188
H. B. Slama et al.
The nutrient acquisition enhancement includes typically: atmospheric nitrogen
fixation, siderophores production, and phosphorus solubilization. Although, some
workers have also studied the potassium and zinc solubilization (Bhatt and Maheshwari 2020). Sulphur oxidation studies have recently been observed from the nonrhizospheric bacteria (Dhiman et al. 2020). Modulation of hormone levels may
entail the synthesis or regulation of one or more plant phytohormones such as
auxins (Gaby and Buckley 2011; Verma et al. 2014) mostly indole acetic acid (IAA),
gibberellins, and cytokinins. Some PGPB reduces ethylene levels by synthesizing 1aminocyclopropane-1-carboxylate (ACC) deaminase enzyme, which cleaves ACC,
the precursor of ethylene in all higher plants (Zahir et al. 2011). Maheshwari and
colleague have been reported the role of Bacillus sp. with ACC deaminase production ability and other endophytes such as Sinorhizobium meliloti bear acdS gene
conferring ACC deaminase (Kumar et al. 2012; Maheshwari et al. 2015; Aeron et al.
2014).
8.6.1 Atmospheric Nitrogen Fixation
Nitrogen (N) belongs to the most essential elements that are responsible for plant
growth. Plants are not able to use atmospheric nitrogen directly (Santi et al. 2013).
Therefore, the use of beneficial microbes is an environment-friendly technique that
enhances nitrogen fixation (Gaby and Buckley 2011; Verma et al. 2014) in both
leguminous and non-leguminous plants.
8.6.2 Phosphorus Solubilization
Phosphorus (P) is an important nutrient for plant growth and sustainability. It is
involved in several metabolic processes (Li et al. 2017). Yet, it cannot be easily assimilated by plants because of its insoluble form (Ahemad and Kibret 2014). Microbial
phosphate solubilization is required to enhance phosphate availability by secreting
several organic acids (Haile et al. 2016; Chauhan et al. 2017) and other mechanisms
involved in P-solubilization.
8.6.3 Siderophore Production
Iron is extremely necessary for plants (Curie and Mari 2017; Tsai and Schmidt
2017). The predominant iron form in nature is a ferric ion (Fe
3+ ), it is only sparingly
soluble, which explains the difficulty of its assimilation by plants. Microbes are the
major precursors of iron uptake through specific pathways, including siderophores
production (Ahemad and Kibret 2014; Ahmad et al. 2017).
H. B. Slama et al.
The nutrient acquisition enhancement includes typically: atmospheric nitrogen
fixation, siderophores production, and phosphorus solubilization. Although, some
workers have also studied the potassium and zinc solubilization (Bhatt and Maheshwari 2020). Sulphur oxidation studies have recently been observed from the nonrhizospheric bacteria (Dhiman et al. 2020). Modulation of hormone levels may
entail the synthesis or regulation of one or more plant phytohormones such as
auxins (Gaby and Buckley 2011; Verma et al. 2014) mostly indole acetic acid (IAA),
gibberellins, and cytokinins. Some PGPB reduces ethylene levels by synthesizing 1aminocyclopropane-1-carboxylate (ACC) deaminase enzyme, which cleaves ACC,
the precursor of ethylene in all higher plants (Zahir et al. 2011). Maheshwari and
colleague have been reported the role of Bacillus sp. with ACC deaminase production ability and other endophytes such as Sinorhizobium meliloti bear acdS gene
conferring ACC deaminase (Kumar et al. 2012; Maheshwari et al. 2015; Aeron et al.
2014).
8.6.1 Atmospheric Nitrogen Fixation
Nitrogen (N) belongs to the most essential elements that are responsible for plant
growth. Plants are not able to use atmospheric nitrogen directly (Santi et al. 2013).
Therefore, the use of beneficial microbes is an environment-friendly technique that
enhances nitrogen fixation (Gaby and Buckley 2011; Verma et al. 2014) in both
leguminous and non-leguminous plants.
8.6.2 Phosphorus Solubilization
Phosphorus (P) is an important nutrient for plant growth and sustainability. It is
involved in several metabolic processes (Li et al. 2017). Yet, it cannot be easily assimilated by plants because of its insoluble form (Ahemad and Kibret 2014). Microbial
phosphate solubilization is required to enhance phosphate availability by secreting
several organic acids (Haile et al. 2016; Chauhan et al. 2017) and other mechanisms
involved in P-solubilization.
8.6.3 Siderophore Production
Iron is extremely necessary for plants (Curie and Mari 2017; Tsai and Schmidt
2017). The predominant iron form in nature is a ferric ion (Fe
3+ ), it is only sparingly
soluble, which explains the difficulty of its assimilation by plants. Microbes are the
major precursors of iron uptake through specific pathways, including siderophores
production (Ahemad and Kibret 2014; Ahmad et al. 2017).