In the realm of agricultural microbiology, Bacillus velezensis has emerged as a remarkable microorganism with a wide array of beneficial properties. This Gram – positive, aerobic, and endospore – forming bacterium, first isolated from a river in Spain in 2005 and named after the river, is now recognized for its significant contributions to plant health and soil quality improvement.
Antagonistic Activity and Production of Secondary Metabolites
Bacillus velezensis exhibits strong antagonistic capabilities against various plant pathogens, mainly due to its ability to produce a diverse range of secondary metabolites. These metabolites play crucial roles in inhibiting the growth and pathogenicity of harmful microorganisms.
- Cyclic Peptides: Compounds like iturin A and bacillomycin L are among the cyclic peptides synthesized by Bacillus velezensis. They possess broad – spectrum antifungal activity. By disrupting the integrity of fungal cell membranes, they cause the leakage of intracellular substances, ultimately leading to cell death. For example, in the control of soil – borne fungal diseases such as those caused by Rhizoctonia solani, the application of Bacillus velezensis – containing products has shown promising results, with the cyclic peptides effectively suppressing the growth of the pathogen.
- Antimicrobial Lipopeptides: Fengycin B and plipastatin A are some of the antimicrobial lipopeptides produced by this bacterium. These lipopeptides, with their unique hydrophilic and lipophilic properties, work in synergy with cyclic peptides. They enhance the damage to fungal cell membranes, further impeding the growth of fungi. In the case of protecting crops from Penicillium spp. infections, the combined action of these lipopeptides and other metabolites from Bacillus velezensis has been demonstrated to be highly effective.
- Bacitracin – like Compounds: Bacillomycin D and mycosubtilin are examples of bacitracin – like compounds produced by Bacillus velezensis. These complex – structured compounds can bind to sterols in fungal cell membranes, altering the membrane’s fluidity and permeability. This disruption in membrane function restricts the growth and development of fungi, such as Aspergillus spp., which are common pathogens in agricultural settings.
- Surfactin – like Compounds: Surfactin and bacilysin, surfactant – like compounds produced by Bacillus velezensis, have both surface – active and antibiotic properties. They can inhibit the growth of both Gram – positive and Gram – negative bacteria. In agricultural applications, this ability helps in controlling bacterial diseases that affect plants, such as those caused by Pseudomonas spp. and Xanthomonas spp.
Promotion of Plant Growth and Health
- Hormone Secretion: Bacillus velezensis secretes hormones like indole – 3 – acetic acid (IAA) and gibberellins (GA). IAA plays a vital role in promoting root growth, cell elongation, and differentiation. Gibberellins, on the other hand, are involved in various aspects of plant growth, including stem elongation, seed germination, and flowering. For instance, in tomato plants treated with Bacillus velezensis, an increase in root length and biomass has been observed, which can be attributed to the IAA secretion by the bacterium.
- Enzyme Secretion: The bacterium secretes enzymes such as phosphatase and nitrifying enzymes. Phosphatase helps in the solubilization of phosphorus in the soil, making it more available for plant uptake. Nitrifying enzymes are involved in the nitrogen cycle, enhancing the conversion of ammonia to nitrate, which is a more accessible form of nitrogen for plants. In nutrient – poor soils, the application of Bacillus velezensis can significantly improve the availability of phosphorus and nitrogen, leading to enhanced plant growth.
- Induction of Systemic Resistance: Bacillus velezensis can induce systemic resistance in plants. When the bacterium interacts with the plant, it triggers a series of physiological and biochemical responses in the plant, leading to the activation of its defense mechanisms. This induced resistance makes the plant more tolerant to various biotic and abiotic stresses. For example, plants pre – treated with Bacillus velezensis show increased resistance to drought stress and are less susceptible to pest attacks.
Applications in Pest and Disease Control
- Control of Plant Pathogens: The antibiotics and growth – inhibiting substances produced by Bacillus velezensis can effectively suppress the growth and reproduction of many plant pathogens. In the case of root rot diseases caused by Fusarium oxysporum, the application of Bacillus velezensis – based biocontrol agents has been shown to reduce the incidence of the disease. The bacterium competes with the pathogen for nutrients and space, and its metabolites directly inhibit the growth of the pathogen.
- Control of Soil Nematodes: Bacillus velezensis produces proteases that can break down the outer shells and body walls of nematodes. This property makes it an effective agent for controlling soil nematodes such as root – knot nematodes (Meloidogyne spp.) and stem nematodes (Ditylenchus spp.). In nematode – infested soils, the introduction of Bacillus velezensis can lead to a reduction in nematode populations, thereby protecting plant roots from damage.
- Control of Fungi: In addition to producing metabolites that target fungal cell membranes, Bacillus velezensis also secretes enzymes that can dissolve fungal cell walls. This dual – action mechanism makes it highly effective in controlling fungal diseases. For example, in the prevention and control of powdery mildew diseases in crops like strawberries and cucumbers, the use of Bacillus velezensis – containing products has shown excellent results, with significant reductions in disease severity.
Application Methods and Considerations
- Seed Treatment: Soaking seeds in a suspension of Bacillus velezensis can protect the seeds from soil – borne pathogens during germination. It also promotes early root growth and seedling vigor. For example, in vegetable seed treatments, such as those for cucumbers and tomatoes, seed soaking with Bacillus velezensis has been shown to increase germination rates and reduce the incidence of damping – off diseases.
- Soil Drenching: Applying a solution of Bacillus velezensis to the soil around the base of plants is an effective way to control soil – borne pests and pathogens. It can enhance the activity of beneficial soil microorganisms and improve the overall soil ecosystem. When dealing with root – feeding pests like nematodes in potted plants or in agricultural fields, soil drenching with Bacillus velezensis can provide long – term protection to the plant roots.
- Foliar Spraying: Foliar application of Bacillus velezensis can protect the above – ground parts of plants from fungal and bacterial diseases. The bacterium forms a protective biofilm on the leaf surface, preventing the invasion of pathogens. However, factors such as spraying time, concentration, and environmental conditions need to be carefully considered. For example, spraying should be done in the early morning or late afternoon to avoid evaporation and ensure better adhesion of the bacteria to the leaf surface.
Market and Future Prospects
Bacillus velezensis has a promising future in the agricultural market. Its use as a biocontrol agent and plant growth promoter aligns with the growing demand for sustainable and environmentally friendly agricultural practices. In recent years, several commercial products containing Bacillus velezensis have been developed and registered. For example, some companies have launched microbial fertilizers and pesticides with Bacillus velezensis as the active ingredient. These products not only help in reducing the use of chemical pesticides and fertilizers but also improve crop yields and quality. As research on Bacillus velezensis continues to expand, more innovative applications and product formulations are expected to emerge, further enhancing its role in modern agriculture.
In conclusion, Bacillus velezensis is a powerful tool in the agricultural toolbox. Its diverse range of beneficial properties, from disease and pest control to plant growth promotion, make it an invaluable resource for farmers and growers worldwide. By understanding its mechanisms of action and optimizing its application methods, we can fully harness the potential of this remarkable microbe to achieve sustainable and productive agriculture.