Small Talk: The Silent Conversations Between Plants and the Soil Microbiome
If I told you that there were creatures that communicate by releasing different chemicals into their environment, you might think I’m talking about an alien species on an episode of Star Trek. (In fact, it was in at least one episode.) Meanwhile, this method of communication is happening right here on Earth. One example is pheromones, which have a variety of functions including “territory marking” by our pets. However, pheromones are only detectable by members of the same species. In the soil beneath our feet, plants, insects, and microorganisms also “talk” to each other using chemical signals.
Similar to how a telephone conversation can happen over a wired landline or wireless cell phones, there are chemical signals that are sent via direct physical connections (through roots or fungal hyphae) or through the air or soil itself (1). These “wireless” communications can be chemicals secreted from roots or microbes, or tiny volatile organic compounds (VOCs) that can move quickly. In addition to chemical communication, plants can “hear” (that is: detect the sound vibrations of) herbivorous insects walking or munching on their leaves (2). There is even new research that explores electrochemical signals within the soil (3).
This invisible communication is important, especially for organisms that are more or less stuck in one place. They may not have eyes or ears, but they are capable of detecting danger in their environment and responding to it. If a plant is being eaten by a herbivore, it’s able to send out chemical signals with a few different targets: 1) to other plants nearby, 2) to enemies of insect herbivores, and 3) within the plant itself (4). If plants could text, it might look something like this:
When plants receive messages like this, they can respond by activating a defense response. This can look like: creating chemicals that make their leaves taste bad (or are even toxic to the herbivore), recruiting helpful microbes from the soil, or putting out signals that attract predators of the herbivore (1).
What else might plants “talk” about? In addition to herbivores, they have signals for other stressful situations: when a pathogen is detected, if the soil salinity is too high, or if the soil is too dry. Even in ideal growing conditions, plants are sending signals into the soil that lets soil microbes know they’re growing. Depending on the genetics of the plant, its age, and environmental conditions, different chemicals are secreted into the soil (called root exudates) and cultivate an environment distinct from the surrounding soil (called the rhizosphere). This attracts certain microbes from the surrounding soil to come live in or on the roots; the resulting microbiome has a higher number of microbes, but a lower overall diversity than the soil microbiome (5).
In situations where plants are experiencing stress, their root exudates change, and they are able to recruit beneficial microbes. For example, in a nitrogen-limiting environment, legumes release chemicals that are picked up by nitrogen-fixing bacteria called rhizobia. In response, the bacteria release a signal essentially saying “open up, I’m here!”, and the plant begins formation of a root nodule (6). Similarly, helpful bacteria can be recruited during drought stress (7).
So what does all this mean for growers trying to raise a successful crop and cultivate healthy soil? By understanding the chemical signals that plants use to defend against pests and improve nutrient uptake, agricultural biotechnology companies can create new options for pest and nutrient management (8). Overall, having a good understanding of the “conversations” happening in the soil can help facilitate cutting-edge, sustainable practices (9).
About the author: Dr. Tuesday Simmons is the Science Writer at Trace Genomics. She earned her Ph.D. in Microbiology from the University of California, Berkeley, studying the root microbiome of cereal crops.
References
- Sharifi, R., and Ryu, C. (2020). Social networking in crop plants: Wired and wireless cross‐plant communications. Plant, Cell & Environment 44, 1095–1110. https://doi.org/10.1111/pce.13966.
- Kollasch, A.M., Abdul-Kafi, A.-R., Body, M.J.A., Pinto, C.F., Appel, H.M., and Cocroft, R.B. (2020). Leaf vibrations produced by chewing provide a consistent acoustic target for plant recognition of herbivores. Oecologia 194, 1–13. https://doi.org/10.1007/s00442-020-04672-2.
- Truscott, S. (2023). Engineers, plant scientists decoding electrochemical signals of soil health. news.cahnrs.wsu.edu. https://news.cahnrs.wsu.edu/article/engineers-plant-scientists-join-forces-to-decode-electrochemical-signals-of-soil-health/.
- War, A.R., Sharma, H.C., Paulraj, M.G., War, M.Y., and Ignacimuthu, S. (2011). Herbivore induced plant volatiles: Their role in plant defense for pest management . Plant Signaling & Behavior 6, 1973–1978. https://doi.org/10.4161/psb.6.12.18053.
- Santoyo, G. (2021). How plants recruit their microbiome? New insights into beneficial interactions. Journal of Advanced Research. https://doi.org/10.1016/j.jare.2021.11.020.
- Geurts, R., and Bisseling, T. (2002). Rhizobium Nod Factor Perception and Signalling. The Plant Cell 14, S239–S249. https://doi.org/10.1105/tpc.002451.
- Xu, L., Naylor, D., Dong, Z., Simmons, T., Pierroz, G., Hixson, K.K., Young Ho Kim, Zink, E.M., Engbrecht, K., Wang, Y., et al. (2018). Drought delays development of the sorghum root microbiome and enriches for monoderm bacteria. Proceedings of the National Academy of Sciences of the United States of America 115. https://doi.org/10.1073/pnas.1717308115.
- Divekar, P.A., Narayana, S., Divekar, B.A., Kumar, R., Gadratagi, B.G., Ray, A., Singh, A.K., Rani, V., Singh, V., Singh, A.K., et al. (2022). Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection. International Journal of Molecular Sciences 23, 2690. https://doi.org/10.3390/ijms23052690.
- Jamil, F., Mukhtar, H., Fouillaud, M., and Dufossé, L. (2022). Rhizosphere Signaling: Insights into Plant–Rhizomicrobiome Interactions for Sustainable Agronomy. Microorganisms 10, 899. https://doi.org/10.3390/microorganisms10050899.
