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

The enriched soil microbial communities in the mycorrhizosphere are often
organized in the form of biofilms and probably horizontal gene transfer (HGT)
among cohabiting microbial species and between plant and microbe occurs [56].
Many plant-associated Pseudomonas rhizobacteria produce signal molecules for
quorum-sensing regulation, which were absent from soilborne strains [57]. This
indicates that quorum-sensing systems exist and are required in the mycorrhizosphere.
Microbial colonies on root surfaces consist of many populations or strains,
and positive and negative interpopulation signaling on the plant root occur [58],
which may play an important role in the efficiency of the use of biofertilizers.
In addition to the above-described interactions between AMF and rhizobacteria,
certain bacterialike organisms (BLOs) reside in the AM fungal cytoplasm, first
described by Mosse [53]. Khan [59] illustrated AMF spores, collected from semiarid
areas of Pakistan, containing 1–10 small spherical endospores without any
subtending hyphae of their own. Ultrastructural observations clearly revealed their
presence in many field-collected AM fungal isolates. Because of their unculturable
nature, further investigation of BLOs was hampered but current advanced electron
and confocal microscopic and molecular analysis techniques have allowed us to
learn more about their endosymbiotic nature. Minerdi et al. [60] reported the presence
of intracellular and endosymbiotic bacteria belonging to the genus Burkholderia
(a nitrogen-fixing genus) in fungal hyphae of many species of Gigasporaceae. The
authors used genetic approaches to investigate the presence of nitrogen-fixing genes
and their expression in this endosymbiont and found nifHDK genes in the endosymbiont
Burkholderia and their RNA messengers demonstrating that they possess a
molecular basis for nitrogen fixation. This discovery, as stated by Minerdi et al. [60],
indicates that a fungus that improves phosphorus uptake might also fix nitrogen
through specialized endobacteria. Endosymbiont Burkholderia may have an impact
on AMF–PGPR–MHB–plant associations and metabolism. This finding suggests a
new application scenario worth pursuing.
Recent methodological developments in molecular and microscopical techniques
together with those in genomes, bioinformatics, remote-sensing, proteomics and so
on will assist in understanding the complexity of interactions existing between
diverse plants, microbes, climates and soil.
13.2.3
Plant Growth Promoting Rhizobacteria
13.2 Rhizosphere and Microorganismsj249
Rhizobacteria include mycorrhization helper bacteria (MHB) and PGPR, which
assist AMF in colonizing the plant root [61,62], phosphorus solubilizers, free-living
and symbiotic nitrogen fixers, antibiotic-producing rhizobacteria, plant pathogens,
predators and parasites [63]. The most common bacteria in the mycorrhizosphere
are Pseudomonas [64], while different bacterial species exist in the hyphosphere.
Like AMF, rhizobacteria such as pseudomonads are also ubiquitous members of
the soil microbial community and have received special attention as they also exert
beneficial effects on plants by suppressing soilborne pathogens, synthesizing phytohormones
and promoting plant growth [65–68]. Many fluorescent Pseudomonas