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Caterpillars off their food – must be too much sun
There is a growing body of evidence that exposure of plants to UV-B radiation makes them less appetizing for at least some herbivores. One possible reason is that UV-B elicits synthesis of compounds that are also defences against herbivory. This idea has been tested in a series of experiments carried out on two annual species, Nicotiana attenuata and N. longiflora, by Izaguirre et al. (Buenos Aires, Argentina and Jena, Germany, pp. 103–109). Experiments with N. attenuata were carried out in glasshouses; exposure to levels of UV-B typical of growth outdoors was achieved with a UV lamp and appropriate filters. Nicotiana longiflora was grown outdoors and here it was non-UV-B conditions that were obtained by using filters. To study the response to herbivores, a simulated herbivory treatment, already shown to elicit defence responses, was employed. The results were very clear. Although baseline levels of phenolic compounds were different, levels increased significantly in the two species in response to both UV-B exposure and herbivory. Analysis of the phenolic compounds by HPLC and by electrospray ionization MS showed that the variety and relative amounts of phenolic compounds elicited by the two experimental treatments were very similar. The most abundant of the phenolics were chlorogenic acid (three different isomers) and the flavanoid, rutin; polyamines (particularly isomers of dicaffeoylspermidine) were also present. In addition to being UV-absorbent, chlorogenic acid is also a powerful antioxidant, thus protecting plants from the oxidative damage that may follow UV-exposure. In addition, chlorogenic acid is an anti-feedant and there is strong negative correlation in Nicotiana between the concentration of chlorogenic acid and the feeding activity of Manduca sexta caterpillars. It looks very much then as if the responses of Nicotiana to UV-B exposure and to herbivory are indeed achieved at least partly via a common signalling pathway, most probably involving jasmonic acid.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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