symbiotic nitrogen fixers belong to either the alpha Proteobacteria (Rhizobium,
Bradyrhizobium, Microvirga, etc.) or the beta-Proteobacteria (Paraburkholderia
and Trinickia, Estrada-de los Santos et al. 2018) and others.
Bacteria with PGP traits are also known for their abilities to provide plants with
nutrients, especially the macro-elements P, K, and iron. N, P, and K are the most
limiting elements for plant growth, in part because negatively-charged nutrients such
as NO 3
À and PO 4
À are often limiting in soils, which themselves are negatively
charged. On the other hand, cations are usually not limiting in soil. Under nutrient
deficiency conditions, inoculation with PGPB enhances the plant’s acquisition of the
“big three” nutrients, NPK. Several bacteria are known to increase the availability of
P and K for plant uptake. P-solubilizing bacteria (PSB) mineralize insoluble phosphate, which leads to improved P uptake by plant. As reviewed by Alori et al. (2017)
and Kalayu (2019), PSB belong to different bacterial groups including Pseudomonas, Bacillus, Paraburkholderia, Rhizobium, Bradyrhizobium, and others (Ruzzi
and Aroca 2015). The mechanisms of microbial P solubilization rely principally on
the release of organic acids that acidify the soil and also facilitate the mineralization
of organic P through enzymes such as phytases (Kalayu 2019).
P-solubilizing bacteria and mycorrhizal fungi in association with symbiotic
nitrogen fixation are critical for ensuring soil fertility, sustainability, and food
security in many parts of the world. However, most research is still limited in
terms of screening the best performing strains of bacteria and fungi for agriculture.
Bacillus, Pseudomonas, Rhizobium, and the fungi Aspergillus and Penicillium, are
the most efficient P solubilizers to increase P bioavailability in soil. In addition,
many PS microbes function as biocontrol agents against plant pathogens via the
production of antibiotics and antifungal metabolites.
Other soil bacteria solubilize potassium (K), and these are known as potassiumsolubilizing bacteria (KSB). The mechanisms of K solubilization include lowering
soil pH via proton extrusion and production of organic acids, acidolysis, chelation,
and bacterial oxidation-reduction reactions (Uroz et al. 2009; Sattar et al. 2019).
PGPB also influence plant growth via several indirect mechanisms. As reviewed
in many studies, the indirect actions include: biocontrol via the production of
siderophores, enzymes, and antibiotics to suppress pathogen growth as well as
ISR, and the stimulation of stress hormones in plants grown under abiotic stress
conditions (Glick 2012; Goswami et al. 2016; Backer et al. 2018; Ferreira et al.
2019).
PGPB are found as free-living organisms in soil or as symbiotic partners associated with a particular plant species (Glick 2012). Bacteria with PGP traits have been
isolated from different plant tissues including seeds, roots, stems, and leaves. Many
of these endophytes have the potential to be used as PGPB. The combination of
beneficial endophytes and soil bacteria has considerable potential for being used
worldwide because inoculation with them is considered an environmentally safe way
to meet the nutritional demands of plants to achieve high productivity, and at the
same time reduce the use of chemical fertilizers.
For legume plants, co-inoculation of rhizobia with PGP microorganism strains is
a promising practice that leads to improved plant growth and productivity.
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
429
Bradyrhizobium, Microvirga, etc.) or the beta-Proteobacteria (Paraburkholderia
and Trinickia, Estrada-de los Santos et al. 2018) and others.
Bacteria with PGP traits are also known for their abilities to provide plants with
nutrients, especially the macro-elements P, K, and iron. N, P, and K are the most
limiting elements for plant growth, in part because negatively-charged nutrients such
as NO 3
À and PO 4
À are often limiting in soils, which themselves are negatively
charged. On the other hand, cations are usually not limiting in soil. Under nutrient
deficiency conditions, inoculation with PGPB enhances the plant’s acquisition of the
“big three” nutrients, NPK. Several bacteria are known to increase the availability of
P and K for plant uptake. P-solubilizing bacteria (PSB) mineralize insoluble phosphate, which leads to improved P uptake by plant. As reviewed by Alori et al. (2017)
and Kalayu (2019), PSB belong to different bacterial groups including Pseudomonas, Bacillus, Paraburkholderia, Rhizobium, Bradyrhizobium, and others (Ruzzi
and Aroca 2015). The mechanisms of microbial P solubilization rely principally on
the release of organic acids that acidify the soil and also facilitate the mineralization
of organic P through enzymes such as phytases (Kalayu 2019).
P-solubilizing bacteria and mycorrhizal fungi in association with symbiotic
nitrogen fixation are critical for ensuring soil fertility, sustainability, and food
security in many parts of the world. However, most research is still limited in
terms of screening the best performing strains of bacteria and fungi for agriculture.
Bacillus, Pseudomonas, Rhizobium, and the fungi Aspergillus and Penicillium, are
the most efficient P solubilizers to increase P bioavailability in soil. In addition,
many PS microbes function as biocontrol agents against plant pathogens via the
production of antibiotics and antifungal metabolites.
Other soil bacteria solubilize potassium (K), and these are known as potassiumsolubilizing bacteria (KSB). The mechanisms of K solubilization include lowering
soil pH via proton extrusion and production of organic acids, acidolysis, chelation,
and bacterial oxidation-reduction reactions (Uroz et al. 2009; Sattar et al. 2019).
PGPB also influence plant growth via several indirect mechanisms. As reviewed
in many studies, the indirect actions include: biocontrol via the production of
siderophores, enzymes, and antibiotics to suppress pathogen growth as well as
ISR, and the stimulation of stress hormones in plants grown under abiotic stress
conditions (Glick 2012; Goswami et al. 2016; Backer et al. 2018; Ferreira et al.
2019).
PGPB are found as free-living organisms in soil or as symbiotic partners associated with a particular plant species (Glick 2012). Bacteria with PGP traits have been
isolated from different plant tissues including seeds, roots, stems, and leaves. Many
of these endophytes have the potential to be used as PGPB. The combination of
beneficial endophytes and soil bacteria has considerable potential for being used
worldwide because inoculation with them is considered an environmentally safe way
to meet the nutritional demands of plants to achieve high productivity, and at the
same time reduce the use of chemical fertilizers.
For legume plants, co-inoculation of rhizobia with PGP microorganism strains is
a promising practice that leads to improved plant growth and productivity.
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
429