Plant Growth Promoting Rhizobacteria or PGPR is a group of bacteria that can be found in the rhizosphere or it can be said that bacteria that colonize the roots of the plants that enhance plant growth. It is observed that the rhizosphere is the zone of maximum microbial activity. It is the rhizosphere region of the plant from where most of the essential mico and macro- nutrients are extracted. The different species of Bacteria, Fungi, Actinomycetes, Protozoa, and Algae can be found in the rhizosphere region, Bacteria being the most abundant.
The term PGPR was introduced by Kloepper and Schroth. They concluded that PGPR are not only associated with the roots to exert beneficial effects on plant development but also have positive effects on controlling phytopathogenic microbes. Therefore, PGPR is one of the active ingredients in biofertilizer.
Based on the interactions , PGPRs are of 2 different types :
- SYMBIOTIC or INTRACELLULAR BACTERIA (iPGPR) live inside plants and exchange metabolites directly by biofertilization, stimulation of plant growth, rhizoremediation, and plant stress control. While performing direct growth promotion, they behave as Biofertilizers.
- FREE – LIVING or EXTRACELLULAR BACTERIA (ePGPR) live outside plant cells and indirectly increases plant growth by reducing the impact of disease, by Antibiosis, induction of systemic resistance, and competition for nutrients and niches. While performing indirect plant growth promotion, they behave as Biopesticide.
PGPR shows an important role in sustainable agriculture industry.
Undoubtedly, there is an increased demand of crop production now-a-days and also a significant reduction of synthetic chemical pesticides and fertilizers which is a big challenge. So, the use of PGPR has been proven to be one of the best ways of increasing crop yields by facilitating plant growth.
PGPR show synergistic and antagonistic interactions with microorganisms within the rhizosphere and in bulk soil, which indirectly boosts the plant growth rate. PGPR also works as a biofertilizer for agricultural sustainability.
Agriculture is one of the human activities which leads to the increasing amount of chemical pollutants with the excessive and continuous use of chemical fertilizers and pesticides. This causes further environmental damage causing a serious risk for human health.
For e.g. N2O is excessively released by continuous use of nitrogen fertilizers which causes Greenhouse effect and finally Global Warming. Farmers apply a high concentration of nitrogen fertilizers in the form of ammonium nitrate to increase crop yield but continuous use of nitrogen fertilizers decreases the Biological Nitrogen Fixation in soil.
For Sustainable Agriculture fulfillment, crops which are produced must be grown with disease resistance, salt tolerance, drought tolerance, heavy metal stress tolerance, and better nutritional value. Also, the use of soil microorganisms is the one possible way. Soil microbes increase the nutrient uptake capacity and water use efficiency in the soil. Among these soil microorganisms, PGPRs are mostly used which are capable of performing all the activities without environmental contamination. PGPR include the species of Pseudomonas, Bacillus, Enterobacter, Klebsiella, Azobacter, Variovorax, Azosprillum and Serratia.
But it is found that agricultural industries worldwide still not use the concept of PGPRs. This is due to the inconsistent properties of inoculated PGPR which can greatly influence the crop production.
A PGPR must :
- Possess its survival in soil by tolerating several environmental factors.
- Be compatible with the crops on which it is inoculated.
- Be interactive with the already existing microflora in soil.
- Have a broad spectrum of action.
- Be safe for the environment.
- Enhance plant growth.
Another challenge is that all Rhizobacteria do not possess the same mechanisms which is a major disadvantage for the environment therefore, PGPR still are not used as a biofertilizer in global agricultural productivity.