This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. The work is published in the journal Science. 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 assistant professor Kira Gysel, who, together with professor Kasper Røjkjær Andersen, led the work. "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.
Gysel, together with Simon Boje Hansen and Henriette Rübsam, 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.
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