12 Endophytic Rhizobacteria for Mineral Nutrients …
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has been emphasized in the selection of potential endophytes for enhancement of
Zn uptake in plants (Singh et al. 2018). Other endophytic ZSB include species of
Bacillus, Chryseobacterium, Paenibacillus, Rhodococcus, Staphylococcus, Achromobacter, Acinetobacter, Enterobacter, and Klebsiella (Suman et al. 2016). Recently,
Kushwaha et al. (2019) also observed that endophytic Bacillus strains from pearl
millet exhibited Zn solubilization potential and had multiple roles in stress tolerance
of the plant. The use of such ZSB can increase Zn uptake by filed crops, which would
in turn lead to their improved growth and yield (Suman et al. 2016).
12.2.5 Endophytic Rhizobacteria and Iron Acquisition
in Plants
Iron is the fourth most abundant element in soil and is an important micronutrient
required by plants for many physiological processes (Saha et al. 2016). However,
most agricultural soils are Fe-deficient because the element occurs in the insoluble
ferric (Fe
3+ ) form that is unavailable for plant uptake (Rajkumar et al. 2010; Arora
and Verma 2017; Singh et al. 2019). Some microorganisms have developed a special
Fe acquisition mechanism under these Fe-limiting conditions by producing certain
special metabolites known as siderophores (Maheshwari et al. 2019b).
Siderophores are secondary metabolites with high affinity for Fe
3+ (Goswami
et al. 2016; Arora and Verma 2017), and under Fe-limiting conditions, siderophores
complex with Fe
3+− , a phenomenon which is important for enhancing Fe availability in the rhizosphere (Ferna´ndez-Scavino and Pedraza 2013; Boiteau et al. 2016;
Chhabra and Dowling 2017). It is proposed that once the siderophores bind onto Fe
3+ ,
the acquisition of the bound Fe by plants can occur by the degradation of the chelates
or complexes (Rajkumar et al. 2009). According to Loaces et al. (2011), siderophore
production is a common trait among the free-living PGPR (Souza et al. 2015) and is
rarely reported for the endophytic rhizobacteria. Recent literature suggests that only a
few endophytic bacterial isolates possess this trait (Brigido et al. 2019), investigated
mainly as a bio-control agent against plant pathogens (Suman et al. 2016). In such
cases, the siderophores chelate most of the Fe present in the rhizosphere and prevent
the proliferation of pathogens due to its non-availability in the rhizosphere soil (Mitter
et al. 2013; Olanrewaju et al. 2017). Nevertheless, endophytic rhizobacteria can also
produce these metabolites under Fe-stress and aid in plant Fe acquisition (Ghavami
et al. 2017; Perez-Rosales et al. 2017), and endophytic genera like Pantoea, Bacillus,
Burkholderia, and Pseudomonas can increase the concentration of bioavailable Fe
in plant tissues (Maheshwari et al. 2019a).
Endophytic siderophore producers that include Brevundimonas diminuta, Leifsonia shinshuensis, Sphingomonas parapaucimobilis, Brevundimonas vesicularis,
and Agrobacterium tumefaciens have been identified from pear and peach roots
(Liaqat and Eltem 2016). Bacillus sp., Pseudomonas sp., and Stenotrophomonas sp.
are also recognized among the effective siderophore-producing endophytes (Jasim
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has been emphasized in the selection of potential endophytes for enhancement of
Zn uptake in plants (Singh et al. 2018). Other endophytic ZSB include species of
Bacillus, Chryseobacterium, Paenibacillus, Rhodococcus, Staphylococcus, Achromobacter, Acinetobacter, Enterobacter, and Klebsiella (Suman et al. 2016). Recently,
Kushwaha et al. (2019) also observed that endophytic Bacillus strains from pearl
millet exhibited Zn solubilization potential and had multiple roles in stress tolerance
of the plant. The use of such ZSB can increase Zn uptake by filed crops, which would
in turn lead to their improved growth and yield (Suman et al. 2016).
12.2.5 Endophytic Rhizobacteria and Iron Acquisition
in Plants
Iron is the fourth most abundant element in soil and is an important micronutrient
required by plants for many physiological processes (Saha et al. 2016). However,
most agricultural soils are Fe-deficient because the element occurs in the insoluble
ferric (Fe
3+ ) form that is unavailable for plant uptake (Rajkumar et al. 2010; Arora
and Verma 2017; Singh et al. 2019). Some microorganisms have developed a special
Fe acquisition mechanism under these Fe-limiting conditions by producing certain
special metabolites known as siderophores (Maheshwari et al. 2019b).
Siderophores are secondary metabolites with high affinity for Fe
3+ (Goswami
et al. 2016; Arora and Verma 2017), and under Fe-limiting conditions, siderophores
complex with Fe
3+− , a phenomenon which is important for enhancing Fe availability in the rhizosphere (Ferna´ndez-Scavino and Pedraza 2013; Boiteau et al. 2016;
Chhabra and Dowling 2017). It is proposed that once the siderophores bind onto Fe
3+ ,
the acquisition of the bound Fe by plants can occur by the degradation of the chelates
or complexes (Rajkumar et al. 2009). According to Loaces et al. (2011), siderophore
production is a common trait among the free-living PGPR (Souza et al. 2015) and is
rarely reported for the endophytic rhizobacteria. Recent literature suggests that only a
few endophytic bacterial isolates possess this trait (Brigido et al. 2019), investigated
mainly as a bio-control agent against plant pathogens (Suman et al. 2016). In such
cases, the siderophores chelate most of the Fe present in the rhizosphere and prevent
the proliferation of pathogens due to its non-availability in the rhizosphere soil (Mitter
et al. 2013; Olanrewaju et al. 2017). Nevertheless, endophytic rhizobacteria can also
produce these metabolites under Fe-stress and aid in plant Fe acquisition (Ghavami
et al. 2017; Perez-Rosales et al. 2017), and endophytic genera like Pantoea, Bacillus,
Burkholderia, and Pseudomonas can increase the concentration of bioavailable Fe
in plant tissues (Maheshwari et al. 2019a).
Endophytic siderophore producers that include Brevundimonas diminuta, Leifsonia shinshuensis, Sphingomonas parapaucimobilis, Brevundimonas vesicularis,
and Agrobacterium tumefaciens have been identified from pear and peach roots
(Liaqat and Eltem 2016). Bacillus sp., Pseudomonas sp., and Stenotrophomonas sp.
are also recognized among the effective siderophore-producing endophytes (Jasim