Plants have the ability to defend themselves against herbivores despite being immobile researchers say. Sensing danger, plants often employ defensive tactics such as generating toxins or emitting volatile chemicals that attract pest's natural enemies.
In the June 2007 issue of Plant Physiology researchers reported to have identified a family of small peptide elicitors, or plant defense signals, which help plant's respond to insect attack.
The defense mechanism works as follows: proteins already present in the plant are ingested by the insect attackers. As the insects digest the proteins, they are converted into a peptide elicitor which other plants recognise when the insects feed on them. Detecting the insect-carried elicitor, the plant then launches its defensive chemistry.
Scientists have long known that some plants distinguish different insect attackers, however, this defensive behavior has proven difficult to describe at the molecular level. Few model systems have been utilised to characterise the potential interactions between what researchers estimate to be at least four million insects and 230,000 flowering plant species. In addition, highly active plant defense signals can occur at trace levels, too small to easily detect or isolate.
Dr Eric Schmelz at the United States Department of Agriculture's Centre for Medical, Agricultural and Veterinary Entomology, led a research team that spent three years analysing the biochemical response of cowpea plants, to herbivory and oral secretions of fall armyworm a general crop pest. During the term of the project, the biochemical activity of over 10000 leaves was analysed, testing for plant phytohormone production after exposure to successively fractionated insect oral secretions, among other experiments.
Previously scientists identified and isolated an amino acid called inceptin, that plays a pivotal warning role in cowpea plants being attacked by the fall armyworm. Inceptin is part of a larger, essential enzyme in plants. When the fall armyworm feeds on cowpea, the insect ingests the larger enzyme and breaks it down, releasing inceptin, which then becomes part of the armyworm's oral secretions. When the worm next feeds on cowpea, trace amounts of inceptin re-contact the wounded leaf and alerts other plants to generate a burst of defensive phytohormones.
Dr Schmelz and team, confirm inceptin's role as the dominant peptide in the cowpea's defense to fall armyworm. In addition, the researchers identify two related but less abundant peptide fragments that provoke similar defense responses in cowpea and a third with no apparent effect. They also show that inceptin-related peptides spark a consistent, sequential cascade of phytohormone increases in cowpea, in addition to determining inceptin's structure.
Building on this work, Dr Schmelz is now recruiting a post-doctoral scientist to help the team biochemically purify and identify the inceptin receptor from legumes.
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