plant surface microbiology.pdf

18 Mycorrhizal Development and Cytoskeleton 295
fungus might affect the expression pattern of tubulin genes is the observation
that tua3 transcripts increased in the differentiated cortical cells at the invasion
of an endomycorrhizal fungus. Similarly, in the transgenic tobacco, in
which the Gus expression took place under the promoter of maize tubulin
gene tua3, Gus activity occurred in differentiated cortical cells infected by
endomycorrhizal fungus (Bonfante et al. 1996).As with a-tubulins the expression
of Arabidopsis b-tubulins is relatively high in the root tip and vascular
tissue, but low in root cortical cells (Villemur et al. 1994). The tub6 and tub8
genes are preferentially expressed in the root tip and vascular cylinder while
the high expression of tub4 gene seems to be a unique feature for vascular tissue
(Villemur et al. 1994). It has not been investigated whether the formation
of endomycorrhiza is affecting the expression pattern of b-tubulin genes.
Until now, Eucalyptus globulus is the only ectomycorrhiza forming plant in
which tubulin expression during ectomycorrhiza formation has been studied
at the RNA level. An a-tubulin gene was shown to be upregulated during formation
of symbiosis, and the upregulation of a-tubulin expression paralleled
the increased formation of lateral roots in Eucalyptus seedlings that were in
contact with the fungal mycelium (Diaz et al. 1996). In Pinus sylvestris- Suillus
bovinus and P. contorta–S. variegatus ectomycorrhiza the expression of tubulins
has been analyzed at the protein level (Timonen et al. 1993, 1996; Niini et
al. 1996). Different mobility of plant and fungal a-tubulin allowed their comparison
in one-dimensional (1-D) immunoblots, which suggested that in
mature ectomycorrhiza the fungal a-tubulin dominated (Timonen et al.
1996). The comparison of the amount of plant and fungal a-tubulin during
the development of P. contorta–S. variegates ectomycorrhiza for 60 days also
indicated that the amount of plant a-tubulin decreased gradually, probably
due to the development of fungal sheath around the root (Timonen et al.
1996). No such comparisons could be made between plant and fungal b-tubulin
or actin due to their similar mobility during the electrophoretic separation.
The immunoblots of two-dimensional gels from three root types of P.
sylvestris radicles, main root and first order laterals and short roots as well as
from different developmental stages of P. sylvestris–S. bovinus ectomycorrhiza
revealed more differences in the tubulin protein patterns than 1-D
immunoblots (Niini et al. 1996). Three plant a-tubulins were detected in all
root types, but the pattern in the short roots differed from that in radicles and
first-order laterals. This is an interesting observation, since the formation of
ectomycorrhiza occurs in Pinus short roots probably due to their reduced
growth rate that could be associated with the occurrence of the short-rootspecific
a-tubulin pattern. During the development of ectomycorrhiza the initial
short root-specific a-tubulin pattern gradually changed and two new
a-tubulins were distinguished in mature ectomycorrhiza. Whether the atubulin
protein patterns in P. sylvestris short roots and ectomycorrhiza results
from alterations in the expression of a-tubulin genes or post-transcriptional