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

13.2 Rhizosphere and Microorganismsj251
Duc et al. [85] were the first to report that some pea mutants defective in nodulation
also did not support AM symbiosis. Since the first report of legume nodulation
mutants to be nonmycorrhizal, a number of nodulation-defective mutant legumes
and mutated nonlegume crops have been tested [86]. Many recent reports have
confirmed the similarities between nodulation and mycorrhiza-formation processes
[87,88]. There is enough evidence available now that indicates a positive interaction
between AMF and PGPR [89].
Several reports address the interactions between AMF and Rhizobium species [90],
suggesting that the interaction is synergistic, that is, AMF improve nodulation due to
enhanced phosphorus uptake by the plant. In addition to this principal effect of AMF
on phosphorus-mediated nodulation, other secondary effects include supply of trace
elements and plant hormones, which play an important role in nodulation and N2
fixation. Current research is directed toward understanding the role AMF play in the
expression of Nod genes in Rhizobia [91]. Barea and associates [34,46,47,92] have
made many significant findings in this regard. Synergistic interactions between
AMF and asymbiotic N2-fixing bacteria such as Azobacter chroococum, Azospirillum
spp. and Acetobacter diazotrophicus have also been reported by many researchers [84].
Synergistic interactions between phosphorus-solubilizing bacteria and AMF and
their effect on plant growth have been studied by many researchers during the last
three decades. Duponnois and Plenchette [93] studied the effects of MHB Pseudomonas
monteillii strain HR13 on the frequency of AM colonization of Australian
Acacia species and reported a stimulatory effect. They recommend dual inoculation
to facilitate controlled mycorrhization in nurseries where Acacia species are
grown for forestation. Modern research has provided evidence that the genetic
pathway of AM symbiosis is shared in part by other root–microbe symbioses such
as nitrogen-fixing rhizobia [87].
Many researchers reported unsuccessful attempts to select an appropriate Rhizobium
strain for inoculating legumes owing to the failure of the selected strain(s) to
survive and compete for nodule occupancy with indigenous native strains under low
phosphorus and moisture contents [94]. In this context, the role of AMF as phosphorus
suppliers to legume root nodules appears to be of great relevance. Requena et
al. [95] found a specific AM fungus–Rhizobacterium sp. combination for effective
nodulation and N2 fixation in a mycotrophic legume Anthyllis cytisoides in Mediterranean
semiarid ecosystems in Spain. They reported that Glomus intraradices was
more effective with Rhizobium sp. NR4, whereas G. coronatum was more effective
when coinoculated with strain NR9. Such specificity in interactions between AMF,
Rhizobium and PGPR have been described by various researchers, indicating that it
is important to consider the specific functional compatibility relationships between
AMF, PGPR and MHB and their management when using these symbiotic microbes
as biofertilizers.
New techniques applied in molecular ecology have resulted in the identification of
members of nonculturable Archea in the mycorrhizosphere, but their role in the
hyphosphere is not known [96]. This means, as pointed out by Sen [97], that analyses
of PGPR distribution and activities must now be extended to accommodate Crearchaeotal
microbes as well. Areas such as host–microbe specificity and microbial-linked