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

258j 14 Salt-Tolerant Rhizobacteria: Plant Growth Promoting Traits
increasing the rate of seed germination and seedling emergence, minimizing the
adverse effects of external stress factors, and protecting plants from soilborne pests
and diseases [12]. Understanding the highly complex nature of the microbial adaptation
and response to alterations in the biological, chemical and physical environment
of the rhizosphere remains a significant challenge for plant biologists and
microbiologists [13,14].
Interest in bacterial fertilizers has increased, as their use would substantially
reduce the use of chemical fertilizers and pesticides, which often contribute to
pollution of soil–water ecosystems. Presently, about 20 biocontrol products based
on Pseudomonas, Bacillus, Streptomyces and Agrobacterium strains have been commercialized,
but there still is a need to improve the efficacy of these biocontrol
products [12]. Soil salinity, high temperatures and soil contamination often affect
phytoefficiency of plant growth promoting bacterial inoculants in nature [15]. Damage
to soil and plants in arid and semiarid areas is not easily repairable, because
these areas are fragile and sensitive ecosystems [16]. It is important to study soil
microbial activity in stressed environments to evaluate soil quality and plant productivity
as affected by natural calamities and anthropogenic activities [17–19].
The challenge for the future includes understanding the behavior of microbes in
their natural and often complex habitats, such as the rhizosphere [20]. Microbial
processes and properties in the rhizosphere are crucial to support functional agriculture.
Root-associated bacteria have a great influence on organic matter decomposition,
which, in turn, is reflected in soil nutrient availability for plant growth [21].
The phosphorus- and potassium-solubilizing bacteria may enhance plant nutrient
availability by dissolving insoluble phosphorus and releasing potassium from silicate
minerals [22]. Plant growth promoting bacteria often help increase root surface
area to increase nutrient uptake and, in turn, enhance plant production [23].
The mechanisms and interactions among these microbes are still not well understood,
especially in field applications under different environments. Therefore,
this requires studying plant–microbe interactions, the natural resident microorganisms
and their physiological adaptation in ecologically stressed environments. Understanding
the physiology, adaptation and functions of salt-tolerant bacteria in
stressed environments (e.g. arid regions) may provide valuable information on
plant–microbe interactions to develop such new agricultural technologies as would
improve soil ecology and plant development. At present, however, the interest lies in
the development and application of salt-tolerant plant growth stimulating bacterial
inoculants to improve plant growth and yield; their interactions with host plants; and
biological control of fungal diseases in saline environments. There have been few
reports on microbial diversity and function in saline environments in different
regions of the world [24–26]. However, only a limited number of studies on rhizobacteria
and their physiological characterization in saline arid soils have been undertaken.
This chapter intends to discuss recent developments and advances in our
understanding of the high salt- and temperature-resistant rhizobacteria and their
characteristics, physiology, adaptation and production of metabolites that play a
synergistic role in plant growth and development under fragile and stressed
environments.