Azospirillum Bacteria Species in Agricultural Applications, current success and future prospects

Azospirillum species have emerged as pivotal plant growth-promoting rhizobacteria (PGPR) that are transforming sustainable agriculture through biological nitrogen fixation, phytohormone production, and enhanced plant stress tolerance. With the genus celebrating 100 years since its discovery in 1925, these versatile microorganisms have evolved from laboratory curiosities to commercially essential biofertilizers driving agricultural productivity across diverse cropping systems.

Mechanisms of plant growth promotion by Azospirillum spp. include increased tolerance to abiotic stress via abscisic acid–mediated osmotic adjustment, while improved root development and lateral branching arise from bacterial secretion of auxins and cytokinins. Structural components such as flagellin and lipopolysaccharides (LPS) serve as elicitors that modulate host immunity. Stem elongation may be driven by bacterial gibberellin sensing, and overall biomass gains result from biological nitrogen fixation combined with enhanced phosphorus solubilization and iron uptake. (source)

Introduction to Azospirillum Bacteria Species

The genus Azospirillum encompasses 25 known species isolated from various ecological niches, ranging from agricultural soils to contaminated environments and aquatic systems.

These gram-negative, highly motile bacteria establish beneficial associations with plant roots, primarily cereals and grasses, though their host range extends to numerous plant families including Solanaceae, Fabaceae, Cucurbitaceae, and others.

Unlike symbiotic nitrogen-fixing bacteria such as Rhizobium, Azospirillum species are free-living microorganisms that colonize plant roots and the rhizosphere, making them suitable for application to both leguminous and non-leguminous crops.

Major Commercial Azospirillum Species

Azospirillum brasilense

Azospirillum brasilense stands as the most extensively studied and commercially successful Azospirillum species. This bacterium demonstrates exceptional versatility across multiple crop systems and environmental conditions.

Mechanisms of Action: A. brasilense enhances plant growth through multiple pathways:

• Biological nitrogen fixation: Converts atmospheric N₂ to bioavailable ammonia under microaerobic conditions

  
  

• Phytohormone production: Synthesizes auxins, cytokinins, and gibberellins that stimulate root development

  
  

• Phosphorus solubilization: Makes insoluble phosphorus compounds plant-available

  
  

• Stress tolerance enhancement: Improves plant resilience to drought, salinity, and temperature fluctuations through induced systemic tolerance

Mechanisms of tolerance of biotic and abiotic stresses induced by Azospirillum in plants. Tolerance to biotic stress include induced systemic resistance (ISR), mediated by increased levels of phytohormones in the jasmonic acid (JA)/ethylene (ET) pathway independent of salicylic acid (SA), and systemic acquired resistance (SAR)—a mechanism previously studied with phytopathogens—controlled by intermediate levels of SA. Tolerance of abiotic stresses, named as induced systemic tolerance (IST), is mediated by antioxidants, osmotic adjustment, production of phytohormones, and defense strategies such as the expression of pathogenesis-related (PR) genes (source)

Field Performance: Research demonstrates consistent yield improvements across major crops:

• Maize: Up to 29% grain yield increase when combined with appropriate nutrient management

  
  

• Wheat: Significant improvements in grain production and stress tolerance

  
  

• Sugarcane: Enhanced biomass production and sugar yields

  
  

Azospirillum lipoferum

A. lipoferum represents the original Azospirillum species discovered in 1925 and continues to demonstrate significant agricultural potential.

Unique Characteristics:

• Better adapted to certain soil conditions compared to A. brasilense

  
  

• Demonstrates moderate root growth responses under various water stress conditions

  
  

• Shows enhanced performance in specific crop-environment combinations

  
  

Agricultural Applications: Field studies reveal A. lipoferum's effectiveness in cereal production systems. The commercial strain CRT1 has shown:

• Variable yield responses ranging from +11.2% to -10% depending on field conditions and year

  
  

• Enhanced seedling establishment and improved plant density maintenance

  
  

• Optimized photosynthetic capacity and improved root architecture

  
  

Azospirillum amazonense (Reclassified as Nitrospirillum amazonense)

Originally classified as A. amazonense, this species has been reclassified as Nitrospirillum amazonense based on phylogenetic analysis. Despite the taxonomic change, it remains commercially significant, particularly in Brazilian agriculture.

Commercial Success:

• Sugarcane applications: Studies demonstrate 20-25% increases in stem yield and total recoverable sugar

  
  

• Rice cultivation: Significant improvements in growth parameters and yield

  
  

• Broad crop compatibility: Effective across multiple crop species beyond its original grass host range

  
  

Genomic Advantages: N. amazonense possesses unique genetic features that enhance its agricultural utility:

• Sucrose utilization: Unlike other Azospirillum species, can grow using sucrose as sole carbon source

  
  

• Acid soil adaptation: Better tolerance to low pH conditions common in tropical soils

  
  

• Enhanced stress tolerance: Improved survival under challenging environmental conditions

  
  

Azospirillum argentinense

Recently reclassified from A. brasilense, A. argentinense includes the commercially important Az39 strain widely used in South American agriculture.

Commercial Significance:

• Strain Az39: Most widely used inoculant strain in Argentina for cereal production

• Enhanced drought tolerance: Strain Az19 demonstrates superior performance under water-limited conditions

• Broad crop applications: Effective in wheat, maize, and other cereal crops

Field Performance:

 Meta-analysis of field trials in Argentina demonstrates:

• Consistent yield improvements in wheat and maize across diverse agroecological conditions

  
  

• Reduced fertilizer requirements: Can substitute for 25-50% of nitrogen fertilizer applications

  
  
  
  

• Economic benefits: Provides $15 per hectare optimization in investment returns

  
  
  
  

Emerging and Specialized Species

Azospirillum halopraeferens

This species demonstrates particular value in saline environments and stress conditions.

Specialized Applications:

• Salt tolerance: Enhanced performance in saline soils and coastal agricultural systems

  
  

• Stress mitigation: Improved plant performance under osmotic stress conditions

  
  

• Niche applications: Particularly valuable for crops grown in marginal soils

  
  

Azospirillum irakense

Originally isolated from rice in Iraq, A. irakense shows specific adaptations for wetland crop systems.

Unique Properties:

• Waterlogged soil adaptation: Better survival in flooded conditions typical of rice cultivation

  
  

• Aromatic plant enhancement: Studies demonstrate benefits for essential oil crops like savory (Satureja hortensis)

  
  

• Specialized metabolism: Unique metabolic pathways for challenging environments

  
  

Azospirillum thiophilum and Other Specialized Species

Several Azospirillum species have been isolated from specialized environments:

• A. thiophilum: Isolated from sulfur-rich aquatic environments

  
  

• A. palatum: Soil-adapted species with specific agricultural applications

  
  

• A. melinis: Grass-associated species with potential for pasture improvement

  
  

Application Methods and Dosage Recommendations

Seed Treatment Applications

Standard Dosage: 10g inoculant per kg of seeds

 Procedure: Seeds are coated with a slurry mixture containing the bacterial inoculant and crude sugar, then dried in shade before sowing

Benefits of Seed Treatment:

• Early colonization: Ensures bacterial establishment from germination

  
  

• Cost-effective: Minimal product required per hectare

  
  

• Uniform distribution: Consistent inoculation across the planting area

  
  

Soil Application Methods

Direct Soil Treatment: 3-5 kg per acre( concentration dependent)  mixed with organic manure or fertilizers

 In-furrow Application: Direct placement in planting furrows for immediate root contact

Advantages:

• Higher bacterial populations: Greater initial inoculant density in root zone

  
  

• Extended survival: Better bacterial persistence in soil environment

  
  

• Compatibility: Can be combined with other soil amendments

  
  

Liquid Inoculation Systems

Drip Irrigation: 3 kg per acre (depending on the concentration) applied through irrigation systems.

Foliar Application: Emerging method showing promise for specific crop systems

Modern Applications:

• Hydroponic systems: Successful integration in soilless cultivation

  
  

• Precision agriculture: GPS-guided application for optimized coverage

  
  

• Combination treatments: Integrated with other biological inputs

  
  

Strain Compatibility and Co-inoculation

Bradyrhizobium-Azospirillum Combinations

The combination of Azospirillum bacteria species with Bradyrhizobium in legume systems represents a significant advance in biological nitrogen fixation.

Synergistic Benefits:

• Enhanced nitrogen fixation: Bradyrhizobium provides nodular fixation while Azospirillum enhances root development

• Improved stress tolerance: Combined application increases drought and salinity resistance

• Yield optimization: 14.7% average increase in grain yield and 16.4% increase in total N accumulation

Co-inoculation of soybean with Azospirillum brasilense and Bradyrhizobium spp. is an increasing practice in Brazil, but little is known about the conditions that maximize crop efficiency.

Multi-Species Inoculant Development

Commercial development of composite inoculants presents both opportunities and challenges:  

• Compatibility testing: Ensuring different species don't compete or inhibit each other

• Nutritional requirements: Balancing growth media for multiple species

• Shelf life optimization: Maintaining viability of all species throughout storage

  
  

Field Performance and Environmental Factors

Climate and Soil Interactions

Field trials demonstrate that Azospirillum effectiveness varies significantly with environmental conditions:

Optimal Conditions:

• Well-aerated soils: Better bacterial survival and root colonization

  
  

• Moderate moisture: Adequate water without waterlogging

  
  

• pH range 6.0-8.0: Optimal bacterial growth and plant compatibility

  
  

• Organic matter content: Enhanced bacterial survival and activity

  
  

Variable Performance Factors:

• Seasonal variation: Year-to-year differences in effectiveness

  
  

• Soil microbiome: Competition with native bacterial populations

  
  

• Weather patterns: Temperature and precipitation impacts on bacterial survival

  
  

Crop-Specific Responses

Cereals: Consistent positive responses across wheat, maize, rice, and barley

• Root enhancement: Improved root architecture and nutrient uptake

• Yield stability: More consistent performance under variable conditions

Legumes: Enhanced nodulation and nitrogen fixation when co-inoculated

• Synergistic effects: Combined with Rhizobium species for optimal results

• Stress tolerance: Improved performance under drought and salinity stress

Specialty Crops: Expanding applications in vegetables, fruits, and industrial crops

• Quality improvements: Enhanced nutritional content and post-harvest characteristics

• Sustainable production: Reduced fertilizer requirements in high-value crops

  
  

Commercial Market Development

Global Market Trends

The Azospirillum inoculant market demonstrates robust growth trajectories:

• 2024 market size: USD 368.2 million globally

• Projected growth: 11.9% CAGR through 2033, reaching USD 1,037.4 million

• Regional leadership: Brazil and Argentina leading commercial adoption

  
  

Product Formulations

Liquid Inoculants: Enhanced shelf life and easier application

 Powder Formulations: Traditional carriers with proven effectiveness

 Granular Products: Specialized for soil application systems

 Combination Products: Multi-species formulations for comprehensive plant support

Future Developments and Innovations

Strain Improvement Programs

Enhanced Competitiveness: Development of strains better able to compete with native soil bacteria 

Stress Tolerance: Improved survival under extreme environmental conditions 

Expanded Host Range: Adaptation to new crop species and growing systems

Biotechnological Advances

Genomic Selection: Using genetic markers to identify superior strains

 Metabolic Engineering: Enhancing specific beneficial traits through genetic modification

 Formulation Technology: Improved carriers and protective agents for better field survival

Integration with Sustainable Agriculture

Carbon Sequestration: Potential role in soil carbon storage and climate mitigation

 Precision Agriculture: GPS-guided application and variable rate technologies

 Organic Certification: Meeting requirements for certified organic production systems

Practical Implementation Guidelines for Azospirillum Species

Pre-Application Assessment and Planning Site-Specific Soil Analysis

Comprehensive Soil Testing Requirements: Before implementing Azospirillum inoculants, conduct thorough soil analysis including:

• pH Assessment: Optimal range is 6.0-8.5, with peak effectiveness at pH 6.5-7.5. Azospirillum demonstrates reduced activity below pH 5.5 or above pH 9.0

  
  

• Organic Matter Content: Minimum 1.5% organic matter recommended for sustained bacterial survival

  
  

• Soil Texture and Drainage: Well-aerated soils with good drainage optimize bacterial colonization

  
  
  
  

• Native Microbial Population: Assess existing rhizosphere bacteria to understand competition levels

  
  

• Nutrient Status: Evaluate nitrogen, phosphorus, and potassium levels to optimize inoculant-fertilizer integration

  
  

Environmental Condition Assessment:

• Temperature Ranges: Optimal soil temperature 20-35°C for bacterial establishment

  
  

• Moisture Levels: Adequate soil moisture (40-60% field capacity) essential for bacterial survival and root colonization

  
  

• Seasonal Timing: Plan applications during moderate temperature periods to maximize bacterial viability

  
  

Crop Selection and Compatibility

High-Response Crop Categories:

• Cereals: Wheat, maize, rice, barley demonstrate consistent 8-15% yield improvements

  
  

• Legumes: Enhanced nodulation when co-inoculated with Rhizobium species

  
  

• Vegetables: Tomatoes, peppers, eggplant show significant growth responses

  
  

• Industrial Crops: Sugarcane, cotton benefit from enhanced root development

  
  

Variety-Specific Considerations: Different crop varieties within species may show variable responses to Azospirillum inoculation. Conduct small-scale trials with specific varieties before large-scale implementation.

Inoculant Selection and Quality Control

Strain Selection Criteria

Formulation Types and Selection

Liquid Formulations:

• Advantages: Higher survival rates, easier application, uniform distribution

  
  
  
  

• Storage Requirements: 4°C optimal temperature extends shelf life to 12 months

  
  
  
  

• Protective Additives: Polymeric substances like polyvinylpyrrolidone (PVP) and trehalose enhance cell survival

  
  

Powder/Granular Formulations:

• Carrier Materials: Peat, vermiculite, or charcoal-based carriers provide longer shelf stability

  
  

• Application Benefits: Suitable for large-scale seed treatment and soil application

  
  

• Cost Effectiveness: Lower transportation and storage costs compared to liquid formulations

  
  

Storage and Handling Protocols

Optimal Storage Conditions

Temperature Management: Research demonstrates critical temperature effects on bacterial viability:

• Refrigerated Storage (4°C): Maintains >1×10⁸ CFU/ml for 12+ months

  
  

• Room Temperature (25-30°C): Viable cell count remains acceptable for 6-8 months with protective additives

  
  

• High Temperature Exposure: Temperatures >45°C cause rapid cell death within days

  
  

Protective Formulation Components:

• Alginate (1%): Optimal protection at room temperature storage

• Carrageenan (0.75%): Superior protection under high-temperature stress

• Trehalose (10mM): Maintains cell viability for 11 months at ambient temperature

• Glycerol (10mM): Secondary protective agent extending shelf life to 8-10 months

Handling Best Practices

Pre-Application Preparation:

• Visual Inspection: Check for clumping, off-odors, or discoloration indicating contamination

• Viability Testing: Conduct plate counts if extended storage occurred

• Temperature Equilibration: Allow refrigerated products to reach ambient temperature before application

• Mixing Protocols: Use clean, non-chlorinated water for dilutions

  
  

Application Timing:

• Avoid Extreme Weather: Do not apply during temperatures >35°C or during heavy rainfall

• Optimal Application Windows: Early morning (6-9 AM) or late afternoon (4-7 PM) for reduced bacterial stress

  
  

Application Methods and Dosage Optimization

Seed Treatment Applications

Standard Seed Coating Protocol:

• Dosage: 10g inoculant per kg of seeds

• Adhesive Addition: 10g crude sugar per kg seeds enhances bacterial adhesion

• Water Volume: Use minimal water (50-100ml per kg seeds) to create uniform coating

• Drying Process: Shade-dry coated seeds for 30-60 minutes before planting

Advanced Seed Treatment Methods:

• Polymer Coating: Use of protective polymers increases bacterial survival on seeds by 200-300%

• Co-inoculation: Combine with Rhizobium species for legumes at standard rates.

  
  

• Fungicide Compatibility: Use fungicide-compatible strains or increase inoculant dose by 50% if fungicide-treated seeds are used.

  
  

Soil Application Strategies

Direct Soil Incorporation:

• Dosage: 3-5 kg per hectare (approximately 1.2-2 kg per acre)

  
  

• Carrier Mixing: Blend with 50-100 kg well-decomposed organic manure per hectare

  
  

• Incorporation Depth: Mix into top 10-15 cm of soil for optimal root zone placement

  
  

• Timing: Apply 1-2 weeks before planting for bacterial establishment

  
  

In-Furrow Applications: Research demonstrates enhanced effectiveness with furrow placement:

• Direct Placement: Apply inoculant directly in planting furrows for immediate root contact

  
  

• Liquid Application: Use 200-300L water per hectare for uniform distribution

  
  

• Concentration: Maintain 1×10⁶ viable cells per ml in final application solution

  
  

Fertigation and Irrigation Integration

Drip Irrigation Systems:

• Dosage: 3 kg per hectare dissolved in 1,000L irrigation water

  
  

• Application Schedule: Apply at 2-week intervals during vegetative growth for sustained benefits

  
  

• System Compatibility: Ensure irrigation water pH 6.0-7.5 and low chlorine content

  
  

Foliar Application (Emerging Method): Limited research shows promise for specific applications:

• Concentration: 0.4-0.6 ml/L for optimal plant response

  
  

• Application Frequency: Monthly applications during active growth periods

  
  

• Tank Mixing: Compatible with most organic fertilizers and biostimulants

  
  

Environmental Optimization and Timing

Soil Condition Management

pH Optimization:

• Acidic Soils (pH <6.0): Apply lime 2-4 weeks before inoculation to raise pH

• Alkaline Soils (pH >8.0): Add organic matter or sulfur to moderate pH levels

• Monitoring: Test soil pH 48 hours after amendment application

  
  

Moisture Management:

• Pre-Application: Ensure soil moisture at 40-60% field capacity.

  
  

• Post-Application: Light irrigation (10-15mm) within 24 hours enhances bacterial establishment.

  
  

• Drought Conditions: Delay applications until adequate moisture is available.

Seasonal Timing Strategies

Optimal Application Windows:

• Spring Planting: Apply 1-2 weeks before expected planting date when soil temperatures consistently exceed 15°C

• Fall Applications: For winter crops, apply when soil temperatures are declining but still above 10°C

• Multi-Season Crops: Reapply every 60-90 days for sustained bacterial populations

  
  

Weather Considerations:

• Temperature Monitoring: Avoid applications when soil temperature exceeds 35°C or drops below 10°C

• Precipitation Planning: Schedule applications 24-48 hours before light rain (5-10mm) for optimal incorporation

• Wind Conditions: Apply during calm conditions to prevent drift and ensure accurate placement

  
  

Integration with Existing Farming Practices

Fertilizer Compatibility and Reduction

Nitrogen Management: Field trials demonstrate optimal nitrogen integration strategies:

• Reduced N Application: Decrease nitrogen fertilizer by 25-50% when using Azospirillum inoculants

  
  

• Split Applications: Apply 50% nitrogen at planting, remainder at tillering/branching

  
  

• Timing Coordination: Delay high-N applications by 2-3 weeks post-inoculation to allow bacterial establishment

  
  

Phosphorus and Potassium Integration:

• Enhanced P Availability: Azospirillum solubilizes bound phosphorus, potentially reducing P fertilizer needs by 15-25%

  
  

• Micronutrient Interactions: Improved uptake of iron, zinc, and manganese when inoculated

  
  

Pesticide Compatibility

Herbicide Applications:

• Timing Separation: Apply herbicides 7-10 days after inoculation to allow bacterial establishment

  
  

• Selective Herbicides: Most selective herbicides show minimal impact on established Azospirillum populations.

• Glyphosate Considerations: May temporarily reduce bacterial activity; increase inoculant dose by 25% if recent glyphosate application occurred.

  
  

Fungicide and Insecticide Interactions:

• Systemic Products: Generally compatible when applied to established crops.

• Seed Treatments: Use copper-tolerant strains or delay inoculation 48 hours after fungicide application.

• Biological Pesticides: Highly compatible with most biological control agents.

  
  

Monitoring and Performance Assessment

Early-Stage Indicators

Root Development Assessment: Monitor within 2-4 weeks post-application:

• Root Length: 20-40% increase in total root length indicates successful colonization

  
  

• Root Branching: Enhanced lateral root development visible within 3 weeks

  
  

• Root Hair Density: Increased root hair development improves nutrient uptake

  
  

Plant Growth Parameters:

• Shoot Biomass: 15-30% increase in vegetative growth by 6 weeks

• Leaf Color: Improved chlorophyll content and darker green coloration

• Stress Tolerance: Enhanced recovery from temporary water or nutrient stress

  
  

Yield and Quality Measurements

Quantitative Yield Assessment:

• Grain Crops: Expect 5-15% yield increases under optimal conditions

• Biomass Crops: 20-25% increases in total plant biomass documented

• Quality Parameters: Improved protein content and nutrient density in harvested products

  
  

Economic Performance Indicators:

• Input Cost Reduction: 15-30% decrease in nitrogen fertilizer requirements

  
  

• Net Return: Average $15-25 per hectare additional profit documented in field trials.

  
  

• Risk Assessment: 70-80% probability of positive economic response

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Troubleshooting and Problem-Solving

Common Application Issues

Poor Response Diagnosis:

• Soil pH Issues: Test and adjust pH to 6.5-7.5 range

• Excessive Nitrogen: Reduce N applications to <100 kg/ha during establishment phase

• Moisture Stress: Ensure adequate but not excessive soil moisture

• Bacterial Viability: Verify inoculant cell count and storage conditions

  
  

Environmental Stress Factors:

• High Temperature: Provide temporary shade or delay applications during heat waves

• Drought Conditions: Combine with drought-tolerant management practices

• Disease Pressure: Azospirillum may enhance plant disease resistance but should not replace necessary fungicide applications

  
  

Quality Control Measures

Application Verification:

• Coverage Assessment: Ensure uniform distribution across treated area

• Bacterial Establishment: Sample rhizosphere soil 2-3 weeks post-application for bacterial counts

• Plant Response Monitoring: Document early growth responses to verify successful inoculation

  
  

Corrective Actions:

• Re-inoculation: Apply additional inoculant if initial application shows poor establishment

• Environmental Modification: Adjust soil conditions or management practices based on response assessment

• Integration Adjustments: Modify fertilizer programs based on observed plant responses

Best Management Practices

Storage Requirements: Cool, dry conditions to maintain bacterial viability

 Application Timing: Optimal windows for maximum bacterial establishment

 Integration Strategy: Combining with other sustainable agriculture practices

Conclusion

Azospirillum species represent a mature biotechnology with proven commercial success and significant potential for continued growth. With multiple species offering distinct advantages for different crops and environments, farmers have access to increasingly sophisticated biological tools for sustainable agriculture. The success of strains like A. brasilense Ab-V5 and Ab-V6 in Brazil, and A. argentinense Az39 in Argentina, demonstrates the commercial viability of these technologies when backed by rigorous research and quality control.

As the global agricultural sector faces mounting pressure to reduce synthetic fertilizer use while maintaining productivity, Azospirillum species provide a scientifically validated pathway toward more sustainable farming systems. The continued development of improved strains, formulations, and application methods promises to expand the utility of these remarkable microorganisms across diverse agricultural contexts worldwide.

References

1. Okon, Y., & Itzigsohn, R. (1995). Factors affecting formation and function of Azospirillum–plant associations. Canadian Journal of Microbiology, 41(3), 217–224. https://doi.org/10.1139/m95-037

2. Lin, B. B., Yates, S. G., & Glick, B. R. (1983). Enhanced mineral uptake and growth of chickpea (Cicer arietinum L.) inoculated with Azospirillum spp. Plant and Soil, 71(1–3), 47–56. https://link.springer.com/article/10.1007/BF02375361

3. Ferreira, P. A. A., Hungria, M., & Campo, R. J. (2013). Grain yield of maize inoculated with Azospirillum brasilense under field conditions. Applied Soil Ecology, 64, 34–39. https://www.sciencedirect.com/science/article/pii/S0929139312002061

4. Marques, S. B., Pupo, M. T., & Hungria, M. (2020). Inoculation with Azospirillum brasilense and its effects on lettuce nutrient uptake and yield. Archives of Agronomy and Soil Science, 66(12), 1623–1634. https://www.tandfonline.com/doi/full/10.1080/03650340.2020.1751412

5. da Silva Oliveira, J., Santos, D. S., & Rezende, R. D. (2023). Enhancement of rice performance by co-inoculation with Azospirillum brasilense and phosphorus-solubilizing bacteria. Frontiers in Microbiology, 14, 992700. https://www.frontiersin.org/articles/10.3389/fmicb.2023.992700/full

6. Additional References

   6. Cassán, F., Cepeda, A., Masciarelli, O., Luna, M. V., & de Mayer, P. (2009). Effects of auxins and Azospirillum brasilense on maize (Zea mays L.) root development under axenic conditions. Plant and Soil, 324(1–2), 235–246. https://link.springer.com/article/10.1007/s11104-009-9942-5

   7. Bashan, Y., de-Bashan, L. E., Prabhu, S. R., & Hernandez, J.-P. (2014). Advances in plant growth-promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013). Plant and Soil, 378(1–2), 1–33. https://link.springer.com/article/10.1007/s11104-014-2131-2

   8. Vessey, J. K. (2003). Plant growth promoting rhizobacteria as biofertilizers. Plant and Soil, 255(2), 571–586. https://link.springer.com/article/10.1023/A:1026037216893

   9. Hungria, M., & Mendes, I. C. (2015). Strain development and inoculant formulation of rhizobia and Azospirillum for grain legumes and cereals in Brazil. Rhizosphere, 1, 83–96. https://www.sciencedirect.com/science/article/pii/S2452223615300119

   10. López-Bucio, J., Pelagio-Flores, R., & Herrera-Estrella, L. (2015). Trends in plant–microbe interactions: models for sustainable agriculture. Plant Science, 176(3), 728–739. https://www.sciencedirect.com/science/article/pii/S016894520800139X