A Green Revolution in Plant Tissue Culture
How tiny microbes could replace synthetic chemicals in growing our plants
For decades, plant scientists and commercial growers have relied on synthetic auxins to propagate plants in laboratory settings. These powerful plant hormones trigger root development and callus formation, enabling mass production of identical plants through tissue culture techniques. However, a quiet revolution is underway as researchers discover that certain bacteria naturally produce these same growth-promoting compounds—offering a more sustainable path forward for plant propagation.
Auxins represent a class of plant hormones that fundamentally influence nearly every aspect of plant growth and development. From determining which direction roots grow to facilitating the formation of new roots and shoots, these compounds are master conductors of plant development. In tissue culture laboratories worldwide, synthetic auxins have become indispensable tools, particularly for triggering the formation of callus—a mass of undifferentiated cells that can regenerate into entire new plants.
"The combination of two growth-promoting hormones, auxin and cytokinin, induces callus from plant explants in vitro, [a system] extensively used in both basic research and horticultural applications" 2 .
Historically, the delicate balance of inducing callus formation required precise combinations of synthetic hormones. This hormone-driven process has enabled everything from mass propagation of elite crop varieties to conservation of endangered species.
Carefully sterilized plant material grown on nutrient-rich gels containing precisely measured synthetic auxins.
Expensive chemicals, regulatory hurdles, and environmental footprint of synthetic hormone production 5 .
Enter plant growth-promoting bacteria (PGPB)—microscopic allies that have evolved alongside plants for millions of years. These beneficial bacteria possess the remarkable ability to produce natural auxins identical to those plants produce themselves.
The process begins with a simple amino acid—tryptophan—which bacteria convert into active auxin through multiple biochemical pathways 5 . This bacterial auxin production isn't merely a laboratory curiosity; in nature, it forms the basis of mutually beneficial relationships between plants and microbes.
"Bacterial IAA is a reciprocal signaling molecule in plant-microorganism interactions, where the plant provides exudates containing nutrients and shelter for microorganisms, and the microorganisms provide auxin, which is essential for root development" 5 .
| Bacterial Genus | Auxin Production Capacity | Applications |
|---|---|---|
| Bacillus | 43.78 µg mL⁻¹ (Cyn2 strain) 1 | Enhanced root development in pear rootstocks 5 |
| Acinetobacter | Positive in screening assays 5 | Successful replacement of synthetic auxin in pear tissue culture 5 |
| Azospirillum | High IAA production 7 | Significant root growth promotion in tomato seedlings 7 |
| Methylobacterium | Positive in screening assays 7 | Plant growth promotion in hydroponic systems 7 |
A pioneering 2022 study demonstrated the very possibility this article explores—that bacterial auxins can effectively replace synthetic versions in plant tissue culture 5 . The research focused on pear rootstocks ('OH×F87' and 'PDW' selections), valuable fruit tree varieties known for their challenging propagation.
Researchers began by isolating endogenous bacteria from surface-sterilized pear tissues, recognizing that even carefully maintained tissue cultures often host beneficial microbes.
The collected bacterial strains underwent screening for auxin production using colorimetric assays. Remarkably, 30.36% of tested microorganisms showed positive for auxin production 5 .
The most promising auxin-producing strains were identified through genetic analysis as belonging to the genera Acinetobacter, Bacillus, and Buttiauxella.
Researchers introduced selected bacteria into sterile tissue culture systems containing pear rootstock explants.
The team monitored root development, comparing bacterial treatments against traditional synthetic auxin protocols and control groups without auxins.
The findings were compelling. Pear rootstocks inoculated with auxin-producing bacteria showed comparable root development to those treated with synthetic auxins—and significantly better than untreated controls.
"None of the bacterial isolates was notably more promising, but the general similarity of treatments containing the A. septicus and A. ursingii strains, with the synthetic auxin treatment, suggests the possibility of its use on a large scale" 5 .
The implications are significant: natural bacterial auxins could effectively replace synthetic versions, reducing chemical inputs while maintaining propagation efficiency.
The transition from synthetic to bacterial auxins offers multiple advantages across agricultural and environmental contexts:
For commercial growers, bacterial auxins represent an opportunity to reduce chemical inputs while maintaining—or even improving—propagation success. As one study noted, bacterial treatments can potentially "shorten the rooting period and increase the growth of shoots and number of sprouts during the acclimatization process, which represents a decrease in cultivation time, reduced costs, and an environmentally friendly approach" 5 .
Recent research reveals that the benefits of bacterial auxins extend beyond mere root initiation. A 2025 study demonstrated that auxin-producing bacteria enhanced temperature stress tolerance in Chilean common bean landraces, helping plants cope with the increasingly challenging growing conditions associated with climate change 1 .
The application of bacterial auxins isn't limited to traditional tissue culture. Recent innovations have demonstrated their effectiveness in hydroponic systems, with researchers noting that "plantlets treated with microbial inoculants showed a significant increase in the survival rate compared to the control treatment" 7 .
| Aspect | Synthetic Auxins | Bacterial Auxins |
|---|---|---|
| Production | Chemical synthesis | Microbial fermentation |
| Cost | High production costs 5 | Potentially lower with optimized growth media 5 |
| Environmental Impact | Classified as biochemical pesticides 5 | Natural, biodegradable compounds |
| Regulatory Status | Strictly controlled in some countries 5 | Generally considered safe |
| Additional Benefits | Single compound effect | May provide multiple growth-promoting effects |
For researchers exploring bacterial auxins, several essential tools and methods have emerged as fundamental to the field:
Advanced imaging to verify bacterial colonization in plant tissues 1 .
Nutrient-rich solutions for growing auxin-producing bacteria 7 .
Specifically formulated growth media like Murashige and Skoog (MS) 5 .
Setups for continuous interaction between bacteria and plants 7 .
Carefully surface-sterilized plant tissues for experiments 7 .
| Bacterial System | Auxin Production Measurement | Experimental Context |
|---|---|---|
| B. proteolyticus Cyn1 | 46 µg mL⁻¹ 1 | Isolated from Chilean common bean landraces |
| B. safensis Cyn2 | 43.78 µg mL⁻¹ 1 | Isolated from Chilean common bean landraces |
| Bacterial Consortium | 90.67 µg mL⁻¹ 1 | Combination of Cyn1 and Cyn2 strains |
| Various Pear Endophytes | 30.36% of isolates tested positive 5 | Endophytic bacteria from pear rootstocks |
As research progresses, the potential applications of bacterial auxins continue to expand. The surprising discovery that bacterial consortia sometimes produce significantly more auxin than individual strains—as evidenced by one study where a consortium produced 90.67 µg mL⁻¹ compared to 46 µg mL⁻¹ and 43.78 µg mL⁻¹ from individual strains—suggests that microbial teamwork may unlock even greater potential 1 .
The growing understanding of how environmental factors influence auxin signaling opens new avenues for optimization. Recent research has revealed that "submergence promotes auxin-induced callus formation through ethylene-mediated post-transcriptional control of auxin receptors" —demonstrating the complex interplay between different hormonal pathways.
While questions remain about optimal strains for different plant species, application methods, and large-scale production, the direction is clear: bacterial auxins offer a viable, sustainable alternative to synthetic plant growth regulators.
"This opens the door for further research using new, more promising microbial isolates, and also for lower-cost microbial cultivation techniques, such as low-cost media obtained from agro-industrial residues" 5 .
Bacterial consortia can produce significantly more auxin than individual strains 1 .
The transition from synthetic to bacterial auxins represents more than just a technical improvement in plant tissue culture—it embodies a fundamental shift toward working with nature rather than attempting to dominate it. By harnessing the innate capabilities of beneficial bacteria, we can develop plant propagation systems that are simultaneously more effective, more economical, and more environmentally responsible.