How plants read fungal warning signs
Chitin, 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 fungal attack and trigger strong plant immune responses. But not all fungi are bad. Shorter chitin fragments, meanwhile, are associated with beneficial partnerships, including interactions with symbiotic fungi that exchange essential nutrients with their hosts. What remained unknown was how plants can tell these signals apart.
To find out, 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 found that a receptor called CHIP13 binds long chitin chains much more strongly than short ones. A distinctive part of the receptor, 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 the project leads. "That allows the plant to recognize signals associated specifically with a potential fungal attack and not confuse them with signals from potentially helpful microbes."
Plants also depend on a second receptor, called CERK6, to respond to chitin. The study’s first authors investigated how the two receptors work together. The scientists 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 recognize 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 one of the first authors. "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 another author. "It turns out the two receptors split the work: one recognizes the signal, the other passes it on. Neither can do the job alone."
"We could see this directly in plants too," says the senior author. "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.”
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 the first author. “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 help guide efforts to improve crop resilience while preserving beneficial interactions.
