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CONJUGAL GENE TRANSFER IN THE RHIZOSPHERE OF WATER GRASS (ECHINOCHLORA CRUSGALLI): INFLUENCE OF ROOT EXUDATE AND BACTERIAL ACTIVITY
Citation:
Kroer, N., T Barkay, S. Sorenson, AND D. E. Weber. CONJUGAL GENE TRANSFER IN THE RHIZOSPHERE OF WATER GRASS (ECHINOCHLORA CRUSGALLI): INFLUENCE OF ROOT EXUDATE AND BACTERIAL ACTIVITY. Presented at Biotechnology Risk Assessment: Proceedings of the Biotechnology Risk Assessment Symposium, Pensacola, FL, June 06 - 08, 1995.
Impact/Purpose:
Presentation
Description:
The premise that genetic exchange is primarily localized in niches characterized by dense bacterial populations and high availability of growth substrates was tested by relating conjugal gene transfer of an RP4 derivative to availability of root exudates and bacterial metabolic activity. The rhizosphere of the marsh plant, Echinochlora crusgalli. was used as a model system. Pots containing muffled sand with and without a 2 week old plant were inoculated with 10 to the seventh power to 10 to the eight power donors (Pseudomonas fluorescens) and recipients (Serratia sp.) per gram of sand. To eliminate the indigenous bacterial flora, seeds were sterilized with AgNO3. Production of root exudates was demonstrated by measuring changes in DOC concentration in plant nutrient medium during plant growth. To demostrate bacterial utilization of exudates, incorporation of 14C into donors and recipients from 14C-labelled exudates was determined. Metabolic activity of donors and recipients was measured by incorporation of tritiated leucine. The presence of plants greatly stimulated conjugal transfer. Apparent transfer frequencies (T/D*R) in the rhizosphere were 4-6 and 6-7 orders of magnitude higher than in sand of pots with or without a plant, respectively. Donors and recipients were able to utilize up to 14% of the root exudates (assuming a 50% growth efficiency). Average leucine uptake rates (fmoles leu CFU-1 h-1) were 1.1 x 10 to the negative third power for rhizosphere, 4.5 x 10 to the negative fourth power for sand of pots with plant, and 3.1 x 10 to the negative fourth power for sand of pots without plant. These studies show that the rhizosphere of water grass is a niche were conjugal gene transfer is enhanced possibly due to stimulation of bacterial metabolism by root exudates.