Bacillus subtilis: A Soil-Inhabiting Plant Growth Promoting Rhizobacterium

Bacillus subtilis is a Gram-positive, rod-shaped bacterium that is commonly found in soil. It is a naturally occurring bacterium and is known for its ability to form endospores, which are resistant to heat, radiation, and other harsh conditions. This resilience allows B. subtilis to survive in diverse environments.

B. subtilis is widely recognized for its beneficial effects on plant growth, and it has been extensively studied as a plant growth-promoting rhizobacterium (PGPR). PGPR are beneficial bacteria that live in the rhizosphere, the region of soil around plant roots. Here are some ways in which B. subtilis benefits plant growth:

Biocontrol

 

B. subtilis produces several antimicrobial compounds that contribute to its biocontrol properties and benefit plant growth. Two key types of antimicrobial compounds produced by B. subtilis are:

1. Antibiotics:

   • Bacitracin: Bacitracin is an antibiotic produced by B. subtilis that inhibits the growth of certain Gram-positive bacteria. It interferes with bacterial cell wall synthesis, disrupting the integrity of the cell wall and preventing the growth of pathogenic microorganisms.

   • Subtilin: Subtilin is a bacteriocin, which is a type of antimicrobial peptide produced by B. subtilis. It has antibacterial activity against a range of Gram-positive bacteria, including some plant pathogens. Subtilin can create a hostile environment for competing microbes, reducing the incidence of diseases caused by these pathogens.
     
     

2. Lipopeptides:

   • Surfactin: Surfactin is a lipopeptide produced by B. subtilis with antimicrobial properties. It has the ability to disrupt cell membranes of target microorganisms, leading to cell lysis. This action helps in controlling the growth of pathogenic bacteria and fungi in the plant rhizosphere.

   • Iturin: Iturin is another lipopeptide produced by B. subtilis that exhibits antifungal activity. It forms complexes with fungal cell membranes, leading to membrane permeabilization and cell death. Iturin-producing strains of B. subtilis contribute to the control of soilborne fungal pathogens, protecting plants from diseases.

Induced Systemic Resistance (ISR)

Bacillus subtilis can induce the plant's defense mechanisms, leading to a phenomenon known as induced systemic resistance. This helps plants resist attacks from various pathogens. B. subtilis is able to do so by stimulating the production of both jasmonic and salicylic acids, molecules that play key roles in activating defence mechanisms against pests and pathogens.

Nutrient Solubilization

Bacillus subtilis has the ability to solubilize inorganic phosphates in the soil. Phosphorus is an essential nutrient for plant growth, and in many soils, phosphates may be present in insoluble forms. B. subtilis produces organic acids, such as citric and gluconic acids, which can convert insoluble phosphates into soluble forms that are more easily taken up by plant roots.

B. subtilis also produces siderophores, which are molecules that can chelate iron. Chelation makes iron more available to plants by preventing it from forming insoluble precipitates in the soil. Iron is a critical micronutrient for plants, and increased availability supports healthier growth.

Production of Plant Growth Hormones

Bacillus subtilis can produce auxins, which are a type of plant growth hormone. Auxins play a crucial role in stimulating cell elongation and division, promoting root development, and influencing various aspects of plant growth and development. The production of auxins by B. subtilis is one of the mechanisms through which this bacterium can positively influence the growth of plants by enhancing root architecture and overall plant vigour.

Enhanced Stress Tolerance

Bacillus subtilis enhances plant stress tolerance through multiple mechanisms. By inducing the expression of stress-responsive genes, it stimulates the synthesis of antioxidant enzymes, aiding in the scavenging of reactive oxygen species during stress conditions. Additionally, B. subtilis contributes to osmoprotection by promoting the accumulation of compatible solutes, maintaining cell turgor pressure and preventing dehydration under stress. Modulating abscisic acid levels, this bacterium helps regulate stomatal closure during drought stress. Improved nutrient uptake facilitated by B. subtilis supports overall plant health, and biofilm formation on plant surfaces provides a protective barrier against pathogens and environmental stressors.

Summary

In summary, Bacillus subtilis, a Gram-positive bacterium commonly found in soil, exhibits remarkable resilience due to its ability to form heat- and radiation-resistant endospores. Acknowledged as a plant growth-promoting rhizobacterium (PGPR), B. subtilis offers diverse benefits to plants. Its biocontrol properties manifest through the production of antimicrobial compounds, including antibiotics like bacitracin and bacteriocins like subtilin and lipopeptides such as surfactin and iturin. These compounds create a hostile environment for pathogens, safeguarding plants from diseases. Additionally, B. subtilis induces systemic resistance, stimulates nutrient solubilization, and produces auxins, enhancing root development and overall plant growth. Furthermore, the bacterium contributes to stress tolerance by modulating stress-responsive genes, synthesizing antioxidant enzymes, promoting osmoprotection, and regulating abscisic acid levels. The improvement of nutrient uptake and the formation of protective biofilms on plant surfaces further reinforce B. subtilis' role in promoting plant health and resilience in challenging environmental conditions.