Auxin: The Universal Language
- zackljones
- Aug 30
- 6 min read
I'll start this piece with a story about how I came to write this post. I do a bit of Uber driving to make some extra cash, but also just to get out of the house and see what's going on. I usually find it therapeutic after spending years isolated with small children. Every once and awhile, I happen to meet some really interesting people. This was the case about a month ago when I met maybe the most interesting person yet.
Professor Dolf Weijers
I usually drive around downtown Fort Collins which is also home to the land grant university, Colorado State University. This night I got a ride request for "Dolf" outside a popular college bar. The only Dolf I had ever heard of prior to this encounter was Dolph Lundgren. Well Dolf and I got to talking after picking him up. He had just flown in from the Netherlands and his kids were in college. He was a bit insulted when I asked if they went to school outside of Holland. He told me he was actually a professor at a university there and he was in town for a plant substances conference at the university.
It turns out that this Dolf was Dolf Weijers, not only a professor, but Chair of Biochemistry at Wageningen University - the #1 university in world for Agricultural Sciences. His area of expertise was plant development, specifically related to the phytohormone auxin (also known as indole acetic acid or IAA). I spent the rest of the ride pelleting him with questions which of course he had answers. Full disclosure - he was also wearing the same glasses as above but they were glowing neon green, and I thought I had picked up someone coming from a rave.
Why do compost bacteria make plant hormones?
One question that has been bothering for awhile now is, "Why do so many different microorganisms (both bacteria and fungi) in vermicompost produce phytohormones?" Auxin production is spread across nearly all major phyla of bacteria in compost. Additionally, there are (usually) no plants growing in compost so why would there be so many phytohormone producers?
Well as professor Weijers explained, auxin isn't really just a plant hormone. Bacteria were producing auxin long before plants were around. It is used a signaling molecule for bacteria and fungi, but it's role in bacterial development is not nearly as deeply understood as in plants. It labeled as a phytohormone because it was first discovered in plants.
Plants Acquired Auxin Synthesis from Bacteria
Some studies postulate that plants, ancient algae, acquired auxin production from "horizontal" gene transfer from bacteria. Usually genes are passed on through "vertical" gene transfer, meaning from one generation to the next. However, it is possible for single cells organisms to pick up genes from the environment, from other bacteria, or by viral infection. These processes are known as horizontal gene transfer and it makes the microbial world a very dynamic place. The diagram below shows how plasmids, which are small, circular pieces of DNA that often carry beneficial traits can be passed on vertically to the next generation of horizontally to the same generation of cells.
This transfer of the auxin synthesis pathway to algae is possibly what made it possible for plants to become terrestrial and move from the oceans to land. Auxin allows for tropic response (plant's ability to grow towards light) and complex anatomy including branching shoots and extensive root systems. While central to plant development, auxin works in concert with other phytohormones like cytokinins, gibberellins, and abscisic acid. I wish more was understood about the role auxin plays in bacterial and fungi, but I am happy to somewhat understand why compost microorganisms produce auxin without the presence of plants. Before it was a phytohormone it was a "bacterial hormone."
Production by Beneficial and Pathogenic Organisms
Auxin is a crucial inter-kingdom signaling molecule in the relationship between microorganisms and plants. It can be produced by beneficial bacteria to form nitrogen fixing nodules in legumes. However, it is also produced by pathogenic organisms as well to produce galls or weaken plant defenses. Plants can interpret auxin signals and facilitate interaction with microorganisms or activate defenses in the case of a pathogen. The duality of this molecule points towards a long coevolution of microorganisms, plants, and even arachnids like mites. Below is an example of mites, or the symbiotic bacteria they host, causing galls on strawberry leaves.
Agrobacterium tumefaciens, the causative agent of crown gall disease, employs a remarkable mechanism where it inserts genes into plant cells. This genetic transformation compels the plant cells to abnormally produce auxins and cytokinins, which in turn stimulate the uncontrolled cell proliferation that leads to gall formation. As you can see, auxin production can be used by organisms for purposes other than beneficial interactions.
Dose Dependence of Auxin
A critical aspect of these beneficial interactions is the dose-dependent nature of auxin's effects. The impact on the plant is closely tied to the quantity of IAA produced by the microbe and the sensitivity of the specific plant tissue. Plant roots, for instance, are highly sensitive to changes in IAA concentration; while optimal levels promote growth, excessively large amounts can inhibit root development.
As you can see in the schematic above, different plant tissues have very different ranges for it's beneficial effects. It seems likely that it would be difficult to over apply it to the stems and leaves of plant, but to exercise caution with roots and buds.
In my mind, phytohormone production by microorganisms in vermicompost is likely the biggest contribution to it plant growth promoting effects. A recent study looked a the root mass of tomatoes grown in sand with varying percentages of vermicompost added as well as auxin concentrations.
Left: room mass of the control (no vermicompost only sand) vs 40% vermicompost. Right: Vermicompost added graphed against root volume.
The authors found that a 40% addition of vermicompost to sand produced the largest increase in root volume as can be seen in the right panel above. You can also start to observe a decrease in root volume when greater than 40% of vermicompost was added. The authors suggest this could be due to an excessive amount if auxin but state more research is needed. I know there are a few difference reason why it might be best to avoid a large proportion of vermicompost in potting mixes but phytohormone levels is not one I've previously heard.
Variability of Phytohormones in Different Composts
The paper I'll leave you with demonstrates that plant phytohormone concentration and types of hormones are variable between different types of compost and an extract (vermicompost no included). In the image below you can see the relative proportions of different types of phytohormones. The green is brassinosteroids which I am not very familiar with but you can see auxins in red and cytokinins in purple.
What is also really important is the total amount of phytohormones detected in these different types of compost. The "organic compost" and "compost hemp chaff and apple pomace" had a near zero amount of of phytohormones. The garden compost had 6.4 ng/mg, the compost with buckwheat husk 47mg/ng, organic compost pellets, 65ng/ml while the dehydrated biohumus leachate had a whopping 2026mg/ng. I personally find the compost with buckwheat husk (also includes grass and fruit pomace) to be the most compelling as it has diversity of different phytohormones at high levels. The authors don't speculate much why the hormone levels are dramatically different between the different compost post types but clearly this study warrants more research into this topic ideally paired with microbiome analysis.
Let me know what you think and comment or ask any questions below!
It was a revelation to learn that bacteria have been producing auxins long before plants appeared, and that plants evolved making use of them. Thank you for this glimpse into our invisible world!