Abstract
BackgroundAphids are agricultural pests of great economical interest. Alternatives to insecticides, using semiochemicals, are of difficult applications. In fact, sex pheromones are of little use as aphids reproduce partenogenetically most of the time. Besides, the alarm pheromone, (E)-ß-farnesene for a great number of species, is difficult to synthesize and unstable in the environment. The search for novel semiochemicals to be used in population control can be efficiently approached through the study of the olfactory system at the biochemical level. Recently odorant-binding proteins (OBPs) have been shown to play a central role in olfactory recognition, thus becoming the target of choice for designing new semiochemicals.Methodology/Principal FindingsTo address the question of how the alarm message is recognised at the level of OBPs, we have tested 29 compounds, including (E)-ß-farnesene, in binding assays with 6 recombinant proteins and in behaviour experiments. We have found that good repellents bind OBP3 and/or OBP7, while non repellents present different spectra of binding. These results have been verified with two species of aphids, Acyrthosiphon pisum and Myzus persicae, both using (E)-ß-farnesene as the alarm pheromone.ConclusionsOur results represent further support to the idea (so far convincingly demonstrated only in Drosophila) that OBPs are involved in decoding the chemical information of odorants and pheromones, and for the first time provide such evidence in other insect species and using wild-type insects. Moreover, the data offer guidelines and protocols for the discovery of potential alarm pheromones, using ligand-binding assays as a preliminary screening before subjecting selected compounds to behaviour tests.
Highlights
Aphids represent one of the major pests in agriculture, and their populations cannot be controlled without the extensive use of insecticides
In a first investigation limited to three odorant-binding proteins (OBPs), we have found that (E)-ßfarnesene and related compounds bind to only OBP3, suggesting that this protein might be involved in the recognition of the alarm pheromone, and that activation of other OBPs could produce a response pattern that is no longer recognised as an alarm signal [26]
Behaviour experiments To investigate whether OBP3 or OBP7 or both could be involved in detecting the signal of alarm pheromones, we performed behaviour experiments with all the compounds used in binding studies, using two species of aphids, A. pisum and M. persicae
Summary
In our previous paper we had suggested that OBP3 could be involved in the perception of (E)-ß-farnesene. The novel concept that we had proposed was that recognition of the alarm pheromone could be based not on a high affinity of this compound for OBP3, but rather on the specific binding to only such OBP among the three tested in that report [26]. The behaviour data reported for two species of aphids, indicate a consistency between repellency and binding characteristics Based on these six OBPs, that are representative of the small aphids repertoire of these polypeptides, we can propose a model that allows us to predict, with reasonable confidence, whether a new chemical compound could be a repellent (for aphids that use (E)-ß-farnesene as the alarm pheromone), based on binding experiments. Given the above limitations and caveats, our data provide for the first time a good correlation between behavioural effects and binding properties of odorants and further
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