Angewandte Chemie International Edition
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For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (10), 1860 - 1862
How do Plants
recognize their Enemies?
set off defensive reactions
When infected by microorganisms or damaged by herbivorous insects, many plants react with characteristic defense mechanisms. A typical reaction is the release of volatile compounds in order to warn other distant plants of the danger or even to lure predators to remove the attacker.
How plants recognize an enemy invasion has not yet been fully researched. It seems that the victim senses chemical substances excreted by the attacking organism. At the Max Planck Institute for Chemical Ecology in Jena a new piece of the puzzle has now fallen into place: low molecular weight peptide-type antibiotics from fungi, called peptaibols, could play a role in triggering plant defense mechanisms. As a model organism, researchers working with Wilhelm Boland used freshly cut lima bean shoots that were placed in a solution of the peptaibol alamethicin. A significant emission of the plant hormone ethylene started after only three hours.
In addition, the treated shoots released a characteristic cocktail of different volatile compounds. This pattern of volatile compounds is normally produced by lima beans as a response to a precursor of the plant hormone jasmonic acid. Does the stimulation of volatile compound emission by peptaibols proceed via an activation of phytohormones? Indeed, an increased endogenous concentration of jasmonic acid was found in the shoots.
Among other things, jasmonic acid stimulates the coiling of tendrils in plants. The researchers were able to see this reaction in shoots of wild hops and peas.
How do peptaibols bring on this reaction? Peptaibols form helical structures that lodge within cell membranes. When several of these coils lodge together, channels or pores are formed, which alter the porosity of the membrane, decreasing the ion gradient between the cell and its surroundings (membrane depolarization). Membrane depolarizations set in motion cascades of cellular signal transmissions, which can lead to the biosynthesis of phytohormones, for example.
Channel forming compounds do not merely play a role in the recognition of the fungal infestation of a plant: researchers have also found pore-forming substances in the salivary secretions of herbivorous insects. Boland suspects that these also substantially participate in the triggering of defense reactions in plants.