ethylene, pyrroloquinoline quinone, cytokinins, abscisic acid, indole acetic acid, and
gibberellins (Perrig et al. 2007). Out of these hormones IAA was reported to play
important role in resistance against salt along with its plant growth promotion (Ali
and Abbas 2003; Kaya et al. 2013; Khalid et al. 2013; Kang et al. 2014). PGPR
therefore through augmentation of this hormone favor plant growth instead of uptake
of the ions responsible for high salinity (Zhang et al. 2008). Along with this Alavi
et al. (2013) reported another molecule spermidine which serves as a protector
molecule in saline stress.
ACC deaminase serves as another key factor involved in supporting the plants in
high salt concentrations. Several rhizospheres inhabiting pseudomonads are known
to have the ability to produce this enzyme along with Bacillus sp. and
Brevibacterium sp. (Siddikee et al. 2010, 2011), Burkholderia sp. (Shaharoona
et al. 2007), and Achromobacter sp. (Mayak et al. 2004; Karthikeyan et al. 2012).
Along with this several root colonizers revealed promising results in lettuce (Kohler
et al. 2009), cucumber (Egamberdieva et al. 2011), sunflowers (Shilev et al. 2012),
rice (Jha et al. 2011), mung bean (Ahmad et al. 2011), wheat (Zahir et al. 2009;
Nadeem et al. 2010), and tomato (Tank and Saraf 2010).
The list of microorganisms associated with salt tolerance directly or indirectly
with plants is many. The reports reveal that the diversity of these organisms varies
with the plant species and the soil or rhizosphere environment. Different organisms
are associated with different plants and the possible mechanism associated with salt
tolerance is mentioned in Table 7.1.
7.3
Drought Resistance
Growing demand for safe drinking water due to overexplosion of population is
creating water scarcity for human consumption itself. Agricultural practices and
even the varieties grown in agricultural fields are reported to be resource consuming
including demand for more water supplies. Along with these there is a drastic change
in the environmental patterns all over the world. Global warming is taking the front
seat in making agricultural systems failure in connection with crop loss due to
photoperiodism or circadian rhythm.
Lack of water is known to induce stomatal closure through which it directly
impacts the rate of photosynthesis and the ability of the plants to acquire nutrients.
Several alterations in response to high salt concentration in plants hold good for
altered temperature. It is reported that the hormonal variation and production of
reactive oxygen species due to drought condition are the main reason for plant
suffering. In this situation, PGPR are also known to provide additional protective
measures such as water retention and soil aggregation near the root zone along with
this PGPR is known to promote the production of enzymes such as
1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, phosphatase to help in
the assimilation of more phosphorus, siderophore production, and also growth
hormones such as indole-3-acetic acid. ACC deaminase serves as a best candidate
to reduce the impact of high salt concentration. It is reported that it removes the
232
M. P. Raghavendra
gibberellins (Perrig et al. 2007). Out of these hormones IAA was reported to play
important role in resistance against salt along with its plant growth promotion (Ali
and Abbas 2003; Kaya et al. 2013; Khalid et al. 2013; Kang et al. 2014). PGPR
therefore through augmentation of this hormone favor plant growth instead of uptake
of the ions responsible for high salinity (Zhang et al. 2008). Along with this Alavi
et al. (2013) reported another molecule spermidine which serves as a protector
molecule in saline stress.
ACC deaminase serves as another key factor involved in supporting the plants in
high salt concentrations. Several rhizospheres inhabiting pseudomonads are known
to have the ability to produce this enzyme along with Bacillus sp. and
Brevibacterium sp. (Siddikee et al. 2010, 2011), Burkholderia sp. (Shaharoona
et al. 2007), and Achromobacter sp. (Mayak et al. 2004; Karthikeyan et al. 2012).
Along with this several root colonizers revealed promising results in lettuce (Kohler
et al. 2009), cucumber (Egamberdieva et al. 2011), sunflowers (Shilev et al. 2012),
rice (Jha et al. 2011), mung bean (Ahmad et al. 2011), wheat (Zahir et al. 2009;
Nadeem et al. 2010), and tomato (Tank and Saraf 2010).
The list of microorganisms associated with salt tolerance directly or indirectly
with plants is many. The reports reveal that the diversity of these organisms varies
with the plant species and the soil or rhizosphere environment. Different organisms
are associated with different plants and the possible mechanism associated with salt
tolerance is mentioned in Table 7.1.
7.3
Drought Resistance
Growing demand for safe drinking water due to overexplosion of population is
creating water scarcity for human consumption itself. Agricultural practices and
even the varieties grown in agricultural fields are reported to be resource consuming
including demand for more water supplies. Along with these there is a drastic change
in the environmental patterns all over the world. Global warming is taking the front
seat in making agricultural systems failure in connection with crop loss due to
photoperiodism or circadian rhythm.
Lack of water is known to induce stomatal closure through which it directly
impacts the rate of photosynthesis and the ability of the plants to acquire nutrients.
Several alterations in response to high salt concentration in plants hold good for
altered temperature. It is reported that the hormonal variation and production of
reactive oxygen species due to drought condition are the main reason for plant
suffering. In this situation, PGPR are also known to provide additional protective
measures such as water retention and soil aggregation near the root zone along with
this PGPR is known to promote the production of enzymes such as
1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, phosphatase to help in
the assimilation of more phosphorus, siderophore production, and also growth
hormones such as indole-3-acetic acid. ACC deaminase serves as a best candidate
to reduce the impact of high salt concentration. It is reported that it removes the
232
M. P. Raghavendra