Rhizobacteria colonize plant roots and form a symbiotic, beneficial relationship with the plant.  These microbes help protect plants from pathogens and stress. They also enhance nutrient uptake and plant hormone production, encouraging growth and health. Thus, plant growth-promoting rhizobacteria (PGPR) are used as biofertilizer.  Recently, researchers  suggested that PGPR be tested on cannabis plants to boost three cultivation outcomes: overall yield, cannabinoid levels, and resistance to disease.
Rhizobacteria could represent a sustainable way to improve commercial cannabis production. The authors emphasize two genera of PGPR, Pseudomonas and Bacillus. Highlighting past studies on rhizobacteria for other plants, the review calls for experimentation.
Only a few studies have examined PGPR on cannabis specifically.  Most notably, Pagnani et al  in 2018 applied blends of rhizobacteria species (Azospirillum brasilense, Gluconacetobacter diazotrophicus, Burkholderia ambifaria, and Herbaspirillum seropedicae) to Finola plants (hemp) grown in a greenhouse. They concluded that rhizobacteria “improves not only growth and plant physiological status, but also plant secondary metabolites accumulation and antioxidant activity…” Furthermore, one of the rhizobacteria blends was able to fully meet plant needs for nitrogen (N).  PGPR is known for nitrogen fixation [1-3], which involves converting atmospheric nitrogen into forms usable by the plant (e.g., NH3, ammonia).
Rhizobacteria can also produce or alter plant hormones. [2,4] For example, in Moringa peregrina (drumstick tree), another medicinal plant, Pseudomonas significantly increased indole 3-acetic acid (IAA).  IAA is a hormone that supports plant growth, development, and immunity.  The same bacterium also produced 1-aminocyclopropane-1-carboxylate (ACC) deaminase. ACC deaminase is an enzyme that lowers ethylene, the hormone that inhibits growth; thus, ACC deaminase encourages greater plant growth. 
Bacillus has been noted to increase IAA in addition to cytokinins.  In lettuce plants, Bacillus subtilis treatment and subsequent cytokinin accumulation led to a 30% increase in shoot and root weight over 8 days.  These hormones are created in plant roots (where the bacteria colonize) and stimulate cell division and growth throughout the plant.
Rhizobacteria can also be used to help defend plants against certain pathogens. In general, they occupy and defend their territory within the plant from other microbial invaders. They have been noted to produce antibiotics and other defensive compounds. In the aforementioned review , researchers highlight Botyrtis, or powdery mildew, a common devastator of cannabis crops. Although it is unclear which species of bacteria may help defend cannabis, rhizobacteria can be used as a safe biological control against powdery mildew in some other plants, including strawberries and peas.
Additional research is needed to understand how different rhizobacteria species (or combinations) affect distinct cannabis cultivars. Even so, the authors of the review conclude that “elements of the phytomicrobiome have the potential to increase the safety, yield and quality of cannabis.” 
- Podile AR, Kishore GK. Plant growth-promoting rhizobacteria. In: Gnanamanickam SS. (ed.) Plant-Associated Bacteria. Springer, Dordrecht; 2007. https://doi.org/10.1007/978-1-4020-4538-7_6
- Lyu D, Backer R, Robinson WG, Smith DL. Plant growth-promoting rhizobacteria for cannabis production: Yield, cannabinoid profile and disease resistance. Front Microbiol. 2019;10:1761. Published 2019 Aug 8. doi:10.3389/fmicb.2019.01761. [Impact Factor: 4.235; Times Cited: 8 (Semantic Scholar)]
- Pagnani G, Pellegrini M, Galieni A, et al. Plant growth-promoting rhizobacteria (PGPR) in Cannabis sativa ‘Finola’ cultivation: An alternative fertilization strategy to improve plant growth and quality characteristics. Ind Crop Prod. 2018;123, 75–83. doi: 10.1016/j.indcrop.2018.06.033. [Impact Factor: 4.244; Times Cited: 22 (Semantic Scholar)]
- Khan AL, Halo, BA, Elyassi A, et al. Indole acetic acid and ACC deaminase from endophytic bacteria improves the growth of Solanum lycopersicum. Electron J Biotechnol. 2016;21:58–64. doi: 10.1016/j.ejbt.2016.02.001. [Impact Factor: 2.894; Times Cited: 85 (Semantic Scholar)]
- Arkhipova TN, Veselov SU, Melentiev AI, et al.Ability of bacterium Bacillus subtilis to produce cytokinins and to influence the growth and endogenous hormone content of lettuce plants. Plant Soil. 2005;272:201–209. https://doi.org/10.1007/s11104-004-5047-x. [Impact Factor: 3.299; Times Cited: 228 (Semantic Scholar)]