Effect of transgenic alfalfa plants with introduced gene for Alfalfa Mosaic Virus coat protein on rhizosphere microbial community composition and physiological profile

The aim of this study was to evaluate the effect of transgenic alfalfa (Medicago sativa L.) plants, in comparison to their non-transgenic counterpart, on the density and physiological profiles of aerobic bacteria in the rhizosphere. Plants of transgenic alfalfa expressing the AMVcp-s gene coding for Alfalfa Mosaic Virus coat protein were cultivated in a climatic chamber. Two methods were used to determine the microbial diversity in rhizospheres of transgenic plants. First, the cultivation-dependent plating method, based on the determination of the density of colony-forming bacteria, and second, a biochemical method using the Biolog™ system, based on the utilization of different carbon sources by soil microorganisms. Statistically significant differences in densities of rhizospheric bacteria between transgenic and non-transgenic alfalfa clones were observed in ammonifying bacteria (GTL4/404-1), cellulolytic bacteria (GTL4/404-1, GTL4/402-2, A5-3-3), rhizobial bacteria (GTL4/402-2), denitrifying bacteria (A5-3-3) and Azotobacter spp. (GTL4/402-2). The highest values of substrate utilization by microbial communities and average respiration of C-sources were determined in non-transgenic alfalfa plants of the isogenic line SE/22-GT2. Carbohydrates, carboxylic acids and amino-acids were the most utilized carbon substrates by both Gram-negative and Gram-positive bacteria. Both, the community metabolic diversity and the utilization of C-sources increased in all alfalfa lines with culture time and regardless of transgenic or non-transgenic nature of lines.