Can plants think? We would have confidently said “No!” just a few decades back. But now we know that even plants have specific information-processing abilities, and this comes in the absence of any neurons at all. In fact, some scientific studies have even demonstrated that plants are able to learn and memorize. Now researchers have started to use terms like learning, intelligence, and behavior when referring to plants.

Although plants do not have brains, they seem to have other structures and mechanisms through which they communicate with each other and receive signals from the outside world. Some scientists believe that plant cells share some common features with neural cells, including the production of substances called neurotransmitters that the brain utilizes for sending the signals. Glutamate is an example of a very well studied excitatory neurotransmitter found in our brain. Interestingly, glutamate cell receptors have been identified in plants, suggesting that glutamate participates in cell-to-cell communication not only in animals and humans but in plants as well. In addition, glutamate was identified as an exogenous signal for regulation of root growth and its morphogenesis (development and structure), suggesting that the neuronal-like activities of glutamate are most evident in root apices. Accordingly, root apices, i.e., root hairs, represent plant parts that are the most similar in appearance to neurons. In other words, root apices (peaks) create branched networks that resemble neuronal networks with extended axons (nerve fibers).

Apart from reacting to neurotransmitters, plants can also generate and transmit signals. As some findings indicate, plants can produce electrical signals (impulses) that control important physiological processes, including photosynthesis, respiration, and movement of lateral organs.

Also, it has been proposed that plants assemble structures similar to neuronal synapses. Synapses are structures that enable transmission of signals from one neuron to another in the brains of animals and humans. As findings further indicate, the activity of so-called plant synapses is regulated by light and gravity. The presence of these synapses could explain how the transportation of nutrients in the plant body goes along the root-shoot axis directed by gravity or light. Also, these synapses might be important determinants in defining plant integrity, i.e., the ability of plant cells to recognize “non-self” and detect “self”.  

Scientists believe that in addition to sensing light, gravity and moisture, plants (more precisely their roots) can also sense toxins and other chemical signals from their neighbors and other plants. Based on the collected information, roots somehow “decide” whether to change their course in order to escape dangerous substances and pathogens.

Plant communication can also be mediated by a complex network that plants form with underground fungi. Fungi that live in the soil form symbiosis with plants: they make physical connections with roots that result in the formation of a huge underground network. This network is not dissimilar to connections formed between brain cells, leading to speculations that it may function as a kind of “underground brain”.  The fungi enhance the plants’ uptake of nutrients (minerals) from the soil, as well as the plant’s tolerance to pathogens. On the other hand, plants supply fungi with the nutrients (such as carbohydrates) that are needed for the formation of so-called mycelial networks (i.e., the networks of fungi filaments). Often, these networks are shared between the roots of different species or separate individuals of same species, forming common mycelial networks.

Emerging evidence suggests that fungi-root networks mediate transmission of signals, allowing chemical signaling between plants. Thus, not only in appearance but also in function these structures resemble the brain. A group of researchers tested the hypothesis that underground fungi networks mediate allelopathy, a phenomenon in which plants limit the growth of neighboring plants by producing certain compounds called allelochemicals. Allelopathy is based on the idea along with water and nutrients that are transported through the common fungal networks, signals (chemicals) that induce plant defense are also transported.

Researchers have demonstrated that common mycelial networks facilitate allelopathic interactions by expanding the zone in which allelochemicals act and by increasing their amounts, i.e., the fungi contribute to their accumulation. Another study has shown that underground fungi networks facilitate inter-plant communication, through which plants defend themselves from pathogens. More precisely, as this study indicated, plants infected with pathogens can induce defense system in neighboring plants and thus increase their resistance to a disease. Through the fungal networks that connect the roots of neighboring plants, plants infected with pathogens can send chemical signals to non-infected plants to induce their defense mechanisms and thereby prevent the spread of disease. In other words, plants can sense sickness.

Other findings in support of the idea that plants have a degree of intelligence are based on claims that plants can sense music and react differently to different types of music (preferring classical music to rock and roll). Although this and similar ideas remain rather speculative, research on this topic progressed and new findings emerged. In certain circumstances, plants do express sophisticated behavior. They respond to competitors and pathogens and have evolved to adapt to different soils and conditions for growth. Thus, it could be assumed that plants can process information, memorize it, learn, and make decisions accordingly. Indeed, some scientists believe that plants have the ability to store biological information and recall it in order to express an adequate response. A group of researchers indicated that some plants can learn from experience. For instance, the mimosa plant can learn which stimuli to respond to in order to defend itself and folds its leaves as a sign of defense.

As these research findings indicate, the presence of a brain and neurons is not a mandatory requirement for the learning process. Plants seem to express behavioral traits common to organisms of higher organization, although through less conventional pathways that need to be further studied.

References

Davenport, R. (2002). Glutamate receptors in plants. Annals of Botany. 90(5): 549-557. DOI: 10.1093/aob/mcf228

Baluska F. (2010). Recent surprising similarities between plant cells and neurons. Plant Signaling and Behavior. 5(2): 87-89. PMCID: PMC2884105

Baluska, F., Volkmann, D., Menzel, D. (2005). Plant synapses: actin-based domains for cell-to-cell communication. Trends in Plant Science. 10(3): 106-111. DOI: 10.1016/j.tplants.2005.01.002

Babikova, Z., Gilbert, L., Bruce, T.J., Birkett, M., Caulfield, J.C., Woodcock, C., Pickett, J.A., Johnson, D. (2013). Underground signals carried through common mycelial networks warn neighbouring plants of aphid attack. Ecology Letters. 16(7): 835-843. DOI: 10.1111/ele.12115

Barto, E.K., Hilker, M., Müller, F., Mohney, B.K., Weidenhamer, J.D., Rillig, M.C. (2011). The fungal fast lane: common mycorrhizal networks extend bioactive zones of allelochemicals in soils. PLoS One. 6(11): e27195. DOI: 10.1371/journal.pone.0027195

Song, Y.Y., Zeng, R.S., Xu, J.F., Li, J., Shen, X., Yihdego, W.G. (2010). Interplant communication of tomato plants through underground common mycorrhizal networks. PLoS One. 5(10): e13324. 10.1371/journal.pone.0013324

Gagliano, M., Renton, M., Depczynski, M., Mancuso, S. (2014). Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia. 175(1): 63-72. DOI: 10.1007/s00442-013-2873-7

Garzón, F. C. (2007). The Quest for Cognition in Plant Neurobiology. Plant Signaling & Behavior, 2(4), 208–211. PMCID: PMC2634130

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