A challenge to claims for mycorrhizal-transmitted wound signaling

KEYWORDS:

• Electrical conductivity
• mycorrhiza
• wound signaling
• Donnan potentials
• proximity electrodes

A recent paper from Thomas and Cooper¹ purports to show the transmission of electrical wound signals from plant to plant via a mycorrhizal fungus. As the authors note, the investigations were designed to address the relationship between the filamentous network of mycorrhizal fungi and plant roots, a relationship that led to notions of a “wood wide web”, communication between trees through fungal connections, and the “Mother Tree” concept². Although the arguments for meaningful communication and material exchange between plants through fungal networks are largely discredited^3,4, they nonetheless continue to fuel imagination. Indeed, electrical excitability in fungi has been known for decades⁵, but the claims by Thomas and Cooper of electrical transmission that depends on the living fungi does not hold up to careful scrutiny.

Quite apart from the issue of what constitutes communication^6,7, a singular flaw in the work is the lack of evidence for a biological origin of the voltage transients. Missing also are meaningful controls for their conduction via the biological net of the fungal filaments. Thomas and Cooper¹ made use of extracellular (‘proximity’) electrodes, rather than recording directly from the fungal and plant cells. Some recordings were made from the substrate underlying the fungi (agar or wood), others from external wall matrix around plant tissues or the sporocarps of fungi. The difficulty here is that proximity electrodes measure electrical field potentials, voltages that arise passively both from biological and non-biological sources, and such measurements therefore cannot distinguish between these sources⁸. Proving any biological origin requires the mechanism(s) to be identified, for example by pharmacological manipulations or by measurement of actively generated potentials at the cellular level^9,10.

One obvious source of such non-biological electrical potentials is the substrate itself. Agar and other aqueous gels incorporate fixed charges that affect ionic mobilities and result in so-called Donnan potentials^11,12. Donnan potentials arise also at interfaces such as between an agar substrate and an open, electrolyte-filled glass capillary (microelectrode). In this case, diffusion of ions gives rise to a voltage between the gel and the solution inside the electrode^10,13–15 that can easily exceed many tens of millivolts. These electrical potentials will vary when the gel is compressed or disturbed, when local ion concentrations change, and when ions diffuse.

Thomas and Cooper¹ use agar gels, together with the salt ‘reservoirs’ of fungal mycelia and plant tissues, and salt solution-filled microelectrodes that will have destroyed the plant tissues around the points of insertion. These configurations inevitably produce Donnan potentials, both with the cell wall matrix and remains of the dead cells, and with the agar gel itself. Such potentials will be conducted passively by any electrically conductive medium between their site of origin and distal positions, with a loss of amplitude according to distance and specific electrical resistance of the conductor (capacitive effects can be ignored, because of the slow time course of such voltage fluctuations).

What controls were included in the study of Thomas and Cooper?¹ They established an elegant arrangement with blocks of agar separated by an air gap. With this arrangement, filaments of the fungus that bridged the gap were able to carry electrical potentials between the two agar blocks and the plants secured in them. Thomas and Cooper also showed that a thread soaked in salt solution similarly conducted the electrical potentials across the air gap. In other words, a biological ‘bridge’ of fungal filaments was not essential for conduction of the electrical ‘signal’. Indeed, as expected, both the fungus and the thread transmitted experimentally-induced potentials with attenuation at the remote recording site consistent with a physical, not a biological, mechanism of transmission. Surely, these observations should have sent alarm bells ringing. They show that the signal propagates passively through any conducting structure, be it an inert, electrolyte-soaked material or organic matter.

Is there a cellular origin for the electrical potentials and for the mechanism of conduction? Clear and unequivocal evidence is missing. Yet there are a number of ways that such evidence might be gathered. For example, the experiments could have included measurements with intracellular microelectrodes [cf. Meharg et al¹⁶; Blatt et al¹⁷; Gradmann et al¹⁸] placed, nondestructively, in the plant cells and in the fungal filaments on either side of the bridge. Such recordings would have shown directly whether the potentials – or at least a component thereof – arose from cellular activities and changes in ion flux across the cell membranes. Intracellular recordings would also have established a biological basis for electrical transmission. Alternatively, a strong argument could have been made for true, fungal specificity if experiments had included controls with inhibitors of an all-or-none transmission by action potentials⁸ or with metabolic inhibitors, such as carbon monoxide, azide and cyanide, that greatly reduce ATP in fungi and plants and inhibit the primary H⁺-ATPases powering membrane transport^19–22.

In summary, we commend the experiments of Thomas and Cooper¹ for their innovative design. However, we caution readers who may not be familiar with the basic concepts of electrophysiology or with the physics of gels in aqueous solutions. As they stand, these experiments do not satisfy the most elemental of tests needed to justify the claims of biological signal transmission by a fungus. We hope, too, that these comments might be useful as a guide for future studies.

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Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

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Funding

The author(s) reported there is no funding associated with the work featured in this article.