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

100j 5 Diversity and Potential of Nonsymbiotic Diazotrophic Bacteria in Promoting Plant Growth
the microbial cells begin to release small signaling molecule mediated sensing
response pathways. This effect has been defined as quorum sensing [206]. These
microbially derived signal molecules are placed into two main categories: (i) amino
acids and short peptide pheromones commonly utilized by Gram-positive bacteria
[207,208] and (ii) fatty acid derivatives such as acyl homoserine lactone (AHL)
utilized by Gram-negative bacteria. Cellular processes regulated by QS in bacteria
are diverse and includes genetic competence development. Quorum-sensing signals
and identical two-component regulatory systems are used by plant-interacting
bacteria (mutualistic or pathogenic associations) to coordinate, in a cell density
dependent manner or in response to changing environmental conditions, the
expression of important factors for host colonization and infection. The success
of invasion and survival within the host also requires that rhizobia and pathogens
suppress and/or overcome plant defense responses triggered after microbial recognition,
a process in which surface polysaccharides, antioxidant systems, ethylene
biosynthesis inhibitors and virulence genes are involved [209]. The role of
AHL and AHL analogues was also reported in Rhizobium–legume symbiosis and
Pseudomonas fluorescens [210,211]. Similarly, QS systems are widespread mechanisms
of gene regulation in both pathogenic and plant associative bacteria, thus
requiring in-depth investigation in modulating various PGP traits and plant–
bacteria interactions.
5.7
Critical Gaps in PGPR Research and Future Directions
The inoculated PGPR may release various secondary metabolites as plant growth
promoting substances. The bioproduction of these substances in contact with roots
is most likely subject to direct uptake by plant roots before being catabolized by soil
microbes or being immobilized in soil. It has been demonstrated that these microbially
derived plant growth promoting substances can promote plant growth and
development. Therefore, there is a need to provide evidence and their role. This can
be explored by monitoring the synthesis of PGP substances in the rhizosphere by
developing analytical techniques for the separation and detection of PGP substances
such as plant growth regulator in the soil and screening of microbes for the production
of PGP substances in the absence or presence of a precursor.
In vitro activities exhibited by various PGPR may not give the expected results
under field conditions. The failure of PGPR to produce the desired effects after seed/
seedling inoculation is frequently associated with their inability to colonize plant
roots. The process of root colonization is complex. Several traits associated with the
survivability, tolerance, competence with indigenous rhizospheric microorganisms,
expression of root colonizing traits and so on are important [49]. In many agroclimatic
situations such as harsh climates, population pressure, land constraints and
decline of traditional soil management practices, reduced soil fertility often exists.
Therefore, considering the varied agroclimatic conditions, continuous research is
needed to develop region-specific bioinoculants with rhizospheric competence and