The influence of fertilizer level and spore density on arbuscular mycorrhizal colonization of transgenic Bt 11 maize (Zea mays) in experimental microcosms

Abstract

Crop plants genetically modified for the expression of Bacillus thuringiensis (Bt) insecticidal toxins have broad appeal for reducing insect damage in agricultural systems, yet questions remain about the impact of Bt plants on symbiotic soil organisms. Here, arbuscular mycorrhizal fungal (AMF) colonization of transgenic maize isoline Bt 11 (expressing Cry1Ab) and its non-Bt parental line (Providence) was evaluated under different fertilizer level and spore density scenarios. In a threeway factorial design, Bt 11 and non-Bt maize were inoculated with 0, 40, or 80 spores of Glomus mosseae and treated weekly withor 'High' (1.87 g L À1 ) levels of a complete fertilizer and grown for 60 days in a greenhouse. While no difference in AMF colonization was detected between the Bt 11 and Providence maize cultivars in the lower spore/higher fertilizer treatments, microcosm experiments demonstrated a significant reduction in AMF colonization in Bt 11 maize roots in the 80 spore treatments when fertilizer was limited. These results confirm previous work indicating an altered relationship between this Bt 11 maize isoline and AMF and demonstrate that the magnitude of this response is strongly dependent on both nutrient supply and AMF spore inoculation level.

Field Evaluation of Arbuscular Mycorrhizal Fungal Colonization in Bacillus thuringiensis Toxin-Expressing (Bt) and Non-Bt Maize

“…Maize plants were harvested at three different physiological time points (60,90, and 130 days after sowing) to examine temporal differences in AMF colonization in each line of Bt and non-Bt maize and to evaluate potential differences in yield at the end of the season. Because we used the same Bt and non-Bt maize genotypes as in previous studies, we hypothesized that results from this field experiment would support findings of our greenhouse studies (10,11) and demonstrate that AMF colonization is lower for the Bt maize lines than for their non-Bt parental controls under field conditions. While we acknowledge that there are differences in soil properties and likely differences in AMF communities between our greenhouse and field studies, previous greenhouse studies, conducted in independent laboratories with different soils and different sources of AMF inocula (e.g., field soil and pure spores of Glomus mosseae), demonstrated an altered relationship between Bt maize and AMF (9)(10)(11)(12), providing evidence that AMF colonization can be reduced in Bt maize under at least some environmental conditions.…”

“…The symbiosis between maize and AMF can vary strongly depending on experimental or environmental conditions (10), and more generally the plant-AMF relationship can fluctuate along a parasitism-mutualism continuum (39). Arbuscular mycorrhizal fungi are considered parasitic when the net cost of the symbiosis exceeds net benefits for the plant and are mutualistic when both partners benefit from the relationship, although there have been some recent discussions of the use of these terms (40,41).…”

“…Under low-fertilizer conditions, AMF associations with Bt maize were significantly lower than those with the non-Bt parental (P) maize (10,11). When residual effects of the cultivation of Bt maize were tested with a subsequently planted crop (Glycine max; soybean), there was no difference in AMF colonization of G. max grown to maturity in Bt or non-Bt preconditioned soil (11).…”

“…Recent greenhouse studies demonstrated that AMF associations were reduced in multiple lines of Bt maize (9)(10)(11)(12) and that differences in AMF colonization between Bt and non-Bt maize can vary as a result of experimental and environmental conditions, such as spore density and fertilizer level (10). Under low-fertilizer conditions, AMF associations with Bt maize were significantly lower than those with the non-Bt parental (P) maize (10,11).…”

“…Plants supply carbon to the fungi, and fungi provide the plant with nutrients such as nitrogen and phosphorus and can improve drought tolerance (8). Recent studies have suggested that some types of Bt crops may have a negative impact on AMF (9)(10)(11)(12), although the mechanism is not yet known. Although there is no evidence for a direct effect of B. thuringiensis proteins on soil fungi, AMF may be uniquely sensitive to genetic changes within a plant because of their reliance on a host plant.…”