Study: Root Bacteria Boosted CBGA 27% and Cut THC 16% in Hydroponic Cannabis Plants

Rhizobacteria.

A new study published by Plant Physiology from researchers at Leiden University in the Netherlands found that plant growth-promoting rhizobacteria, or PGPR, can colonize marijuana roots and meaningfully shift how the plant produces and preserves cannabinoids, including increasing the key precursor CBGA while reducing THC. Researchers tested four PGPR taxa—Bacillus, Pseudomonas, Flavobacterium, and Burkholderia—using two high THCA, drug-type Cannabis sativa cultivars: Amnesia Haze and Gorilla Glue. Plants were grown in a controlled, soilless hydroponic system designed to keep nutrients consistent across all groups, with a single shared reservoir and identical fertigation schedules. The bacteria were applied twice during early growth by placing a measured inoculum directly onto the Rockwool at the base of the stem, while control plants received a sterile mock solution.

At harvest, the researchers confirmed that all four bacterial groups successfully penetrated the root system and colonized internal root tissues. Despite that colonization, the study found no meaningful changes in visible plant performance. Height, biomass, flower weight, and harvest index differed by cultivar, but inoculation itself did not drive statistically significant differences in those phenotypic measures. In other words, the bacteria altered chemistry without obviously changing how the plants looked or yielded under the study’s conditions.

Where the inoculation did show clear effects was in cannabinoid metabolism. Across both cultivars and all PGPR taxa, bacterial inoculation was linked to a significant +27.37% increase in cannabigerolic acid (CBGA), the precursor cannabinoid that sits upstream of both THCA and CBDA in the biosynthetic pathway. At the same time, Δ9-THC measured in the dried flower material fell by −15.76% in inoculated plants compared to controls.

The study also reported significant shifts in key ratios that signal changes in metabolic flow. The CBGA-to-THCA relationship moved toward greater CBGA accumulation, indicating that inoculated plants kept more of the precursor rather than converting it as extensively into THCA. The THCA-to-CBDA relationship also shifted, reflecting a relative move toward CBDA production. The researchers noted that these changes were broadly shared across both cultivars and across the different bacterial taxa, even if some groups appeared to have stronger effects in certain comparisons.

Another finding with practical implications involved decarboxylation. The study found that PGPR treatments reduced in vivo and post-harvest decarboxylation of THCA into Δ9-THC, preserving the acidic cannabinoid profile. That conclusion was supported by shifts in the THCA/THC ratio in inoculated plants compared to controls, indicating less conversion to THC under the same post-harvest handling.

The researchers emphasized that the hydroponic design was intentional. By using a soilless system, sterile starting materials, controlled access to the growth chamber, and consistent mineral nutrition delivered in excess, the experiment was structured to reduce the odds that differences were simply driven by nutrient availability or uncontrolled soil microbiology. In that context, the cannabinoid shifts observed were tied to bacterial colonization itself rather than a fertilizer advantage.

While the work was performed under controlled conditions and used two THCA-dominant cultivars, the findings point to a potentially scalable concept for growers and product developers: manipulating the root microbiome may offer a way to steer cannabinoid profiles and better preserve acidic cannabinoids without changing genetics or relying on post-harvest interventions.