plant surface microbiology.pdf

7 The Functional Groups of Micro-organisms and Biotech Products 127
formation and behaviour of rhizobia nodules. Such changes could influence
the competition between rhizobia and other rhizobacteria. If bacteria selectively
favoured in the rhizoplane enhanced rhizobia competitiveness, then
nodulation would be favoured (Linderman 1992).
In general, soil productivity and nutrient cycling are influenced by soil
microbial populations. The relationships among functional groups of
microorganisms of C, N and P cycling, and their influence on the plant
growth, are potential indicators to evaluate disturbance in the soil environment.
A corresponding rise in the input of Bt and its toxins into soil systems can
be expected with the increased use of B. thuringiensis-based insecticides,
whether by direct spraying, in insect cadavers, or in transgenic plant material
or microorganisms (Addison 1993). Very little attention has been paid to the
effects that B. thuringiensis might have on the indigenous soil assemblages,
and the information that is available is often confusing. Petras and Casida
(1985) reported that endogenous soil bacteria, actinomycetes, fungi and
nematodes increased moderately compared with the control when using a
spore and crystal suspension of B. thurigiensis subsp. kurstaki isolated from
Dipelr, a commercial preparation. Pruett et al. (1980) inoculated B. thuringiensis
subsp. galleriae into clay soil and reported that bacterial populations
increased 2 weeks after inoculation and were still increasing at the end of the
135-day experiment.
In contrast to the above studies, Atlavinyté et al. (1982) reported a decrease
in indigenous soil microbiota when B. thuringiensis subsp. galleriae was inoculated
into the soil. Bacterial numbers had decreased 50 % and actinomycetes
by 90 % after 45 days, and in contrast, fungal populations had increased by
300–500 % compared with the control.
The influence of B. thuringiensis subsp. kurstaki and its protein on functional
groups of soil assemblages was assessed for the first time in our laboratory,
and we discuss our findings as follows.
In non-sterile soil B. thuringiensis vegetative cells seemed to be unable to
compete with the indigenous microorganisms in non-sterile soil. Under these
conditions, the number of cells decreased drastically, sporulation occurred
quickly and the number of spores was stable, approximately four log unit for
at least 45 days. The cell number decrease was greater in non-sterile soil than
in sterile soil conditions (Villas Bôas et al. 2000). The same results were found
by Thomas et al. (2000). Their results suggested that, although the soils used
were of different types and composition, B. thuringiensis apparently did not
show biological activity after spores had been released into the environment
and could persist for several years (Pruett et al. 1980; Pedersen et al. 1995).
However, some species of Bacillus genera such as B. megaterium, B. subtilis, B
cereus suppressed pathogen fungi and/or bacteria and saprophyte fungi populations
in microcosm soil (Reddy and Rhae 1989; Halverson et al. 1993;
Young et al. 1995; Kim et al. 1997). In many cases, the results showed a great