This is the report of the first open field release of genetically modified
microorganisms (GMMs) in Italy. It covers ten years of monitoring, and
follows in-field GMM dynamics from strain release to disappearance below
detection limits, as well as assessment of impact on resident
microorganisms. The bacteria released belong to the nitrogen fixing legume
endosymbiont Rhizobium leguminosarum bv. viciae, and were engineered with non-agronomically-proficient
traits, in order to assess their behavior and fate without GMM-specific
positive feedback from the plant. A DNA cassette containing mercury
resistance and ß-galactosidase genes was introduced in either
plasmid-borne or chromosomally integrated versions, in order to test the
resulting strain stability. A synthetic promoter was used to drive the
lacZ gene, conferring high catabolic activity to the GMM. Two different
wild-type Rhizobium backgrounds were tested, comparing a non-indigenous vs. an
indigenous, highly competitive strain. The latter had much greater
persistence, since it was able to survive and establish at technically
detectable levels for over four years after release. Selection factors, such
as reiterated presence of the plant host, or lactose substrate supply,
enhanced long-term survival to different extents. The lactose treatment
showed that even a single trophic supplementation can surpass the benefits
of symbiotic interaction for a period of several years. Concerning impact,
the GMMs did not alter substantially the other soil community general
microbiota. However, there were some significant differences in microbiota
as a consequence of the Rhizobium inoculation. This effect was observed with either
the WT or GMM, and was more evident in the release of the indigenous Rhizobium.
Moreover, as the indigenous GMM had its parental, dominant wild-type in the
same soil, it was possible to evaluate to what extent the GMM version could
result in parent displacement (“self-impact”), and how much the two
rhizobia would additively contribute to nodulation.