure waves in the case of wounding, and by elicitors in the case of pathogen infestation.
3. If a plant is wounded, the vascular bundles are almost always affected. If air enters into the xylem, which is under negative pressure, a positive pressure wave is triggered across the whole plant. A depolarisation wave, decreasing with increasing distance from the damaged site, follows the change of pressure. The depolarisation wave activates intracellular signal chains, initially the formation of systemin from prosystemin, followed by activation of a lipase which releases linolenic acid from biomembranes. The free linolenic acid undergoes many metabolic reactions, initiated by lipoxygenase. One of the products is jasmo-nate, a stress hormone, but traumatin, a wound stress hormone, which activates cell division in the damaged tissue, may also be produced.
4. Jasmonate, like many phytohormones, has pleiotropic effects; it induces the expression of various genes, which, insofar as they are known, encode proteins serving to counteract stress. These genes are active even in those
■ Fig. 1.10.13. Intraspecific allelopathy In Kalanchoe (Kalanchoe daigremontiana). A Same age daughter plantlets were placed In a sand soil at regular Intervals from the mother plant. After 150 days the height of the daughter plants clearly shows the Inhibitory effect of the mother plant on their growth. B Quantification shows that the Inhibition of the young plants depends on the distance from the mother plant. (After Bär et al. 2000)
parts of the plant that have not been exposed to stress or develop after the stress. Because of this systemic reaction, this phenomenon is also called "immunisation".
5. Plants distinguish in their reaction between simple mechanical wounding and damage because of herbivory. In damage caused by her-bivory, the regurgitant of phytophagous insects or their larvae is very important. This digestive fluid contains fatty acids conjugated to amino acids as elicitors. Synthesis of proteinase inhibitors, as well as synthesis of volatile signal compounds of the group of ter-penes, is predominantly induced. Additionally, non-volatile defensive substances are formed after induction by jasmonate, e.g. the alkaloid nicotine, which is toxic for many insects and warm-blooded animals.
6. In addition to the transduction of the wound signal by jasmonates, there is also a jasmo-nate-independent path which activates the defence reactions.
7. Compatible and incompatible reactions are distinguished in pathogen attacks. In compatible reactions, there is a strong infestation, whilst the incompatible reactions indicate resistance. Resistance may be constitutive, directly acquired or acquired by "immunisation", the so-called systemic acquired resistance. Salicylic acid derived from phenolic metabolism is the signal substance inducing the defence system in infected or in non-infected plant parts.
8. The most frequently occurring form of an incompatible reaction is the "hypersensitive response": In addition to the synthesis of PR proteins (pathogenesis-related proteins, mainly lytic enzymes) and low molecular weight antibiotically active phytoalexins, the hypersensitive response comprises the programmed cell death of the infected cell, thereby depriving the pathogen from its resources. The hypersensitive reaction can be recognised by small, well-defined lesions.
9. Hypersensitive reactions are triggered by so-called elicitors, which may be derived from the pathogen, but also from the plant cell wall attacked by the pathogen. They bind specifically to the receptors which are encoded by R(esistance) genes. The specificity of elicitors is based on the avr genes (aviru-lence genes) of the pathogen.
10. Inhibition of growth and development of plants by neighbouring plants is called allelopathy, if it is based on excretion of incompatible chemical substances and not on simple competition. Classical examples are inhibition of germination and growth caused by rotting leaves of walnut trees, and the inhibition of growth of grasses by volatile compounds from Lamiaceae and Asteraceae.
Intraspecific allelopathy is demonstrated by Kalanchoe daigremontiana, where the mother plant inhibits growth of plantlets developing at the edges of its leaves by secretion of a broad spectrum of phenolic compounds from the roots.
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