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

48j 3 Physiological and Molecular Mechanisms of Plant Growth Promoting Rhizobacteria (PGPR)
increasing its size and weight, branching number and the surface area in contact with
soil. All these changes lead to an increase in its ability to probe the soil for nutrient
exchange, therefore improving plant nutrition and growth capacity [31]. Another important
result of inoculation with auxin-producing bacteria is the formation of adventitious
roots, which derive from the stem. The auxins induce the stem tissues to redifferentiate
as root tissue. All the above effects can vary considerably depending on the auxin
concentration that reaches the root system, including an excess that could be inhibitory.
The production of hormones such as gibberellins or cytokinins is still not well
documented owing to the small number of bacteria able to produce these plant
growth regulators [18,50,53]. There is little information regarding microorganisms
that produce gibberellins, although it is known that symbiotic bacteria existing
within nodules in leguminous plants to fix nitrogen (rhizobia) are able to produce
gibberellins, auxins and cytokinins in very low concentrations when the plant is
forming the nodule and there is a high cell duplication rate [4]. However, the
production of gibberellins by PGPR is rare, with only two strains being documented
that produce gibberellins, Bacillus pumilus and Bacillus licheniformis [32]. These
bacteria were isolated from the rhizosphere of A. glutinosa and have shown a capacity
to produce large quantities of gibberellins GA1, GA3, GA4 and GA20 in vitro. These
types of hormones are the largest group of plant regulators, including more than 100
different molecules with various degrees of biological activity. The common structure
of these diterpenic growth regulators is a skeleton of 19–20 carbon atoms, and
there is a clear relationship between structure and biological effect. The reason for
the pronounced effect of gibberellins is that these hormones can be translocated
from the roots to the aerial parts of the plant. The effects in the aerial part are notable,
and more so when the bacteria also produce auxins that stimulate the root system,
enhancing the nutrient supply to the sink generated in the aerial part.
Ethylene is another growth regulator whose levels alter PGPR, consequently affecting
physiological processes in the plant. Ethylene is fundamentally related to plant
growth and defense systems and is also implicated in stress response. Factors such as
light, temperature, salinity, pathogen attack and nutritional status cause marked variations
in ethylene levels. The influence of abiotic factors on ethylene levels was deduced
before those of biotic factors [1,44]. This hormone mediates in stress response and
adaptive processes, thus being decisive for plant survival. It also mediates other processes
not related to stress such as ripening, root growth and seed germination.
Although ethylene is important as a growth regulator for normal plant development,
there are examples in which ethylene does not appear to have a significant role; for
example, mutant plants impaired in ethylene synthesis can survive. Application of
ethylene synthesis inhibitors to several plant species makes the plant more sensitive to
pathogen attack and abiotic stress [38]. The ethylene effect has been known for centuries.
In China and Russia, pear and banana were stored in rooms where wood or
incense was burned (producing ethylene in the combustion) to accelerate ripening
[1]. In the nineteenth century, Russian researcherNeljubov deduced the ethylene effect
in plants using different combustion gases [2]. It is often preferable to delay or reduce
ethylene synthesis, slowing down ripening and thus extending the lifetime of the
fruit [30]. As ethylene levels decrease, root systems increase their growth, with the