Researchers have discovered a new way plants defend themselves against disease. In a study published in Cell, a team led by Prof. Liu Zhiyong found that a wheat immune protein, called WAI3, forms an eight-part complex that helps trigger the plant’s defenses by allowing calcium to flow into cells. This is the first time an immune complex of this kind — known as an octameric resistosome — has been identified in plants.
The study also showed that this defense mechanism is not limited to wheat. The researchers found evidence that a similar immune protein in Arabidopsis, a widely used model plant, works in much the same way. This suggests that the mechanism may be shared across very different plant species, from crops such as wheat to smaller research plants, making the finding important for plant biology more broadly, according to a press release.
Plant immunity works differently from animal immunity. Instead of relying on a centralized system of specialized immune cells, each plant cell can detect danger and respond on its own, while also sending signals throughout the plant. One important group of plant immune proteins is known as NLRs. These proteins sit inside cells and detect pathogen molecules that are delivered into the plant to interfere with normal cellular functions.
NLRs come in different forms, including one group known as CCG10-NLRs. Although other NLR proteins had already been shown to form immune complexes called resistosomes, scientists had not yet understood how CCG10-NLRs are activated.
To investigate this question, the researchers studied a wheat mutant called M3045, derived from the line Zhongke 331. This mutant shows constant immune activity and poor growth. While harmful for the plant, it gave the researchers a useful way to examine how wheat immune responses are switched on.
Using map-based cloning, the team identified the gene responsible: Wheat Autoimmunity 3 (WAI3), which encodes a CCG10-NLR protein. They found that a single change in the protein causes it to become permanently active.
The researchers then produced the WAI3 protein in Nicotiana benthamiana and used cryo-electron microscopy to determine its structure. They found that activated WAI3 assembles into an eight-part resistosome. This structure is different from previously known plant resistosomes, which had been seen as five-part or six-part complexes.
Further experiments showed that the WAI3 resistosome can trigger calcium entry in plant cells, helping activate immune responses. However, it did not function the same way in animal cells, suggesting that it may rely on additional plant-specific factors.
The team also used the WAI3 structure to study RPS2, a related immune protein in Arabidopsis thaliana. Although they were unable to fully resolve its structure, they showed that activated RPS2 also forms an eight-part resistosome and triggers calcium influx in plants.
Overall, the study identifies a previously unknown plant immune structure and shows that this mechanism is conserved across both monocot and dicot plants. It also highlights wheat as a useful model for discovering general principles of plant immunity.
