Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonize the root zone, acting as biostimulants, biofertilizers and biocontrol agents that influence plant responses to their environment.
Rhizobacteria inhabit the rhizosphere—the region of soil immediately surrounding roots, as well as the sticky root surfaces themselves. This zone regulates key factors such as nutrient availability and microbial interactions that support plant health and survival.
Although soilless systems lack a traditional rhizosphere, root surfaces remain bioactive, allowing PGPR to colonize them in hydroponic conditions. This provides benefits such as enhanced growth and health, increased yields and reduced reliance on fertilizers and pesticides.
PGPR species and inoculation
The most common way to apply PGPR is to add concentrated bacterial suspension to the growing medium. PGPR are ineffective if they do not successfully colonize the roots. Several factors influence colonization success in hydroponic systems, starting with the bacterial species.
Several PGPR species are used commercially as biocontrol agents, including Agrobacterium, Azospirillum, Azotobacter, Bacillus, Burkholderia, Delftia, Paenibacillus, Pantoea, Pseudomonas, Rhizobium and Serratia.[1] Some species may be better suited for hydroponic cannabis cultivation than others. An optimal PGPR species should meet the following criteria:[2]
- Rhizosphere adaptability: Remains environmentally safe and effectively colonizes the rhizosphere under diverse conditions.
- Effective root colonization: Establishes itself in significant numbers on plant roots after inoculation.
- Broad-spectrum activity: Provides a wide range of benefits, such as improved plant growth, nutrient uptake and pathogen protection.
- Compatibility, not competition: Coexists harmoniously with other beneficial bacteria in the rhizosphere.
- Stress tolerance: Withstands harsh conditions like heat, desiccation, radiation and oxidative treatments.
- Rhizosphere adaptability: Remains environmentally safe and effectively colonizes the rhizosphere under diverse conditions.
- Effective root colonization: Establishes itself in significant numbers on plant roots after inoculation.
- Broad-spectrum activity: Provides a wide range of benefits, such as improved plant growth, nutrient uptake and pathogen protection.
- Compatibility, not competition: Coexists harmoniously with other beneficial bacteria in the rhizosphere.
- Stress tolerance: Withstands harsh conditions like heat, desiccation, radiation and oxidative treatments.
Beyond selecting an appropriate strain, other factors to consider that influence PGPR colonization include:[3]
- Inoculation makeup, concentration and timing
- Growing medium, particularly its mechanical matrix properties and porosity
- Management practices
- Fertilizer regime
- pH and cation exchange capacity
One advantage for hydroponic growers is that PGPR do not have to compete with indigenous soil bacteria. Since PGPR tend to colonize more effectively when microbial competition is lower, growers should consider the physical nature of their hydroponic system when introducing PGPR.[4]
Benefits of PGPR
While the full effects of PGPR vary among bacterial and plant species, research indicates a range of typical benefits:
- Phytohormone production: Stimulate root and shoot development by producing hormones like auxins and cytokinins, enhancing plant growth.[5]
- Signal transduction: Influence plant signaling pathways to enhance nutrient uptake and optimize growth conditions for better crop performance.[6]
- Nitrogen fixation and phosphorus solubilization: Convert atmospheric nitrogen into plant-usable forms, reducing fertilizer dependence. Release organic acids and enzymes that solubilize phosphorus, improving availability.[7]
- Improved biocontrol: Inhibit pathogens and activate plant phytohormones involved in defense mechanisms. Reduce pathogen populations by outcompeting them for space and nutrients.[8]
- Secondary metabolite production: Correlate with increased cannabinoid and terpene accumulation, depending on the microbial species.[9]
- Stress tolerance: Produce stress-alleviating compounds that improve plant resilience to abiotic stresses such as drought, salinity and temperature fluctuations.[10]
PGPR in cannabis
Cannabis is highly susceptible to various phytopathogens, with powdery mildew being particularly problematic in hydroponic cultivation. This fungal disease thrives in the high-humidity conditions typical of indoor environments, affecting all phases of the crop life cycle. Powdery mildew can cause premature leaf senescence and reduce photosynthesis, leading to lower yields and diminished flower quality, among other crop problems.
The PGPR species Bacillus subtilis has demonstrated significant protection against powdery mildew in crops like cucurbits by producing antifungal compounds and activating plant defense responses.[11] The Bacillus species supports plant health in multiple ways, including by:[12]
- Excreting the phytohormone cytokinin, which promotes root and shoot development.
- Stimulating phytohormone synthesis by enhancing IAA production, supporting plant growth.
- Suppressing pathogens by releasing antimicrobial compounds.
- Inducing systemic resistance, strengthening plant defenses against disease.
- Altering secondary metabolite biosynthesis, contributing to disease resistance and improved plant quality.
- Excreting the phytohormone cytokinin, which promotes root and shoot development.
- Stimulating phytohormone synthesis by enhancing IAA production, supporting plant growth.
- Suppressing pathogens by releasing antimicrobial compounds.
- Inducing systemic resistance, strengthening plant defenses against disease.
- Altering secondary metabolite biosynthesis, contributing to disease resistance and improved plant quality.
Studies on cannabis inoculated with PGPR report significant benefits, including increased yields:[13]
- One study compared the effects of three species—Mucilaginibacter, Bacillus and Pseudomonas—applied during the vegetative and flowering phases. Mucilaginibacter produced the best results, increasing flower dry weight by almost 25% and boosting CBD and THC levels by just over 11% each when applied in the vegetative phase. When inoculated during the flowering phase, it increased terpene accumulation by 23%.[14]
- Another study by the same researchers found that Pseudomonas produced the highest flower fresh weight when inoculated at the vegetative phase and increased the plant’s photosynthetic rate.[15]
- Other research showed that a PGPR consortium of Azospirillum brasilense, Gluconacetobacter diazotrophicus, Burkholderia ambifaria and Herbaspirillum seropedicae improved growth, enhanced the physiological status of hemp plants and increased secondary metabolite accumulation and antioxidant activity.[16]
Conclusion
While effective colonization depends on various factors, inoculating cannabis with PGPR is an environmentally friendly disease management strategy that can reduce reliance on chemical fertilizers and fungicides. Since hydroponic systems lack the indigenous bacteria found in natural rhizospheres, growers may have greater success introducing beneficial bacteria that typically thrive in soil.
Emerald Harvest Team
[1] Glick, Bernard R. 2012. “Plant Growth-Promoting Bacteria: Mechanisms and Applications.” Scientifica (Cairo). 2012: 063401. https://doi.org/10.6064/2012/963401.
[2] Basu, Anirban, Priyanka Prasad, Subha Narayan Das, et. al. 2021. “Plant Growth Promoting Rhizobacteria (PGPR) as Green Bioinoculants: Recent Developments, Constraints, and Prospects.” Sustainability 13 (3): 1140. https://doi.org/10.3390/su13031140.
[3] Soderstrom, Linus. 2020. “Plant-Growth Promoting Rhizobacteria in Soilless Cannabis Cropping Systems: Implications for Growth Promotion and Disease Suppression.” Accessed January 10, 2025. https://stud.epsilon.slu.se/16079/11/soderstrom_l_200923.pdf.
[4] Ibid.
[5] Chieb, Maha, and Emma W. Gachomo. 2023. “The Role of Plant Growth Promoting Rhizobacteria in Plant Drought Stress Responses.” BMC Plant Biology 23 (1): 407. https://doi.org/10.1186/s12870-023-04403-8.
[6] Ibid.
[7] De Andrade, Luana Alves, Carlos Henrique Barbosa Santos, Edvan Teciano Frezarin, Luziane Ramos Sales, and Everlon Cid Rigobelo. 2023. “Plant Growth-Promoting Rhizobacteria for Sustainable Agricultural Production.” Microorganisms 11 (4): 1088. https://doi.org/10.3390/microorganisms11041088.
[8] Lyu, Dongmei, Rachel Backer, W. George Robinson, and Donald L. Smith. 2019. “Plant Growth-Promoting Rhizobacteria for Cannabis Production: Yield, Cannabinoid Profile and Disease Resistance.” Frontiers in Microbiology 10: 1761. https://doi.org/10.3389/fmicb.2019.01761.
[9] Lyu, Dongmei, Rachel Backer, Fabrice Berrue, Camilo Martinez-Farina, Joseph P. M. Hui, and Donald Lawrence Smith. 2023. “Plant Growth-Promoting Rhizobacteria (PGPR) with Microbial Growth Broth Improve Biomass and Secondary Metabolite Accumulation of Cannabis sativa L.” Journal of Agricultural and Food Chemistry 71 (19): 7268-7277. https://doi.org/10.1021/aces.jafc.2c06961.
[10] Chieb, Maha, and Emma W. Gachomo. 2023. “The Role of Plant Growth Promoting Rhizobacteria in Plant Drought Stress Responses.” BMC Plant Biology 23 (1): 407. https://doi.org/10.1186/s12870-023-04403-8.
[11] Garcia-Guiterrez, Laura, Houda Zeriouh, Diego Romero, Jaime Cubero, Antonio Vicente, and Alejandro Perez-Garcia. 2012. “The Antagonistic Strain Bacillus subtilis UMAF6639 Also Confers Protection to Melon Plants Against Cucurbit Powdery Mildew by Activation of Jasmonate-and Salicylic Acid-Dependent Defence Responses.” Microbial Biotechnology 6 (3): 264-274. https://doi.org/10.1111/1751-7915.12028.
[12] Etesami, Hassan, Byoung Ryong Jeong, and Bernard R. Glick. 2023. “Potential Use of Bacillus spp. as an Effective Biostimulant Against Abiotic Stresses in Crops—A Review.” Current Research in Biotechnology 5: 100128. https://doi.org/10.1016/j.crbiot.2023.100128.
[13] Hasan, Asma, Baby Tabassum, Mohammad Hashim, and Nagma Khan. 2024. “Role of Plant Growth Promoting Rhizobacteria (PGPR) as a Plant Growth Enhancer for Sustainable Agriculture: A Review.” Bacteria 3 (2): 59-75. https://doi.org/10.3390/bacteria3020005.
[14] Lyu, Dongmei, Rachel Backer, Fabrice Berrue, Camilo Martinez-Farina, Joseph P. M. Hui, and Donald Lawrence Smith. 2023. “Plant Growth-Promoting Rhizobacteria (PGPR) with Microbial Growth Broth Improve Biomass and Secondary Metabolite Accumulation of Cannabis sativa L.” Journal of Agricultural and Food Chemistry 71 (19): 7268-7277. https://doi.org/10.1021/aces.jafc.2c06961.
[15] Lyu, Dongmei, Rachel Backer, and D.L. Smith. 2024. “Productivity Effects of Single Plant Growth Promoting Rhizobacterium Inoculation on Cannabis sativa L. Morphological Development and Flower Yield.” Authorea. https://doi.org/10.22541/au.170668537.78254140/v1.
[16] Pagnani, Giancarlo, Marika Pellegrini, Angelica Galieni, et. al. 2018. “Plant Growth-Promoting Rhizobacteria (PGPR) in Cannabis sativa ‘Finola’ Cultivation: An Alternative Fertilization Strategy to Improve Plant Growth and Quality Characteristics.” Industrial Crops and Products 123: 75-83. https://doi.org/10.1016/j.indcrop.2018.06.033.