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

196j 10 Principles, Applications and Future Aspects of Cold-Adapted PGPR
Hydrogenophaga, Enterobacter, Serratia and Azospirullum [3]. The major application of
PGPR strains is plant growth improvement in agriculture, horticulture, forestry and
environmental restoration. The mode of action of PGPR strains can be classified into
two major categories: indirect and direct. The indirect mode includes antibiotic
production, reduction of iron availability to phytopathogens and synthesis of cellwall
lysing enzymes. The direct mode involves provision of bioavailable phosphorus
for plant uptake, nitrogen fixation, sequestration of iron for plant uptake by siderophores,
production of plant hormones such as auxin, cytokinin and gibberellins
and lowering of plant ethylene production [4].
To utilize PGPR for growth promotion, it is inevitable that PGPR must colonize
and survive in the rhizosphere of the host plants. Colonization of PGPR in the
rhizosphere is influenced by a number of factors such as soil temperature and type,
predation by protozoa, production of antimicrobial compounds by other soil microorganisms,
bacterial growth rate and utilization of exudates. Of these abiotic and
biotic factors, the most important is soil temperature. Kemp and coworkers [3]
studied the influence of soil temperature on the leaching of inoculated rhizobacteria
in soil microcosms and showed that the whole process is favored at low temperature
(15 C) than at higher temperature (35 C).
The role of seed and root exudates as the source of nutrients for microorganisms
has been demonstrated as an important factor responsible for their colonization.
The biosynthesis of antagonistic compounds by PGPR strains might also play an
important role in establishing bacterial populations in the rhizosphere [12]. It has
also been reported that PGPR strains isolated from the native rhizosphere colonize
faster and show maximum increase in germination and yield as compared to PGPR
strains isolated from nonrhizospheric soil or rhizosphere of other plants [5]. Therefore,
for better colonization and higher plant growth promotion at low temperature,
PGPR strains must be tolerant to cold.
10.2
Cold Adaptation of PGPR Strains
Cold survival often requires organisms to exhibit a wide range of flexible behavior
and physiological adjustments, including adaptive features in their membranes,
protein structure and genetic responses to thermal shifts. Compared to its mesophilic
counterparts, a cold-active enzyme tends to have reduced activation energy,
leading to a high catalytic efficiency, which may possibly be attributed to an enhanced
local or overall flexibility of the protein structure. Membranes appear to
incorporate specific lipid constituents to maintain fluidity and critical ability to
transport substrates and nutrients under very cold, rather rigidifying, conditions
[6]. Production of antifreeze proteins and accumulation of compounds that
inhibit ice recrystallization (IR) can also be part of the adaptive response in some
bacteria [7,8]. Various mechanisms used for cold adaptation of PGPR are discussed
below.