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Michael Kaldorf et al.
1995). Similarly, the effects of transgenic potatoes producing the lectin GNA
on nontarget soil organisms could not be attributed to the formation of the
lectin GNA itself (Griffiths et al. 2000). So far, only one GMP producing an
antibacterial substance, namely T4 lysozyme-producing potatoes with
enhanced resistance to Erwinia carotovora, has been investigated in detail for
nontarget effects on soil bacteria. Most soil bacteria were lysozyme-sensitive
when tested in laboratory experiments with pure cultures (de Vries et al.
1999). In addition, increased killing of Bacillus subtilis was observed on the
root surface of T4 lysozyme-producing potatoes from the field, and this effect
was ascribed directly to the lysozyme release by the roots (Ahrenholtz et al.
2000). Nevertheless, the production of lysozyme had only a minor influence
on the bacterial phyllo- and rhizosphere communities (Heuer and Smalla
1999; Heuer et al. 2002), which was considered negligible relative to natural
factors by the authors. Further studies with the same system which focused on
potentially beneficial plant-associated microbes like auxin-producing bacteria
or bacteria antagonistic to the pathogenic E. carotovora did not reveal correlations
between the transgenic character of plants and the pheno- or genotypic
features of bacterial isolates (Lottmann and Berg 2001). Thus, up to now,
there is no direct evidence from field experiments that the primary product of
transgene expression is responsible for significant changes in the soil microbial
community in any GMP. Instead, secondary effects of GMP generation,
like somaclonal variation and changes in general plant metabolism induced
by the transgene insertion or expression, may contribute to a major part of
the effects described above.
3 Impact of Genetically Modified Plants on Symbiotic
Interactions
The question whether the genetical transformation of plants might reduce
their ability to form mutualistic symbioses with microorganisms has been
addressed in a surprisingly small number of studies.
Biological nitrogen fixation accounts for about 65 % of the nitrogen utilized
in agriculture worldwide (Vance and Graham 1995). The ability to
reduce atmospheric nitrogen to ammonia (nitrogen fixation) is restricted to
prokaryotes. Beside free-living and plant-associated bacteria, members of the
Rhizobiaceae living symbiotically in the typical root nodules of legumes such
as alfalfa, clover, pea, and soybean are the agriculturally most important
group of nitrogen fixing organisms. The symbiotic interaction between rhizobia
and legumes requires a sequential signal exchange between both partners,
and therefore, exhibits a high degree of host specificity (Bothe 1993). Transgenic
plants have been used as a tool to investigate the host recognition of rhizobia
(Diaz et al. 1989, 2000). For example, the transfer of lectin genes between
different legumes has been shown as a possible way to modify host specificity