9 Phosphate Solubilization by Endophytes from the Tropical Plants
211
development of plants because it improves the entire root system, which consequently
improves the shoot (Raven et al. 2001). However, phosphates applied to agricultural
soils are rapidly immobilized and rendered inaccessible to plants. Due to this rapid
immobilization, many agricultural soils have large reservoirs of phosphates in inaccessible forms (Rodríguez and Fraga 1999; Rodríguez et al. 2006). In this scenario,
the ability of microorganisms to solubilize phosphorus is determined by their ability
to release metabolites such as organic acids, which chelate the cation bound to the
phosphate through their hydroxyl and carboxyl groups, converting the inaccessible
phosphate to a soluble form (van der Heijden et al. 2008). Even in a phosphorus-rich
soil, only a small fraction of the phosphorous is available to plants because most
of it is found in insoluble forms, and plants are not able to absorb it (Gyaneshwar
et al. 2002). Some kinds of the microorganism may be used to improve plant nutrient
utilization because they solubilize phosphate, making it available for absorption
by the plant. Among these microorganisms are phosphate-solubilizing bacteria and
fungi that participate in the phosphorus cycle and facilitate the conversion of insoluble
phosphorus to soluble forms via the secretion of organic acids and phosphatases, thus
making phosphorus available to the plant (Oliveira et al. 2003). Besides, phosphatesolubilizing microorganisms could play an important role in supplying phosphate
to plants in an eco-friendly and sustainable way (Oliveira et al. 2009; Gomes et al.
2014).
Phosphate-solubilizing microorganisms are found in soil, especially in rhizospheric microbial populations, and their numbers vary depending on the type of soil
(Mohammadi 2012). These microorganisms release low molecular-weight organic
acids that solubilize mineral phosphates and reduce the pH of the soil (Pérez et al.
2007; Gomes et al. 2014). Additionally, they have been widely tested as biofertilizers
and inoculants to increase crop yield through phosphate solubilization (Karpagam
and Nagalakshmi 2014; Baliah et al. 2016; Gurikar et al. 2016).
Endophytes represent a group of microorganisms that can colonize plants without
inducing the host defense pathway (Azevedo et al. 2000). Thus, the distinction between free-living soil microorganisms, the rhizosphere population, and the
symbionts of a host plant may represent a true continuum, with microbes able to
move between the soil, the rhizosphere, and the inside of the plant as endophytes
(Farrar et al. 2014; de Abreu et al 2017). In this case, several species of Bacillus
and Pseudomonas use nutrient niches in the rhizosphere and change from a freeliving condition to an endophytic state (Rosenblueth and Martínez-Romero 2006;
Gaiero et al. 2013). In this way, endophytes can be transported from the seeds into
the roots and tissues, reducing the need for continuous inoculation (Johnston-Monje
and Raizada 2011). However, the ability of endophytic microorganisms to solubilize phosphates in tropical and subtropical soils has not been sufficiently studied (de
Abreu et al. 2017). Endophytes, as phosphate solubilizers, are more competitive than
free-living or facultative microorganisms inside the host plant since the endophyte–
plant interaction is the result of an evolutionary process that is controlled by genes
of both organisms (Rosenblueth and Martínez-Romero 2006).
Endophytes can increase the availability of P for the plants by solubilizing precipitated phosphates, using mechanisms like acidification, chelation, ion exchange, and
211
development of plants because it improves the entire root system, which consequently
improves the shoot (Raven et al. 2001). However, phosphates applied to agricultural
soils are rapidly immobilized and rendered inaccessible to plants. Due to this rapid
immobilization, many agricultural soils have large reservoirs of phosphates in inaccessible forms (Rodríguez and Fraga 1999; Rodríguez et al. 2006). In this scenario,
the ability of microorganisms to solubilize phosphorus is determined by their ability
to release metabolites such as organic acids, which chelate the cation bound to the
phosphate through their hydroxyl and carboxyl groups, converting the inaccessible
phosphate to a soluble form (van der Heijden et al. 2008). Even in a phosphorus-rich
soil, only a small fraction of the phosphorous is available to plants because most
of it is found in insoluble forms, and plants are not able to absorb it (Gyaneshwar
et al. 2002). Some kinds of the microorganism may be used to improve plant nutrient
utilization because they solubilize phosphate, making it available for absorption
by the plant. Among these microorganisms are phosphate-solubilizing bacteria and
fungi that participate in the phosphorus cycle and facilitate the conversion of insoluble
phosphorus to soluble forms via the secretion of organic acids and phosphatases, thus
making phosphorus available to the plant (Oliveira et al. 2003). Besides, phosphatesolubilizing microorganisms could play an important role in supplying phosphate
to plants in an eco-friendly and sustainable way (Oliveira et al. 2009; Gomes et al.
2014).
Phosphate-solubilizing microorganisms are found in soil, especially in rhizospheric microbial populations, and their numbers vary depending on the type of soil
(Mohammadi 2012). These microorganisms release low molecular-weight organic
acids that solubilize mineral phosphates and reduce the pH of the soil (Pérez et al.
2007; Gomes et al. 2014). Additionally, they have been widely tested as biofertilizers
and inoculants to increase crop yield through phosphate solubilization (Karpagam
and Nagalakshmi 2014; Baliah et al. 2016; Gurikar et al. 2016).
Endophytes represent a group of microorganisms that can colonize plants without
inducing the host defense pathway (Azevedo et al. 2000). Thus, the distinction between free-living soil microorganisms, the rhizosphere population, and the
symbionts of a host plant may represent a true continuum, with microbes able to
move between the soil, the rhizosphere, and the inside of the plant as endophytes
(Farrar et al. 2014; de Abreu et al 2017). In this case, several species of Bacillus
and Pseudomonas use nutrient niches in the rhizosphere and change from a freeliving condition to an endophytic state (Rosenblueth and Martínez-Romero 2006;
Gaiero et al. 2013). In this way, endophytes can be transported from the seeds into
the roots and tissues, reducing the need for continuous inoculation (Johnston-Monje
and Raizada 2011). However, the ability of endophytic microorganisms to solubilize phosphates in tropical and subtropical soils has not been sufficiently studied (de
Abreu et al. 2017). Endophytes, as phosphate solubilizers, are more competitive than
free-living or facultative microorganisms inside the host plant since the endophyte–
plant interaction is the result of an evolutionary process that is controlled by genes
of both organisms (Rosenblueth and Martínez-Romero 2006).
Endophytes can increase the availability of P for the plants by solubilizing precipitated phosphates, using mechanisms like acidification, chelation, ion exchange, and