Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.
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