e U.S. Department of Agriculture's Agricultural Research Service in Gainesville, Florida, led a research team that spent three years systematically analyzing the biochemical response of cowpea (Vigna unguiculata), a legume, to herbivory and oral secretions of fall armyworm (Spodoptera frugiperda), a general crop pest. During the extensive project, the researchers conducted over 10,000 leaf bioassays, testing for plant phytohormone production after exposure to successively fractionated insect oral secretions, among other experiments. Painstakingly collected just a few microliters at a time, the team tested approximately one full liter of caterpillar secretions.

As previously reported, the scientists identified and isolated an 11 amino acid peptide, inceptin, that plays a pivotal warning role in cowpea plants being attacked by the fall armyworm. Inceptin is part of a larger, essential enzyme, chloroplastic ATP synthase, in plants. When the fall armyworm feeds on cowpea, the insect ingests ATP synthase 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 recontact the wounded leaf and alerts plants to generate a burst of defensive phytohormones.

In the June issue of Plant Physiology, Schmelz and his USDA collaborators, including Sherry LeClere, Mark Carroll, Hans Alborn, and Peter Teal, take the analysis further. They confirm inceptin's role as the dominant (and most stable) peptide in the cowpea's defense to fall armyworm. In addition, the researchers identify two related but less abundant peptide fragments (Vu-GE+In and Vu-E+In) that provoke similar defense responses in cowpea and a third (Vu-In-A) with no apparent effect. They also show that inceptin-related peptides spark a consistent, sequential cascade of phytohormone increases in cowpea, beginning with jasmonic acid, followed by ethylene and, lastly, salicyclic acid. Finally, the rese
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Contact: Dr. Eric Schmelz
Eric.Schmelz@ARS.USDA.GOV
352-374-5858
American Society of Plant Biologists
9-May-2007

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