plant surface microbiology.pdf

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Rüdiger Hampp and Andreas Maier
Pioneering work in this field has been carried out by Garbaye (for a review
see Garbaye 1994). Experiments carried out with Picea abies, Pinus nigra,
Pinus sylvestris, Pseudotsuga menziesii, and Quercus robur (Garbaye et al.
1992) indicated that soil bacteria can stimulate the inoculation of roots with
ectomycorrhiza-forming fungi, thereby also reducing the adverse effect of
pathogens. Both effects resulted in a better seedling growth, and thus the term
“helper bacteria” was coined. For more recent work, see Dunstan et al. (1998)
and Poole et al. (2001).
5 Bacteria Associated with Sporocarps and Ectomycorrhiza
Twenty seven bacterial species were isolated from both the sporocarps of Suillus
grevillei and the ECMs of S. grevillei/Larix decidua (Varese et al. 1996). The
genera Pseudomonas, Bacillus, and Streptomyces were predominant. From
sporocarps of white truffles (Tuber sp.), bacterial strains such as Micrococcus,
Moraxella, Staphylococcus and Pseudomonas could be isolated (Citterio et al.
1995). Gram-positive bacteria seldom stimulated in vitro fungal growth.
Among gram-negative bacteria, Pseudomonas strains enhanced growth.
Streptomyces significantly inhibited the fungus. Bacterial supernatants were
not effective.Volatiles enhanced fungal growth to some extent, but not significantly.
Most of the bacteria isolated produced siderophores.
A distinction between the different structures of ECM showed that bacteria
primarily occurred on the surface of the mantle and in the interhyphal spaces
(Schelkle et al.1996),but also deep within the mantle (Foster and Marks 1967).
Bacteria of subclasses of proteobacteria (containing plant-growth-promoting
rhizobacteria such as Burkholderia, Azospirillum, Acetobacter and Herbaspirillum)
were detected in high numbers on mantle surfaces (Mogge et al.
2000). The two most common fungi on beech, Lactarius vellereus and Lactarius
subdulcis, were associated with members of the a- and b-subclasses of the
proteobacteria. These bacteria have been shown to be abundant in winter and
early spring (Timonen et al. 1998).
Electron microscopy of ECM with Pinus sylvestris and S. bovinus and Paxillus
involutus (Nurmiaho-Lassila et al. 1997) also revealed bacteria on the
mantle surface and at inter- and intracellular locations in the mantle and the
Hartig net (S. bovinus). Fungal strands were colonized only by a few bacteria,
while the outermost external fine hyphae had extensive monolayers of bacteria
attached.
ECM with P. involutus were mostly devoid of bacteria, while the external
mycelium supported bacteria (Nurmiaho-Lassila et al.1997).From their observations,
the authors conclude that single ECM fungi create defined mycorrhizosphere
habitats with distinct populations of bacteria. Knowing that several
different types of ECM can be formed on the same root in close vicinity,a large
local biodiversity of ECM-specific bacterial populations could be postulated.