The term “plant growth-promoting rhizobacteria” [PGPR] as introduced by Kloepper and Schroth (1978) is used to define bacteria that are found in the rhizosphere and help in plant growth, nutrition and disease control and thereby crop productivity. Bacteria are the most abundant species present in the rhizosphere followed by actinomycetes, fungi, algae, and protozoa that colonize rhizosphere. PGPR can live in a symbiotic association, or as free-living microbes.
Mechanisms of action of PGPR :
PGPR promote plant growth through direct and indirect mechanisms
A) Direct mechanisms
a) Production of phytohormones
Phytohormones are the plant growth hormones or phytostimulants that influence plant growth. They include auxin [indole-3-acetic acid [IAA], gibberellic acid [GA], cytokinins, and ethylene. A variety of PGPR species possess the ability to produce phytohormones that helps in growth promotion of crop plants.
b) Nitrogen fixation
- Nitrogen is one of the major and important nutrients required by all living organisms for a variety of cellular, synthesis and metabolic functions. Although it is present abundantly (78%) in the atmosphere, its direct availability to plants and animals is restricted.
- Nitrogen-fixing PGPR plays a significant role in providing nitrogen in the soluble forms to the plants and from the plant the soluble nitrogen reaches to human etc.
- Several bacteria living freely in the soil or in symbiotic association with plants convert (fix) elemental nitrogen into a soluble form that can be easily taken up by plants.
- Symbiotic Nitrogen Fixers – Rhizobium and Frankia sp. Azorhizobium, Bradyrhizobium, and Cvanorhizobium.
- The efficient strain of Rhizobium can fix 40-200 kg/ha N in the soil. Free Living Nitrogen Fixers -Azotobacter sp., Azospirillum sp., BGA, Acetobacter sp., and Azolla sp. are known as best free-living nitrogen-fixing PGPR. They can fix 40-100 kg/ha N in the soil.
c) Sulfur Solubilization
- Although sulfur is present in large quantities (95%) in soil, it is not easily available to crop plants as it is present in organically bound nature in the form of sulfate esters or sulfonates, which are unavailable to the plant and need conversion into inorganic forms via microbial desulfurization.
- Sulfur-oxidizing PGPR like Thiobacillus thioxidans and T. novelis, oxidize insoluble sulfur into a soluble form through multienzyme complex responsible for cleaving the S group from the aromatic ring. S solubilizing PGPR plays a significant role in sulfur nutrition of crop plants.
d) Potash Solubilization
- Potassium (K) is an essential macronutrient for plant growth; it plays important roles in several metabolic processes such as protein synthesis, photosynthesis, and enzyme activation, etc.
- Potassium-solubilizing microorganisms (KSMs) present in soil are capable to solubilize insoluble ‘unavailable forms of the K into soluble/available form.
- The main mechanism of K’ solubilization includes acidolysis, chelation, exchange reactions, and the production of organic acid. KSM can increase crop yield by 20-25%.
e) Phosphate(P) solubilization
- P is the second important key element after nitrogen as a mineral nutrient in terms of quantitative plant requirement.
- It is abundantly available in soil in insoluble forms thus not available for plants. Phosphate-solubilizing microorganisms (PSM) such as Bacteria, molds, yeasts, actinomycetes, Cyanobacteria, etc sp solubilize insoluble P into soluble P that is easily absorbed by plants.
f) Iron Nutrition
Iron is an essential element for the survival of almost all cell types, it is a fourth most abundant element present in the soil but it exists in an insoluble and therefore unavailable form. In order to sequester and solubilize the iron soil, PGPR produces a variety of iron-chelating molecules referred to as siderophores.
g) Zinc solubilization
Zinc is one of the important micronutrients required for growth and reproduction of plants. Zn deficiency in plants is due to its low solubility in soils. Several PGPR including Pseudomonas sp. Bacillus sp., Burkholderia sp., Klebsiella sp., Enterobacter sp. are known as Zn solubilizers.
B) Indirect Mechanisms
a) Antibiotics Production
Antibiotics production is one of the most studied biocontrol strategies displayed by PGPR. Many antibiotics such as amphisin, 2,4-diacetylphloroglucinol, oomycin-A, phenazine, pyoluteorin, pyrrolnitrin, tensin, tropolone, the cyclic polypeptides (oligomycin, kanosamine, zwittermicin A, and xanthobaccin) are produced by PGPR strains namely Pseudomonas strains, Bacillus, Streptomvces and Stenotrophomonas sp.
b) Nutrients and Niche Competition
In order to establish as a dominant species in the soil, PGPR must be able to compete favorably for the available nutrient and space. This is a vital strategy needed for limiting disease incidence and severity. Rapid and abundant colonization of rhizosphere makes the rhizosphere unavailable for phytopathogens.
c) Induced Systemic Resistance (ISR) and Systemic Acquired Resistance (SAR)
PGPR trigger inducement of a defense system in plants that is capable of fighting phytopathogens. SAR is a defense mechanism activated in the plant following the primary infection, while ISR involves an increase in physical and mechanical strength of the cell wall and modulation of physical and biochemical reactions of the cell wall to environmental stress. ISR by PGPR is mediated through the production of salicylic acid. siderophores. lipopolysaccharide, flagella, N-acyl homoserine lactone (AHL) molecules.
d) Production of hydrolytic enzymes
The production of lytic enzymes such as chitinases, cellulases, lipases, 8-1-3 glucanases, and proteases by rhizobacteria has been suggested to be a vital form of biocontrol. These hydrolytic enzymes degrade a wide range of compounds of phytopathogens.
e) Hydrogen cyanide (HCN)
Hydrogen cyanide is principally produced by Pseudomonas sp. Its cyanide ion of HCN inhibits most metallo enzymes, especially copper-containing cytochrome c oxidases
f) Use of Hypovirulent Strains
Hypovirulence is a reduced virulence found in few strains of phytopathogens. Application of such hypovirulent strains of phytopathogens has helped in reducing the effect of virulent phytopathogenic strains. Rhizoctonia solani, Gaeumannonnveces hramini var. Tritici and Ophiostoma ulmi has been used as hypovirulent strains to reduce the severity of plant diseases caused by virulent strains (Sayyed et al. 2013).
g) Detoxification or Degradation of Virulence Factors.
Detoxification of virulence factors of pathogens is another important mechanism of biocontrol. In this mechanism, biocontrol strain produces a protein that reversibly binds the toxin leading to irreversible detoxification. Biocontrol strain of Pseudomonas sp is known to detoxify albicidin toxin produced by Yanthomonas albilineans
PGPR with biocontrol potential can be parasites or predators of the pathogens. Mycoparasites, such as Coniothyvrium minitans and Sporidesmium sclerotivorum inhibit the growth of Sclerotinia sp. and other sclerotia forming fungi. The mycoparasitic fungus produces cell wall degrading enzymes such as beta 1,3-glucanase, chitinase, acid phosphatase, acid proteases, and alginate lyase, etc.