7
rhizobacteria (PGPR) and plant growth-promoting fungi (PGPF), utilize almost
similar mechanisms for promoting plant growth (Glick 2012). However, the functioning and proliferation of rhizospheric bacteria may be influenced by the alteration in the soil pH, moisture, temperature, nutrient status, and the presence of
competing soil bacteria (Santoyo et al. 2016).
PGPRs belong to the diverse group of bacterial strains that reside in the rhizospheric region, at the plant–soil interface, and can stimulate plant growth and development in a variety of ways (Dubey et al. 2017). Many gram-positive and
gram-negative plant growth-stimulating bacteria have been found to colonize in the
plant rhizosphere and confer beneficial effects that can be correlated with their ability to form biofilm, chemotaxis, and production of exopolysaccharides, phytohormones, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase (Nautiyal et al.
2013). Plant growth-stimulating bacteria enhance plant biomass and health through
distinct processes such as plant growth enhancement, improved nutrient use efficiency, and stress tolerance. In general, phytostimulators are known to improve
plant growth by producing the myriads of phytohormones such as auxins, cytokinins, and gibberellins. Secretion of indole-3-acetic acid (IAA) and ACC deaminase
are considered as direct mechanisms (Glick 2012). ACC deaminase inhibits ethylene synthesis by degrading the ACC (a precursor of ethylene) in plants, which
becomes active against the various biotic (bacteria, fungus, insects, viral strains)
and abiotic (high temperature, drought, flood, salinity, heavy metal, pesticide contamination) stresses (Glick 2014). In high concentrations, ethylene can lead to plant
growth inhibition or even death. Biofertilizer bacterial strains fix N, solubilize P,
mobilize K, and chelate with Fe and Zn to form siderophores. Biocontrol agents
fight against phytopathogens and save the plants from diseases by nutrient competition, induced systemic resistance, and antimicrobial chemicals (Dubey et al. 2017).
The following genera of PGPR, such as Azotobacter, Azospirillum, Bacillus,
Burkholderia, Chromobacterium, Erwinia, Enterobacter, Flavobacterium,
Klebsiella, Micrococcus, Rhizobium, Pantoea, and Pseudomonas, as well as
Serratia, are well-known biocontrol agents with the potential to protect the plants
from myriads of phytopathogens (Glick 2012; FigueroaLópez et al. 2016).
Endophytes are microorganisms that colonize host tissues and establish a relationship wherein both partners obtain benefits from their interactions. Some
microbes inhabit within the plants in the form of endophytes but do not harm them
by establishing symbiotic, mutualistic, commensalistic, and trophobiotic relationships (Bertani et al. 2016). The long-term co-evolution of plants and endophytic
bacteria has resulted in an intimate ecosystem that helped plants to adapt and survive in multiple (biotic and abiotic) stress situations and enhanced the ecological
balance of the natural ecosystem. Endophytic bacteria start inhabiting host plant
tissues mainly by the root system; however, inflorescences, shoots, and sometimes
cotyledons are also reported as a route for entry. Once inside the plant, bacteria can
colonize at the primary tissue for entry as well as systemically throughout the plant
(Bulgarelli et al. 2013). Endophytic bacteria have one main advantage over rhizospheric bacteria in that once these are established inside the plant tissues, they are
2.2 Plant Growth-Promoting Microorganisms (PGPMs)
rhizobacteria (PGPR) and plant growth-promoting fungi (PGPF), utilize almost
similar mechanisms for promoting plant growth (Glick 2012). However, the functioning and proliferation of rhizospheric bacteria may be influenced by the alteration in the soil pH, moisture, temperature, nutrient status, and the presence of
competing soil bacteria (Santoyo et al. 2016).
PGPRs belong to the diverse group of bacterial strains that reside in the rhizospheric region, at the plant–soil interface, and can stimulate plant growth and development in a variety of ways (Dubey et al. 2017). Many gram-positive and
gram-negative plant growth-stimulating bacteria have been found to colonize in the
plant rhizosphere and confer beneficial effects that can be correlated with their ability to form biofilm, chemotaxis, and production of exopolysaccharides, phytohormones, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase (Nautiyal et al.
2013). Plant growth-stimulating bacteria enhance plant biomass and health through
distinct processes such as plant growth enhancement, improved nutrient use efficiency, and stress tolerance. In general, phytostimulators are known to improve
plant growth by producing the myriads of phytohormones such as auxins, cytokinins, and gibberellins. Secretion of indole-3-acetic acid (IAA) and ACC deaminase
are considered as direct mechanisms (Glick 2012). ACC deaminase inhibits ethylene synthesis by degrading the ACC (a precursor of ethylene) in plants, which
becomes active against the various biotic (bacteria, fungus, insects, viral strains)
and abiotic (high temperature, drought, flood, salinity, heavy metal, pesticide contamination) stresses (Glick 2014). In high concentrations, ethylene can lead to plant
growth inhibition or even death. Biofertilizer bacterial strains fix N, solubilize P,
mobilize K, and chelate with Fe and Zn to form siderophores. Biocontrol agents
fight against phytopathogens and save the plants from diseases by nutrient competition, induced systemic resistance, and antimicrobial chemicals (Dubey et al. 2017).
The following genera of PGPR, such as Azotobacter, Azospirillum, Bacillus,
Burkholderia, Chromobacterium, Erwinia, Enterobacter, Flavobacterium,
Klebsiella, Micrococcus, Rhizobium, Pantoea, and Pseudomonas, as well as
Serratia, are well-known biocontrol agents with the potential to protect the plants
from myriads of phytopathogens (Glick 2012; FigueroaLópez et al. 2016).
Endophytes are microorganisms that colonize host tissues and establish a relationship wherein both partners obtain benefits from their interactions. Some
microbes inhabit within the plants in the form of endophytes but do not harm them
by establishing symbiotic, mutualistic, commensalistic, and trophobiotic relationships (Bertani et al. 2016). The long-term co-evolution of plants and endophytic
bacteria has resulted in an intimate ecosystem that helped plants to adapt and survive in multiple (biotic and abiotic) stress situations and enhanced the ecological
balance of the natural ecosystem. Endophytic bacteria start inhabiting host plant
tissues mainly by the root system; however, inflorescences, shoots, and sometimes
cotyledons are also reported as a route for entry. Once inside the plant, bacteria can
colonize at the primary tissue for entry as well as systemically throughout the plant
(Bulgarelli et al. 2013). Endophytic bacteria have one main advantage over rhizospheric bacteria in that once these are established inside the plant tissues, they are
2.2 Plant Growth-Promoting Microorganisms (PGPMs)