References

184 A. Deubel and W. Merbach
comparison with the one inside the fungal cells. This indicates that fungi
especially can make this important organic P compound available. This
corresponds with the results of Hayes et al. (2000) who reported a limited
useofphytate-Pbysterilegrowingplants.
4
Interactions Between Microorganisms
and Higher Plants from Competition to Symbiosis
Interactions between microorganisms and higher plants extend from directly
detrimental effects of plant pathogens to directly beneficial effects
in the case of symbiosis. Rhizosphere microorganisms can be competitors
for limited nutrients like P. As a result of higher nucleic acid contents, they
have higher P concentrations than higher plants. Some microbes are able to
incorporate high P amounts in the form of polyphosphates, an energy reservoir
for limited oxygen conditions. Microorganisms often have a higher P
uptake efficiency than plant roots. On the other hand, this incorporated P
can become available to plants as the microbes die. Oberson et al. (2001)
reported a rapid microbial P turnover under different land-use conditions.
Plant hormone production by rhizosphere microorganisms can influence
root architecture, the development of root hairs and the affinity of
roots for phosphate, indirectly affecting the P uptake.
In particular, different forms of mycorrhiza have great importance for
the P nutrition of higher plants. Most agricultural crops are potential host
plants for arbuscular mycorrhizal (AM) fungi. In addition to an exudation
of carboxylates, phosphatases and plant hormones, mycorrhiza increase
the exploitation of the soil volume by the hyphal network, which increases
theactiveadsorptionsurfaceandspreadsbeyondthephosphatedepletion
zone (Lange Ness and Vlek 2000; Martin et al. 2001). Mycorrhizal hyphae
have a higher affinity for phosphate as expressed in the Michaelis-Menten
equation by a lower Km value and absorb P at lower solution concentrations
than roots do (Lange Ness and Vlek 2000). AM fungi store phosphate in
the form of orthophosphate, polyphosphate and organic P in their vacuoles
and transfer it to the roots of the host plant (Ezawa et al. 2002).
5
Phosphorus-Mobilizing Microorganisms as Biofertilizers
A great interest in the use of microorganisms as biofertilizers exists especially
in areas with a low P availability as a result of an unfavorable soil pH.
In addition, inoculates are used to improve the fertilizer efficiency of rock