The significance of that number lies in what IAA actually does inside a plant. Auxins are master regulators of plant development, governing cell elongation, root initiation, and the plant’s overall architectural response to its environment. When roots encounter IAA produced by bacteria living in the rhizosphere, the thin layer of soil hugging the root surface, they typically respond by producing more lateral and adventitious roots. A denser root system means a larger absorptive surface for water and mineral nutrients, which in turn translates into faster growth and improved stress tolerance. For decades, agronomists have known that a subset of soil and root-associated microbes, collectively called plant growth-promoting rhizobacteria or PGPR, can act as miniature hormone factories for their plant hosts. What has been harder to pin down is exactly how much hormone each strain can make and which environmental levers push production to its maximum.

The research team approached this question with a carefully controlled experimental design. ESU181 cultures were grown in Luria-Bertani medium, a standard nutrient broth for laboratory bacterial culture, under a matrix of conditions: three pH levels of 6.5, 7.5, and 8.5, crossed with two temperatures of 30 and 37 degrees Celsius. Half of the cultures received L-tryptophan supplementation, while the other half served as unsupplemented controls. The choice of L-tryptophan was not arbitrary. This amino acid is the principal precursor in the most common bacterial IAA biosynthesis routes, which convert tryptophan through a series of enzymatic intermediates into the finished hormone. By supplying the precursor directly, the researchers effectively handed the bacterium abundant raw material, and the microbe responded accordingly. IAA concentrations were then measured every 24 hours across the four-day incubation period using Salkowski’s reagent, a classic colorimetric assay in which IAA reacts to form a pink-red complex whose intensity can be quantified spectrophotometrically.

The results formed a consistent and statistically robust pattern. Across every combination of pH and temperature tested, cultures supplemented with L-tryptophan produced significantly more IAA than their unsupplemented counterparts, with the differences reaching statistical significance at a p-value below 0.05. This confirms that ESU181 relies heavily on the tryptophan-dependent pathway for its hormone output, a trait shared with many high-performing PGPR strains documented in earlier literature. Temperature also mattered: 37 degrees Celsius, the warmer of the two settings, consistently promoted higher IAA synthesis than 30 degrees. Likewise, the mildly acidic pH of 6.5 outperformed both neutral 7.5 and alkaline 8.5 conditions throughout the experiment. The winning combination of pH 6.5, 37 degrees Celsius, and tryptophan supplementation delivered the maximum yield of 6.936 micrograms per milliliter at the 96-hour mark.

Equally instructive was the temporal dimension of the data. IAA concentrations did not spike early and plateau; instead, they climbed progressively across the entire 96-hour incubation, reaching their highest levels at the very end of the monitoring window. This steady accumulation suggests that hormone production in ESU181 is coupled to bacterial growth and metabolic activity over time rather than being a short-lived burst. From a bioprocess engineering standpoint, that behavior is encouraging, because it implies that extending fermentation time or optimizing nutrient delivery could push yields even further. It also hints that in a natural soil setting, a sustained population of ESU181 cells around a root system could provide a continuous, low-level auxin supply to the plant, which is precisely the kind of prolonged interaction that benefits crop development.

Why does the origin of this particular strain matter? Einkorn wheat is an ancient, largely unhybridized cereal that has been cultivated in Anatolia and the Fertile Crescent for roughly ten thousand years. Its root microbiome represents a reservoir of bacteria that co-evolved with the crop in local soils, often without the heavy inputs of synthetic fertilizers and pesticides that characterize modern intensive agriculture. The same research group had previously reported that bacteria isolated from Triticum monococcum roots can improve the wheat hologenome, the collective genetic capacity of the plant and its associated microbes, in agricultural settings. ESU181 emerged from that exploration as a standout IAA producer, and the new study now provides the quantitative optimization data needed to evaluate its practical potential. Strains adapted to local edaphic conditions frequently colonize local crops more effectively than imported inoculants, giving native isolates a distinct advantage in biofertilizer applications.

The broader context is the global search for alternatives to synthetic nitrogen fertilizers and agrochemicals. Conventional fertilizer production is energy-intensive and a major source of greenhouse gas emissions, while overapplication degrades soils and pollutes waterways. Biofertilizers built on plant growth-promoting bacteria offer a different model: living formulations that colonize the rhizosphere and deliver growth benefits through hormone production, nutrient mobilization, and disease suppression. Bacillus species are particularly attractive for this role because they form resilient endospores that survive harsh conditions, tolerate formulation and storage, and establish themselves readily in soil. B. subtilis in particular has a long track record in agricultural biotechnology, appearing in commercial biocontrol and plant-health products, and it is generally regarded as safe for environmental release. An optimized IAA-producing strain within this species therefore fits neatly into existing industrial and regulatory frameworks.

The Turkish team’s optimization data also speaks to a persistent challenge in translating laboratory PGPR research into field performance: environmental sensitivity. Hormone production by rhizobacteria in the wild is hostage to soil temperature, pH, and the availability of tryptophan or related precursors, all of which fluctuate with season, soil type, and cropping history. By mapping exactly where ESU181 performs best, the study provides a rational basis for designing delivery systems that recreate those favorable conditions, whether through formulation chemistry, co-application with tryptophan-releasing organic amendments, or selection of cropping environments that match the strain’s preferences. The finding that a mildly acidic, warmer environment maximizes output is consistent with the physiology of B. subtilis, which grows vigorously at body-like temperatures and prefers near-neutral to slightly acidic conditions. Earlier studies on other Bacillus isolates, including work on B. velezensis from pear rhizosphere and immobilized B. cereus cells, reported similar benefits of tryptophan supplementation, reinforcing the emerging consensus that precursor availability is often the limiting factor in bacterial auxin production.

The authors reported no specific funding for the work and declared no conflicts of interest, and the study involved no human or animal subjects. Their conclusions are measured but forward-looking: ESU181 demonstrates strong IAA-producing capacity under optimized conditions, particularly in the presence of supplemented tryptophan, and the strain may be leveraged as a promising candidate for biofertilizer development, offering a sustainable strategy to promote plant growth through microbial hormone production. The next steps for such a candidate typically involve greenhouse and field trials to verify that laboratory-scale hormone output translates into measurable gains in crop growth and yield, followed by formulation work to stabilize the bacterium for commercial distribution. For now, the study adds a valuable data point to the growing catalog of native microbial resources being tapped for sustainable agriculture, and it underscores a simple but powerful idea: the roots of an ancient wheat variety, growing quietly in Anatolian soil, may hold microbial partners capable of helping modern crops thrive with fewer chemical inputs.

Subject of Research: Optimization of indole-3-acetic acid biosynthesis by the plant growth-promoting bacterium Bacillus subtilis strain ESU181

Article Title: Impact of temperature, pH, and L-tryptophan supplementation on indole-3-acetic acid biosynthesis in Bacillus subtilis strain ESU181

Article References: Top, N. B., Ünüvar, Ö. C., & Ünlü, E. S. (2026). Impact of temperature, pH, and L-tryptophan supplementation on indole-3-acetic acid biosynthesis in Bacillus subtilis strain ESU181. 3 Biotech, 16(10), Article 451. https://doi.org/10.1007/s13205-026-05083-5

Image Credits: AI Generated

DOI: 10.1007/s13205-026-05083-5

Keywords: Bacillus subtilis, indole-3-acetic acid, auxin, plant growth-promoting rhizobacteria, biofertilizer, L-tryptophan, Einkorn wheat, rhizosphere, sustainable agriculture, microbial biotechnology, pH optimization, temperature optimization