In another experiment conducted with plants Hordeum secalinum and Plantago
winteri which grow in natural salt meadow, Cardinale et al. (2015) isolated plant
growth promoting Curtobacterium flaccumfaciens from the rhizosphere of these
plants. They observed significant growth promoting activity to the tune of 300%
compared to control in barley seeds treated with isolated strain. It was shown to
possess nitrogen fixing ability, auxin synthesis, ACC deaminase activity, and calcium and phosphate mobilization required for plant growth promotion.
Another interesting conclusion these scientists claim is while isolating the strains
with a specific growth promoting activities mentioned earlier, we may land up in
only a few strains with these abilities. Hence, they reported that the screening of
organisms in pure culture may not be suitable with selected criteria of plant growth
promotions, in turn it is important to screen them thoroughly in consortia and it will
lead to understanding of indirect approach these strains use to promote plant growth.
Further these mechanisms may be novel and will add more insights into our
understanding on plant–microbe interaction in depth.
Electron transport chain in mitochondria and chloroplast is severely affected due
to the accumulation of these salt ions. Since two components required for the
development of superoxides that is molecular oxygen and electrons both are
involved in the electron transport chain, there is more possibility of generation of
free radicals due to its abnormal regulation of electron flow. Oxygen in presence of
electrons as an electron acceptor generates reactive oxygen species (ROS). This in
turn facilitates the formation of other free radicals such as hydroxyl ions,
peroxynitrile ions, and hydrogen peroxide, which are strong oxidizing agents and
affect plant metabolism severely even compared to ROS (Grob et al. 2013). To
overcome this problem plant responds in terms of several enzymes such as glutathione reductase, glutathione and ascorbate peroxidase, superoxide dismutase, and
proline catalase. These enzymes involve in scavenging free radicals (Asada 1999;
Noreen et al. 2010). Sharma and Sharma (2017) reported the involvement of
microbes in scavenging these radicals. They observed an increase in phenol content
and also other defensive enzymes in the plants treated with PGPR in response to high
salt and drought. Another interesting observation reported by these scientists is the
parallel increase in these defensive systems with the addition of silicon compared to
control.
The above observation is in line with the need of metal ions as a cofactor for the
activation of several enzymes. Different types of superoxide dismutases (SOD) are
available in plant systems; they are Mn-SOD, Fe-SOD, and Cu/Zn-SOD (Alscher
et al. 2002). The metal ions mentioned along with SOD need to be supplied to make
the enzyme active. This indirectly connects the rhizosphere microflora with the stress
responses. There are several microorganisms involved in mineralizing the metals
ions and supporting its uptake by the plant system through ascent of sap. It means if
Mn, Fe, Cu, and Zn are not supplied in sufficient quantity plants fail to respond to
stresses due to the nonavailability of active defensive enzymes. Conversely, PGPR
are reported to improve physiology of plants and antioxidant potential in providing
nutrients which are not capable of direct assimilation by plants from the environment. They are also involved in increasing the activity of several hormones such as
7 Microbe-Mediated Mitigation of Abiotic Stress in Plants
231
winteri which grow in natural salt meadow, Cardinale et al. (2015) isolated plant
growth promoting Curtobacterium flaccumfaciens from the rhizosphere of these
plants. They observed significant growth promoting activity to the tune of 300%
compared to control in barley seeds treated with isolated strain. It was shown to
possess nitrogen fixing ability, auxin synthesis, ACC deaminase activity, and calcium and phosphate mobilization required for plant growth promotion.
Another interesting conclusion these scientists claim is while isolating the strains
with a specific growth promoting activities mentioned earlier, we may land up in
only a few strains with these abilities. Hence, they reported that the screening of
organisms in pure culture may not be suitable with selected criteria of plant growth
promotions, in turn it is important to screen them thoroughly in consortia and it will
lead to understanding of indirect approach these strains use to promote plant growth.
Further these mechanisms may be novel and will add more insights into our
understanding on plant–microbe interaction in depth.
Electron transport chain in mitochondria and chloroplast is severely affected due
to the accumulation of these salt ions. Since two components required for the
development of superoxides that is molecular oxygen and electrons both are
involved in the electron transport chain, there is more possibility of generation of
free radicals due to its abnormal regulation of electron flow. Oxygen in presence of
electrons as an electron acceptor generates reactive oxygen species (ROS). This in
turn facilitates the formation of other free radicals such as hydroxyl ions,
peroxynitrile ions, and hydrogen peroxide, which are strong oxidizing agents and
affect plant metabolism severely even compared to ROS (Grob et al. 2013). To
overcome this problem plant responds in terms of several enzymes such as glutathione reductase, glutathione and ascorbate peroxidase, superoxide dismutase, and
proline catalase. These enzymes involve in scavenging free radicals (Asada 1999;
Noreen et al. 2010). Sharma and Sharma (2017) reported the involvement of
microbes in scavenging these radicals. They observed an increase in phenol content
and also other defensive enzymes in the plants treated with PGPR in response to high
salt and drought. Another interesting observation reported by these scientists is the
parallel increase in these defensive systems with the addition of silicon compared to
control.
The above observation is in line with the need of metal ions as a cofactor for the
activation of several enzymes. Different types of superoxide dismutases (SOD) are
available in plant systems; they are Mn-SOD, Fe-SOD, and Cu/Zn-SOD (Alscher
et al. 2002). The metal ions mentioned along with SOD need to be supplied to make
the enzyme active. This indirectly connects the rhizosphere microflora with the stress
responses. There are several microorganisms involved in mineralizing the metals
ions and supporting its uptake by the plant system through ascent of sap. It means if
Mn, Fe, Cu, and Zn are not supplied in sufficient quantity plants fail to respond to
stresses due to the nonavailability of active defensive enzymes. Conversely, PGPR
are reported to improve physiology of plants and antioxidant potential in providing
nutrients which are not capable of direct assimilation by plants from the environment. They are also involved in increasing the activity of several hormones such as
7 Microbe-Mediated Mitigation of Abiotic Stress in Plants
231