Wheat immune protein structure solved – an important tool in the battle for food security
Scientists from the Max Planck Institute for Plant Breeding Research and the University of Cologne in Germany, along with their Chinese colleagues, have discovered how wheat protects against deadly pathogens. Their findings, published in the journal Nature, can be exploited to make important plant species more disease-resistant.
As the staple food of 40% of the world’s population, the importance of wheat for food security is hard to overstate.
The resilience of crops to a changing climate and resistance to infectious diseases will be determinants of food stability in the future. In the case of wheat, one of the most economically important pathogens is stem rust, an evil fungus that can have devastating effects on crops.
Although stem rust has been afflicting wheat since pre-Christian times, thanks to the efforts of breeders and plant pathologists, any major outbreaks in the major wheat growing regions of the world could have been prevented during the past 50 years of the 1920s. century. Unfortunately, that rosy picture shattered in 1998, with the emergence of a new, highly virulent species of wheat stem rust in Uganda. Ug99, as it is called, can attack up to 80% of wheat varieties worldwide, in some cases leading to a complete loss of yield in affected fields. In their quest to provide crops with resistance against new and emerging plant pathogens, crop scientists and plant breeders often search the wild species of some of our staple crops in search of genes that may provide effective immunity. The emergence of Ug99 gave particular impetus to these efforts and led to the identification of Sr35, a gene that protects against Ug99 when introduced into bread wheat.
Now, scientists led by Jijie Chai and Paul Schulze-Lefert of the University of Cologne and the Max Planck Institute for Plant Breeding Research in Cologne, Germany, and Yuhang Chen of the Chinese Academy of Sciences, in China, have deciphered the structure of the SR35 wheat protein. This allowed them to explain how Sr35 protects a spell against Ug99.
Sr35 is an example of a nucleotide-binding leucine-rich repeat (NLR) receptor (NLR) within plant cells that detects the presence of invading pathogens. Activation of the NLR is triggered by the recognition of pathogenic ‘effectors’, which are small proteins that are transported into plant cells by invading microorganisms in order to weaken the plant. Each NLR is generally associated with one type of effector.
When Sr35 is activated, five receptors assemble into a large protein complex, which the researchers call the “Sr35 resistosome.” These resistors have the ability to act as channels in the plant cell membrane. This channel activity triggers powerful immune responses culminating in plant cell suicide at the site of infection, a kind of self-sacrifice to protect the rest of the plant.
In this study, researchers succeeded for the first time in solving the structure and describing the immune function of an antigen from a cultured species.
The scientists began by synthesizing both Sr35 and its corresponding effector Ug99 in insect cells, a strategy that allowed them to isolate and purify large amounts of Sr35 resistors, and used cryogenic electron microscopy, a technique in which samples are frozen at extremely cold temperatures allowing their identification. of biomolecular structures at atomic resolution. “In the structure of Sr35, we were able to identify the protein fragments important for recognition of the Ug99 effector,” says Alexander Vorderer, who led the study. “With this idea, I hope we can create new NLRs that can be applied in the field. To protect elite wheat cultivars from Ug99 and thus contribute to the global food security.”
Armed with their knowledge of the structure of the Sr35 resistosome, Alexander Vorderer and co-authors Ertong Lee and Aaron W. Lawson set out to determine if they could now repurpose nonfunctional receptors from sensitive elite cultivars of barley and wheat to recognize Ug99. responder. They land on two proteins that, although similar to Sr35, do not recognize Ug99. When they swapped Sr35 elements known to come into contact with the Ug99 effector, the scientists were able to convert these proteins into receptors for the Ug99 effector.
According to Paul Schulze-Levert, “This study also demonstrates how nature has used a common design principle to construct immune receptors. At the same time, these receptors have evolved in such a way that they have retained the flexibility to generate new receptor variants that can provide immunity to other microbial pathogens such as viruses, bacteria or nematodes.” »
Jijie Chai notes that the knowledge gained in this study “opens up the possibility of improving crop resistance by designing plant resistance proteins that recognize a range of different pathogens.”