Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 181

community composition between Bt and non- Bt rice straw samples as determined by
T-RFLP, although there were some differences in fungal community composition at the
early stage of rice root decomposition (Lu et al., 2010a). In a greenhouse study, Tan et al.
(2010) reported that neither actively growing Bt maize nor the incorporation of Bt maize
biomass (leaves and straw) had a negative effect on fungal community structure in soil as
determined by PCR-DGGE and sequences of 18S rRNA genes. Although Xue et al. (2011)
found a minor effect (1 of 16 comparisons) of Bt maize biomass buried in litterbags on fun-
gal decomposer communities as determined by T-RFLP, the differences were mostly due to
environmental factors (i.e., litterbag placement, recovery year, and plot history) and were
not a result of Cry3Bb protein in the Bt maize.
In growth chamber experiments, Saxena and Stotzky (2001a) found no difference
in the numbers of selected culturable Zygomycetes, Ascomycetes, Deuteromycetes, and
yeasts in rhizosphere soils cultivated with Bt and non- Bt maize. However, Blackwood and
Buyer (2004) reported that soils cultivated with Bt maize expressing Cry1Ab and Cry1F
reduced the presence of eukaryotic PLFA in bulk soils compared with soils cultivated
with non- Bt maize, although it was not clear which groups of eukaryotes were affected. In
field soils cultivated with Bt and non- Bt maize, Icoz et al. (2008) and Oliveira et al. (2008)
found no consistent effect of Bt maize cultivation on culturable fungi. Similarly, there was
no difference in fungal-to-bacterial ratio reported between field soils cultivated with Bt
maize and non- Bt maize as determined by PLFA and culturing methods (Xue et al., 2005).
Li et al. (2011) reported a seasonal variation in numbers of CFUs of culturable fungi in soils
cultivated with Bt and non- Bt cotton over a 3-year field study, but there was no negative
effect of Bt cotton cultivation on soil fungi.
Although it is not surprising that Bt proteins in transgenic plant material have little or
no direct effect on saprotrophic fungi, it was hypothesized that the higher lignin content
reported in the biomass of some Bt cultivars (Saxena and Stotzky, 2001b; Stotzky, 2004;
Flores et al., 2005; Poerschmann et al., 2005) might take longer for fungi to degrade, thus
leading to accumulation of Bt plant residue in the soil over time. This has turned out not
to be true most of the time. Although one study showed that some Bt plants, including
maize, canola, potato, rice, and tobacco, decomposed less in soil and linked this effect
to the higher (although not always significantly higher) lignin content in each of the Bt
cultivars tested (Flores et al., 2005), several subsequent studies have reported that Bt plant
residue does not generally decompose more slowly than non- Bt plant material (Lehman
et al., 2008; Tarkalson et al., 2008; Kravchenko et al., 2009; Wu et al., 2009). Moreover, some
studies have not even been able to detect a difference in lignin content between Bt and
non- Bt cultivars (Jung and Sheaffer, 2004; Mungai et al., 2005; Lang et al., 2006). These con-
trasting reports may be the result of differences in age of the plants, detection techniques,
or cultivar type, highlighting the importance of a plant line-specific, multidetection, mul-
tiple sampling time approach to determine more accurately the effects of the composition
of Bt crop material on organisms in the soil ecosystem.
Parasitic and pathogenic fungi are also prevalent in soil, and although not desir-
able in agricultural systems, they may also be affected by changes in the physiology of
crop plants. However, during the plant selection process when GM plants are developed
and tested, it is unlikely that genotypes that are more susceptible to disease would be
released for commercial application. One study that examined the effects of Bt crops on
fungal pathogens found that genetically engineered Bt potato had no negative effect on
soilborne pathogens in the rhizosphere, including Fusarium sp., Pythium sp., Verticillium
dahliae , potato leaf roll virus, and potato virus Y, under field conditions (Donegan et al.,
1996). When fungal growth and survival of the plant pathogen Fusarium graminearum and