The study, published in the journal BioRisk, set out to answer a deceptively simple question: can a well-chosen soil bacterium, applied at the right concentration and the right physiological moment, reliably stimulate the early development of Zea mays? The answer, according to the data, is a resounding yes. When corn seeds were treated with a 0.2 percent suspension of Pseudomonas putida 1046, the length of the radicle, the embryonic root that anchors the seedling, increased by between 22.87 and 28.33 percent compared with untreated controls. The coleoptile, the protective sheath that encases the emerging shoot, grew even more impressively, with increases ranging from 35.96 to 49.56 percent. The number of lateral roots, which determine how efficiently a young plant can forage for water and nutrients, rose by between 5.41 and 16.67 percent.

These numbers matter because germination is the make-or-break phase of a crop’s life cycle. A seedling that emerges faster and establishes a larger root system has a decisive head start against drought, pests, and competition from weeds. The researchers quantified this advantage using a standard agronomic metric called the vigour index, calculated by multiplying the combined mean radicle and coleoptile lengths by the percentage of germination. The treated corn seeds registered vigour index values of 2125 for the 16-hour bacterial culture and 2721 for the 48-hour culture, figures that represent a substantial improvement over the untreated seeds and that signal a seedling population with markedly enhanced early-stage performance.

What makes the study technically interesting is the attention paid to the metabolic status of the bacterial culture itself. The researchers grew Pseudomonas putida 1046 for either 16 hours or 48 hours on a rotary shaker at 28 degrees Celsius, then prepared water suspensions at three concentrations: 0.1, 0.2, and 0.4 percent, each containing roughly 10 to the eighth colony-forming units per millilitre. Corn seeds of the commercial hybrid Dekalb DKC 5830 HD were surface-sterilized with a dilute sodium hypochlorite solution, immersed in the bacterial suspensions, and cultured for five days at 25 degrees Celsius. The growth parameters of radicle length, coleoptile length, and lateral root number were then measured and compared with untreated controls.

The results revealed a clear optimum. The 0.1 percent suspension produced only mild or negligible effects on most indices, while the 0.4 percent suspension failed to outperform the 0.2 percent treatment despite its higher bacterial load. The 0.2 percent suspension, in contrast, consistently delivered the strongest stimulation across all three growth indicators. This non-linear dose response is a hallmark of PGPR biology: too few bacteria fail to colonize the seed surface adequately, while too many can trigger competition or quorum-dependent effects that blunt the stimulatory signal. The identification of a sweet spot at 0.2 percent has direct implications for the economics of bioinoculant production, since a less concentrated suspension is cheaper to manufacture and easier to apply at scale.

The physiological age of the culture proved equally consequential. The 16-hour culture, representing a metabolically active population in logarithmic growth, showed stronger promotion of lateral root formation, with a 16.67 percent increase compared with only 5.41 percent for the 48-hour culture. Lateral roots matter enormously because they expand the root surface area available for nutrient assimilation. However, the 48-hour culture slightly outperformed the 16-hour one in radicle and coleoptile elongation, registering 28.33 percent versus 22.87 percent for the radicle and higher values for the coleoptile as well. The researchers noted that the shorter cultivation period offers a defined technological advantage, since producing a 16-hour culture requires less time and energy than a 48-hour one, and the modest difference in performance can be compensated by the efficiency gains.

Correlation analysis strengthened the case for a genuine causal relationship between the bacterium and the plant response. The researchers calculated correlation coefficients between the bacterial suspension concentration and each of the four plant growth parameters, including the vigour index. For the 16-hour culture, the coefficients fell in the range of 0.9 to 1.0, indicating a very strong positive relationship: as the bacterial concentration increased from control to 0.2 percent, the plant growth parameters rose in near-lockstep. The coleoptile length showed a coefficient of 0.99982, essentially a perfect correlation, while the radicle length registered 0.98927. For the 48-hour culture, the coefficients were somewhat weaker but still substantial, with the lateral root number showing the weakest relationship at 0.36147.

Beyond the germination experiments, the team probed the biochemical machinery that might explain the observed effects. Using standardized ApiZYM and Api 20 NE detection systems, they established that Pseudomonas putida 1046 produces alkaline phosphatase, acid phosphatase, and naphthol-AS-BI-phosphohydrolase, enzymes directly involved in phosphate solubilization and mineralization. Phosphate is a critical macronutrient that is often locked in insoluble mineral forms in soil, and bacteria that can liberate it provide plants with a readily available nutrient source. The strain also demonstrated the ability to produce siderophores of the hydroxamate type, small iron-chelating molecules that bacteria secrete to scavenge iron from the environment. Siderophores benefit plants by enhancing metal accumulation and have putative applications in phytoremediation, the use of plants to clean contaminated sites.

Additional biochemical analysis revealed that the strain possesses the biosynthetic capacity to produce three key phytohormones: gibberellic acid, indole acetic acid, and jasmonic acid. Indole acetic acid, the most abundant member of the auxin family, is particularly significant because it drives root initiation and elongation and promotes the differentiation and proliferation of plant tissues. The researchers speculate that enhanced hormone biosynthesis may trigger the activity of hydrolytic enzymes such as alpha-amylases, which break down starch reserves in the seed and thereby accelerate early germination. This hormone-dependent mechanism would explain why the vigour index rises so sharply in treated seeds, since promoted auxin synthesis feeds directly into the root and shoot development that the index measures.

To confirm that the effect was not unique to corn, the researchers repeated the experiments with soybean, Glycine max, another globally significant crop grown for food, protein, and oil. The same tendency for enhanced germination, promoted by increased lateral root number and enlarged epicotile length, was confirmed in the soybean model system, suggesting that Pseudomonas putida 1046 exerts its growth-promoting influence across a spectrum of crop species. This cross-species activity enlarges the potential market for the strain as a bioinoculant and reinforces the argument that PGPR-based biofertilizers can serve as versatile alternatives to chemical fertilizers, which contaminate soil, water, and air while microbial inoculants enrich rather than pollute the soil they inhabit.

The broader context of the research is the steady intensification of agriculture and the mounting environmental costs of chemical inputs. Fertilizers and pesticides have enabled remarkable gains in crop yield over the past century, but they have also degraded soil structure, polluted waterways, and contributed to air contamination. PGPR offer an attractive path toward sustainable agriculture because they are a natural component of healthy soil ecosystems. They support root and shoot development, positively influence plant morphogenesis, flowering, and photosynthetic efficiency, and execute biocontrol against soil-borne and seed-borne pathogens. Pseudomonas species deserve special attention among PGPR genera because of their well-established growth-promoting mechanisms, which include superior root colonization, enzyme and metabolite production, phytohormone and siderophore synthesis, mineral and phosphate solubilization, and plant protection from both biotic and abiotic stress.

The commensal relationship between Pseudomonas putida strains and their plant hosts is a sophisticated biological partnership. Plant roots exude sugars, amino acids, and organic acids that feed the bacteria, while the bacteria reciprocate by producing hormone precursors and antibiotics that suppress pathogen growth and by supporting nutrient immobilization in the rhizosphere. This exchange is influenced by environmental factors such as temperature, humidity, pH, and nutrient availability, which is why the researchers emphasize that any biofertilization program must consider both the physiological and economic aspects of the process. The plant growth stimulating effect has to be checked and proved for each microorganism and its plant counterpart, since the interaction is highly specific and context-dependent.

The optimal treatment parameters identified by the study, comprising the application of a 0.2 percent suspension of the 16-hour grown Pseudomonas putida 1046 strain for five days, represent a practical recipe that balances biological efficacy with production economics. From an industrial standpoint, the use of a less concentrated suspension is more feasible and cost-effective, and the shorter cultivation period reduces the time and resources required to prepare the inoculant. These considerations are critical for translating laboratory results into field applications, where the cost of bioinoculant production must compete with conventional fertilizers that benefit from established supply chains and economies of scale.

The findings contribute to a growing body of literature showing that PGPR can significantly enhance seed emergence across diverse crop cultures. Previous studies have reported similar improvements in canola, wheat, sunflower, and potato, where seed emergence was registered at up to 100 percent of untreated controls. The consistency of these results across species and bacterial strains suggests that PGPR-based biofertilization is not a niche phenomenon but a broadly applicable agricultural technology. However, the researchers caution that greater understanding of the interaction between bacteria and host plants requires further study, particularly regarding the molecular signals that mediate the partnership and the environmental conditions that optimize it.

As the world grapples with the dual challenges of feeding a growing population and preserving the environmental integrity of agricultural landscapes, the humble Pseudomonas putida 1046 offers a glimpse of what sustainable intensification might look like. A bacterium that can boost corn germination by nearly 30 percent, stimulate lateral root formation, solubilize phosphate, and chelate iron, all while being produced in a simple nutrient broth over 16 hours, is a powerful tool in the arsenal of sustainable agriculture. The next step will be field trials that test whether the laboratory results hold up under real-world conditions of variable soil, weather, and management practices. If they do, this unassuming soil bacterium may soon find itself at the center of a new generation of environmentally friendly biofertilizers that enrich rather than pollute the ground they inhabit.

Subject of Research: Study of the plant growth-promoting capacity of Pseudomonas putida 1046 in a model plant system

Article Title: Study of the plant growth-promoting capacity of Pseudomonas putida 1046 in a model plant system

Article References: Georgieva, G., Nedeva, T., Badalova, M., Deleva, V., & Savov, V. (2023). Study of the plant growth-promoting capacity of Pseudomonas putida 1046 in a model plant system. BioRisk, 20, 115-128. https://doi.org/10.3897/biorisk.20.97581

Image Credits: AI Generated

DOI: 10.3897/biorisk.20.97581

Keywords: plant, growth-promoting, capacity, Pseudomonas, putida, model, system, scientific research