plant surface microbiology.pdf

30 Axenic Culture of Symbiotic Fungus Piriformospora indica 595
shape and granulated dense bodies were observed. Many cells contained more
than one nucleus. Chlamydospores were formed from thin-walled vesicles at
the tips of the hyphae. The chlamydospores appeared singly or in clusters and
were distinctive due to their pear-shaped appearance (Fig. 1). The chlamydospores
were (14-) 16–25 (-33) mm in length and (9-) 10–17 (-20) mm in
width. Figure 2 shows the maize root colonization. The cytoplasm of the
chlamydospores was densely packed with granular material and usually contained
8–25 nuclei (Fig. 2, inset).Very young spores had thin, hyaline walls. At
maturity, these spores had walls up to 1.5 mm thick, which appeared two-layered,
smooth and pale yellow. Neither clamp connections nor sexual structures
could be observed.
3 Taxonomy of the Fungus
Different kinds of substrates were tested to induce sexual development, such
as young and mature leaves of Cynodon dactylon and pollen grains, oat meal,
potato, carrot or tomato dextrose agar and soil-on-agar culture methods. No
apparent adverse affect was seen on cultivation in light. It is not necessary to
grow the fungus in the dark. Growth under light and dark conditions did not
promote sexuality. May be the fungus is heterothallic in nature and one has to
work for compatible strains. Since all these efforts did not lead to the desired
results, there were only a few features to characterize the fungus morphologically
and group it according to the classical species concept. In order to obtain
more information about the systematic position of the new fungus, the ultrastructures
of the septal pore and the cell wall were examined. The cell walls
were very thin and multilayered structures. The septal pores consisted of
dolipores with continuous parenthosomes. The dolipores were very prominent,
with a multilayered cross wall and a median swelling mainly consisting
of electron-transparent material. The electron-transparent layer of the cross
walls extended deep into the median swellings, but did not fan out. In median
sections of the septal pores, the parenthosomes were always straight and had
the same diameter as the corresponding dolipore. Parenthosomes were flat
discs without any detectable perforation. The parenthosomes consisted of an
electron-dense outer layer, which showed an inconspicuous dark line in the
median region. The parenthosomes were in contact with the ER membranes,
which were mostly found near the dolipore (Verma et al. 1998).
The ultrastructural data proof that P. indica is a menber of the Hymenomycetes
(Basidiomycota). Studies on the moleclar phylogeny will help to
reveal the closest relatives of this species (Fig. 3).
Interestingly, immunological characterization showed a strong cross-reactivity
with the members of Zygomycota (Glomerales) instead of species of
Basidiomycota (Table 1). This aspect needs further critical appraisal.