Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

248j 13 Microbial Dynamics in the Mycorrhizosphere with Special Reference to Arbuscular Mycorrhizae
producing plant growth regulating hormones and enzymes. Others alter the plant
nutrient availability and biochemical reactions undertaken by the plant.
AMF impart differential effects on the bacterial community structure in the
mycorrhizosphere [29,43]. AMF improve phosphorus nutrition by scavenging available
phosphorus through the large surface area of their hyphae. Plant growth
promoting rhizobacteria (PGPR) may also improve plant phosphorus acquisition
by solubilizing organic and inorganic phosphorus sources through phosphatase
synthesis or by lowering soil pH [44]. Garbaye [45] defined mycorrhizal helper
bacteria (MHB) as bacteria associated with mycorrhizal roots and mycorrhizal fungi
which collectively promote the establishment of mycorrhizal symbioses .
There is growing evidence that diverse microbial populations in the rhizosphere
play a significant role in sustainability issues [46,47] and that the manipulation of
AMF and certain rhizobacteria such as PGPR and MHB is important. Vivas et al.
[48] used a dual AM fungus–bacterium inoculum to study the effect of the drought
stress induced in lettuce grown in controlled-environment chambers. Their results
showed that there was a specific microbe–microbe interaction that modulates the
effectivity of AMF on plant physiology. The authors concluded that plants must
have mycorrhizal associations in nutrient-poor soils and that mycorrhizal effects
can be improved by coinoculation with MHB such as Bacillus spp. Results of the
study by Vivas et al. show that coinoculation of selected free-living bacteria isolated
from adverse environments and AMF can improve the formation and function of
AM symbiosis, particularly when plant growth conditions are also adverse. Both
AMF and PGPR complement each other in their role in nitrogen fixation, phytohormone
production, phosphorus solubilization and increasing surface absorption.
Behl et al. [49] studied the effects of wheat genotype and Azobacter survival on
AMF and found that the genotype tolerant to abiotic stresses had higher AMF
infection and noticed a cumulative effect of plant–AMF–PGPR interaction. Similar
observations were made by Chaudhry and Khan [50,51] who studied the role of
symbiotic AMF and PGPR nitrogen-fixing bacterial symbionts in sustainable plant
growth on nutrient-poor heavy metal contaminated industrial sites and found that
the plants surviving on such sites were associated with nitrogen-fixing rhizobacteria
and had a higher arbuscular mycorrhizal infection, that is, a cumulative and
synergistic effect.
The MHB cannot be ignored when studying mycorrhizal symbioses in their
natural ecosystems. They are quite common and, as Garbaye [45] said, they are
found every time they are sought and seem to be closely associated with the mycorrhizal
fungi in the symbiotic organs. They are adapted to live in the vicinity of AMF
as high frequencies of MHB populations have been isolated from the mycorrhizae.
Some MHB isolates also promoted ectomycorrhizae formation in four conifers [52],
indicating that the MHB effect is not plant specific. But various researchers have
shown that MHBs are fungus selective [45]. Mosse [53] showed that cell wall degrading
enzyme producing Pseudomonas sp. enhanced the germination of AM
fungal spores of Glomus mosseae and promoted the establishment of AM on clover
roots under aseptic conditions. These observations were later supported by other
workers [54,55].