Relationship status between plants and fungi: it’s complicated. Some fungi cause disease, others help plants acquire nutrients and grow. To survive, plants need to be able to tell them apart. However, how they read the chemical signals fungi release was not very well understood – until now. Researchers at Aarhus University in Denmark, together with colleagues at the University of Copenhagen and CNRS-CERMAV in Grenoble, have uncovered how a plant receptor recognises long chains of chitin, a signal linked to fungal attack. Their findings, published in Science on October 15th, 2026, reveal a highly specialised receptor system that selectively recognises molecular fungal signals based on their size. They performed their research at the light sources PETRA III at DESY and MAX IV in Lund, Sweden.

Kira Gysel from Aarhus University and her colleagues have discovered a new immune mechanism in plants which could, in the long term, also be applied in agriculture. Photo: Emma Steer, Aarhus University

CChitin, a chain of repeating sugar molecules, is the main building block of fungal cell walls. When fungi grow or are broken down, fragments of chitin chains of different lengths end up in the soil, and plants detect them to monitor the microbes around them.

Long chitin fragments are commonly associated with attacks of harmful fungi and trigger strong plant immune responses. But not all fungi are bad: shorter chitin fragments are commonly associated with beneficial partnerships, for example when symbiotic fungi exchange essential nutrients with their hosts. What remained unknown was how plants can distinguish the good short chitin fragments from the bad long ones. “We knew that plants could tell harmful and beneficial fungi apart, and that the length of chitin plays a role,” says Kira Gysel, Assistant Professor at Aarhus University, who led the study together with Professor Kasper Røjkjær Andersen. “But we had no idea how a receptor could tell a long chain from a short one. This work unlocks a new mechanism in plant immunology.”

A receptor that measures molecular length

To do this, the team combined structural biology, which reveals the three-dimensional shape of proteins, with biochemistry and experiments in living plants to investigate the receptors that detect chitin at the surface of plant root cells. They did parts of their work at the EMBL beamlines P13 and P14 at PETRA III which specialise in crystallography. The X-rays revealed the 3D structure of a plant receptor recognising chitin in atomic detail, giving crucial clues to how the mechanism works.

This receptor, called CHIP13, binds long chitin chains much more strongly than short ones. A distinctive part of CHIP13, which the team calls the "bridge domain," helps it grip the chain at several points, so that only chains long enough to span them are held tightly. This bridge domain is a hallmark of a whole class of related receptors.

"The central question was how plants tell long chitin chains from short ones. We learned that CHIP13, and specifically its bridge domain, works like a measuring tape," says Kira Gysel. "That allows the plant to recognise signals associated specifically with a potential fungal attack and not confuse them with signals from potentially helpful microbes."

A division of labour

Plants also depend on a second receptor, called CERK6, to respond to chitin. The researchers investigated how the two receptors work together and found that in the presence of chitin, CHIP13 recruits CERK6, and the two form a signaling complex that activates the plant's immune system, with a clear division of labor between the two proteins. Experiments in which either receptor was blocked confirmed that the plant needs both to mount a defense response.

"CHIP13 can recognise or 'see' the long chitin fragments associated with dangerous fungi in the soil. It then physically comes together with CERK6, and only CERK6 can sound the alarm for the rest of the plant," explains Gysel. "Both proteins have very specific jobs, and both are needed for this core pillar of the plant immune system.”

“We were facing a puzzle: how can CERK6 be essential for the plant's response to chitin, when it can barely hold on to it?” says Simon Boje Hansen from Aarhus University, one of the authors of the paper. "It turns out the two receptors split the work: one recognises the signal, the other passes it on. Neither can do the job alone."

"We could see this directly in plants too," says Aarhus scientist Henriette Rübsam, also a co-author of the study. "It was only when chitin was bound to CHIP13 that CHIP13 could call on CERK6. Importantly, we saw that this was also true in a second legume plant, showing that this mechanism is preserved across different species.”

A window into how plants perceive the world  

As groundbreaking as it is, this research is more than just discovering the structure and mechanism of these proteins. It provides valuable new insights into how plants perceive their environment and protect themselves from biological threats in the soil.

“This research is a window into the complex pattern recognition system that plants use to interpret the microbial world surrounding them,” reflects Gysel. “If we can better understand this, we can start to think about designing receptors that read new signals altogether, expanding what plants are capable of sensing and responding to."

Insights such as these help scientists understand how plants distinguish between different microbes and could make a big difference in future agriculture by guiding efforts to improve crop resilience while preserving beneficial interactions.

“Some of the most interesting questions in plant immunity involve larger receptor complexes that are very difficult to crystallise and often give only tiny or poorly diffracting crystals,” Gysel explains. “An even more brilliant light source like a future PETRA IV could make it feasible to study these, using techniques such as serial crystallography, where we can collect structural snapshots from such tiny crystals.

Original publication

Gysel, Hansen, Rübsam et al., "Structural basis for size-selective perception of chitin in plants," Science (2026). DOI: 10.1126/science.adz4600