Several biotic and abiotic factors critically influence the growth of a plant in soil. The layer of soil in immediate proximity to the plant roots is highly metabolically active. This soil layer is directly influenced by the root exudates released by the plants and the bacterial communities present in close association with the plant. This soil area is termed as rhizosphere and the bacteria harbouring the rhizosphere are rhizobacteria. The microorganisms colonizing the rhizosphere are categorized based on the way they interact with the plant roots. Some bacteria exert a negative effect while others have a positive influence on plant growth. Hence, the bacteria inhabiting the root rhizosphere which affect the plant growth positively are planting growth-promoting rhizobacteria, abbreviated as PGPR. The term was coined for the very first time by J. W. Kloepper in the 1970s. The use of PGPR in agriculture has been on a constant increase over few decades. PGPRs tend to release higher quantities of growth-promoting substances in the soil which directly and indirectly influence influences the overall plant morphology.
Major classes of PGPR
PGPR can be classified based on their location, function and activities.
- Based on location: PGPR can be extracellular PGPR (ePGPR) and intracellular PGPRs (iPGPR). ePGPR are the bacteria that live outside the plant cells in the rhizosphere and do not form root nodules. iPGPR resides inside the plant cells in small specialized structures called root nodules where they fix nitrogen.
- Based on function: PGPR can be planted growth-promoting, controlling and stress homeo-regulating. Plant growth-promoting (PGP) bacteria improve plant growth by increasing their nutrient uptake and by secreting plant growth-promoting compounds. Biocontrol bacteria protect the plants against pathogen attack whereas stress homeo-regulating bacteria will release compounds that mitigate the effect of different stress on plant growth and development.
- Based on activities: PGPR can act as biofertilizers, Phyto stimulators. Rhizoremediators, and/or biopesticides. PGPR acting as a biofertilizer improve crop yields by increasing plant nutrient availability, and by improving soil quality and fertility. PGPR which enhances plant growth through the synthesis of phytohormones is termed Phyto stimulators. PGPR with the ability to degrade organic pollutants is termed as rhizoremediators whereas PGPR which tend to suppress phytopathogens are termed as biopesticides.
Mechanisms of PGPR
The mechanisms of plant growth promotion exhibited by PGPR are mainly directed to altering the microbial community in the rhizospheres’ niche of the plants. This is mainly accomplished by the action of several plant growth-promoting substances secreted by PGPR. These growth-promoting compounds will directly or indirectly influence plant growth.
The major PGP mechanisms that directly influence the growth of plants are:
- Improving acquisition of nutrients like nitrogen, phosphorous and, other essential minerals
- Production of phytohormones and related compounds
- Nitrogen fixation
- Iron sequestration by the production of siderophores.
The major PGP mechanisms that indirectly influence the growth of plants are achieved through the inhibition of the action of pathogenic bacteria and fungi through mechanisms like:
- Production of ACC deaminase
- Production of enzymes that degrade the cell wall
- Production of antibiotics
- Production of hydrogen cyanide at threshold levels
- Generation of induced systemic resistance
- Quorum sensing
Even though no single organism can exhibit all the aforementioned PGP traits, a single PGPR may often harbour the machinery for more than one PGP trait. The presence of multiple types of PGPR in the soil with differing and varying PGP traits act in combination to promote plant growth. Several PGPR bacteria capable of promoting plant growth through the employment of more than one mechanism listed above have been commercialized and used as “Bio-fertilizers”.
Examples of PGPR
Examples of intracellular PGPR include members of rhizobia like Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, Allorhizobium and Azorhizobium whose major role in PGP is to improve plant nutrition through mechanisms of nitrogen fixation and phosphate solubilization. Examples of extracellular PGPR include members of Bacillus, Agrobacterium, Pseudomonas, Erwinia, Serration, Burkholderia, Azospirillum, Actinomycetes, Streptomycetes, etc. which improve plant growth by acting as phytostimulators, rhizoremediators and as biopesticides.
Conclusion and future perspectives
PGPR research in the last 3-4 decades has seen a drastic improvement in the precise understanding of how PGPR improves plant growth. The chemical and molecular basis behind the different mechanisms employed by PGPR is also being studied in great detail. A clear understanding of such an interaction has resulted in the commercialization of several strong PGPR strains for use as bio-fertilizers. However, in most cases the interaction between the plant and PGPR becomes unstable. The PGP results obtained in vitro are not successfully translated in vivo. This variability in the PGPR performance can be attributed to several abiotic and biotic factors. Strategies and techniques resulting in the production of efficient PGPR strains and their genetic manipulation for maximum advantage need to be devised. At the end of the day, the success of these PGPR will depend on its ability to survive in the rhizosphere amidst the indigenous microflora to improve its chances of survival, effectiveness and performance.
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