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

agricultural and industrial purposes, since some of the defensive compounds are
molecules with pharmacological activity.
In A. thaliana, the SAR and ISR responses are regulated by distinct pathways. The
former response, SAR, is associated with an increase in salicylic acid levels and the
translation of an ankirine-type protein called NPR1, located in the nucleus, which
induce the transcription of the pathogenetic related genes. These genes codify the PR
proteins and are responsible for systemic resistance in the plant [39,56]. In ISR
response, salicylic acid levels are not altered but are mediated by two growth regulators,
ethylene and jasmonic acid, which act as signal transductors and not as stress
hormones. In ISR, the NPR1 protein is also involved, but here is induced the expression
of other proteins different from PRs [17]. Responses arising from SAR and ISR
lead to plant protection against different pathogen spectra, but there are spectra that
overlap. The ability of a PGPR to induce systemic resistance depends on the plant–
beneficial bacteria–pathogen system, a highly specific response. However, both SAR
and ISR responses can coexist in the same plant at the same time [59]. Thus, the use of
PGPR or PGPR mixes that are able to trigger both responses at same time would result
in an important advance in the improvement of pest defense systems.
3.3
Conclusions
It may be concluded that PGPR with molecular mechanisms related to plant nutrition
should be used in the appropriate soil; for example, phosphate-solubilizing bacteria
will exhibit their effect in a soil with low phosphorous content and siderophoreproducing
bacteria will exhibit their effect in a soil deficient in available iron. If not,
the bacteria will be ineffective. Furthermore, the activities expressed by the bacteria are
inducible and not usually expressed in soils rich in nutrients. However, PGPR that can
alter hormone balance in the plant are very efficient at improving plant fitness.
Some PGPR may interact with plant root receptors and have exhibited notable
effects on the secondary metabolism of the plant. These include defensive
metabolism, providing the plant with protection against pathogens and, furthermore,
improving resistance to abiotic stress conditions or inducing the synthesis of
molecules of pharmacological interest. Nonetheless, for PGPR to be used
effectively in agriculture, it will be necessary to study each plant–PGPR–soil
system individually.
3.4
Future Prospects
3.4 Future Prospectsj51
The future in PGPR research should be directed toward the selection of PGPR or
PGPR mixes to help solve current agricultural problems such as the use of highly
contaminating pesticides and fertilizers and cultivation in low-fertility soils.
In other contexts, PGPR applications may lead to the creation of functional foods,
that is, foods having a beneficial effect on human health.