Bacterial and fungal diversity effects and the activity of biocontrol agents in the rhizosphere of crop plants

Bacterial biocontrol agents have previously been identified as alternatives to agrochemicals for the control of crop diseases. The advent of genetic engineering and the identification of the modes of biocontrol that bacteria possess has allowed the potential modification of successful inocula to improve their efficacy in biocontrol. A plant growth promoting rhizobacterium (PGPR) Pseudomonas fluorescens SBW25 (WT) was modified to contain the phenazine-l-carboxylic acid biosynthesis pathway. The genetically modified biological control agent (GM-BCA) 23.10 was previously shown to have improved biocontrol activity when compared to WT. The work described in this thesis assessed the natural microbial community structure and function in the rhizosphere of pea, wheat and sugar beet under field conditions and the potential impact of the release of a GM-BCA may have on these communities, using laboratory based mesocosms. In addition to this, an investigation in to the production and conservation of HCN biosynthesis was undertaken on global collection of biocontrol strains to assess the potential of HCN biosynthesis as another trait for genetic modifications. A detailed field study of the indigenous rhizosphere communities was undertaken using culturable microbiological techniques, carbon source utilization and the development of molecular community profiling approaches on bacteria and fungi. A dominating effect of the plant species was shown and specific enrichment was identified on soil communities with significant effect also seen relating to plant growth stage. These data provided a robust baseline data set for comparisons to mesocosm experiments assessing the potential ecological impact that the release of a GM-BCA may have on the environment. This mesocosm study demonstrated plant growth stage effect dominated shifts in community profile with transient perturbations of indigenous community structure and function identified as a result of the inoculation. The significant change with greatest persistence was the replacement of a large proportion of 7-proteobacteria communities by the inocula