44
A. Dahiya et al.
and water content, and the presence of soil bacteria that may compete for binding
sites and nutrients on host plant root surfaces (Glick 2012; Sindhu and Dadarwal
2000). Endophytic bacteria have been shown to impart several beneficial effects on
their plant host directly or indirectly. They can benefit plants directly by assisting
plants in getting nutrients and improve plant growth by modulating growth-related
hormones, which can help plants grow better under normal and stressed conditions
(Ma et al. 2016) and indirectly by the improvement of plant growth due to inhabition
of phytopathogens (Miliute et al. 2015).
Nitrogen, phosphorus, and potassium are usually available in limited quantities
for plant growth in agricultural soils. The endophytic bacteria help their host plants
in getting increased amounts of limiting plant nutrients (Glick 2012). Endophytic
bacteria have been found to contribute toward solubilization of phosphate, potassium,
and zinc, and also provide fixed nitrogen and plant growth-promoting substances
to the host plants under a wide range of environmental conditions. These bacteria
release certain organic acids such as citric acids, oxalic acid, gluconic acid, lactic
acid, fumaric acid, etc., which contribute toward solubilization of bound phosphorus
in soil. Numerous genera with the ability to solubilize phosphorus and synthesize
auxins include Pseudomonas, Bacillus, Rhizobium, Xanthomonas, Serratia, Piriformospora, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Flavobacterium, Erwinia, Enterobacter, and Paenibacillus (Whitelaw 2000; Fraga et al. 2001;
Mota et al. 2008; Ribeiro and Cardoso 2012).
Phosphorous is the second most abundant plant nutrient after nitrogen, which is
crucial for enzymatic reactions responsible for different plant physiological processes
(Sindhu et al. 2014; Ahemad 2015). The majority of the soil phosphorus is insoluble, therefore, it cannot support the plant growth due to its unavailability. Moreover, almost 75% of phosphorus applied as fertilizer forms complexes with iron,
aluminum, and calcium in the soil and becomes unavailable for the plants (Ezawa
et al. 2002). Phosphorus deficiency causes stunted growth, forms dark leaves, causes
inhibition of flowering, and adversely affects the development of the root system.
Phosphorus compounds in the soil can be present in bound forms either as: (i) inorganic compounds, (ii) organic compounds of the soil humus, and (iii) organic and
inorganic P compounds associated with the cells of living matter. Mineral compounds
of P usually contain aluminum (Al), iron (Fe), manganese (Mn), and calcium (Ca)
and vary in different kinds of soils. For example, under acidic conditions phosphorus
forms a complex with Al, Fe, and Mn, where as it reacts very strongly with Ca in
alkaline soils (Khan et al. 2014). Thus, the applied P fertilizers are easily precipitated into insoluble forms, i.e., CaHPO 4 , Ca 3 (PO 4 ) 2 , FePO 4 , and AlPO
4− , and are
not efficiently taken up by the plants, which lead to an excess application of P
fertilizer to achieve maximum plant productivity (Omar 1998). These agricultural
practices disrupted natural ecological nutrient cycling, health hazards, environmental
disturbance, and damage to biological communities.
Many endophytic microorganisms enhance the availability of phosphorus for
plants by solubilization of precipitated phosphates (Nautiyal et al. 2000; Zhao et al.
2015; Adhikari and Pandey 2019) (Table 3.1). These microorganisms increase phosphorus availability in the soil either by secreting acid phosphatase that can mineralize
A. Dahiya et al.
and water content, and the presence of soil bacteria that may compete for binding
sites and nutrients on host plant root surfaces (Glick 2012; Sindhu and Dadarwal
2000). Endophytic bacteria have been shown to impart several beneficial effects on
their plant host directly or indirectly. They can benefit plants directly by assisting
plants in getting nutrients and improve plant growth by modulating growth-related
hormones, which can help plants grow better under normal and stressed conditions
(Ma et al. 2016) and indirectly by the improvement of plant growth due to inhabition
of phytopathogens (Miliute et al. 2015).
Nitrogen, phosphorus, and potassium are usually available in limited quantities
for plant growth in agricultural soils. The endophytic bacteria help their host plants
in getting increased amounts of limiting plant nutrients (Glick 2012). Endophytic
bacteria have been found to contribute toward solubilization of phosphate, potassium,
and zinc, and also provide fixed nitrogen and plant growth-promoting substances
to the host plants under a wide range of environmental conditions. These bacteria
release certain organic acids such as citric acids, oxalic acid, gluconic acid, lactic
acid, fumaric acid, etc., which contribute toward solubilization of bound phosphorus
in soil. Numerous genera with the ability to solubilize phosphorus and synthesize
auxins include Pseudomonas, Bacillus, Rhizobium, Xanthomonas, Serratia, Piriformospora, Burkholderia, Achromobacter, Agrobacterium, Micrococcus, Flavobacterium, Erwinia, Enterobacter, and Paenibacillus (Whitelaw 2000; Fraga et al. 2001;
Mota et al. 2008; Ribeiro and Cardoso 2012).
Phosphorous is the second most abundant plant nutrient after nitrogen, which is
crucial for enzymatic reactions responsible for different plant physiological processes
(Sindhu et al. 2014; Ahemad 2015). The majority of the soil phosphorus is insoluble, therefore, it cannot support the plant growth due to its unavailability. Moreover, almost 75% of phosphorus applied as fertilizer forms complexes with iron,
aluminum, and calcium in the soil and becomes unavailable for the plants (Ezawa
et al. 2002). Phosphorus deficiency causes stunted growth, forms dark leaves, causes
inhibition of flowering, and adversely affects the development of the root system.
Phosphorus compounds in the soil can be present in bound forms either as: (i) inorganic compounds, (ii) organic compounds of the soil humus, and (iii) organic and
inorganic P compounds associated with the cells of living matter. Mineral compounds
of P usually contain aluminum (Al), iron (Fe), manganese (Mn), and calcium (Ca)
and vary in different kinds of soils. For example, under acidic conditions phosphorus
forms a complex with Al, Fe, and Mn, where as it reacts very strongly with Ca in
alkaline soils (Khan et al. 2014). Thus, the applied P fertilizers are easily precipitated into insoluble forms, i.e., CaHPO 4 , Ca 3 (PO 4 ) 2 , FePO 4 , and AlPO
4− , and are
not efficiently taken up by the plants, which lead to an excess application of P
fertilizer to achieve maximum plant productivity (Omar 1998). These agricultural
practices disrupted natural ecological nutrient cycling, health hazards, environmental
disturbance, and damage to biological communities.
Many endophytic microorganisms enhance the availability of phosphorus for
plants by solubilization of precipitated phosphates (Nautiyal et al. 2000; Zhao et al.
2015; Adhikari and Pandey 2019) (Table 3.1). These microorganisms increase phosphorus availability in the soil either by secreting acid phosphatase that can mineralize