36
A. Dahiya et al.
cause any noticeable symptoms to their host plants (Santoyo et al. 2016). These
microbes can exist within the aboveground and underground parts of crop plants
and even in the seeds, which can be isolated from surface-disinfested plant tissue or
extracted from inside the plant (Hallmann et al. 1997; Chebotar et al. 2015). These
endophytic microorganisms use the plant endosphere as a unique protective ecological niche that provides a safe and unperturbed environment under fluctuating conditions (Senthilkumar et al. 2011). Most of the endophytic microorganisms possess a
biphasic life cycle that alternates between plants and soil environments. Thus, endophytic microorganisms are a specialized group of rhizosphere microbes that have
acquired the ability to invade their plant host (Reinhold-Hurek and Hurek 2011).
They share all the important traits consistent with the host plant growth promotion
of beneficial rhizosphere microbes. Endophytes can communicate and interact with
the plant more efficiently than rhizospheric bacteria (Ali et al. 2012; Coutinho et al.
2015). Usually, the plant growth promotion effects observed due to the inoculation
of endophytic microbes to host plants are greater than those provided by many rhizospheric microorganisms. These may benefit crop plants directly by enhancing nutrient
availability and may improve plant growth by modulating growth-related hormones
under normal as well as stressed environmental conditions (Ma et al. 2015). Indirectly, endophytic microbes may improve plant growth by inhibiting the growth of
phytopathogens using mechanisms like production of antibiotic, siderophores, lytic
enzyme, and by priming plant immunity (Luo et al. 2012; Coutinho et al. 2015;
Miliute et al. 2015; Maheshwari et al. 2017).
Phosphorus is the second most important nutrient for the growth and development
of plants after nitrogen. Plants require approximately 30 μmol l
−1 of soil phosphorus
for maximum productivity, but its availability is only about 1 μmol l
−1 in many soils.
Therefore, the unavailability of phosphorus in many soils has been recognized as a
major growth-limiting factor in agricultural and horticultural systems (Daniels et al.
2009). On the other hand, the efficiency of applied phosphorus rarely exceeds 30%
due to fixation as calcium, iron, or aluminum phosphates in soil (Sharma et al. 2013).
Some of the phosphorus is also lost as a result of run-off and leaching (Sashidhar
and Podile 2009).
Thus, phosphorus deficiency in soil is traditionally overcome by adding either
phosphatic fertilizers (Khan et al. 2007) or it may be incorporated as leaf litter,
plant residues, or animal remains. The phosphatic fertilizers are the world’s secondlargest bulk chemical used in agriculture on earth (Goldstein 2007). After the addition
of chemical phosphatic fertilizers, the extremely reactive soluble phosphate anions
(H 2 PO 4
− , HPO 4
2− ) may form metal complexes with Ca in calcareous soils (Lindsay
et al. 1989) and Fe
3+ and Al
3+ in acidic soils (Norrish and Rosser 1983). Thus,
a large portion, i.e., 75–90% of added P fertilizer in agricultural soils is precipitated/immobilized rapidly by iron, aluminum, manganese, and calcium complexes
depending on soil type, soil pH, and existing minerals (Bünemann et al. 2006; Vu
et al. 2008; Miller et al. 2010). Moreover, the phosphatic fertilizers are being prepared
from phosphate-containing rocks, which is a non-renewable resource. The current
global reserves of rock phosphates may be depleted in the next 50–100 years (Cordell
et al. 2009). Therefore, endophytes having phosphate solubilization ability could
A. Dahiya et al.
cause any noticeable symptoms to their host plants (Santoyo et al. 2016). These
microbes can exist within the aboveground and underground parts of crop plants
and even in the seeds, which can be isolated from surface-disinfested plant tissue or
extracted from inside the plant (Hallmann et al. 1997; Chebotar et al. 2015). These
endophytic microorganisms use the plant endosphere as a unique protective ecological niche that provides a safe and unperturbed environment under fluctuating conditions (Senthilkumar et al. 2011). Most of the endophytic microorganisms possess a
biphasic life cycle that alternates between plants and soil environments. Thus, endophytic microorganisms are a specialized group of rhizosphere microbes that have
acquired the ability to invade their plant host (Reinhold-Hurek and Hurek 2011).
They share all the important traits consistent with the host plant growth promotion
of beneficial rhizosphere microbes. Endophytes can communicate and interact with
the plant more efficiently than rhizospheric bacteria (Ali et al. 2012; Coutinho et al.
2015). Usually, the plant growth promotion effects observed due to the inoculation
of endophytic microbes to host plants are greater than those provided by many rhizospheric microorganisms. These may benefit crop plants directly by enhancing nutrient
availability and may improve plant growth by modulating growth-related hormones
under normal as well as stressed environmental conditions (Ma et al. 2015). Indirectly, endophytic microbes may improve plant growth by inhibiting the growth of
phytopathogens using mechanisms like production of antibiotic, siderophores, lytic
enzyme, and by priming plant immunity (Luo et al. 2012; Coutinho et al. 2015;
Miliute et al. 2015; Maheshwari et al. 2017).
Phosphorus is the second most important nutrient for the growth and development
of plants after nitrogen. Plants require approximately 30 μmol l
−1 of soil phosphorus
for maximum productivity, but its availability is only about 1 μmol l
−1 in many soils.
Therefore, the unavailability of phosphorus in many soils has been recognized as a
major growth-limiting factor in agricultural and horticultural systems (Daniels et al.
2009). On the other hand, the efficiency of applied phosphorus rarely exceeds 30%
due to fixation as calcium, iron, or aluminum phosphates in soil (Sharma et al. 2013).
Some of the phosphorus is also lost as a result of run-off and leaching (Sashidhar
and Podile 2009).
Thus, phosphorus deficiency in soil is traditionally overcome by adding either
phosphatic fertilizers (Khan et al. 2007) or it may be incorporated as leaf litter,
plant residues, or animal remains. The phosphatic fertilizers are the world’s secondlargest bulk chemical used in agriculture on earth (Goldstein 2007). After the addition
of chemical phosphatic fertilizers, the extremely reactive soluble phosphate anions
(H 2 PO 4
− , HPO 4
2− ) may form metal complexes with Ca in calcareous soils (Lindsay
et al. 1989) and Fe
3+ and Al
3+ in acidic soils (Norrish and Rosser 1983). Thus,
a large portion, i.e., 75–90% of added P fertilizer in agricultural soils is precipitated/immobilized rapidly by iron, aluminum, manganese, and calcium complexes
depending on soil type, soil pH, and existing minerals (Bünemann et al. 2006; Vu
et al. 2008; Miller et al. 2010). Moreover, the phosphatic fertilizers are being prepared
from phosphate-containing rocks, which is a non-renewable resource. The current
global reserves of rock phosphates may be depleted in the next 50–100 years (Cordell
et al. 2009). Therefore, endophytes having phosphate solubilization ability could