Summary
Using transgenic crops expressing Bacillus thuringensis-derived insecticidal proteins versus
conventional crops involves a reduced need for conventional insecticides, providing benefits
for human health and the environment. At the same time, our understanding of soil nutrient
transformations and soil-plant-microbe interactions in Bt-crop versus conventional crop
systems is incomplete. Thus, we studied potential adverse effects of Bt-maize on beneficial,
plant and soil associated microbial communities through the analysis of specific microbial
groups performing important functions in the soil-plant-microbial-system.
Three successive vegetation periods were considered: In the first year of the study, three
transgenic, Bt-toxin expressing maize lines, including DK3945 MON 88017, DK 3945 MON
89034 and the stacked event DK 3945 MON 88017x89034, were planted alongside with the
isogenic line DK315 (=DK3945) and the DKC3420, DKC5143 and Antares conventional lines
on two different soils (Fischamend and Tulln). Rotational cropping of horse bean (Vicia faba
L.) was done in the second year and repeated maize cultivation (using only the isogenic and
conventional lines) was done in the third year. In the first year, four replicate plants per maize
line and soil were grown individually in pots, which were all situated in a containment tunnel
to prevent pollen translocation. Plants were sampled for community analysis of bacterial and
fungal endophytes from maize roots; and endophytic bacteria were isolated.
50 bacterial endophyte isolates were analyzed from each of the seven maize varieties, which
comprised in total 143 IGS-types with affiliations to the Actinobacteria, Alpha-, Beta-, and
Gamma-Proteobacteria, Firmicutes (Bacilli), and Cytophaga-Flavobacterium-Bacteroides
phylogenetic groups. Isolates gained from all seven maize lines were similar in phylogenetic
diversity and richness, and they performed similarly regarding plant-beneficial functions.
Isolates mainly among the Actinobacteria and Alpha- and Gammaproteobacteria showed 1aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, while mainly members of
the Alphaproteobacteria were IAA producers. In cultivation-independent analysis, both
bacterial and fungal endophytic communities were differentially composed according to the
respective host maize lines. Evidently, the endophytic communities derived from Bt-maize
lines were not specifically discriminated against those from conventional and isogenic lines,
and they did not diverge regarding phylotype diversity. PCA (phenazine-1-carboxylic acid)
and Phl (2,4-diacetylphloroglucinol) genes were PCR amplified from the endophytic
communities derived from all maize lines. AcdS genes, encoding ACC deaminase, however,
were present in too low copy number for reliable detection with available primers.
The dynamics in soil nitrogen (N) cycling was monitored in the soils following the harvest of
the maize, when residues from the various maize lines and from the rotational crop were
mineralized. The soil mineral N content exhibited a pronounced temporal dynamics, but
overall soil ammonium and nitrate concentrations did not differ in Fischamend versus Tulln
soils or among maize lines. N availability measured as N mineralization potential was
generally higher in Tulln than Fischamend soils but did not differ among soils growing
different maize lines in the first year of the study, including Bt- and non-Bt lines. Neither did
soils growing contrasting maize varieties differ in bacterial or archaeal amoA or nirS copy
numbers, indicating that nitrifying and denitrifying microorganisms were equally abundant.
Thus, no differential effects on microbial N cycling were evident in soils following cultivation
of Bt- versus non Bt-maize varieties.