plant surface microbiology.pdf

122
Galdino Andrade
Some studies have assessed B. thuringiensis spore and vegetative cell survival
in the soil. The soil permanence of the protein crystal released by Bttransgenic
maize root exudates has also been assessed (Saxena et al. 1999).
The analysis of the soil stability of B. thuringiensis indicated that the bacterium
was not active, and that the number of cells inoculated either in the
vegetative or in the spore form decreased rapidly a few hours after inoculation.
However, the soil can be considered its natural deposit, as the spores are
released into the soil after the death of an insect and persist in this form for
several years until they find another host insect. The B. thuringiensis toxin
produced by Bt-maize is transferred to the soil by root exudates, pollen and
other plant parts. Tapp et al. (1995a) reported that the B. thuringiensis crystal
is adsorbed by the clay minerals in the soil, and thus may be protected from
biodegradation action of hydrolytic enzymes such as proteases, and may
remain active in the soil for several months.
Pesticides have protocols to evaluate non-target effects where many organisms
are used as biological indicators. Similar test protocols could be
extended to plant bio-insecticide manufacturers. The soil biological process is
poorly understood, and care should be taken to prevent further negative
impacts on soil ecosystems from genetically modified organisms. Although
these might be non-target events, all effects of Bt-plants on the soil environment
must be well understood.
2 General Aspects of Bacillus thuringiensis
The B. thuringiensis bacterium is a Gram positive rod, aerobic, chemoheterotrophic,
with perithiquious flagella that sporulates when the environmental
conditions are not favourable. The bio-insecticide protein is formed
when the sporulation event is activated and cells should be isolated from soil
or infected insects.
Meadows (1993) suggested three hypotheses for the natural habitat of B.
thuringiensis based on isolation studies. The bacterium could be an insect
pathogen, a component of the normal flora of the phylosphere tree species, or
a soil microorganism.
According to the first hypothesis, the release of the crystal would be a strategy
to kill the insect larvae, which would permit spore germination and vegetative
cell multiplication.
The second hypothesis was suggested by a study where great quantities of
B. thuringiensis were found in tree species, and could be disseminated by the
wind or rain, and the soil would only be a deposit where B. thuringiensis did
not multiply (Smith and Couche 1991).
These studies have shown that B. thuringiensis is widely distributed in soils
throughout the world. Wide distribution even in localities which cannot be
correlated with the presence of insects reinforces the hypothesis that the bac-