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

262j 14 Salt-Tolerant Rhizobacteria: Plant Growth Promoting Traits
nutrients exuded by plant roots, proliferate and efficiently colonize the root system
and be able to compete with endogenous microorganisms [63].
Colonization can be considered as the delivery system of the microbes beneficial
factors [8]. It is reported that plant growth promoting rhizobacteria are able to
colonize the rhizosphere, the root surface or even the superficial intercellular spaces
of plants [64]. Colonization of plant roots by bacteria depends on biotic and abiotic
factors such as dynamics of microbial population, plant characteristics and soil
types. To persist and reproduce, bacteria introduced into a soil should be able to
rapidly adapt to soil conditions.
Survival strategies of bacteria such as Bacillus species depend on the physiological
adaptation of the introduced cells –adaptation to nutrient-limited conditions and/or
other physicochemical limiting conditions, efficient utilization of root-released compounds
or specific interactions with plants [65]. In general, Bacillus endospores have
a greater survival capacity in dry soil than Pseudomonas cells [66]. Although soil type
can influence the composition of fluorescent Pseudomonades in the rhizosphere [67],
the action of aggressive colonizers appears to be independent of soil type [68].
Results from a number of studies show that the density of a bacterial population
associated with pea roots was negatively correlated to an increase in temperature
[68,69]. It is worth mentioning that [15,70] the rifampicin-resistant mutants of
P. denitrificans PsD6 and Bacillus amyloliquefaciens BcA12 bacteria isolated from
arid saline soils were able to establish more easily in both wheat (80 and 99.9%,
respectively) and pea (93 and 99.9%, respectively) rhizosphere than in bulk soil and
phyllosphere. Effects were more pronounced in peas than in wheat (Table 14.2).
A study on wheat root colonization and survival of an Arthrobacter spp. illustrated
that the coryneforms are well adapted to long-term survival in both rhizosphere and
bulk soil [71]. Efficient colonization and/or physiological adaptation to soil conditions
is necessary for soil bacterial inoculants to survive under adverse environments
[65]. However, microbial colonization of plant roots is often affected by biotic and
abiotic factors such as root exudates, competition, nutrients, pH, electrical conductivity
and temperature [72,73]. Information on the effects of different factors on
rhizosphere microflora may help to understand the rhizosphere microbial dynamics
in soil. It is suggested that the persistent nature of introduced bacterial inoculants in
nutrient-limited habitats is closely related to their ability to resist starvation [74]. A
direct relationship between starvation resistance and the microbe s ability to survive
Table 14.2 Bacterial colonization in soil, rhizosphere (RS) and
phyllosphere (PS) of wheat and peas.
Wheat Peas
Bacterial strain Soil RS PS Soil RS PS
P. denitrificans PsD6 762b 6524a 1.4c 812.5b 11909a 7.3c
B. amyloliquefaciens BcA12 152.8b 912a 31.3c 60.6b 308a 4.8c
Mean values followed by different letters in the same row for each crop were significantly
different at P 0.05.