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

still viable and showed inhibition of IR. IR can be mediated by polysaccharides such
as xanthum gum or by AFPs that have been reported in few bacteria, for example,
Antarctic Moraxella, [38] Arctic Rhizobacterium [39,40], cold-acclimated Micrococcus
and Rhodococcus [41]. Therefore, it was hypothesized that IR activity can potentially
contribute to overall viability of a microbial consortium under Chinook conditions.
10.3
Mechanism of Plant Growth Promotion at Low Temperature
When deciding on the type of bacterial strains to be used with a plant for a given
climatic condition, understanding of the mechanism of plant growth promotion is
essential; for example, the overwintering ability of PGPR is fundamental when
considering them to use in colder climates. There is evidence that Pseudomonas
sp. are able to overwinter in sufficient quantities on roots of winter wheat [42,43].
The principal mechanisms of plant growth promotion in colder regions include
phytostimulation and frost injury protection.
10.3.1
Phytostimulation
10.3 Mechanism of Plant Growth Promotion at Low Temperaturej201
Phytostimulators are chemical compounds produced by a number of bacteria that
directly enhance plant growth. Different genera of bacteria such as Proteus mirabilus,
Pseudomonas vulgaris, Klebsiella pneumoniae, Bacillus cereus, Escherichia coli and so on
produce auxin cytokinins, gibberellins and abscisic acid [30]. Quantitatively, auxins
are the most abundant phytohormone secreted by PGPR strains such as Azospirullum
and is the major factor that stimulates root generation and enhances root
growth. ABA and ethylene have been shown to play an essential role in plant stress
signaling [43]. A more direct correlation is evident between the level of ABA and the
increasing freeze tolerance [44,45]. Exogenous application of ABA can increase
freeze tolerance in both woody and herbaceous plants [46,47]. Several studies have
convincingly demonstrated that exogenous application of ABA increases cold tolerance.
Application of ABA at room temperature increased cold resistance in callus
explants of tobacco, cucumber, winter wheat and alfalfa. Therefore, the PGPR strains
capable of producing ABA can be used for protecting seedlings and plants from
freeze injuries and hence can contribute to growth enhancement at low
temperature.
Another important factor in phytostimulation is lowering the plant ethylene level,
which gets elevated during stress in plants. Higher concentrations of ethylene are
inhibitive to plant growth. Any factor or stimulus that causes changes in endogenous
levels of ethylene in plants leads to enhanced growth and development [43]. It has
been discovered that certain microorganisms contain an enzyme, ACC deaminase,
that hydrolyzes ACC into ammonia and a-ketobutyrate [48]. Hall et al. [49] reported
that a soil isolate P. putida GR12-12 contains a gene for ACC deaminase, which
hydrolyzes ACC, the immediate precursor of ethylene synthesized in plant tissues,