POPULATION DYNAMICS OF BIOCONTROL AGENTS AND PATHOGENS IN THE SOIL AND RHIZOSPHERE
Department of Plant Science, Macdonald Campus of McGill University, Ste. Anne de Bellevue, Quebec H9X 3V9, Canada
Understanding the dynamics between the pathogen and biocontrol agent (BCA) in the soil or in an infection site such as the rhizosphere is crucial for predicting the success of biological control. This is especially true for biological control that uses the strategy of reducing the initial inoculum prior to infection. By studying the population density fluxes over time, several questions can be answered. How much inoculum of the BCA must be added to reduce the pathogen below the damage or economic threshold? When and where should the BCA be added? How long does the BCA persist in the soil or in the formulated product? Can the BCA colonize the infection site and maintain high population densities to restrict infection by the pathogen? Does the BCA colonize internal plant tissues? The first question can be addressed by applying epidemiological models of dose-response relationships of the pathogen and BCA over a wide range of inoculum levels. Several recent models will be discussed, including models that also incorporate host resistance and environmental factors.
The most widely used methodology for population dynamic studies is still traditional plating techniques. However, the use of marked strains has greatly facilitated this technique. In addition to antibiotic resistance markers in bacteria, genetically engineered markers have been developed, such as LacZ for lactose utilization, InaZ for the production of ice nucleation protein and Lux for bioluminescence. Immunological techniques such as ELISA and immunofluorescence colony staining have also been applied to the detection and quantification of bacteria and, to a lesser extent, fungi. BCA fungi can also be marked by transformation with the ß-glucuronidase (GUS) gene and the hygromycin B resistance genes, providing a way to assess the populations and visualize the fungus in situ in the plant. However, the ecological fitness of transformed strains must be established. The use of reporter genes downstream from the promoters involved in biocontrol functions can give insights into the expression of biocontrol genes under natural conditions, without having to directly measure the gene product such as siderophores or antibiotics.
Molecular techniques such as DNA hybridization (dot-blot, reverse dot blot) and PCR techniques such as RAPD and AFLP provide a sensitive means of detecting the presence of pathogens and BCAs without culturing. However, quantification of the original DNA and extrapolation to the original population is problematic. The use of these techniques for large ecological studies are limited by the labour-intensive steps of DNA extraction from plant or soil samples and the need to run the amplified DNA on electrophoresis gels. However, simplified DNA extraction techniques and the use of fluorogenic probes in the PCR process (TaqMan) may reduce these obstacles. PCR techniques may also be used to target functional genes involved in biocontrol, such as genes for antibiotic production. In this way, population density of BCAs can be quantifyd based on function rather than on taxa.