Rice blast remains one of the most persistent diseases threatening global food security. The disease, caused by the Magnaporthe oryzae, is directly responsible for more than 30% of rice yield losses each year. The pathogen can also cause blast in other cereal crops such as wheat and barley.
Currently, deployment of durable disease resistance strategies in the field is limited by the speed of recognition in nature, while pathogenic fungi such as M. oryzae are constantly evolving to circumvent these new resistances. Bioengineering of plant immune receptors such as leucine-rich repeat (NLR) has emerged as an emerging approach to generate novel disease resistance traits to combat the growing threat of plant pathogens to global food security, and in theory, can be developed on demand.
Pathogens secrete proteins called effectors into host cells to manipulate plant metabolism and promote infection. Plants are able to recognize these effectors using immune receptors called NLRs. However, it is not always easy to define a receptor that can naturally recognize a specific effector, and even if such a receptor exists, the pathogen's effector may mutate and evolve to escape recognition. Researchers study the interactions between pathogen effectors and plant receptors to understand how each pathogen works, and it also allows researchers to modify natural plant receptors to change their recognition specificity.
Mark J. Banfield's team published their latest research results in PNAS, titled "Bioengineering a plant NLR immune receptor with a robust binding interface toward a conserved fungal pathogen effector".
In this article, the researchers focused on engineering NLR immune receptors in rice to enable them to stably bind a wider family of conserved effectors from the rice blast pathogen. By identifying a conserved family of effectors, this engineered immune receptor established a proof of principle that could provide more robust and durable rice blast resistance in agriculture in the future. This may make it more difficult for pathogens to evolve to evade recognition. The concept of host-target-based immune receptor engineering may also be applicable to other plant diseases that rely on delivering effectors to host cells to exert pathogenicity.
By replacing the heavy metal associated (HMA) domain in the rice NLR Pikm-1 with the corresponding domain from the rice protein OsHIPP43, a natural target of Pwl2 effectors, the researchers succeeded in changing the response profile of the receptor, allowing it to recognize and respond to Pwl2 and its broader family of Pwl effectors. The researchers collected X-ray diffraction data to investigate the details of the interaction between the two proteins. The crystal structure of the complex revealed an extensive contact interface between Pwl2 and OsHIPP43.
Figure 1. Crystal structure of the Pwl2/OsHIPP43 complex. (Zdrzałek, et al., 2024)
Interestingly, the researchers performed experiments to show that the newly synthesized proteins were able to recognize different Pwl effectors.
To explore the limits of the chimeric protein, they generated a series of site-directed mutations against Pwl2 based on the crystal structure and performed new experiments to test the recognition specificity of the alterations. In many cases, the protein still recognized the effector, showing the robustness of the system.
The results of this study demonstrate the potential of host-target-based NLR engineering to develop new resistance traits that may be less easily overcome by pathogen evolution. This research may have far-reaching implications for future crop protection and the stability of the global food supply.
