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C. C. V. Velloso et al.
7.1 Introduction
Cereals play an important role in the world’s agriculture economy, considering their
area sown and annual production volume, being used as food, feed, and industry.
Although maize, wheat, and rice are the most important to feed the world, other
crops such as sorghum and millet are also relevant, especially in Asia and Africa
(FAO 2019). The world’s population today is around 7 billion and is estimated to
reach 9 billion by 2050. Challenges to feed the ever-growing population coupled with
global climate change increase the need for sustainable and environmentally sound
agricultural production (FAO 2019; Naumann et al. 2018; Ngumbi and Kloepper
2016). However, the productivity enhancement system is highly dependent on chemical inputs, especially nitrogen, phosphate, and potassium fertilizers (Ladha et al.
2016), which not only increases the production cost (Haygarth et al. 2014; Kvaki´ c
et al. 2018) but cause environmental adverse impacts such as groundwater pollution,
soil degradation, micronutrient deficiency, eutrophication of water sources, toxicity to different beneficial organisms and plummeting of microbiota biodiversity,
and overall management of ecology (Sharma and Singhvi 2017; Maheshwari and
Annapurna 2017).
Endophytic bacteria, microorganisms that spend at least part of their life cycle
inside plants without causing apparent damage, have emerged as an economically and
environmentally sustainable alternative to traditional methods. Microbial inoculants,
characterized as products that contain strains of beneficial microorganisms to the
plant growth and development, such as plant growth-promoting bacteria (PGPB)
play an important role in the production of sustainable crops, reducing environmental
impact and human health hazards. Various names have been given to PGPB according
to their efficacy and use in plant ecosystem. Maheshwari (2010) has coined the term as
plant growth- and health-promoting bacteria. Such organisms are capable to provide
better adaptability and survival under biotic and abiotic stress conditions, and have
the potential to mitigate the excessive use of pesticides and fertilizers in agriculture
(Alori et al. 2017; Alori and Babalola 2018; Bashan et al. 2014; Singh et al. 2016).
These are able to stimulate plant growth at different stages of development using
direct mechanisms as phytohormones, enzyme production and nutrient uptake, and
indirect mechanisms including biological control and induced systemic resistance
(Nazir et al. 2018; Saini et al. 2015; Varma et al. 2017).
Various workers consider the plant microbiome, i.e., the collective genomes of
microorganisms living in association with plants, as a second genome, due to its
close proximity between both partners and its impact on the host plant. It comprises
a broad and diverse group of microorganisms, although most of them belong to relatively small phylogenetic group, comprising mainly of Firmicutes and Proteobacteria.
The most important genera of Proteobacteria are Rhizobium, Agrobacterium, and
Sphingomonas (α-proteobacteria); Burkholderia (β-proteobacteria); Enterobacter,
Klebsiella, Pantoea, and Pseudomonas (γ-proteobacteria). Among all, Bacillus is
the most common genus within the Firmicutes group followed by Paenibacillus and
Staphylococcus (Wallace and May 2018; Rodriquez et al. 2019).
C. C. V. Velloso et al.
7.1 Introduction
Cereals play an important role in the world’s agriculture economy, considering their
area sown and annual production volume, being used as food, feed, and industry.
Although maize, wheat, and rice are the most important to feed the world, other
crops such as sorghum and millet are also relevant, especially in Asia and Africa
(FAO 2019). The world’s population today is around 7 billion and is estimated to
reach 9 billion by 2050. Challenges to feed the ever-growing population coupled with
global climate change increase the need for sustainable and environmentally sound
agricultural production (FAO 2019; Naumann et al. 2018; Ngumbi and Kloepper
2016). However, the productivity enhancement system is highly dependent on chemical inputs, especially nitrogen, phosphate, and potassium fertilizers (Ladha et al.
2016), which not only increases the production cost (Haygarth et al. 2014; Kvaki´ c
et al. 2018) but cause environmental adverse impacts such as groundwater pollution,
soil degradation, micronutrient deficiency, eutrophication of water sources, toxicity to different beneficial organisms and plummeting of microbiota biodiversity,
and overall management of ecology (Sharma and Singhvi 2017; Maheshwari and
Annapurna 2017).
Endophytic bacteria, microorganisms that spend at least part of their life cycle
inside plants without causing apparent damage, have emerged as an economically and
environmentally sustainable alternative to traditional methods. Microbial inoculants,
characterized as products that contain strains of beneficial microorganisms to the
plant growth and development, such as plant growth-promoting bacteria (PGPB)
play an important role in the production of sustainable crops, reducing environmental
impact and human health hazards. Various names have been given to PGPB according
to their efficacy and use in plant ecosystem. Maheshwari (2010) has coined the term as
plant growth- and health-promoting bacteria. Such organisms are capable to provide
better adaptability and survival under biotic and abiotic stress conditions, and have
the potential to mitigate the excessive use of pesticides and fertilizers in agriculture
(Alori et al. 2017; Alori and Babalola 2018; Bashan et al. 2014; Singh et al. 2016).
These are able to stimulate plant growth at different stages of development using
direct mechanisms as phytohormones, enzyme production and nutrient uptake, and
indirect mechanisms including biological control and induced systemic resistance
(Nazir et al. 2018; Saini et al. 2015; Varma et al. 2017).
Various workers consider the plant microbiome, i.e., the collective genomes of
microorganisms living in association with plants, as a second genome, due to its
close proximity between both partners and its impact on the host plant. It comprises
a broad and diverse group of microorganisms, although most of them belong to relatively small phylogenetic group, comprising mainly of Firmicutes and Proteobacteria.
The most important genera of Proteobacteria are Rhizobium, Agrobacterium, and
Sphingomonas (α-proteobacteria); Burkholderia (β-proteobacteria); Enterobacter,
Klebsiella, Pantoea, and Pseudomonas (γ-proteobacteria). Among all, Bacillus is
the most common genus within the Firmicutes group followed by Paenibacillus and
Staphylococcus (Wallace and May 2018; Rodriquez et al. 2019).