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

3.2 PGPR Grouped According to Action Mechanismsj43
Soviet Union [9]. They are free-living organisms that inhabit the rhizosphere but
do not establish a symbiosis with the plant. Although they do not penetrate the
plant s tissues, a very close relationship is established; these bacteria live sufficiently
close to the root such that the atmospheric nitrogen fixed by the bacteria
thatisnotusedfortheirownbenefit is taken up by the plant, forming an extra
supply of nitrogen. This relationship is described as an unspecific andloose
symbiosis.
Free nitrogen-fixing bacteria belong to a wide array of taxa; among the most
relevant bacterial genera are Azospirillum, Azotobacter, Burkholderia, Herbaspirillum
and Bacillus [61]. Biological nitrogen fixation is a high-cost process in terms of
energy. Bacterial strains capable of performing this process do so in order to fulfill
their physiological needs and thus little nitrogen is left for the plant s use. However,
growth promotion caused by nitrogen-fixing PGPR was attributed to nitrogen fixation
for many years, until the use of nitrogen isotopes showed additional effects.
This technique showed that the benefits of free nitrogen-fixing bacteria are due more
to the production of plant growth regulators than to the nitrogen fixation [9]. Production
of plant growth regulators is discussed below.
Azotobacteraceae is the most representative of bacterial genera able to perform
free nitrogen fixation. Various reports describe the benefits of Azotobacteraceae on
several crops [43]. According to data provided by the FAO [21], amounts of nitrogen
supplied to soil are low; Bhattacharya and Chaudhuri [11] report that the amount
ranges between 20 and 30 kg per hectare per year.
Azotobacter is the genus most used in agricultural trials. The first reports appeared
in 1902 and it was widely used in Eastern Europe during the middle decades of the
last century [29]. As previously suggested, the effect of Azotobacter and Azospirillum
is attributed not only to the amounts of fixed nitrogen but also to the production of
plant growth regulators (indole acetic acid, gibberellic acid, cytokinins and vitamins),
which result in additional positive effects to the plant [48].
Application of inoculants in agriculture has resulted in notable increases in crop
yields, especially in cereals, where Azotobacter chroococcum and Azospirillum brasilense
have been very important. These two species include strains capable of releasing
substances such as vitamins and plant growth regulators, which have a direct
influence on plant growth [5,19,29,43,48,60]. According to González and Lluch [29],
the production of these substances by Azotobacter strains is seriously affected by
nitrogen availability, which affects auxin and gibberellin production; but when
nitrate is available, auxin release is impaired while gibberellin synthesis is enhanced.
As mentioned above, the amount of nitrogen from free fixation available to the
plantislowbecauseitisusedefficiently by the bacteria. Three strategies have been
proposed to address this low-yield problem: (i) glutamine synthase bacterial
mutants, (ii) formation of paranodules and (iii) facilitating the penetration of plant
tissues by nitrogen-fixing bacterial endophytes that enhance colonization in a low
competition niche. Regarding the first strategy, mutations target the glutamine
synthase gene, focusing on achieving low efficiency in retaining the fixed nitrogen
so that it is released for the plant. The main problem with these types of mutants is
that they are not very effective in colonizing the root system [9]. To overcome this