12 Endophytic Rhizobacteria for Mineral Nutrients …
273
other essential PGP functions including the solubilization abilities for other important
plant nutrients. Nevertheless, there are studies that demonstrate the existence of K
solubilizing endophytes.
Potassium solubilizing endophytic rhizobacteria have been identified from wheat
(Verma et al. 2013, 2015), more recently, from pearl millet (Kushwaha et al. 2019),
maize (Jha 2019), and other crops (Dhiman et al. 2019). Rhizobia are the best-studied
endophytes and are widely known for symbiotic N 2 fixation in leguminous plants
(Santoyo et al. 2016). However, of late, these novel rhizobacteria have also been
shown to solubilize K in plant rhizospheres. For instance, K solubilization by rhizobia
in rice has recently been reported by Patel et al. (2017a). Thirumal et al. (2017)
demonstrated 5 rhizobial cultures associated with K solubilization in vitro. These
new discoveries suggest that apart from enhancing N nutrition in plants, rhizobia can
also be exploited for their K solubilizing abilities to enhance K availability in plant
rhizosphere.
Indigenous KSB are currently in the limelight for sustainable cropping systems
and environmental conservation and have emerged as one of the viable technologies
for mitigating K-deficiency in soils (Meena et al. 2015). Potassium solubilization
indeed holds a lot of potential for PGP and the K solubilizing abilities of endophytic rhizobacteria are worth exploring. According to Meena et al. (2018), KSB are
precious bio-resources that can mitigate K-deficiency in agricultural soils but their
experimental evidence at the field level is still inadequate. Such processes may need
to be exploited in detail so as to increase their usability.
12.2.3 Endophytic Rhizobacteria and Phosphorus
Acquisition in Plants
Phosphorus is the second most important plant nutrient after N (Goswami et al.
2016). Although soils contain P reserves, most of this is available in insoluble forms
and inaccessible to plants (Verma et al. 2019).
This non-availability is recognized as a major plant growth-limiting factor in
agricultural systems (Sharma et al. 2013). The P solubilization potential of soil
microorganisms is one of the most essential traits of PGPR for enhancing P-nutrition
acquisition in plants (Walia and Shirkot 2012; Ouattara et al. 2019). While P solubilizing rhizobacteria are widely investigated, recent literature maintains that only a
few endophytic rhizobacteria possess this ability (Brigido et al. 2019).
Nevertheless, there is mounting evidence on the role of endophytes in P solubilization and mobilization compared to their widely reported rhizospheric counterparts
(Ji et al. 2014; Oteino et al. 2015; Walitang et al. 2019). PSB can proliferate both in
plant rhizospheres and endosphere (Hui et al. 2011), and according to Suman et al.
(2016), P solubilization is a common trait among endophytic bacteria. However, the
P solubilizing bacteria (PSB) still tend to be more abundant in plant rhizospheres in
comparison to plant cells and tissues (Chen et al. 2006; Mwajita et al. 2013; Mehta
273
other essential PGP functions including the solubilization abilities for other important
plant nutrients. Nevertheless, there are studies that demonstrate the existence of K
solubilizing endophytes.
Potassium solubilizing endophytic rhizobacteria have been identified from wheat
(Verma et al. 2013, 2015), more recently, from pearl millet (Kushwaha et al. 2019),
maize (Jha 2019), and other crops (Dhiman et al. 2019). Rhizobia are the best-studied
endophytes and are widely known for symbiotic N 2 fixation in leguminous plants
(Santoyo et al. 2016). However, of late, these novel rhizobacteria have also been
shown to solubilize K in plant rhizospheres. For instance, K solubilization by rhizobia
in rice has recently been reported by Patel et al. (2017a). Thirumal et al. (2017)
demonstrated 5 rhizobial cultures associated with K solubilization in vitro. These
new discoveries suggest that apart from enhancing N nutrition in plants, rhizobia can
also be exploited for their K solubilizing abilities to enhance K availability in plant
rhizosphere.
Indigenous KSB are currently in the limelight for sustainable cropping systems
and environmental conservation and have emerged as one of the viable technologies
for mitigating K-deficiency in soils (Meena et al. 2015). Potassium solubilization
indeed holds a lot of potential for PGP and the K solubilizing abilities of endophytic rhizobacteria are worth exploring. According to Meena et al. (2018), KSB are
precious bio-resources that can mitigate K-deficiency in agricultural soils but their
experimental evidence at the field level is still inadequate. Such processes may need
to be exploited in detail so as to increase their usability.
12.2.3 Endophytic Rhizobacteria and Phosphorus
Acquisition in Plants
Phosphorus is the second most important plant nutrient after N (Goswami et al.
2016). Although soils contain P reserves, most of this is available in insoluble forms
and inaccessible to plants (Verma et al. 2019).
This non-availability is recognized as a major plant growth-limiting factor in
agricultural systems (Sharma et al. 2013). The P solubilization potential of soil
microorganisms is one of the most essential traits of PGPR for enhancing P-nutrition
acquisition in plants (Walia and Shirkot 2012; Ouattara et al. 2019). While P solubilizing rhizobacteria are widely investigated, recent literature maintains that only a
few endophytic rhizobacteria possess this ability (Brigido et al. 2019).
Nevertheless, there is mounting evidence on the role of endophytes in P solubilization and mobilization compared to their widely reported rhizospheric counterparts
(Ji et al. 2014; Oteino et al. 2015; Walitang et al. 2019). PSB can proliferate both in
plant rhizospheres and endosphere (Hui et al. 2011), and according to Suman et al.
(2016), P solubilization is a common trait among endophytic bacteria. However, the
P solubilizing bacteria (PSB) still tend to be more abundant in plant rhizospheres in
comparison to plant cells and tissues (Chen et al. 2006; Mwajita et al. 2013; Mehta