plant surface microbiology.pdf

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Marjatta Raudaskoski, Mika Tarkka and Sara Niini
modifications of a-tubulin proteins, will be resolved when the number and
the degree of transcript accumulation of a-tubulin encoding genes in P.
sylvestris short roots have been clarified. In the immunoblots from the different
root types and ectomycorrhiza, three different b-tubulins were identified.
For the expression of P. sylvestris b-tubulins no alterations comparable to
those in a-tubulin were observed.
The expression of the tubulin genes in endo- and ectomycorrhizal fungi
has also obtained some attention. Fragments encoding 267 amino acids from
the central part of the b-tubulin gene have been cloned from several endomycorrhizal
fungi including Glomus mossae, G. geosporum, G. coronatum, G.
clarum, Gigaspora rosea, Acaulospora laevis, andScutellospora castanea (M.
Stommel and P. Franken, public databases). The deduced amino acid
sequences of the fragments suggest clearly that there are at least two b-tubulin
encoding genes in the endomycorrhizal fungi. Comparison of the deduced
amino acid sequences of the b-tubulin genes in and between different species
indicates that in each species the b-tubulin amino acid sequences are more
similar to b-tubulins of other species than to those within the same species.
The G. rosea b-tubulin transcripts accumulate in dormant and germinated
spores, in extraradical hyphae and in pea mycorrhiza (Franken et al. 1997;
Bütehorn et al. 1999), while a- and b-tubulin protein was detected in the
hyphae of G. mossae elicited by the host plant (Åström et al. 1994). None of
these experiments have yet shown whether the expression of either the btubulin
gene is associated with the formation of endomycorrhiza or some
other specific stage in the life cycle of the endomycorrhizal fungi.
The immunoblots of a- and b-tubulins from the ectomycorrhizal fungus S.
bovinus and from its ectomycorrhiza with P. sylvestris indicated the presence
of three a- and two b-tubulin polypeptides (Niini et al. 1996) in nonsymbiotic
hyphae and ectomycorrhiza. From the filamentous homobasidiomycete
Schizophyllum commune that is closely related to S. bovinus,three a- and two
b-tubulins have also been identified by 2-D gel electrophoresis. Until now,
only two a- and one b-tubulin encoding genes have been isolated from S.
commune in spite of several attempts (Russo et al. 1992; Raudaskoski unpublished
data). The higher number of polypeptides than tubulin encoding genes
suggests that fungal a- and b-tubulins are targets for posttranslational modifications.
Recently, a b-tubulin encoding cDNA highly similar to that of S.
commune has been isolated from S. bovinus. By using the encoding region of
the S. bovinus b-tubulin gene as a probe, a high, but similar amount of b-tubulin
mRNAs were detected both in nonsymbiotic and symbiotic hyphae (Lahdensalo
et al., unpublished). Both in vegetative and ectomycorrhizal hyphae of
S. bovinus the tubulin polypeptides occurred in doublet patterns (Niini et al.
1996), which are thought to be due to allelic differences between tubulins of
the haploid genomes present in the dikaryotic hyphae of S. bovinus. This
needs to be certified by cloning and further analysis of S. bovinus tubulin
genes.