Wednesday, February 26, 2025

Hidden allies

Endophytic fungi inside the leaves strengthen the chemical defenses of black poplars and influence the interactions between insect populations living on the trees

Max Planck Institute for Chemical Ecology

image:
Ants and aphids on a poplar leaf: When aphids infest plants, ants are often also present. The ants do not prey on the small insects, but are interested in their honeydew. In return, they even protect the aphids from predators. Observations showed that fewer ants colonized the poplars when endophytic *Cladosporium fungi* could be detected inside the leaves. This may be due to the defense substance stachydrine, which the fungi produce and which the aphids excrete with their honeydew.
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Credit: Christin Walther, Christian-Albrechts-Universität zu Kiel, Germany

Endophytes: A plant’s friends or foes

Endophytes are microorganisms that live inside plants. Some of these organisms, mostly bacteria or fungi, make the plants sick, while others have no harmful effect on the plants or are even beneficial. Previous studies of endophytic fungi living inside certain grasses have shown that these fungi provide the grasses with a defense against predators. However, little has been known about whether this is also true for trees. The current study investigated the influence of an endophytic fungus of the genus Cladosporium on the herbivore defense of the black poplar Populus nigra, as well as the effects on the insect communities that live on poplars.

"We investigated the influence of the endophyte on the chemical defense of black poplars and the consequences for the food preference and fitness of herbivorous insects, as well as the composition of the insect community on young trees in the field. We already knew from previous studies that plant pathogenic rust fungi are beneficial to herbivorous insects. Now we were interested in an endophytic fungus whose role in poplar-insect interactions is still largely unexplored," says first author Christin Walther, explaining the starting point of the study.

The researchers found that colonization with the fungal endophyte boosted the poplars' natural defenses and those induced by insect damage. In laboratory experiments, the larvae of the gypsy moth Lymantria dispar, a generalist on various trees, preferred uninfected leaves and performed better there. "We were able to show that Cladosporium helps the plant to defend itself against predators. To do this, the fungus alters the plant's chemical defense profile and also supports it with a self-produced defense substance," says Christin Walther.

Using the state-of-the-art chemical analysis method known as metabolomics, which analyses the metabolic products in the leaves, the scientists were able to show that infection with the fungus changes the poplar metabolome, i.e. the substances produced in the poplar. Specific defense substances such as salicinoids (salicin, nigracin) and phenolic acids (caffeic acid) were produced at higher levels in the presence of the fungus. Together with tannins, salicinoids are the most important natural defense substances produced by poplars to protect themselves. Salicinoids are particularly effective against generalist insects that are not only specialized on poplar, but are also pests of other plants. As well as deterring pests, they also have a negative effect on the fitness of insects that feed on chemically defended leaves.

Cladosporium defends poplars with its own defense substance

The analysis revealed that the fungus itself produces an effective defense compound, the alkaloid stachydrine. Tests with stachydrine showed that the endophyte's active substance fended off both generalists, such as the brush moth Orgyia antiqua, and poplar specialists, such as the poplar leaf beetle Chrysomela tremulae.

"It was particularly exciting for us to see that the fungus can produce an alkaloid that we can detect in the plant matrix and thus use as a marker for the presence of the fungus. We even found this substance in the leaves of old trees in the field," says study leader Sybille Unsicker, who headed the "Plant-Environment Interactions" group at the Max Planck Institute for Chemical Ecology and has been a professor at Kiel University since 2023.

Combination of chemical analyses, behavioral experiments and field studies

The scientists took a holistic approach to their research. In addition to chemical analyses of laboratory plants with and without the endophytic fungus and associated feeding experiments, they investigated in the field how Cladosporium affects the chemical ecology of black poplars.

Another interesting result of this study is an observation that the researchers made in the field. The endophytic fungus also seems to alter the interactions between aphids and ants that colonize young black poplars. The ant-aphid-relationship is a mutualism, a symbiosis that benefits both the ants, who drink the aphids' honeydew, and the aphids, who are protected from predators by the ants. Plants that had previously been infected with the endophyte were more often colonized by aphids, but less often by ants. This is probably due to the concentration of stachydrine in their honeydew. "The fungus even seems to be able to manipulate the mutualistic relationship between ants and aphids. Further research is needed to clarify exactly what is happening," says Christin Walther.

Trees are metaorganisms

Studying a tree's microbiome, the entirety of the microorganisms associated with the tree, is important for understanding the complexity of the co-evolution of trees and other organisms with which they interact. The current study is just the beginning of a broad field of research that raises many other exciting questions. For example, further studies will look at how the fungus spreads through the plant and whether it protects the plant not only from insect damage, but possibly also from pathogens. Another interesting question is whether the relationship between black poplar and Cladosporium is symbiotic, and whether the fungus benefits from living with the tree, or whether there are conditions under which it turns from a partner to a threat to the tree.

"There is a growing body of evidence in favor of considering trees as holobionts, as communities of different organisms. A tree is never a single individual, but a complex metaorganism colonized by microorganisms and arthropods in all its organs. The exciting question for us is how microorganisms contribute to the co-evolution of trees and herbivorous insects. Our results are certainly only a small contribution to understanding these complex interactions," concludes Sybille Unsicker.

Several endophytic fungi have been isolated from a poplar leaf. The current study investigates the influence of an endophyte of the genus Cladosporium on the chemical defense of black poplar and on the coexistence of insects on the tree.

Credit

Sybille Unsicker, Christian-Albrechts-Universität zu Kiel, Germany

Journal

Ecology Letters

DOI

10.1111/ele.70007

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

A fungal endophyte alters poplar leaf chemistry, deters insect feeding and shapes insect community assembly

Article Publication Date

26-Feb-2025

Scientists build robot to track plant-fungal trade networks, revealing nature’s underground supply chains

New research uses advanced robotics to track the hyper-efficient supply chains formed between plants and mycorrhizal fungi as they trade carbon and nutrients across the complex, living networks that help regulate the Earth’s atmosphere and ecosystems.

SPUN (Society for the Protection of Underground Networks)

video:
Video of high-speed flows and bi-directional movement inside mycorrhizal fungus Rhizophagus irregularis. False color for contrast
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Credit: Loreto Oyarte Gálvez - VU Amsterdam/AMOLF

By tracking half a million fungal highways and the traffic flows within them, researchers describe how plants and symbiotic fungi build efficient supply chains
The team built an imaging robot that allowed them to gather 100 years’ worth of microscopy data in under 3 years
Work advances our understanding of how fungi move billions of tons of CO2e into underground ecosystems each year

New research publishing in the journal Nature on February 26, 2025 uses advanced robotics to track the hyper-efficient supply chains formed between plants and mycorrhizal fungi as they trade carbon and nutrients across the complex, living networks that help regulate Earth’s atmosphere and ecosystems.

Travelling waves, traffic flows, and navigating pathfinders

Understanding plant-fungal trade is urgent because these fungal networks draw down around 13 billion tons of CO2 per year into the soil -- equivalent to ~1/3 of global energy-related emissions. More than 80% of plant species on Earth form partnerships with mycorrhizal fungi, in which phosphorus and nitrogen collected by fungi is exchanged for plant carbon. Despite their global importance, scientists did not understand how these brainless organisms construct expansive and efficient supply chains across their underground networks.

Using a custom-built imaging robot, the international research team of 28 scientists discovered that the fungi construct a lace-like mycelial network that moves carbon outward from plant roots in a wave-like formation. To support this growth, fungi move resources to-and-from plant roots using a system of two-way traffic, controlling flow speed and width of these fungal highways as needed. To seek further resources, the fungi deployed special growing branches as microscopic ‘pathfinders’ to explore new territory, appearing to favor trade opportunities with future plant partners over short-term growth within immediate surroundings. The researchers describe how these behaviors appear to be coordinated by simple, local “rules” that prevent the fungus from “over-building” and define a unique ‘travelling wave strategy’ for growth, resource exploration, and trade.

“We’ve been mapping the decentralized decision-making processes of mycorrhizal fungal networks, exposing a hyper-efficient blueprint for an underground supply chain,” said Evolutionary Biologist and co-author Dr. Toby Kiers of Amsterdam’s Vrije Universiteit. “Humans increasingly rely on AI algorithms to build supply chains that are efficient and resilient. Yet mycorrhizal fungi have been solving these problems for more than 450 million years. This is the kind of research that keeps you up at night because these fungi are such important underground circulatory systems for nutrients and carbon.”

Advanced robotics to track fungal decision-making

Discovering these new fungal behaviors was only possible because the team built an imaging robot that ran 24/7 in Amsterdam, allowing measurements of how the fungi reshaped their trade routes over time and space. “We discovered that these fungi are constantly adapting their trade routes, adding loops to shorten paths so they could efficiently deliver nutrients to plant roots” said Dr. Thomas Shimizu, co-author and Biophysicist from the physics institute AMOLF in Amsterdam.

Similar to navigation apps tracking congestion, the team then measured “traffic flows” at specific coordinates in the fungal road system, quantifying how fast resources were flowing to and from the root, tracking more than 100,000 particle flows. “By using our robot instead of a human being, we cut the lab time from a century to around three years”, added Shimizu.

“Robotics is making it possible to study fungal behavior in unprecedented detail, and at an unprecedented scale,” said co-author Dr. Merlin Sheldrake. “These techniques open the door to future work to understand the ways that these living, sensing, networks regulate ecosystem function and the Earth’s nutrient cycles”.

Data critical for understanding carbon draw down

The data collected are becoming increasingly important as atmospheric CO2 increases. Scientists want to understand how fungal networks control flows of carbon belowground. Kiers, also Executive Director of the Society for the Protection of Underground Networks (SPUN), the non-profit organization mapping Earth’s mycorrhizal networks adds, “Because these fungal networks are key entry points of carbon into global soils, we can now explore what triggers fungi to increase carbon flows underground.”

As in human supply chains, the efficiency of mycorrhizal fungal supply-chains depends on the ability of a network to produce and deliver goods to the right place, at the right time, at the lowest possible cost. Dr. Howard Stone, co-author and Professor of Mechanical and Aerospace Engineering at Princeton University adds “Understanding how these fungal networks adjust internal flows and resource trading to build supply chains in response to environment stimuli will be an important direction for future research”

Whether and how designers of human-built supply chains can learn from these principles evolved by plants and fungi over hundreds of millions of years is an exciting frontier. The team is now in the final stages of building a new robot which will increase data collection by a further 10x, allowing them to explore how fungal networks respond to rapid environmental change, including increases in disturbance and rising temperatures.

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See the authors discuss their work in a video, here. Fungal images and flow videos for download here.

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Full paper available: https://www.dropbox.com/scl/fi/m8t6bh7bw1qb0chh25l18/Travelling-Fungal-Wave-Nature-2025.pdf?rlkey=0gbll8fim8x5ui2qdmjgck4sm&st=ra7o8vzg&dl=0

“A travelling-wave strategy for plant–fungal trade” Nature https://www.nature.com/articles/s41586-025-08614-x

Research funded by the Human Frontier Science Program (HFSP), Netherlands Organization for Scientific Research (NWO), the Grantham Foundation, the Paul Allen Foundation, and the Schmidt Family Foundation

Network of arbuscular mycorrhizal fungal network with a muti-nucleate reproductive spore imaged with a fluorescent dye and confocal microscopy. The image, derived from hundreds of z-stacks, uses pseudo-coloration to indicate depth, with blue being closer and red farther from the observer.