PGPR works against the broad spectrum of root and foliar pathogens benefiting the plant growth
Several bacterial species which are found in the plant rhizosphere, at root surfaces or in association with roots, can improve the quality of plant growth directly or indirectly, are known as ‘Plant Growth Promoting Rhizobacteria’ (PGPR). It effects plant growth by production of phytohormones like Indole-3-acetic acid (IAA), solubilization of inorganic phosphates and suppression of disease-causing microbe.
PGPR works against the broad spectrum of root and foliar pathogens benefiting the plant growth. Treatment with PGPR increases the rate of germination, root and shoot growth, total biomass of the plants, early flowering, fodder and fruit yields. Phytochrome production like auxins, cytokinin and gibberellin, enhancing plant nutrition by solubilization of minerals like phosphorus and iron, production of siderophores and enzymes, directly or indirectly contributes in growth promotion of plant.
Many bacteria are used as PGPR, including Azospirillum, Enterobacter, Bacillus and Pseudomonas. Out of them Pseudomonas species have been identified as efficient bacteria which plays a major role in growth promotion and resistance against disease. Due to their characteristics and environmentally friendly traits, they can survive in normal conditions as well stressful conditions.
- Solubilization of Phosphorus
Phosphorus present in the chemical fertilizers is in the soluble form which can be easily available for plant uptake, but they react with soil and becomes less available due to immobilization into insoluble forms. With the help of acid and alkaline phosphatase enzyme, PGPR can dissolve insoluble phosphorus, making it available for plant uptake. They are called as Phosphate Solubilizing Bacteria (PSB). These bacteria improve the nutrient availability for the plant, resulting in growth and yield of the crop. Solubilization and mineralization of phosphorus is the main physiological trait in soil biogeochemical cycles as well as in plant growth promotion.
- IAA production
Production of IAA affects cell division, root initiation, cell enlargement and division, tissue differentiation, increase the rate of xylem and root development, control processes of vegetative growth, affects photosynthesis, pigment formation, biosynthesis of various metabolites and resistance to stressful conditions. The level of IAA synthesized by the plant is important to determine whether bacterial IAA stimulate or supresses the plant growth, additional IAA that is taken up from the bacteria could alter the IAA level, resulting in plant growth promotion or suppression. Increase in auxin level, affects the root nodulation. Microbial biosynthesis of IAA is enhanced by the roots which secretes tryptophan resulting in positive effect on plant growth, increase in accessibility of more nutrients from soil and increasing the number of beneficial microbes controlling disease causing pathogens.
- Stress study
Studies revealed, Pseudomonas can survive in the adverse conditions like dry and saline environment, as they develop their internal pressure above the surrounding environment by accumulating osmolytes in their body. It can also tolerate temperature up to 40⁰ C, due the formation of biofilm which provides a protective mode of growth and enhance the bacterial survival under adverse conditions.
- Siderophore
Siderophore are small, high-affinity iron chelating compounds, secreted by PGPR to transport iron across cell membranes. Iron is an essential micronutrient for plants as it serves as a cofactor of many enzymes and required in several physiological processes like nitrogen fixation, photosynthesis and respiration. Pseudomonas species synthesize siderophore molecules which have high affinity for ferric iron act as chelating agents which binds the molecules and make the insoluble form to soluble form, that facilitates iron uptake by plants.
Limitations
Despite a number of characteristics and better performance of Pseudomonas species, there are certain limitations regarding the use of bacteria on commercial basis.
- High concentration of IAA production can inhibit the root and shoot growth of plants; therefore, the bacterial strain should be kept in mind for obtaining better results.
- Being a well-known strain for degrading hydrocarbon material, it is also a pathogenic bacterium causing infections like asepsis which leads to high mortality rate. Therefore, care must be taken in its use.
- It is recommended to be used with other microbial inoculation for enhancing plant growth, as it has been observed that single inoculation is not effective or less effective for the particular purpose.
Conclusion
PGPR are one of the most suitable choices for plant growth promotion and in disease management over the use of chemical fertilizers and pesticides. Physiological characteristics of Pseudomonas species under adverse conditions have worth in sustainable agriculture and effective performance verifies their positive role in environmental protection. Due to their negative impact in one condition could be effective against other aspects of environment. Overall, they are an environmentally friendly microbial population that can be used to tackle many problems if used wisely.
Rashmi Chauhan, Quality Control Officer, Indore Biotech Inputs & Research Pvt. Ltd. Indore (M.P.)
