Pathogen-induced plant chemical defense: Effect on insect herbivores and parasitoids

The study presented here is the first in which the production of volatile compounds by a single host-plant system in response to both insect herbivores and pathogens has been evaluated simultaneously. Additionally, it is the first study on the effect of previous pathogen infection on the production of plant volatiles in response to insect damage.; In my peanut-white mold-beet armyworm system, beet armyworm (BAW) feeding and oviposition preference and larval performance were all enhanced by the fungal-infection on peanut plants. This may indicate an interference of the fungal infection with the plant's direct chemical defenses against the herbivores. Peanut plants released a specific set of volatile chemical in response to white mold infection. This volatile profile differs qualitatively and quantitatively from signals emitted in response to BAW damage. Previous infection on the plant by the white mold fungus does not interfere with the emission of volatiles by the infected plant in response to BAW attack. The white mold-derived compound 3-octanone and the plant-produced volatile methyl salicylate were only recovered from plants infected with S. rolfsii. Thus, the presence of these compounds could potentially be used in the future for the detection of white mold-infected plants in the field. Although BAW preference and performance were enhanced by white mold infection on peanut plants, it does not appear that previous infection of the plant has any negative effect on the host searching ability of its parasitoid, Cotesia marginiventris.; Finally, I found that pepper plants under simultaneous attack by leaf spot bacteria, Xanthomonas campestris pv. vesicatoria (pepper race 3) (XCVP3) and BAW were able to produce a volatile profile qualitatively similar to, but quantitatively greater than, that produced by plants under insect damage alone during the 4 days after bacterial inoculation. However, after 4 days from bacterial inoculation, the amount of volatiles emitted by XCVP3-infected plants in response to BAW infestation was significantly lower than that produced by healthy plants in response to BAW damage. Analyses of the levels of signaling hormones jasmonic acid (JA) and salicylic acid (SA) in these plants revealed that the reduction in volatile production in response to insect damage was preceded by a significant increase in conjugated SA, and consequent release of methyl salicylate by plants under the combined bacterial/insect attack. (Abstract shortened by UMI.)...