plant surface microbiology.pdf

a single copy gene of A. muscaria (SCIV038, Nehls et al. 2001) revealed PCR
fragments in any case, indicating that no inhibitors of the PCR reaction were
present in the genomic DNA preparation. The reason for false positives was
most probably the low herbicide concentration in the growth medium.A concentration
of 200 mg/ml Basta (as used in this study) results in fungal background
growth. This relatively low Basta-concentration was chosen to recognize
also lateral transfer of the resistance gene lacking its heterologous
promoter. In this case, the bar-gene might integrate behind a weak A. muscaria
promoter, resulting in only a weak herbicide resistance.
The 35,000 mycorrhizas investigated in this study represent, of course, only
a limited sample number. Nevertheless, since each mycorrhiza does contain a
large number of competent fungal hyphae in direct contact with plant epidermal
cells that die under the selection conditions, this sample number is large
enough to reveal that HGT from the tree to the fungal partner is a quite rare
or maybe completely missing event in EM symbiosis, at least under axenic
conditions.
5 Conclusions
11 Interactions of Microbes with Genetically Modified Plants 191
Taken together, many of the studies cited above demonstrate that transgenic
plants can induce changes in soil microorganism communities. Nevertheless,
the importance of these findings is unclear as in most studies, the modifications
in the rhizosphere of GMPs were not compared to the natural variance
in the rhizosphere of different plant breeds generated by conventional methods.
For example, the mycorrhization capacity of modern wheat varieties with
high pathogen resistance has been shown to be reduced (Hetrick et al. 1992).
Such potentially negative effects would be considered unacceptable in the
case of any GMP introduced into agriculture.
Concerning the investigations on HGT, there is some evidence for the possibility
of HGT not only between bacteria, but also between plants and
microorganisms. In soil, HGT must be a rare event, as several attempts to
detect HGT in field experiments failed. Despite the missing evidence for HGT
in the field, the possibility of HGT should be kept in mind for risk assessment.
The question to be answered in a case-to-case consideration is whether a possible
rare HGT of the introduced genes from GMPs to microorganisms might
cause specific problems. This is unlikely if the transgene itself is common in
nature. For example, a natural transfer of the rolC gene from Agrobacterium to
other bacteria seems much more likely than a HGT from the rolC transgenic
aspen mentioned above to microorganisms. On the other hand, artificial
genes generated by genetic engineering might have a high risk potential when
released into nature.