Plants communicate through an underground web of fungi

Life depends on cooperation, not competition. Mutually beneficial symbiotic relationships exist throughout the biosphere. Plants don’t exist in isolation, but as part of a super organism or holobiont that also contains microorganisms in a microbiome¹. Microbial networks of fungi, bacteria, archaea, viruses, protists, and algae, as well as nematodes, arthropods, and protozoa form a nutrient web in the soil². They work together with plants in complex adaptive systems to drive biogeochemical cycles and influence every aspect of ecosystem structure and function. These interacting networks coevolved and are carefully regulated. The loss of key species can trigger shifts in this system to alternative stable states. The interactions between microbiomes and plants are fundamental to life on earth. They led to the chemical weathering of rock, the migration of ancient plants from the ocean to land about 360 million years ago and the subsequent co-evolution of highly specialized gymnosperm and angiosperm trees and eventually animal life. There is also an interface between the roots and soil that is called the rhizosphere microbiome. It is very diverse and active. Plants send 10–90% of their products of photosynthesis underground into the rhizosphere to support carbon, nutrient, and water cycles. The smallest microbiome is below ground in the tropical forest biome and the largest is in the grassland and tundra biomes².

Fungi are in such relationships with plants and animals. A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as mushrooms. Mycorrhiza can be thought of as the roots of the fungus, with mushrooms being the fruit. Hyphae are the long, filamentous part of a fungus. They are the vegetative part of the fungus, where growth occurs. They form a mycelium and underground mycorrhizal networks (MRNs). Fungi are found in marine environments and in any terrestrial environment in which moisture and a carbon source coexist³. They can penetrate limestone, mollusk shells and other things containing carbonates substrates⁴. They can exploit mineralized organic matter, attack their hosts, or engage in symbiotic relationships. They leave specific boring traces, which can be identified in fossils⁴. Fungi can be devastating to agricultural crops, both in the field and during their storage³. They kill immunocompromised patients in numbers that rival the deaths from malaria. At the same time, fungi are sources of food, chemicals and biofuels. About three trillion trees on Earth survive through symbiosis with an underground network of fungi^2,5,6. Scientists have mapped a wood wide web on a global scale, using a database of more than 28,000 tree species living in more than 70 countries⁷. A global map of the symbiotic status of forests was made using a database of over 1.1 million forest inventory plots that collectively contain over 28,000 tree species. This provided a quantitative understanding of microbial symbioses at the global scale, and showed the critical role of microbial mutualisms in shaping the distribution of plant species⁵.

Networks of fungi (MRNs) link trees through their roots and facilitate communication between them². The fungi forage nutrients in the soil and exchange them with nutrients derived from photosynthesis by plants⁸. This mutualistic association between fungi and plant roots is perhaps the most prevalent interaction between species in the biosphere. It includes over 90% of plant species and several groups of soil fungi^2,9.

The plant and microbial species that inhabit these biomes have coevolved sophisticated communication systems^2,10. Information is exchanged between organisms both within and among kingdoms and species. There are bidirectional signal molecules such as auxins that are used in signal perception, signal transduction, and to activate defense genes. Plants benefit from the fungus because it is energetically less expensive to invest in hyphal growth than root growth to acquire soil nutrients. Fungal hyphae grow faster, have smaller diameters for accessing tight soil pores, and branch more profusely. As mycorrhiza evolved, communication coevolved between the highly active plant root apex and the fungal symbiont. Mycorrhizal fungi can link the roots of different plant hosts, forming MRNs. In MRNs in forests, trees are nodes and interconnecting fungal hyphae are links. The topology is similar to that of neural networks in the human brain. They have scale-free patterns and small-world properties that provide both local and global efficiencies that are important for intelligence and memory-based learning. The biochemical signals that transmit between trees through the fungal linkages are thought to provide resources to receivers, especially among regenerating seedlings. Some of these signals appear to have similarities with neurotransmitters^2,10.

Most mycorrhizal symbioses are generic. Each plant species can associate with a diverse suite of fungal species. At the same time, a fungal species colonizes many plant species. However, some of the associations are highly specialized, where some plant and fungal species only associate with a single partner species, with the potential to form exclusive, specific networks. In forests, networks of ectomycorrhizal fungi (EMF) connect gymnosperm and some angiosperm trees as well as woody shrubs in temperate and boreal forest biomes. Networks of the arbuscular mycorrhizal fungi (AMF) connect angiosperm trees along with many herbs and grasses in the tropical forest biome, as well as some conifers in temperate forests. Ectomycorrhizal fungi are predominantly in the Basidiomycota and Ascomycota phyla, while the endomycorrhizal AMF are predominantly in the Glomeromycota phylum. Some exceptional plant families and genera are capable of forming viable symbioses with EMF and AMF simultaneously and serve as key hubs linking together ectomycorrhizal and arbuscular mycorrhizal networks².

Plants have the cognitive capacity to perceive, process, and communicate with other plants, organisms, and the environment. They can remember and use this information to learn, adjust their behaviors, and adapt accordingly. So, plants can make decisions and take actions. These characteristics of intelligence are often only ascribed to humans or perhaps animals. The fact that plants can perceive, communicate, remember, learn, and behave could transform the ways that humans perceive, empathize with, and care for trees and the environment².

Plants can also communicate by emitting and sensing volatile organic compounds, natural grafting of roots from the same species, electrostatic interactions and acoustic communication¹¹. Plants can also communicate with each other using electric signals that travel through their roots in the soil. Electrostimulation can activate ion channels and plant movement while repairing cellular damage and enhancing growth. In human neurons, exponentially decreasing electrical potentials are called electrotonic potentials. Such potentials also exist in plants. They can induce action potentials, as done in neurons⁹. Fungi also communicate through electrical potentials¹². Spikes of electrical activity are used by fungi to communicate and process information in mycelium networks. These networks transform information contained in the interaction of spikes and trains of spikes, similar to neurons. Fungi respond to mechanical, chemical and optical stimulation by changing the pattern of its electrical activity and, by modifying characteristics of their spike trains. Electric currents are also used in the interactions between mycelium and plant roots during formation of mycorrhiza. Researchers compared the complexity of the fungal spike train and compared it to the complexity of words used in European written texts. They found that there was a fungal language that is more complex in some ways than many European languages¹².

Communication and interactions between plants and fungi through common MRNs can improve yields of agricultural crops¹³. These MRNs mediate the exchange of nutrients and minerals. Growing two or more crops simultaneously on the same land can improve yields and improve ecosystems. MRNs make this easier by providing necessary nutrients, such as potassium. For example, when potatoes and onions were grown together common MRNs enabled them to obtain the potassium and other nutrients that they needed¹³. Common MRNs can also help with bioremediation of contaminated soils¹⁴. They can transfer cadmium (a toxic metal) from a contaminated plant (such as maize) to another plant (such as soybeans). This showed that common MRNs can transfer toxic heavy metals from main food crops to heavy metal hyperaccumulators via intercropping¹⁴.

In many cultures walking among the trees in a forest or living off the land by working the soil is thought to connect one with Gaia or Mother Nature. This connection and feeling of unity is similar to that which one can feel with mindfulness and meditation. Hopefully, this article helps to provide a solid, scientific basis for this understanding.