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B. N. Aloo et al.
12.1 Introduction
Agricultural activities are quickly gaining momentum to feed the rapidly growing
population across the globe. One of them is excessive use of chemicals established as
an effective tool to increase crop productivity of different crops. However, conventional agricultural practices have a lot of undesirable outcomes as the chemical inputs
have commonly been linked to land degradation, environmental pollution, global
warming, climate change, etc. (Steffen et al. 2015; Di Benedetto et al. 2017). For
many decades, researchers all over the world have focused on alternative crop fertilization mechanisms such as the use of plant growth-promoting bacteria (PGPB) to
replace the contemporary fertilization practices (Smith et al. 2016). In fact, these
are free-living bacteria with unique capabilities of stimulating plant growth, either
directly or indirectly through different mechanisms (Archana et al. 2013; Ahemad
and Kibret 2014; Kumar et al. 2014). Glick (2014) and later, Baliyan et al. (2018)
described the exploitation of such organisms as a viable and environment friendly
technology befitting for sustainable crop production in eco-safe ways.
Among them, endophytes are organisms that spend all or part of their lives in
plant cells or tissues with different degrees of dependence without harming their
hosts (Compant et al. 2010; Hardoim et al. 2015; Brader et al. 2017; Lata et al. 2019)
and can be recovered from surface-sterilized plant tissues (Santoyo et al. 2016). As
many plant species as exist on earth host bacterial endophytes (Ryan et al. 2008),
and several endophytic bacteria like the Proteobacteria, Firmicutes Actinobacteria,
and Bacteroidetes have putative PGP functions (Rosenblueth and Martinez-Romero
2006; Bulgarelli et al. 2013; Hardoim et al. 2015; Liu et al. 2017). Endophytic
bacteria have been isolated from various plant parts including stems, roots, seeds,
leaves, fruits, ovules, tubers, nodules, etc. (Benhizia et al. 2004; Pandey et al. 2018).
Nevertheless, below ground potential i.e., plant roots harbor the greatest populations
of these bacteria in comparison to aerial parts (Rosenblueth and Martinez-Romero
2006; Taghavi et al. 2010), at approximately 10
4 –10
6 per g of root tissue (Compant
et al. 2010; Bulgarelli et al. 2013).
Depsite occupying different ecological niches, endophytic bacterial populations employ PGP mechnaisms similar to those of free-living rhizosphere bacteria
(Compant et al. 2005). The common PGP mechanisms can either be direct such as
nitrogen-fixation, solubilization of nutrients, production of siderophores and phytohormones or indirect such as the suppression of plant pathogens and diseases (Suman
et al. 2016; Lata et al. 2018). Diverse PGP bacterial endophytes have been explored
and applied for crop yield enhancement under nutrient-poor conditions (Rosenblueth
and Martinez-Romero 2006; Liu et al. 2017). Several studies demonstrate their positive effects in different food and cash crops such as the banana (Musa spp.) (Patel
et al. 2017b), maize (Zea mays) (Alves et al. 2015), tomato (Lycopersicon esculentum) (Upreti and Thomas 2015), groundnut (Arachis hypogaea) (Dhole et al.
2016), and many more outlined by various workers (Hardoim et al. 2015; Pandey
et al. 2018; Maheshwari 2018). Literature documents that endophyte-elicited PGP
activities culminate into increased seed germination rates, biomass, chlorophyll, N
B. N. Aloo et al.
12.1 Introduction
Agricultural activities are quickly gaining momentum to feed the rapidly growing
population across the globe. One of them is excessive use of chemicals established as
an effective tool to increase crop productivity of different crops. However, conventional agricultural practices have a lot of undesirable outcomes as the chemical inputs
have commonly been linked to land degradation, environmental pollution, global
warming, climate change, etc. (Steffen et al. 2015; Di Benedetto et al. 2017). For
many decades, researchers all over the world have focused on alternative crop fertilization mechanisms such as the use of plant growth-promoting bacteria (PGPB) to
replace the contemporary fertilization practices (Smith et al. 2016). In fact, these
are free-living bacteria with unique capabilities of stimulating plant growth, either
directly or indirectly through different mechanisms (Archana et al. 2013; Ahemad
and Kibret 2014; Kumar et al. 2014). Glick (2014) and later, Baliyan et al. (2018)
described the exploitation of such organisms as a viable and environment friendly
technology befitting for sustainable crop production in eco-safe ways.
Among them, endophytes are organisms that spend all or part of their lives in
plant cells or tissues with different degrees of dependence without harming their
hosts (Compant et al. 2010; Hardoim et al. 2015; Brader et al. 2017; Lata et al. 2019)
and can be recovered from surface-sterilized plant tissues (Santoyo et al. 2016). As
many plant species as exist on earth host bacterial endophytes (Ryan et al. 2008),
and several endophytic bacteria like the Proteobacteria, Firmicutes Actinobacteria,
and Bacteroidetes have putative PGP functions (Rosenblueth and Martinez-Romero
2006; Bulgarelli et al. 2013; Hardoim et al. 2015; Liu et al. 2017). Endophytic
bacteria have been isolated from various plant parts including stems, roots, seeds,
leaves, fruits, ovules, tubers, nodules, etc. (Benhizia et al. 2004; Pandey et al. 2018).
Nevertheless, below ground potential i.e., plant roots harbor the greatest populations
of these bacteria in comparison to aerial parts (Rosenblueth and Martinez-Romero
2006; Taghavi et al. 2010), at approximately 10
4 –10
6 per g of root tissue (Compant
et al. 2010; Bulgarelli et al. 2013).
Depsite occupying different ecological niches, endophytic bacterial populations employ PGP mechnaisms similar to those of free-living rhizosphere bacteria
(Compant et al. 2005). The common PGP mechanisms can either be direct such as
nitrogen-fixation, solubilization of nutrients, production of siderophores and phytohormones or indirect such as the suppression of plant pathogens and diseases (Suman
et al. 2016; Lata et al. 2018). Diverse PGP bacterial endophytes have been explored
and applied for crop yield enhancement under nutrient-poor conditions (Rosenblueth
and Martinez-Romero 2006; Liu et al. 2017). Several studies demonstrate their positive effects in different food and cash crops such as the banana (Musa spp.) (Patel
et al. 2017b), maize (Zea mays) (Alves et al. 2015), tomato (Lycopersicon esculentum) (Upreti and Thomas 2015), groundnut (Arachis hypogaea) (Dhole et al.
2016), and many more outlined by various workers (Hardoim et al. 2015; Pandey
et al. 2018; Maheshwari 2018). Literature documents that endophyte-elicited PGP
activities culminate into increased seed germination rates, biomass, chlorophyll, N