plant surface microbiology.pdf

7 The Functional Groups of Micro-organisms and Biotech Products 129
might be explained by degrading enzymes produced (Cody 1989) or another
compound that would attack the fungus cell wall or inhibit the fungal growth.
Probably, these enzymes and other metabolites are produced at a vegetative
phase when B. thuringiensis multiplies, nevertheless, this does not happen in
the soil (Thomas et al. 2000; Vilas Bôas et al. 2000). However, much of the cell
contents were released during the cell/spores lysis after inoculation and could
have a suppressor effect on soil microbiota.
The B. thuringiensis effect on plant growth was observed only in plants
inoculated with Cry+ strain and ICP.Although the Cry+ strain inhibited mycorrhizal
colonisation, plant growth was not affected, possibly because soil fertility
status and nitrogen fixation were not affected by B. thuringiensis inoculum.
The same results were found by Reddy and Rhae (1989) with B. subtilis
and other rhizobacteria.
AM fungi were suppressed by B. thuringiensis inoculum due to the use of
spores as inoculum, but due to the fact that B. thuringiensis is found in low
numbers in the rhizosphere, it is difficult to explain the inhibitory effect
mechanism. Other authors (Andrade et al. 1995; Bethlenfalvay et al. 1997)
found the same inhibitory effect by Bacillus spp. on mycorrhizae fungi
colonising pea plants. Some strains of Bacillus spp. can probably suppress the
release of AM fungi and other soil microorganism cellular contents, but this
subject needs more investigation to conclude the mechanisms involved.
The present data provide evidence that B. thuringiensis inoculum does not
produce an effect on plant growth when soil fertility is involved. However, B.
thuringiensis var. Kurstaki HD1 demonstrated inhibitory effects on some
functional groups of microorganisms that could be involved in deleterious
effects in the field when the nutritional condition is oligotrophic. However,
the cumulative effect of protein crystal was not evaluated. It should also be
emphasised that the accumulative effect of the protein crystal due to successive
cultivation of Bt-transgenic plants has not yet been assessed, but some
authors have suggested that there may be a deleterious effect on the microbiota
(Saxena et al. 1999) and macrofauna (Donegan et al. 1997).
The groups of soil functional microorganisms may be either positively or
negatively affected by B. thuringiensis products, whether produced by bacteria
or transgenic plants. Up to now, the results obtained by microbial ecologists
are still preliminary, and it is clear that exhaustive studies should be carried
out before releasing these plants into the environment. The dynamic of
the functional groups of microorganisms in the presence of these plants must
be understood. In addition, the accumulative effect of the crystal on these
microorganism groups should be assessed together with their subsequent
effects on the bio-geochemical cycles. Confidence that Bt-plants will not damage
the environment when released for intense cultivation will be obtained
after the positive or negative effects they may have on the environment are
established.