Film director James Cameron imagined that on the planet Polyphemus's moon Pandora - the setting on which the action of the movie Avatar takes place - all organisms were connected. In one scene in the film, Dr. Grace Augustine (played by actress Sigourney Weaver) warns the marine and protagonist that in this natural satellite the resources are managed thanks to "some type of electrochemical communication between the roots of the trees."
The environmental plea for this film, released in 2009, included the main idea of Suzanne Simard, a scientist at the University of British Columbia in Vancouver (Canada), who in 1997 published part of her doctoral thesis in the magazineNature on how plants interact with each other. According to their studies, forests become complex systems where species exchange nutrients, send warning signals and interact with the environment with greater or lesser success.
Those responsible for this collaboration are the mycorrhizal networks,
that is, the symbiosis between fungi and plant roots
The expert has been spreading her work around the world for 20 years with the same premise: those responsible for this collaboration are the mycorrhizal networks, that is, the symbiosis between fungi and plant roots. This connection, which is also known as the Hartig network, enables the exchange of nutrients, water and carbon with and between the plant species to which they are connected.
"Most plant systems grow on this symbiotic association in which the fungus supplies the plant with inorganic compounds such as nitrogen or phosphorus that it needs to feed and grow, and the plant provides the fungus with sugars resulting from photosynthesis", explains the scientific information about these networks, which some researchers have called the 'internet of plants' due to their similarity to internet nodes.
Despite the acceptance by the entire scientific community about the relevance of the interactions that occur in mycorrhizae, the controversy begins when Simard refers to these connections as ‘wisdom of the forest’. For this reason, other researchers have shed light on this network of underground pipes of roots and hyphae (cylindrical filaments of the body of fungi), which can be kilometers long and appear in all climatic systems.
Trees that exchange carbon
In this sense, a study by the journalScience showed, after five years of research, that some specimens of European spruce more than 120 years old in Swiss forests transferred carbon to other trees, both to their peers and to those of different species.
"A forest is more than a collection of individual trees.
They no longer just compete for resources, they share them. They act collectively ”, says the author
“It was a surprise to find interspecific transfer. Until now this has only been reflected in seedlings, but not in adult specimens ”, says Tamir Klein, geochemist at the University of Basel (Switzerland) and main author of the work, for whom at first the results were the result of a calculation error.
To check this, Klein stepped down from the 12-meter-high crane from which he had previously watered the treetops with a network of tubes into which he injected carbon-13, a type of element denser than that normally found in the air. "This allowed us to distinguish it from the usual material and trace its transfer from the leaves, where photosynthesis was carried out, until it was transported to the branches, stems and fine roots of other trees," he details.
Once on the ground, the Israeli researcher dug into the ground with his team until he reached the mycorrhiza network to verify that the tagged isotope had traveled from the tagged specimen to the closest trees of different species. “This is very relevant as it allows us to understand that a forest is more than a collection of individual trees. They no longer just compete for resources, they share them. They act collectively ”, asserts the expert.
In these same forests, ecologist Kevin Beiler, a researcher at the University of Eberswalde (Germany) and a disciple of Simard, mapped the links between mycorrhiza species in a forest and Douglas firs (Pseudotsuga menziesi) through their genetic connections.
“I used DNA microsatellite markers to verify the genes of the fir and the fungus at each point where the root cells and hyphae joined. I also collected the DNA from each tree and compared it with the samples I obtained from the roots close to each specimen, ”says Beiler to Sinc.
The results of this first sampling, published in theJournal of EcologY, they overwhelmed the researcher, who observed how the roots of each Douglas fir were attached to probably "more than 1,000 species of mycorrhizal fungi," he says. In order to study the unapproachable network, he opted to analyze the connections between the mycelia of the two fungi that were most often attached to the roots of fir trees.
"I discovered that the oldest trees were those with the most connections, while the youngest specimens were not so closely linked to the rest of the forest", specifies the German scientist, who was one of the first to coin the term 'internet of plants '(Wood Wide Web) to this mycorrhiza network with a study inNew Phytologist.
Connections to overcome threats
These networks, similar to the ones we use in our home Wi-Fi, are in danger of “disconnecting” due to massive felling of trees. But in the face of other threats, such as increased carbon dioxide emissions, the pipes that connect the trees play an essential role, especially considering that forests absorb about 30% of these emissions.
"Trees can take advantage of the fertilization effect of carbon and,
as they grow and reproduce rapidly, they absorb a greater amount of CO2 atmospheric"
A multidisciplinary team of scientists, with the collaboration of the Spanish biologist César Terrer, from Imperial College London, reviewed inScience 83 studies on the fertilization capacity of large plant ecosystems related to the increase in CO2atmospheric.
"A good part of the articles contradicted each other, but we found a point in common: the limiting factor of nitrogen," he tells Sinc Terrer. Here a special type of mycorrhizae came into play, the ectomycorrhizae, which are hyphae offungi associated with coniferous species such as boreal forests or alpine regions, similar to birches or pines that have been cited in the rest of the research in the article.
"Ectomycorrhizae have special enzymes that allow plants to access the inorganic nitrogen in the soil, produced by bacteria and microorganisms, in exchange for carbohydrates that plants produce in photosynthesis. Thus, trees can take advantage of the fertilization effect of carbon and, while growing and reproducing rapidly, they absorb a greater amount of CO2 atmospheric ”, says the scientist, for whom this capacity does not depend only on the presence of nitrogen in the soils, but on the association of plants with this type of fungi.
“Species were observed that despite growing in soils where there was less nitrogen, the trees developed more and therefore absorbed more carbon as they were more linked to ectomycorrhizae than other plants that were born in soils with more nitrogen, but without the presence of this network ”, asserts the Spanish researcher.
However, according to Terrer, much of the carbon uptake experiments have been done in soils where the main limiting factor is nitrogen and we do not know the patterns in phosphorus-limited ecosystems. "This would indicate that the forests of the Amazon could not absorb more carbon in the future," warns the researcher.
In another study ofScience, published last January, University of British Columbia scientist Jonathan A. Bennett focused on the relationships of 550 populations of 55 North American tree species. His team collected seeds and seedlings of the dominant species in the area, as well as soil samples close to the oldest trees.
"The main hypothesis was that adult specimens, having grown for decades in the same place, have established many interactions with other soil organisms, including both mycorrhizal fungi and pathogens," says the American researcher.
The results confirmed it: the ectomycorrhiza networks were thicker the closer they are to an elderly specimen. "These generate a kind of pod around each seed, a kind of armor with which the fungi protect the small roots of the seedlings from pathogens," the expert tells Sinc.
The ‘market’ between plants and fungi
However, despite the importance of the networks that link plants and fungi, scientists are still unclear on how the trade of nutrients between them is regulated. For Marcel van der Heijden, an ecologist at the University of Utrecht (Holland), these are not mutual transfers, that is, they benefit both participants equally, nor is it clear which species dominates the exchanges.
"It is impossible to address the full range of interactions that occur in arbuscular symbiosis networks in nature, but it seems that not all respond to the dynamics of the biological market," says the Dutchman, who refers to a perspective similar to the economic , in which the fungi would provide more nutrients to the plants which in turn provide them with more carbon.
"In our review of studies on arbuscular mycorrhizae, we concluded that both plants and fungi can regulate the delivery of resources and favor one or the other symbionts", Van der Heijden points out in a study published inNature Plants in which five different exchange dynamics were established, ranging from parasitism to the identification of the plant of its most beneficial partner.
“In the symbiosis, not only carbon is exchanged for phosphorus or nitrogen, but fungi also provide plants with other nutrients such as copper, iron or zinc, and chemical compounds to resist stress situations, such as attack by pathogens or droughts. ”, Asserts the scientist.
Not all the scientific community agrees
on the latest interactions of plants
But not all the scientific community ends up agreeing on the latest interactions indicated by the Dutch. The idea that plants are capable of sending alarm or help signals to their peers raises many doubts. However, there are studies that point towards this.
Sending out alarm signals
One of them is the one published inFrontiers in Plant Science by Ren Sen Zeng, an agricultural engineer at Fujian Agricultural University, China. Zeng's team grew pairs of tomato plants in pots. In some samples, plants were allowed to form mycorrhizal networks, while in others this symbiosis was limited.
When the fungal webs had finished forming, the leaves of one plant from each pair were sprayed withAlternaria solani, a fungus that causes blight disease in agricultural crops. To prevent the plants from interacting with other chemical compounds in the environment, they were surrounded with hermetically sealed plastic bags.
After 65 hours, Zeng infected the plant that was healthy in each pair, but the specimens that were attached to a mycorrhizal network showed resistance to the fungus, being less likely to get sick; And when they did, stress levels were significantly lower.
Another research similar to that of Zeng is carried out by a team of scientists from the University of Aberdeen (Scotland), led by David Johnson. For your study, published inInsights & Perspectives, Fava beans were selected, plants that also associate with each other with arbuscular fungal networks.
Some samples were exposed to aphids, species of insects whose pests are a threat to agricultural and forestry crops, and gardening. In the study, these organisms fed on the leaves of the broad bean plant they were able to access. "Those that were connected through the mycelia (mass of hyphae of the fungus) excreted chemical defenses against aphids, while those that were not connected could not react," points out Sinc Johnson.
Thus, forests act as an organism, a huge structure that is articulated under the ground through a network in which a prominent cast of actors invisible to the human eye interact, but who can determine the future of the climate. Understanding its operation is the challenge that science still faces
Do plants send messages through the air?
Plant species not only receive stimuli through their roots. "It has been found that plants detect volatile organic compounds (VOCs) with chemical receptors in their leaves, which in turn transmit signals that end up giving rise to changes in gene expression", Josep Peñuelas, a researcher at the CSIC at the Center for Ecological Research and Forest Applications (CREAF).
"Plants and other organisms through evolution have developed a kind of language, biochemical pathways, which they have used to communicate and act as a consequence of the message received," says the scientist, who gives as an example the variation in emissions of these compounds, when the plant is sprayed with antibiotics.
Peñuelas highlights as the most obvious interactions those that cause the exchange of CO2 and water. "But the emissions of VOCs are given by the hundreds with environmental implications without which the general functioning of the biosphere is not understood," he adds. An example of this is the pollination of flowers, whose tissues emit these compounds to attract the attention of insects that will carry their pollen.