Agriculture Reference
In-Depth Information
2013
). Phosphate-solubilizing
Pantoea cypripedii
and
Enterobacter aerogenes
used together increased P uptake by 53 % in chickpea crop compared to control
(Singh et al.
2013
).
11.5 PGPR and Stress Alleviation
Different studies have suggested that such microorganisms can also divulge some
degree of tolerance to the plants thus imparting resistance to these plants. Tolerance
can be defined as microbe's intrinsic property to encounter stressful conditions,
whereas resistance is microorganism's ability to withstand stressful conditions by
certain mechanisms. Some of the experimental evidence indicates that micro-
organisms with tolerance/resistance abilities can help plants to successfully adapt
to different stressed situations. Therefore, the organisms endowed with tolerance/
resistance abilities can be used effectively as beneficial inoculants for enhancing
crop production in stressed/derelict soils (Khan et al.
2011
; Milosevic et al.
2012
).
Some of the mechanisms by which PGPR ameliorate stress situations are discussed
in the following section and are illustrated in Fig.
11.2
.
11.5.1 Mechanisms and Role of PGPR in Stress Alleviation
11.5.1.1 Exopolysaccharide Secretion
Microorganisms belonging to different functional groups for example rhizobia
secrete exopolysaccharides (EPS), which provide resistance to cell against different
stressors and thus protect the microorganism from stress. The EPS also improve the
soil structure by forming macroaggregates with soil, which further increase the
water retention ability of soil (Alami et al.
2000
). Macroaggregates uphold equili-
brium in aerobic and anaerobic conditions in soil and also ascertain gradual uptake
of nutrients from soil. In case of salt stress, these aggregates help by binding cations
making them unavailable to plants (Haynes and Swift
1990
). The rhizobacteria
have the ability to form biofilms by secreting polysaccharides and proteins, the
matrix so formed limits the diffusion of compounds like plant growth hormones and
nutrients from the plant's vicinity, thus promoting plant growth by alleviating stress
conditions (Timmusk et al.
2013
).
11.5.1.2 Accommodation: Accumulation and Sequestration of Metals
Plant growth-promoting rhizobacteria produce metal-chelating agents, known as
siderophores, an iron-chelating agent, which can make the required iron available to
plants and hence prevent plants
from becoming chlorotic and indirectly
