Importance and mechanism of plant growth promoting rhizobacteria (PGPRs)
Department of Plant Pathology, S.K.N. University of Agriculture, Jobner, Jaipur(Raj.)India 303328


PGPRs are those Bacteria that colonize plant roots following inoculation onto the seed and that enhance plant growth. It also causes indirect plant growth promotion or biological control. Its play vital role for disease control. Such bacteria found in and on rhizoplane and in rhizosphere.

PGPRs are characterized by the following features

  • They must be proficient to colonize the root surface.
  • They must survive, multiply and compete with other microorganism.
  • They must promote plant growth.
    PGPRs promote plant growth by direct and indirect means.
    Direct mechanism:- In direct mechanisms it enhances plant growth in the absence pf pathogen.
1.Nitrogen fixation:

  • There are number of PGPRs, which are able to fix atmospheric nitrogen and make it more accessible to plants.
  • Examples: Rhizobium form nodules on roots of leguminous plants such as soyabean, peanut and alfalfa.
  • Azospirillum is a free living N2 – Fixer that can fertilize wheat, sorghum and maize.
2.Phosphate solubilization:

  • Low levels of soluble phosphate can limit the growth of plants as it is not available to support plant growth.
  • Some PGPR solubilize phosphate from either organic or inorganic bound phosphates, which facilitating plant growth.
  • Example: Azospirillum spp., and Acinetobacter calcoaceticus.
3. Siderophore production:

  • Iron is a vital nutrient for almost all forms of life.
  • Despite its abundances, particularly in soils and natural aquifers, however, its bioavailability is relatively low.
  • Many microorganisms possess high affinity iron uptake system mediated by the action of low molecular weight iron chelators termed as siderophores.
  • Eg.- Pseudomonas fluorescens.
4. Modulating phytohormone levels :

  • Phytohormones play key role in plant growth and development and the response of plants to their environment.
  • Under stress conditions, plants attempt to adjust the levels of their endogenous plant hormones in order to decrease the negative effects of the environmental stresses.
  • Example: Bacillus spp., Pseudomonas spp. , Rhizobium spp. Indole acetic acid production. Indirect Mechanism:- Indirectly by decreasing the inhibitory effects of various pathogens on plant growth and development in the forms of biocontrol agents.
  • Antibiotic and lytic enzymes:
  • The production of one or more antibiotics is the mechanism most commonly associated with the ability of plant growth promoting rhizobacteria to act as antagonistic agents against phytopathogens.
  • Some PGPR produce enzyme including chitinases, proteases and lipases that can lyse a portion of the cell walls of many pathogenic fungi like Botrytis cinerea, Sclerotium rolfsii and Fusarium oxysporium.
  • Competition:-
  • PGPR competes with the pathogen for nutrients and niches in the rhizosphere which helps to limit disrase incidence and disease incidence and severity.
  • Pseudomonas fluorescens PCL 1751 suggests that it controls the disease ( Fusarium oxysporium f.sp. radicis - lycopersici) through the competition mechanism for nutrients and niche.
  • Induced systemic resistance:-
  • PGPR can stimulate induced systemic resistance in which the plant can better defend itself against pathogenic bacteria, fungi and viruses.
  • Induced resistance is the state of an enhanced defensive ability developed by plants when appropriately stimulated.
  • ISR involves jasmonate and ethylene signaling which stimulate the host plant defense responses against a broad range of plant pathogens.
  • Pseudomonas fluorscense : ISR and antifungal activity.

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