The Science of Plant Growth Promoting Rhizobacteria (PGPR)
Introduction
Plant Growth Promoting Rhizobacteria (PGPR) are a group of bacteria that can be found in the rhizosphere, a region of soil that is directly influenced by root secretions and associated soil microorganisms. These bacteria have the ability to enhance plant growth and productivity, not only by direct mechanisms such as nitrogen fixation or phosphorus solubilization, but also indirectly by suppressing plant diseases and promoting plant health.
Characteristics of PGPR
PGPR are characterized by their ability to colonize plant roots and enhance plant growth. They can be free-living in the soil or can form symbiotic relationships with plants. PGPR include a wide variety of bacterial species and strains, including those from the genera Pseudomonas, Bacillus, Rhizobium, Azospirillum, and others.
Mechanisms of Plant Growth Promotion
PGPR promote plant growth through a variety of mechanisms, which can be broadly classified into direct and indirect mechanisms.
Direct Mechanisms
Direct mechanisms of plant growth promotion by PGPR include nitrogen fixation, phosphorus solubilization, production of plant growth regulators, and improvement of plant nutrition.
Nitrogen Fixation
Many PGPR have the ability to fix atmospheric nitrogen into a form that is usable by plants. This process, known as nitrogen fixation, is carried out by bacteria that possess the enzyme nitrogenase, which catalyzes the conversion of atmospheric nitrogen (N2) into ammonia (NH3).
Phosphorus Solubilization
Phosphorus is an essential nutrient for plant growth, but it is often present in soils in forms that are not readily available to plants. Some PGPR have the ability to solubilize insoluble forms of phosphorus, making it available for plant uptake.
Production of Plant Growth Regulators
PGPR can produce plant growth regulators, such as auxins, cytokinins, and gibberellins, which can stimulate plant growth and development.
Improvement of Plant Nutrition
PGPR can enhance plant nutrition by increasing the availability of nutrients in the soil, improving root development, and enhancing the plant's ability to uptake nutrients.
Indirect Mechanisms
Indirect mechanisms of plant growth promotion by PGPR include suppression of plant diseases and enhancement of plant health.
Suppression of Plant Diseases
PGPR can suppress plant diseases by producing antimicrobial compounds, competing with pathogenic microorganisms, and inducing systemic resistance in plants.
Enhancement of Plant Health
PGPR can enhance plant health by improving plant tolerance to abiotic stresses, such as drought, salinity, and heavy metals, and by enhancing the plant's immune system.
Applications of PGPR
Due to their ability to promote plant growth and suppress plant diseases, PGPR have been widely used in agriculture as biofertilizers, biopesticides, and biostimulants.
Biofertilizers
PGPR-based biofertilizers are used to enhance soil fertility and plant nutrition. They can be used as an alternative to chemical fertilizers, reducing the environmental impact of agriculture and improving soil health.
Biopesticides
PGPR-based biopesticides are used to control plant diseases. They offer a sustainable and environmentally friendly alternative to chemical pesticides.
Biostimulants
PGPR-based biostimulants are used to enhance plant growth and productivity. They can stimulate plant growth, improve plant tolerance to abiotic stresses, and enhance plant health.
Future Perspectives
The use of PGPR in agriculture has great potential for sustainable and environmentally friendly farming. However, more research is needed to fully understand the complex interactions between PGPR, plants, and the soil environment, and to develop effective and reliable PGPR-based products for agriculture.