Plant Growth-Promoting Rhizobacteria (PGPR)
What is PGPR?
Plant growth-promoting rhizobacteria are the soil bacteria that colonize plant roots. They have the ability to:
- Reduce the insects and disease damage.
- Facilitate plant growth directly or indirectly.
- Decrease the global dependence on hazardous agricultural chemicals.
- Stimulate plant growth through mobilizing nutrients in the soil.
Image Source: Paula Garcia-Fraile
History of PGPR
- Kloepper and Schroth (1981) first time use the term to demonstrate the microbial population in the rhizosphere which is beneficial, colonize the roots of plants and show plant growth-promoting activity.
- The concept of Rhizobacteria was the first time given by Hiltner (1904) to detect the zone of soil that surrounds the roots where the microbial population are accelerated by root activities.
Characteristics of PGPR
The ideal characteristics of PGPR are that they have the ability to
- Colonize the root surface
- They can survive, multiply and compete with other microbiota.
- Promote plant growth.
Classification of PGPR
The classification of PGPR r depends on functional activities because the can act as biofertilizers, biostimulator, rhizomediator, and biopesticides.
- Extracellular PGPR (ePGPR)
- Intracellular PGPR (iPGPR)
Extracellular PGPR may exist in the rhizosphere and in the rhizoplane or in the spaces of cells of the root cortex. The bacterial genera such as Agrobacterium, Arthobacter, Bacillus, Caulobacter, Erwinia, Micrococcus, Pseudomonas and Serratia.
They locate inside the specialized nodular structure of root cells. They belong to the family of Rhizobacteriaceae includes Allorhizobium, Bradyrhizobium, Mesorhizobium, and Frankia.
Mechanism of action of PGPR
- Direct plant growth promotion (Biofertilizer activity)
- Indirect plant growth promotion (Biocontrol activity)
It facilitates nutrient uptake and nutrient availability. Direct mechanism includes
- Nitrogen fixation
- Phosphate solubilization
- Potassium solubilization
- Siderophore production
- Phytohormone production
PGPR fix atmospheric nitrogen and provides it to plant. Nitrogen is changed to ammonia by nitrogen-fixing microorganism using a complex enzyme system known as nitrogenase which consists of dinitrogenase and dinitrogenase reductase.
PGPR provides nitrogen to plant by two mechanisms
- SYMBIOTIC NITROGEN FIXATION: This is a mutual relationship between microbe and plant such as Rhizobacteria and Frankia.
- NON SYMBIOTIC NITROGEN FIXATION: Is carried out by free-living, associative and endophytes such as Cyanobacteria, Azotobacteria
- Rhizobial attachment with root cells
- Extraction of nod factors by rhizobia causing the root hair curling
- Rhizobia penetrate root hair by form an infection thread through which they penetrate the cortical cells and form bacterial state and thereby nodules are formed.
Phosphate is another key element in the nutrition of plants. Phosphorous is abundantly available in the soils but the amount of available forms to plant is generally low as in the majority of soil phosphorus is present. The plant absorbs phosphorus in two basic forms monobasic ion and dibasic ions. Organisms bind with phosphate solubilizing activity and termed as phosphate solubilizing microorganisms and provide the available form of phosphorus to plants and thus constitute to chemical phosphatic fertilizers.
PGPR are able to solubilize the potassium through the production and secretion of organic acid. Potassium solubilization PGPR are Acidothiobacillus, Burkholderia and Pseudomonas.
They are microbial chelating compounds. Kloepper (!980) first time describe the importance of siderophore production by PGPR in the enhancement of plant growth for example Bacillus, Azotobacter
Phytohormone such as auxin, gibberellins, cytokines effect on cell proliferation
INDOLE ACETIC ACID PRODUCTION: IAA produced by PGPR. It has a positive effect on root growth and helps in cell division and differentiation. Biosynthesis of various metabolites and resistance the stressful condition. Pseudomonas, Enterobacter, and Klebsiella plays role in indole acetic acid production.
CYTOKINES AND GIBBERALINES: PGPR also produce cytokines and gibberellins for plant growth promotion such as Azotobacter spp, Bacillus subtilis.
Indirect mechanism includes
- ANTIBIOTIC PRODUCTION
- HYDROLYTIC ENZYME PRODUCTION
- INDUCED SYSTEMIC RESISTANCE
- EXOPOLYSACCHARIDE PRODUCTION
PGPR produce antibiotics against phytopathogens. It is a biocontrol mechanism. A variety of antibiotics have been identified including compounds such as phenazine, tropolone.
PGPR produce enzyme such as chitinase, dehydrogenase, beta-glucanase, lipase, etc. they exhibit hypersensitivity activity, attacking pathogens by secreting cell wall hydrolases. It protects from biotic and abiotic stresses.
INDUCED SYSTEMIC RESISTANCE
It is defined as the physiological state of enhancing defensive capacity in response to specific environmental stimuli and as a result plant, innate defenses are potentiated against subsequent biotic changes. Many individual bacterial components induce systemic resistance such as lipopolysaccharide, siderophore, and volatile compound, etc.
EPS production is important in biofilm formation and colonization.it helps to hold free phosphorus and circulating essential nutrients. The function performed by EPS producing microbe is shielding from desiccation.
Role of PGPR in agriculture
- PHYTOSTIMULATORS: They are phytohormones which stimulate the growth of plants. The best example is the hormone auxin.
- ABIOTIC STRESS TOLERANCE: Abiotic stress are due to heavy metals in soil, nutrient deficiencies, and salinity. PGPR microbes act as stress relievers. Indigenous and native microbes are effective and competitive.
- BIOFERTILIZERS: They are defined as preparations containing living cells or latent cells of efficient strains of microorganisms that help the uptake of nutrients by their interaction in the rhizosphere when applied through the soil. PGPR play role in creating proper rhizosphere for plant growth.
- BIOPESTICIDES: PGPR produce certain enzymes such as chitinase, lipase, protease, etc. exhibit hyper parasitic activity, attacking pathogens by excreting cell wall hydrolases.
Harmful aspects of PGPR
Harmful aspects of PGPR are given below:
- Cyanide act as a growth inhibitor for some plants.
- A high level of auxin inhibits root growth.
- Rhizhobitoxine produce by Bradyrhizobiumelkanii may have a negative effect on nodulation.
- Rhhizobitoxine also induces foliar chlorosis in soybeans.
Future research and developmental strategies
- Future research in rhizosphere biology will rely on the development of molecular and biotechnological approaches to increase our knowledge of rhizosphere biology.
- Research on nitrogen fixation and phosphate solubilization by PGPR is progress on but research done on potassium solubilization is little.
- Higher yield and cost-effective PGPR products use by agriculture farmers.
- Fresh alternatives should be explored for the use of bioinoculants.
- The application of multi-strain bacterial consortium over single inoculation could be an effective approach for reducing the harmful effects of stress on plant growth.
- PGPR are economically and environmentally beneficial for plant growth promotion.
- PGPR may function as biofertilizers bio inoculant and other growth-promoting activities.
- In the future, they might replace chemical fertilizers and pesticides which have many hazardous effects on agriculture.
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