ARS | Publication request: Rhizosphere bacterial communities associated with insect root herbivory of an invasive plant, Euphorbia esula/virgata

Submitted to: Biocontrol Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 4, 2007
Publication Date: July 1, 2008
Citation: Caesar, A.J., Caesar, T. 2008. Rhizosphere bacterial communities associated with insect root herbivory of an invasive plant, Euphorbia esula/virgata. In: Sforza, R., Bon, M.C., Evans, H.C., Hatcher, P.E., Hinz, H.L. and Rector, B.G., editors. Proceedings of the XII International Symposium on Biological Control of Weeds, April 22-27, 2007, Montpellier, France. p. 13-19.

Interpretive Summary: The question being pursued by this research was whether herbivory would cause a different population makeup or structure of root zone bacterial species than on roots of plants free of herbivory. This constitutes basic analysis of how insects may affect root zone microbes through stimulation of root exudates or leakage from damaged roots. This information in turn may help determine whether the biotic after effects of biocontrol may determine patterns of plant succession. Root zone-dwelling bacteria which were present in the highest population levels (termed predominant by us) on the exotic and invasive perennial leafy spurge were identified. Communities of predominant bacteria species found on roots of spurge from stands with detectable adults of the leafy spurge flea beetle Aphthona were compared to such communities occurring on roots from spurge stands without the flea beetle. Bacteria were identified by the distinct profile of fatty acids found in the cells of each species. The method also allowed comparison among isolates of single species based on whether they were from roots of spurge with and without flea beetle herbivory. Some species were associated exclusively with insect presence and there were distinctions within a single species of based on their origin from herbivory affected stands or not.

Technical Abstract: The invasive perennial plant of Eurasian origin, Euphorbia esula/virgata L., has been successfully controlled over large areas in North America with a synergism between larvae of Aphthona spp. and soilborne plant pathogens. However, a multitude of sites is not yet under control. Studies are needed on how flea beetle root herbivory may alter the microbial ecology of the rhizosphere of E. esula/virgata and how the resulting rhizosphere community may affect the synergism. Studies were undertaken at Theodore Roosevelt National Park from 2001-2003 to identify the predominant culturable prokaryotic species found in the rhizospheres of E. esula/virgata. The hypothesis was that distinct rhizosphere communities of E. esula/virgata would be associated with root herbivory by the flea beetle Aphthona compared to rhizospheres of E. esula/virgata from stands without insect presence. Stands with and without resident populations of Aphthona spp. were assayed by spiral plating root washes of E. esula/virgata and selecting colonies from the most dilute portion of the spiral (deemed as predominant). Fatty acid methyl ester (GC-FAME) analysis was performed on the resulting pure cultures to identify the isolates and further characterize community structures using principle component analysis. Pseudomonas syringae van Hall, P.cichorii (Swingle) Stapp, Erwinia chrysanthemii Burkholder, all plant pathogens, were associated exclusively with herbivory by Aphthona flea beetles. Conversely, Variovorax Willems et al. 1991 and Aquaspirillum Hylemon et al. 1973 spp. were a greater proportion of predominant species from roots without Aphthona present. There were also differences in the occurrence of the root pathogen antagonistic Pantoea agglomerans Gavini et al., 1989 and Stenotrophomonas maltophilia (Hugh 1981) Palleroni and Bradbury 1993.