plant surface microbiology.pdf

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Michael Kaldorf et al.
chitinases, one acidic and two basic glucanases, as well as three PR proteins of
unknown function had no detectable influence on AM colonization. Only one
of the PR proteins tested, an acidic, extracellular b-1,3-glucanase of class II,
reduced the mycorrhization of transgenic tobacco roots by G. mosseae significantly.
This observation demonstrates that mycorrhiza formation could be
affected in GMPs expressing antifungal PR proteins. Therefore, a case-to-case
investigation of GMPs with increased fungal pathogen resistance seems to be
necessary to exclude negative effects on AM formation.
In addition to the quantity of mycorrhizal colonization, the structural and
functional diversity of mycorrhizal fungi colonizing the root system might be
influenced in GMPs. Probably due to methodical difficulties, this question has
not been addressed for AM fungi. Compared to the rather uniform morphology
of AM fungi, which makes their morphological identification quite difficult,
EM fungi exhibit many morphological and anatomical characters that
could be used for their characterization (Agerer 1991). In combination with
PCR-RFLP and sequence analysis of the ITS region within the fungal rDNA
(Buscot et al. 2000), EM communities can be described with a sufficient resolution
to compare the mycorrhization of transgenic and nontransgenic trees,
even in the field.A release experiment with transgenic aspen carrying the rolC
gene from Agrobacterium rhizogenes (Fladung and Muhs 2000) was accompanied
by a detailed analysis of the EM status of the trees. Although rolC modified
the hormonal balance in the trees, and therefore, might have affected
their mycorrhization ability, no significant difference in the degree of mycorrhization
was observed in the transformed aspen. The structure of the EM
community of the different aspen lines was similar in the first two years of the
experiment (Kaldorf et al. 2002), but in the third and fourth years, a significantly
reduced EM diversity was observed on the rolC transgenic aspen compared
to controls (Kaldorf et al. 2001). In addition, one EM morphotype
formed by Phialocephala fortinii appeared to be significantly less represented
on the transgenic line “E2/5” compared to all other transgenic and control
lines (Kaldorf et al. 2002). This reduced compatibility for one mycobiont represents
the first example of a clone-specific effect concerning mycorrhization
of transgenic plants.
4 Horizontal Gene Transfer
Three potential pathways have been proposed for the spread of GMPs or the
transgenes introduced into these plants. Two of these pathways, the establishment
of self-sustaining GMP populations and the introgression of genes into
wild populations, regarded as the major risks of GMPs for natural plant communities
(Wolfenbarger and Phifer 2000), do not involve plant/microorganism
interactions. The third possibility is the horizontal transfer of genes from
GMPs to microorganisms, which might lead to bacterial or fungal strains car-