Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 183

AMF arbuscules between Bt cotton (Cry1Ac, Cry2Ab) and non- Bt cotton. As each of these
studies differed in experimental conditions (soil type; plant genotype; species of AMF;
fertilizer level; spore number; location of study, i.e., microcosm, greenhouse, or field; AMF
detection technique; etc.), it has been difficult to identify the primary factors influencing
the patterns of AMF colonization in the different Bt cultivars.
When the environmental and experimental factors that may influence the symbiotic
relationship between Bt maize and AMF were evaluated in a greenhouse study, Cheeke
et al. (2011) found that significant differences in AMF colonization were only observed
between Bt maize (Event Bt 11) and its non- Bt isoline (Providence) when fertilizer levels
were limited and AMF spore density was high (80 spores of G. mosseae /pot). Under these
experimental conditions, Bt maize roots had less than 50% of the AMF colonization of
the non- Bt isoline ( Figure 8.1 ) . This study demonstrated that under circumstances where
AMF would be most likely to colonize and benefit the host plant (high spore density, low
fertilizer conditions), mycorrhizal colonization was limited in the Bt maize. This study
also showed that plant response to AMF is dynamic, and AMF colonization levels by G.
mosseae can vary, even within the same Bt or non- Bt maize cultivar, depending on the level
of fertilizer application or number of spores added to soil ( Figure 8.1 ) (Cheeke et al., 2011).
In a follow-up greenhouse study in which AMF colonization levels by fungi from field-
collected soil were evaluated in nine paired lines of Bt and non- Bt maize, Bt maize plants
overall had lower levels of AMF colonization in roots than the non- Bt maize plants when
fertilizer was limited (Cheeke et al., 2012).
The effect of Bt plant material incorporated into soil on AMF has also been investi-
gated. In a greenhouse study, Castaldini et al. (2005) observed that 4 months after Bt maize
biomass (Event Bt 11) was incorporated into soil, the percentage of root colonization by
AMF in Medicago sativa (alfalfa) was significantly lower in soil containing Bt maize residue
Figure 8.1 Percentage colonization by the AMF species Glomus mosseae in Bt maize (Event Bt 11)
and non- Bt maize (parental isoline, Providence) inoculated with 0, 40, or 80 spores of G. mosseae and
grown for 60 days in a greenhouse with weekly treatments of no (0 g/L), low (0.23 g/L), or high (1.87
g/L) complete fertilizer. Striped bars represent the means (±SEM) of Bt 11 plants, and solid bars
represent the means (±SEM) of non- Bt maize plants. * P ≤ 0.05, n = 5 for each bar.