Effect of a Sinorhizobium meliloti Strain with a Modified putA Gene on the Rhizosphere Microbial Community of Alfalfa

Abstract

The success of a rhizobial inoculant in the soil depends to a large extent on its capacity to compete against indigenous strains. M403, a Sinorhizobium meliloti strain with enhanced competitiveness for nodule occupancy, was recently constructed by introducing a plasmid containing an extra copy of a modified putA (proline dehydrogenase) gene. This strain and M401, a control strain carrying the same plasmid without the modified gene, were used as soil inoculants for alfalfa in a contained field release experiment at León, Spain. In this study, we determined the effects of these two strains on the indigenous microbial community. 16S rRNA genes were obtained from the rhizosphere of alfalfa inoculated with strain M403 or strain M401 or from noninoculated plants by amplification of DNA from soil with bacterial group-specific primers. These genes were analyzed and compared by restriction fragment length polymorphism and temperature gradient gel electrophoresis. The results allowed us to differentiate between alterations in the microbial community apparently caused by inoculation and by the rhizosphere effect and seasonal fluctuations induced by the alfalfa plants and by the environment. Only moderate inoculation-dependent effects could be detected, while the alfalfa plants appeared to have a much stronger influence on the microbial community.

Assessing the Risk of Biological Control Agents on the Indigenous Microbial Communities: Serratia plymuthica HRO-C48 and Streptomyces sp. HRO-71 as Model Bacteria

“…Similar results were obtained in other culture dependent (Walsh et al, 2001) and cultureindependent (Van Dillewijn et al, 2002;Viebahn et al, 2003) studies after the introduction of genetically modified bacteria on various plants. In these studies, seasonal shifts of community composition, but only minor, transient shifts due to the beneficial microorganisms, were detected.…”

Proline utilization system is required for infection by the pathogenic α-proteobacterium Brucella abortus

“…In S. meliloti, a DputA strain has a weakened ability to form root colonies [35] and, moreover, strains complemented with additional copies of putA are more competitive at establishing a symbiotic relationship with alfalfa [36,37]. The Put system has also been linked to infectivity in both Helicobacter hepaticas and Helicobacter pylori [28,29,38], and the Put system plays a role in regulating the cell cycle within Ehrlichia chaffeensis [39].…”

Impact of rhizobial inoculants on rhizosphere bacterial communities of three medicinal legumes assessed by denaturing gradient gel electrophoresis (DGGE)

“…DGGE has been employed to investigate the microbial communities within the different plant rhizospheres and to monitor the effects of microbial inoculants on the rhizosphere communities. van Dillewijn et al (2002) determined the effect of Sinorhizobium meliloti on the rhizosphere microbial community of its host plant, alfafa (Medicago sativa), and found that the plant appeared to have a much stronger influence on the microbial community compared with a S. meliloti inoculation. Schwieger and Tebbe (2000) studied the impact of rhizobial inoculant, S. meliloti L33, on the microbial communities of M. sativa rhizosphere and found that both plant species and inoculant had an effect on the rhizosphere structure.…”