Agriculture Reference
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assays, although there were seasonal variations not related to the
Bt
or non-
Bt
cultivars.
When soil enzyme activities (phosphatase activity, dehydrogenase activity, respiration,
and methanogenesis) and microbial community composition were compared in the rhi-
zosphere of
Bt
rice, non-
Bt
parental rice, and non-
Bt
parental rice treated with the insecti-
cide Triazophos at multiple sampling times, there were generally no significant negative
effects detected on soil enzyme activity or microbial community structure as determined
by DGGE and T-RFLP (Liu et al., 2008). There were, however, seasonal variations in the
selected enzyme activities and microbial community composition in the rhizosphere over
the course of the 2-year experiment (Liu et al., 2008).
There was no negative effect of 4 years of cultivation with
Bt
spruce (engineered to
express Cry1Ab) on nitrogen-fixing bacteria compared with non-
Bt
white spruce trees as
determined by molecular sequencing of a region of the nitrogenase reductase gene from
genomic DNA extracted from rhizosphere soil (Lamarche and Hamelin, 2007). There
were also minimal differences in culturable aerobic bacteria in rhizosphere soil cultivated
with
Bt
potato, non-
Bt
russet potato treated with insecticide (Di-Syston), and non-
Bt
rus-
set potato treated with microbial
Bt
(M-Trak) (Donegan et al., 1996). When the microflora
colonizing the leaves of these potato plants were compared over multiple time points
(0, 21, 42, 63, and 98 days), Donegan et al. (1996) found few significant differences across
potato cultivars.
These, and other, results indicate that, in general, the insecticidal
Bt
proteins, either
purified or expressed in transgenic
Bt
plants, have no significant negative effects on most
soil bacteria. However, in the few studies in which effects of cultivation of
Bt
plants on soil
microbes were observed (e.g., Donegan et al., 1995; Wu et al., 2004a, 2004b; Castaldini et
al., 2005; Rui et al., 2005; Xue et al., 2005; Fang et al., 2007; Sun et al., 2007; Chen et al., 2011),
differences in physiological properties within plants resulting from the genetic insertion
may be implicated (e.g., Donegan et al., 1995; Rui et al., 2005). Genetic alterations, as a
result of the insertion of
Bt
genes, that produce a change in plant root exudates or quality
of plant material, for example, may influence microbial growth and species composition
in the rhizosphere or affect the degradation time or quality of
Bt
plant litter. In this way,
microbial communities could be affected by the cultivation of transgenic
Bt
crops without
being negatively affected by
Bt
proteins directly. Fluxes in microbial community structure,
however, can also be influenced by soil type, temperature, season, plant type, and other
biotic and abiotic factors (e.g., Griffiths, 2000; Lottman, 2000; Kowalchuk, 2002; Dunfield
and Germida, 2003; Zwahlen, Hilbeck, Gugerli, et al., 2003; Blackwood and Buyer, 2004;
Icoz and Stotzky, 2008b). Thus, where an impact of the cultivation of a
Bt
crop on soil bac-
teria has been detected, the ecological significance has often been difficult to assess.
cultivation of Bt crops on soil bacteria
Many of the studies evaluating nontarget effects of
Bt
crops or
Bt
proteins on soil bacte-
1% of bacterial taxa are thought to be culturable (e.g., Handelsman and Tiedje, 2007), this
methodology could influence the results of many of these studies. Thus, differences in
laboratory techniques may also have a role in the different outcomes of similar studies
evaluating the effects of
Bt
crops on microbial communities (e.g., plating vs. DGGE vs.
metabolic analysis). To evaluate the nontarget effects of the cultivation of
Bt
crops on soil
bacteria, multiple detection methods should be employed as most microbes are not cul-
turable and could be better identified, quantified, or characterized using a combination
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