Indigenous microflora and bean responses to introduction of genetically modified pseudomonas fluorescens strains into soil contaminated with copper

Abstract

A laboratory study was carried out to evaluate survival of genetically altered strains of Pseudomonas fluorescens in the rhizosphere of bean and in bulk soil, and the effect on indigenous microflora. The strains were released into uncontaminated soil and soil contaminated with copper. Numbers of plasmid RP4 donor, recipient and transconjugant cells were determined after 10 and 28 days. The impact on indigenous microflora was examined in terms of changes in total numbers of heterotrophic and Gram‐negative bacteria, and shifts in microbial community structure. Microbial community structures were determined using a method based on the r‐ and K‐strategists concept. The reaction of bean seedlings to the introduced strains in uncontaminated soil and soil contaminated with copper was also studied. Higher copper dose (2.5 mg g^‐1) markedly decreased the numbers of donor and recipient cells while transconjugant cells were not found at all. The donor carrying plasmid RP4 survived better than the recipient in the rhizosphere and bulk soil. The introduced strains of P. fluorescens did not cause significant changes in the numbers of indigenous heterotrophic and Gram‐negative bacteria in the rhizosphere and bulk soil. The numbers of these bacteria increased in response to soil contamination with copper. In the rhizosphere, the introduced strains of P. fluorescens did not affect the structure of indigenous heterotrophic and Gram‐negative bacteria belonging to slow growing K‐strategists. Soil contamination with copper decreased the growth of indigenous microflora. In the bulk soil, short‐term shifts towards fast growing r‐strategists was observed. However, K‐strategists dominated in soil contaminated with copper, especially in samples also inoculated with P. fluorescens strains. The introduction of P. fluorescens strains into the soil did not affect pheno‐typic diversity (eco‐physiological index) of indigenous microflora in the rhizosphere and bulk soil. Changes in bean shoots and roots were observed after 28 days. The data suggest that the genetically altered pseudomonads introduced did not affect the stability of indigenous microflora in the rhizosphere and bulk soil, nor plant growth.

Key words:

• community structure
• introduced pseudomonads
• soil contamination