Plants use underground networks to ‘eavesdrop’ on their neighbors 

A recent study led by the University of Oxford challenges the widely held notion that plants actively communicate with each other to warn of impending threats.

Instead, the findings suggest that plants are more likely to passively “eavesdrop” on their neighbors through shared fungal networks rather than engage in altruistic signaling.

Underground communication network

Beneath the soil, plants are connected by an intricate fungal network popularly referred to as the “wood wide web.” This network emerges from the symbiotic relationships between plant roots and mycorrhizal fungi. 

In this partnership, plants supply fungi with carbon while the fungi provide essential nutrients to the plants. These networks also enable the transmission of resources and information among plants.

Previous studies have demonstrated that when one plant is attacked by herbivores or pathogens, neighboring plants connected to the same fungal network often strengthen their defenses. 

However, it has remained unclear whether these responses result from active warnings sent by the attacked plant or passive cues interpreted by its neighbors.

Questioning altruism in plant behavior

The idea that plants actively warn their neighbors poses a puzzle for evolutionary biologists. Altruistic behavior is generally rare in nature and typically occurs when it offers mutual benefits. For plants competing for resources like sunlight and nutrients, warning their neighbors would seem counterintuitive.

To address this paradox, an international research team from the University of Oxford and Vrije Universiteit Amsterdam developed mathematical models to explore whether plants would evolve to signal threats to one another. 

The results suggest that it is highly unlikely for plants to develop active warning mechanisms, as such behavior would not confer significant evolutionary advantages.

Plants communicate in deceptive ways

The study revealed that plants might even engage in deceptive signaling to gain an advantage over their competitors. 

“Our results indicate that it is more likely that plants will behave deceptively toward their neighbors, rather than altruistically,” said lead author Thomas Scott from the Department of Biology at the University of Oxford. 

“For instance, plants may signal that a herbivore attack is occurring, even when no herbivore is present. Plants can gain a benefit from dishonest signaling because it harms their local competitors, by tricking them into investing in costly herbivore defense mechanisms.”

The findings challenge the idea of plants as cooperative entities and instead suggest that what appears to be altruistic behavior may often serve competitive or self-interested purposes.

Why do neighboring plants still respond?

If plants are not actively warning their neighbors, why do connected plants upregulate their defenses when one is attacked? The researchers proposed two alternative explanations.

First, plants might unintentionally emit cues when under stress, similar to humans blushing when embarrassed. These involuntary emissions could be picked up by neighboring plants as signals of a threat, even though the emitting plant gains no direct benefit from sharing this information.

Second, the fungal networks themselves could play an active role in transmitting warnings. Mycorrhizal fungi rely on healthy plants for their survival, as plants provide them with carbohydrates. By relaying signals about potential threats, the fungi may protect their host plants and ensure the stability of the network. 

“Maybe it is the fungal networks themselves that are sending the warning signals,” Scott said. “Mycorrhizal fungi rely on the plants on their network for carbohydrates, so it’s important to keep these plants in good condition. Perhaps the fungi are listening in on their plant partners, detecting when one has been attacked, and warning the others to prepare themselves.”

Nosy plants that eavesdrop

Co-author Toby Kiers from Vrije Universiteit Amsterdam offered another perspective, likening plants to neighbors gossiping about a shared environment:

“There is no dispute that information is transferred. Organisms are constantly detecting and processing information about their environment. The question is whether plants are actively sending signals to warn each other. Maybe just like gossiping neighbors, one plant is simply eavesdropping on the another.”

This reinterpretation highlights the possibility that plants are more passive participants in these networks, interpreting cues rather than engaging in intentional communication.

Implications for plant ecology and conservation

The study’s findings have significant implications for our understanding of plant behavior and ecosystem dynamics. Recognizing that plants may rely more on passive information gathering than active signaling shifts the focus of research to the role of fungal networks in mediating plant interactions. 

This knowledge could be valuable in predicting how plant communities will respond to environmental changes such as habitat loss or climate change.

Understanding these dynamics also has potential applications in agriculture and conservation. For instance, manipulating fungal networks might enhance crop resilience or help restore ecosystems by improving communication between plants in stressed environments.

Future research directions

While the study provides compelling theoretical support for the eavesdropping hypothesis, further empirical research is needed to validate these models. 

Experiments could focus on isolating the role of fungi in transmitting signals and exploring whether different plant species vary in their ability to interpret these cues.

Additionally, studying how plants and fungi interact in diverse ecosystems could reveal whether certain environments favor passive communication over active signaling. 

Expanding the research to include underrepresented plant species and regions would also provide a more comprehensive understanding of these interactions.

Rethinking how plants communicate

This study fundamentally challenges the idea of plants as altruistic communicators, suggesting instead that their behavior may be driven by competition, self-interest, and passive eavesdropping. 

By highlighting the role of fungal networks, the research opens new avenues for exploring the intricate relationships that sustain plant communities.

As Scott and his team continue to refine their models and expand their investigations, the hidden dynamics of plant communication promise to reveal even more about the complex strategies plants use to survive and thrive in their environments.

The study is published in the journal Proceedings of the National Academy of Sciences (PNAS).

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