plant surface microbiology.pdf

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Rüdiger Hampp and Andreas Maier
fluorescent pseudomonad strains (Founoune et al. 2002). After 3 months of
co-culture, the bacterial inoculants disappeared, showing how difficult such
experiments are to interpret. Obviously, the amount of inoculum supplied can
also play an important role (Frey-Klett et al. 1999).
7 Possible Mechanisms of Interaction
As pointed out by Schelkle and Peterson (1996), and in addition to the production
of antibiotics, protective or “helper” effects could be due to competition
for nutrients in the rhizosphere. The formation of siderophores, for
example could be such a mechanism. Siderophores are iron chelators which
make iron available for uptake by the bacteria. As these compounds are
species-specific, Fe-chelates can only be taken up by those bacteria that are
able to produce them. Protective bacteria synthesizing siderophores could
thus out-compete pathogens with regard to Fe (Neidhardt et al. 1990). Similar
mechanisms are known for ECM fungi (Watteau and Berthelin 1990).
Siderophore release from bacteria into the mycorrhizosphere could also
improve absorption of Fe by the mycorrhizal fungus.
Protection from pathogens could, however, also be a mass effect, simply
due to the large number of nonpathogenic bacteria that accumulate in the rhizosphere
due to the high nutrient supply. However, as outlined by Garbaye
(1994), there can be many more mechanisms, such as an improved receptivity
of the root for mycorrhizal infection, a modification of the rhizospheric soil,
improvement of the root-fungus recognition, stimulation of germination of
fungal propagules, as well as an enhancement of fungal growth in the rhizosphere
(see also Brule et al. 2001) which would increase the probability of contact
between fungus and root (compare Dunstan et al. 1998).
In nutrient-poor acidic forest soils modification by micro-organisms
should be an important factor; the C-rich environment provided by the plant
is attractive for soil micro-organisms, leading to the formation of functionally
compatible microbial communities. These are jointly able to co-mobilize soil
nutrients such as P and N in and around the vegetative mycelium. In addition,
N-fixing bacterial species including Bacillus spp. are possibly present in the
mycorrhizosphere of forest trees (Li et al. 1992) as the vegetative mycelium
represents a niche that is ideally suited for the selection and enrichment of
associative N-fixing bacteria (Sen 2000).
In many of the possible mechanisms, phytohormones such as IAA could
play an important role. A study on the rooting of derooted shoot hypocotyls
of spruce showed that Laccaria bicolor and Pseudomonas fluorescens BBc6
(MHB) both increased the number of roots formed per rooted hypocotyl
(Karabaghli et al. 1998). The same effect was caused by the addition of IAA
alone (control). Both organisms produced IAA in pure culture.