Agriculture Reference
In-Depth Information
of molecular and metabolic tools. Risk assessment studies should also be conducted at
multiple levels (laboratory, greenhouse, and field), and under different experimental and
environmental conditions, as it is well known that bacteria and other soil organisms can
be easily influenced by a multitude of biotic and abiotic factors that may not be associ-
ated with GM plants. As relatively few significant negative impacts of
Bt
crops have been
reported for soil bacteria, it may be more useful to focus future research efforts on other
soil organisms that are more closely associated with roots (e.g., mycorrhizal fungi) or nar-
rowing the scope of bacterial research to investigate the effects of transgenic crops on
nitrogen-fixing bacteria, many of which can form symbiotic relationships with plant roots
and have clear benefits for plant health and ecosystem function by providing nutrients
to plants and protection against plant pathogens (e.g., reviewed in Sessitsch et al., 2002;
Dobbelaere et al., 2003; Hayat et al., 2010).
Soil fungi, including saprotrophic, parasitic/pathogenic, and mycorrhizal fungi, are
another group of soil organisms that may be affected by cultivation of transgenic
Bt
crops
clers, plant symbionts, and plant pathogens. Saprotrophic fungi (also called saprophytic
fungi) are the primary group of soil organisms that degrade organic material in agricul-
tural fields, whereas parasitic/pathogenic fungi and mycorrhizal fungi are found in close
association with living plant roots in the rhizosphere.
Saprotrophic fungi are free-living soil organisms that obtain their nutrients from
dead organic material, such as leaves, wood, and other plant and animal materials, and
are responsible for recycling a significant amount of the carbon in the soil ecosystem. To
date, most studies have determined that there is no significant negative effect of purified
Bt
proteins,
Bt
crop cultivation, or
Bt
plant material on saprotrophic fungi or culturable
fungi (
Table 8.4
) (e.g., Saxena and Stotzky, 2001a; Koskella and Stotzky, 2002; Ferreira et
al., 2003; Icoz et al., 2008; Oliveira et al., 2008). For example, when soil was amended with
purified Cry1Ab or Cry1Ac
protein, there was no difference in the population levels of
culturable fungi using selective plating compared with control soil (Donegan et al., 1995).
There was also no effect of the addition of purified
Bt
proteins (Cry1Ab, Cry3A, or Cry4)
on the growth of representative culturable fungi (
Cunninghamela elegans, Rhizopus nigricans,
Aspergillus niger, Fusarium solani, Penicillium
sp
., Saccharomyces cerevisiae,
and
Candida albi-
cans
) in pure and mixed cultures (Koskella and Stotzky, 2002).
Where significant effects have been reported, they have often been minor or transient.
When soybean was grown in soil inoculated with a strain of
Bacillus thuringiensis
bacteria
that expressed Cry1Ab protein
(Cry
+
), a
Bt
mutant strain that did not express Cry pro-
tein (Cry
-
), purified insecticidal crystal protein (ICP), or no treatment (control), there was
no significant difference in the culturable fungal populations between rhizosphere soils;
however, there was a transient increase in some functional groups (saprophytic, amylo-
lytic, cellulolytic, and proteolytic fungi) between the treatments as determined by selective
plating, at the beginning of the experiment (Ferreira et al., 2003). Donegan et al. (1995) also
reported a transient increase in culturable fungi in soil amended with
Bt
cotton based on
selective plating methods. However, Flores et al. (2005) and Saxena and Stotzky (2001a)
found no difference in culturable fungi between soils amended with
Bt
and non-
Bt
maize
in soil microcosms. There were also no negative effects on the numbers of culturable fungi
in flooded soils amended with
Bt
versus non-
Bt
rice straw in laboratory experiments (Wu
et al., 2004b). In litterbag decomposition field studies, there was no difference in fungal
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