Agriculture Reference
In-Depth Information
than in
M. sativa
plants grown in soil amended with non-
Bt
maize biomass (Castaldini et
al., 2005). Although no mechanism for these results was identified, a change in nutrient
composition of
Bt
plant biomass, lignin content, or time required for degradation of trans-
genic plant material could potentially contribute to indirect effects on AMF in the soil.
Interestingly, when soybean was grown in soil inoculated with a strain of
Bacillus thuringi-
ensis
that expressed Cry1Ab protein
(Cry
+
), a
Bt
mutant bacterial strain that did not express
Cry protein (Cry
-
), purified ICP, or no treatment (control), Ferreira et al. (2003) found that
there was no effect of the addition of purified ICP on AMF, but that Cry
+
and Cry
-
bacterial
strains inhibited AMF compared to the control treatment.
cultivation of Bt crops on soil fungi
In general, there appear to be no negative effects of
Bt
plants,
Bt
proteins, or the cultiva-
tion of
Bt
crops on most free-living fungal species, including saprophytic and pathogenic
fungi. However, AMF appear to be sensitive to some
Bt
plant lines, perhaps because of
their symbiotic relationship with host plants. As the effect does not appear to be protein
specific, each
Bt
plant line containing a different insertion event should be screened for its
ability to form mycorrhizal relationships. Currently, there are at least 15 different lines of
Bt
maize genetically engineered to include single, stacked, or pyramided resistance traits,
yet few of these
Bt
cultivars have been evaluated for their ability to form symbioses with
AMF. Because of the initial indications that certain cultivars of
Bt
maize are poorly colo-
nized by AMF (Turrini et al., 2004; Castaldini et al., 2005; Cheeke et al., 2011; Cheeke et
al., 2012), and that each
Bt
line may have a different response to mycorrhizal fungi, it is
important to test the ability of each
Bt
line to form associations with AMF under the same
experimental conditions as it may not be possible to generalize about the effects of
Bt
maize on AMF, especially if the effect is not protein specific. As most agriculturally impor-
tant crops are mycorrhizal (excluding species of
Brassica
), even small impacts of
Bt
crops
on their fungal symbionts should be closely monitored, particularly in low-input farming
systems where reliance on a healthy soil community for plant health and nutrition is key.
More research, including screening of multiple
Bt
cultivars for AMF responsiveness and
molecular identification of the taxa of AMF colonizing
Bt
and non-
Bt
plant roots, should
be conducted to determine whether
Bt
plants with lower levels of AMF colonization also
have reduced diversity of AMF in their roots. If this is the case, it would then be pertinent
to evaluate changes in spore abundance and diversity of AMF over time in soils in regions
where
Bt
crops have been cultivated for multiple years.
Protozoa (amoebae, flagellates, and ciliates) and nematodes have an important role in the
soil ecosystem, primarily as grazers. By consuming bacteria and other soil organisms,
they release nutrients into the soil that can be used as a food source by other soil biota
or taken up by plant roots (see Chapter 1 in this volume by Coleman et al.). Protozoa are
unicellular organisms that are motile in both aquatic and soil environments and feed on
bacteria, small fungi, algae, and other protozoa. Nematodes are microscopic worms that
are classified into several trophic groups—bacterial feeders, fungal feeders, and root feed-
ers—primarily based their mouth parts (e.g., whether they have a sharp stylet for pierc-
ing root or fungal tissue). Because of their importance in nutrient turnover in soil food
webs and their close proximity to plant roots (reviewed by Bais et al., 2006), protozoa and
Search WWH ::
Custom Search