Gram-negative bacteria, e.g., Azospirillum and Pseudomonas spp.; (e) Gram-positive PGP bacteria such as Bacillus and Paenibacillus; and also (f) Gram-positive
PGP actinobacteria, e.g., Micromonospora and Streptomyces spp. Fungi that associate with roots are essential for phosphate (P) nutrition especially in the absence of
added fertilizer. The bacteria that support plant growth are commonly known as
Plant Growth Promoting Bacteria (PGPB; Bashan and Holguin 1998), and PGPB are
found as both free-living or symbiotic organisms and also as endophytes that
colonize plant tissues (Glick 2012). Bacteria and fungi together are often called
PGPM (Plant Growth Promoting Microbes). This chapter focuses on the bacteria,
which are mostly isolated from soil. However, PGPB have been isolated from many
plant organs including leaves, flowers, stems, and seeds as well as legume root
nodules, which house diverse PGPB in addition to the dominant rhizobia (MartínezHidalgo and Hirsch 2017). Mycorrhizal fungi are essential for plant growth promotion as well and were probably one of several critical forces leading to plant life
on land.
Bacteria with PGP properties exert their effects on plants both directly and
indirectly. Microbial inoculants for phosphate biofertilizers in agriculture include
both P-solubilizing bacteria (PSB) and mycorrhizal fungi, both of which promote P
nutrition, while N 2 -fixing organisms provide fixed nitrogen (ammonia) for plants.
These two essential plant nutrients are usually not retained in soil and accumulate in
groundwater, resulting in N and P accumulation in waterways and lakes, a common
consequence of the overuse of commercial fertilizers. By contrast, microbial inoculants are cost effective, eco-friendly, and renewable sources of plant nutrients.
Besides P-solubilization and N 2 fixation, soil bacteria sequester iron via
siderophores, produce plant hormones, for example, indole acetic acid (IAA), and
cytokinins, and other growth factors. They also suppress pathogenic microorganisms
that have deleterious effects on plant growth via induced systemic resistance (ISR)
(Glick 2012) and produce various defense compounds. Mycorrhizal fungi perform
many of these same functions but are especially known for their effects on P and Fe
nutrition.
Nitrogen fixation ability is present in several bacterial and archaeal groups.
However, the most studied are the rhizobia, which establish an intimate developmental and physiological relationship with plants to establish nitrogen-fixing nodules. Many soil bacteria and archaea fix N 2 to ammonia, but do not establish novel
structures on plant roots. This type of N 2 fixation is termed free-living or associative
nitrogen-fixation and is an ancestral strategy. An environmental source of fixed N is
extremely important for sustainable agriculture because it ensures that an
eco-friendly source of N-fertilizer, which reduces production costs and mitigates
the negative environmental impacts of chemical fertilizers (Smercina et al. 2019), is
employed. The best-known nitrogen-fixing species that associate with non-legumes
belong to many diverse bacterial genera including Azospirillum, Azotobacter,
Paraburkholderia, Herbaspirillum, and some Paenibacillus, with species belonging
to Azotobacter and Azospirillum being some of the most significant (Goswami et al.
2016). Of the archaea, the methanogens such as Methanococcus fix atmospheric
nitrogen (N 2 ) to ammonia, while other archaea are involved in the soil N cycle. The
428
S. Kouas et al.
PGP actinobacteria, e.g., Micromonospora and Streptomyces spp. Fungi that associate with roots are essential for phosphate (P) nutrition especially in the absence of
added fertilizer. The bacteria that support plant growth are commonly known as
Plant Growth Promoting Bacteria (PGPB; Bashan and Holguin 1998), and PGPB are
found as both free-living or symbiotic organisms and also as endophytes that
colonize plant tissues (Glick 2012). Bacteria and fungi together are often called
PGPM (Plant Growth Promoting Microbes). This chapter focuses on the bacteria,
which are mostly isolated from soil. However, PGPB have been isolated from many
plant organs including leaves, flowers, stems, and seeds as well as legume root
nodules, which house diverse PGPB in addition to the dominant rhizobia (MartínezHidalgo and Hirsch 2017). Mycorrhizal fungi are essential for plant growth promotion as well and were probably one of several critical forces leading to plant life
on land.
Bacteria with PGP properties exert their effects on plants both directly and
indirectly. Microbial inoculants for phosphate biofertilizers in agriculture include
both P-solubilizing bacteria (PSB) and mycorrhizal fungi, both of which promote P
nutrition, while N 2 -fixing organisms provide fixed nitrogen (ammonia) for plants.
These two essential plant nutrients are usually not retained in soil and accumulate in
groundwater, resulting in N and P accumulation in waterways and lakes, a common
consequence of the overuse of commercial fertilizers. By contrast, microbial inoculants are cost effective, eco-friendly, and renewable sources of plant nutrients.
Besides P-solubilization and N 2 fixation, soil bacteria sequester iron via
siderophores, produce plant hormones, for example, indole acetic acid (IAA), and
cytokinins, and other growth factors. They also suppress pathogenic microorganisms
that have deleterious effects on plant growth via induced systemic resistance (ISR)
(Glick 2012) and produce various defense compounds. Mycorrhizal fungi perform
many of these same functions but are especially known for their effects on P and Fe
nutrition.
Nitrogen fixation ability is present in several bacterial and archaeal groups.
However, the most studied are the rhizobia, which establish an intimate developmental and physiological relationship with plants to establish nitrogen-fixing nodules. Many soil bacteria and archaea fix N 2 to ammonia, but do not establish novel
structures on plant roots. This type of N 2 fixation is termed free-living or associative
nitrogen-fixation and is an ancestral strategy. An environmental source of fixed N is
extremely important for sustainable agriculture because it ensures that an
eco-friendly source of N-fertilizer, which reduces production costs and mitigates
the negative environmental impacts of chemical fertilizers (Smercina et al. 2019), is
employed. The best-known nitrogen-fixing species that associate with non-legumes
belong to many diverse bacterial genera including Azospirillum, Azotobacter,
Paraburkholderia, Herbaspirillum, and some Paenibacillus, with species belonging
to Azotobacter and Azospirillum being some of the most significant (Goswami et al.
2016). Of the archaea, the methanogens such as Methanococcus fix atmospheric
nitrogen (N 2 ) to ammonia, while other archaea are involved in the soil N cycle. The
428
S. Kouas et al.