Plant Growth Promoting Rhizobacteria (PGPR)

               Plant growth promoting rhizobacteria (PGPR) are soil bacteria with some beneficial effects on soil properties, plant growth and the environment. Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR). The most intensively researched use of PGPR has been in agricultural and horticulture. Several PGPR formulations are currently available as commercial products for agricultural production. Recently developing areas of PGPR usage include forest regeneration and phytoremediation of contaminated soils. 

               Rhizobacteria are root colonizing bacteria that form symbiotic relationship with many plants. They are an important group of microorganisms used in biofertilizer. PGPRs have different relationship with different species of host plants. The two major classes of relationship are rhizospheric and endophytic. Rhizospheric relationship consists of the PGPRs that colonize the surface of the root often forming root nodules.

PGPR are considered as efficient microbial competitors in the soil-root zone. Genera of PGPR generally include, Arthrobacter, Azospirillum, Bacillus, Bradyrhizobium, Frankia, Pseudomonas, Rhizobium, Serratia, Thiobacillus, and others.

Siderophores, bacteriocins, and antibiotics production as antagonistic activities

The ability of rhizobacteria to produce siderophores and metabolites contributing to antibiosis has been the focus of many studies dedicated to investigating PGPR. The uptake of ferric ion via side-rophore is largely used by pathogenic and non-pathogenic microorganisms from the soil, human body and marine environments. The importance of siderophore is closely related to iron, which is an essential element for different biological processes. On the other hand, bacteria can produce a wide variety of compounds with antimicrobial activity used as defense systems. These include broad-spectrum antibiotics, lactic acid produced by lactobacilli, lytic agents such as lyso-zymes, numerous types of exotoxins and bac-terio-cins, which also have a bactericidal mode of action. Sidero-phores, bacteriocins and antibiotics are three of the most effective and well known mechanisms that an antagonist can employ to minimize or prevent phytopathogenic proliferation.

MECHANISM.

PGPRs enhance plant growth by direct and indirect mechanisms. The direct promotion of plant growth by PGPR entails either providing the plant with a compound that is synthesized by the bacterium, for example phyto-hormones, or facilitating the uptake of certain nutrients from the environment. The indirect promotion of plant growth occurs when PGPR lessen or prevent the deleterious effects of one or more phytopathogenic organisms. This can happen by producing antagonistic substances or by inducing resistance to pathogens.

BIOCONTROL.

Rhizobacteria are also able to control plant diseases that are caused by other bacteria andfungi. Disease is suppressed through induced systematic resistance and through the production of anti fungal metabolites.

Pseudomonas biocontrol strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops.

PGPR ACTION UNDER STRESSED CONDITION.

The PGPR containing ACC deaminase are present in various soils and offer promise as a bacterial inoculum for improvement of plant growth, particularly under unfavourable environmental conditions such as flooding, heavy metals, phytopathogens, drought and high salt. Ethylene is an important phytohormone, but over-produced ethylene under stressful conditions can result in the inhibition of plant growth or death, especially for seedlings. PGPR containing ACC deaminase can hydrolyze ACC, the immediate precursor of ethylene, to F-ketobutarate and ammonia, and in this way promote plant growth. Inoculation of crops with ACC deaminase-containing PGPR may assist plant growth by alleviating deleterious effects of salt stress .

APPLICATIONS OF PGPRs

They stimulate plant growth through mobilizing nutrients in soils and producing numerous plant growth regulators.

PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. 

They protect plants from phytopathogens by controlling or inhibiting them.

PGPR is able to reduce the incidence and severity of disease. Some PGPR inoculated bacteria on seeds before planting can give the defense on the hood of roots of plants. This makes PGPR bacteria capable of reducing the severity of disease.

The most intensively researched use of PGPR has been in agricultural and horticulture several PGPR formulations are currently available as commercial products for agricultural production.

In recent years, the use of PGPR to promote plant growth has increased in various parts of the world. PGPR can affect plant growth by production and release of secondary metabolites (plant growth regulators/phytohormones/biologically active substances), lessening or preventing deleterious effects of phytopathogenic organisms in the rhizosphere and/or facilitating the availability and uptake of certain nutrients from the root environment. PGPR having multiple activities directed toward plant growth promotion via exhibiting bioremediating   potentials by detoxifying pollutants like, heavy metals and pesticides and controlling a range of phytopathogens.

In future they are expected to replace the chemical fertilizers, pesticides and artificial growth regulators which have numerous side effects to sustainable agriculture.