274
B. N. Aloo et al.
et al. 2015; Walia et al. 2017). Generally, the population of endophytic PSB range
between 10
2 and 10
4 bacteria/g of root tissue (Kumar et al. 2013; Saini et al. 2015). A
number of endophytic rhizobacterial populations belonging to Burkholderia, Enterobacter, Pantoea, Pseudomonas, Citrobacter, Azotobacter genera from wheat, rice,
maize, legumes, and sunflower, respectively, are reported to solubilize mineral P in
plate assays, and a vast number of P solubilizing PGPR are documented (Verma
et al. 2013, 2015). In a recent study, Patel et al. (2017b), examined that 36% of
over 50 endophytic rhizobacterial isolates belonging to genera Bacillus, Klebsiella,
Microbacterium, and Enterobacter showed P solubilization. Further reports on P
solubilizing endophytic rhizobacteria are depicted in Table 12.2.
The P solubilizing PGPR can greatly impact plant growth by increasing P availability in the rhizospheric soils but must maintain an intimate relationship with the
host plants (Walia et al. 2017). Numerous studies have highlighted the importance
and mechanisms of P solubilization by PSB (Chhabra and Dowling 2017; Varma
et al. 2017; Walia et al. 2017; Shrivastava et al. 2018; Billah et al. 2019; Goswami
et al. 2019; Rafi et al. 2019; Dheeman et al. 2020). The solubilization of P is purportedly mediated through acidification, chelation, or exchange reactions (Li et al. 2017).
According to Rosenblueth and Martinez-Romero (2006), endophytic PSB are more
competitive than free-living rhizobacteria since the plant-endophyte interactions are
the result of complex evolutionary processes. Moreover, endophytic rhizobacteria can
prevent the adsorption and fixation of P under P-limiting conditions by assimilating
the solubilized P (Khan and Joergersen 2009; Shakeela et al. 2017).
12.2.4 Endophytic Rhizobacteria in Zinc Acquisition
in Plants
Zinc is an important micronutrient required for primary and secondary metabolism
in plants (Goteti et al. 2013; Bhatt and Maheshwari 2020). For instance, Zn is a
cofactor in many enzymes (Hafeez et al. 2013) and it is critical for membrane function, photosynthesis, protein synthesis, and auxin metabolism in plants (Tavallali
et al. 2010). Reports show that Zn deficiency is a common problem worldwide
due to nutrient mining during crop harvesting and increased use of NPK fertilizers
containing lesser amounts of Zn micronutrients (Sharifi and Paymozd 2016; Sindhu
et al. 2019). Synthetic Zn fertilizers are often applied to overcome these deficiencies
at rates of about 25 kg ha
−1 ZnSO4 heptahydrate (equivalent to 5 kg ha
−1 Zn). Nevertheless, these artificial fertilizers are not cost-effective and quickly get converted into
insoluble forms that are inaccessible to plants (Bapiri et al. 2012; Sindhu et al. 2019).
Rhizobacterial Zn solubilization abilities are widely reported phenomenon
(Mishra et al. 2013; Shaikh and Saraf 2017). Reports also exist on endophytic Zn solubilization. For instance, Zn solubilizing bacteria (ZSB) have been reported to enhance
Zn uptake in soybean up to 21% (Sharma et al. 2014), various G. diazotrophicus
strains showed solubilization potential for various Zn compounds (Suman et al. 2016)
B. N. Aloo et al.
et al. 2015; Walia et al. 2017). Generally, the population of endophytic PSB range
between 10
2 and 10
4 bacteria/g of root tissue (Kumar et al. 2013; Saini et al. 2015). A
number of endophytic rhizobacterial populations belonging to Burkholderia, Enterobacter, Pantoea, Pseudomonas, Citrobacter, Azotobacter genera from wheat, rice,
maize, legumes, and sunflower, respectively, are reported to solubilize mineral P in
plate assays, and a vast number of P solubilizing PGPR are documented (Verma
et al. 2013, 2015). In a recent study, Patel et al. (2017b), examined that 36% of
over 50 endophytic rhizobacterial isolates belonging to genera Bacillus, Klebsiella,
Microbacterium, and Enterobacter showed P solubilization. Further reports on P
solubilizing endophytic rhizobacteria are depicted in Table 12.2.
The P solubilizing PGPR can greatly impact plant growth by increasing P availability in the rhizospheric soils but must maintain an intimate relationship with the
host plants (Walia et al. 2017). Numerous studies have highlighted the importance
and mechanisms of P solubilization by PSB (Chhabra and Dowling 2017; Varma
et al. 2017; Walia et al. 2017; Shrivastava et al. 2018; Billah et al. 2019; Goswami
et al. 2019; Rafi et al. 2019; Dheeman et al. 2020). The solubilization of P is purportedly mediated through acidification, chelation, or exchange reactions (Li et al. 2017).
According to Rosenblueth and Martinez-Romero (2006), endophytic PSB are more
competitive than free-living rhizobacteria since the plant-endophyte interactions are
the result of complex evolutionary processes. Moreover, endophytic rhizobacteria can
prevent the adsorption and fixation of P under P-limiting conditions by assimilating
the solubilized P (Khan and Joergersen 2009; Shakeela et al. 2017).
12.2.4 Endophytic Rhizobacteria in Zinc Acquisition
in Plants
Zinc is an important micronutrient required for primary and secondary metabolism
in plants (Goteti et al. 2013; Bhatt and Maheshwari 2020). For instance, Zn is a
cofactor in many enzymes (Hafeez et al. 2013) and it is critical for membrane function, photosynthesis, protein synthesis, and auxin metabolism in plants (Tavallali
et al. 2010). Reports show that Zn deficiency is a common problem worldwide
due to nutrient mining during crop harvesting and increased use of NPK fertilizers
containing lesser amounts of Zn micronutrients (Sharifi and Paymozd 2016; Sindhu
et al. 2019). Synthetic Zn fertilizers are often applied to overcome these deficiencies
at rates of about 25 kg ha
−1 ZnSO4 heptahydrate (equivalent to 5 kg ha
−1 Zn). Nevertheless, these artificial fertilizers are not cost-effective and quickly get converted into
insoluble forms that are inaccessible to plants (Bapiri et al. 2012; Sindhu et al. 2019).
Rhizobacterial Zn solubilization abilities are widely reported phenomenon
(Mishra et al. 2013; Shaikh and Saraf 2017). Reports also exist on endophytic Zn solubilization. For instance, Zn solubilizing bacteria (ZSB) have been reported to enhance
Zn uptake in soybean up to 21% (Sharma et al. 2014), various G. diazotrophicus
strains showed solubilization potential for various Zn compounds (Suman et al. 2016)