Wednesday, April 02, 2008

Rhizosphere bacterial communities associated with insect root herbivory of an invasive plant, Euphorbia esula/virgata


The invasive perennial plant of Eurasian origin Euphorbia esula/virgata 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. Possible effects of rhizosphere bacteria on biocontrol success were investigated by comparing rhizosphere communities of bacteria associated with E. esula/virgata populations with a resident population of Aphthona with those without detectable insect presence. Hypotheses investigated were 1) that insect herbivory accelerates negative feedback or 1A) shifts in prokaryote community structure and that 2) E. esula/virgata rhizospheres at sites with insects but without biocontrol impact harbored distinct communities or constituent species. Sites representative of high biocontrol impact or a low level of impact were assayed by spiral plating root washes of E. esula/virgata, selecting colonies from the most dilute portion of the spiral (deemed as predominant), and with resulting pure cultures obtaining the results of GN or GP Biolog procedures or GC-FAME analysis to identify the isolates and further characterize community structures using principle component analysis. Bacterial species predominant at the unimpacted site included Stenotrophomonas maltophilia, Pseudomonas chlororaphis and P. putida. At biocontrol sites exhibiting rapid stand declines, high percentages were Bacillus spp. and several coryneform species. Some of the isolates from the impacted site tested in on E. esula/virgata caused ca 65% reduction in biomass in combination with Aphthona. Overall, at sites with insect presence, isolates identified as Pseudomonas syringae and Erwinia carotovora were exclusive constituents of rhizosphere communities. Authors : Caesar, Anthony, Caesar, Thecan

No comments: