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

5.7 Critical Gaps in PGPR Research and Future Directionsj101
PGP traits. One possible approach may involve isolation of PGPR from the indigenous
soil–plant system and use them in the same agroclimatic conditions. The
selection of PGPR may be based on number and types of PGP traits present.
Therefore, potential PGPR adapted to particular soil and plant soil conditions and
harboring multiple PGP traits should be selected and evaluated in field conditions.
In order to determine the successful establishment of a PGPR in field conditions, its
identity and activities must be continuously monitored (Table 5.4). Considering the
cost of molecular techniques, more simple and reliable methods must be developed
for rapid detection and monitoring of inoculant strains in the rhizosphere.
The extensive research data generated on plant growth promoting rhizobacteria
clearly indicate that the plant–bacteria interaction leads to rhizosphere colonization
and its influence on plant health is a complex process. Various mechanisms of plant
growth promotion, both direct and indirect, by diazotrophic bacteria and their
interactive effects have been investigated. However, evidence for the contribution
of individual mechanisms of plant growth promotion is less prevalent. What is
urgently needed in this direction is listed in the following:
(i) Develop more productive analytical and bioassay-based techniques for the
identification and uptake of microbial products/nutrients by plant roots.
Table 5.4 Techniques used for the detection and quantification of inoculated PGPR.
Techniques References
Classical and immunological
Selective media containing the appropriate toxic substances
(antibiotics, heavy metals and herbicides)
Immuno fluorescence colony staining approach
(enumeration of colonies marked with antibodies conjugated
with fluorescein isothiocyanate)
Immunomagnetic attraction (enumeration of bacteria captured with a
supermagnet)
Molecular
Specific rRNA probes, coupled with PCR [using probes labeled
with a fluorochrome (hybridization in situ coupled with
confocal laser microscopy)
16S rRNA probe (dot blot hybridization of a directly isolated
nucleic acid mixture)
Marker genes (quantified by colorimetry): lacZ (b-galactosidase,
blue colonies), gusA (b-glucuronidase, indigo colonies), xylE
(catechol 2,3-dioxygenase, yellow colonies), tfdA
(2,4-dichlorophenoxyacetate, red colonies)
Lux (luciferase) (bioluminescence quantified by charge-coupled
device cameras or visualized in planta), GFP (green fluorescent
protein detected in situ with confocal laser microscopy or
epifluorescence microscopy), inaZ (ice-nucleation protein
quantified by freezing assay)
[214,222]
[212]
[213]
[214–216]
[23,217–221]