Agriculture Reference
In-Depth Information
macromolecules like proteins during dehydration and also acts as a scavenger for
hydroxyl radical, thus protecting from osmotic stress (Csonka
1989
; Upadhyay
et al.
2012
).
Antioxidative Enzymes
Another mechanism of PGPR to counteract stress involves the production of ROS
scavengers. Enhanced production of ROS, such as H
2
O
2
, hydroxyl radicals, singlet
oxygen, and superoxide, ensues oxidative damage to DNA, proteins, and lipids.
This response is mainly an outcome of imbalance in production and scavenging of
ROS due to stress condition. Major ROS scavengers include catalase, superoxide
dismutase, and ascorbate peroxidase. PGPR, for instance,
Serratia
sp
.
,
Rhizobium
sp
.
(Han and Lee
2005
),
Bacillus
sp.,
Arthrobacter
sp. (Upadhyay et al.
2012
),
Azospirullum
sp., and
Pseudomonas
sp. (Baniaghil et al.
2013
), are reported to
enhance the production of these antioxidant enzymes responsible for ROS degra-
dation/breakdown, thereby helping plants to ameliorate stress response and also
growth promotion (Kohler et al.
2009
; Carmen and Roberto
2011
).
Induced Systemic Tolerance
Similar to ISR for biotic factors, another term “induced systemic tolerance (IST)”
had been proposed for abiotic stress alleviation by PGPR. IST is defined as physical
and chemical changes elicited by PGPR in response to abiotic stresses such as salt
stress, drought stress, temperature stress, metal stress, or nutrition deficiency (Yang
et al.
2009
). These microbial communities follow different mechanisms such as
production of (1) volatiles to modulate Na
+
homeostasis under salt stress (Farag
et al.
2013
); (2) abscisic acid causing closure of stomata, thus preventing water loss
in drought stress; (3) antioxidant enzymes like superoxide dismutase and catalase,
which degrade the reactive oxygen species, bringing down cell damage
(Selvakumar et al.
2012
); (4) IAA, cytokinins, and other metabolites stimulating
root growth, thus helping nutrient acquisition combating nutrient deficiency; etc.
(Yang et al.
2009
). Some of the PGPR reported for IST include
B. cereus
,
B. subtilis
,
Serratia
sp
.
(Wang et al.
2012
),
Paenibacillus polymyxa
(Timmusk
and Wagner
1999
),
Achromobacter piechaudii
(Mayak et al.
2004
), etc.
