Rice blast disease is a major threat to rice production around the world. Now, a team of researchers from Japan and the UK’s John Innes Centre has discovered that exploiting unusual protein activity in rice could give the crops the edge they need to overcome this devastating disease, paving the way for disease-resistant rice in the future.
Rice fungus causes loss of a third of the harvest
Magnaporthe oryzae, the fungus that leads to rice blast disease, creates lesions on rice plants that reduce the yield and quality of grain. The fungus causes losses of up to a third of the global rice harvest – enough to feed 60 million people each year.
Rice farmers employ various strategies to ward off the fungus, but a sustainable approach is not yet available. Both cost and environmental concerns have limited the success of fungicides, and resistant rice varieties can be low yielding.
Rice blast fungus works by deploying a multitude of proteins, known as effectors, inside cells of the rice plant. As a coping response, rice plants have evolved genes encoding nucleotide binding-leucine-rich repeat proteins or NLRs.
These NLRs are intracellular immune receptors that bait specific proteins or effectors of the rice blast fungus. Once the specific fungal effector binds to the NLR receptor, the rice plant initiates signalling pathways to kill off the rice blast fungus’ effector cell.
“(The cells) die in a very localised area so the rest of the plant is able to survive. It’s almost like sacrificing your finger to save the rest of your body,” said Professor Mark Banfield, group leader at John Innes Centre and lead author of the study, which has been published in the Journal of Biological Chemistry.
Improving plant’s own defences
Previous work showed that fungal effectors AVR-Pia and AVR-Pik have similar structures. In this study the researchers sought to find out whether any rice NLRs, known to bind to one of these pathogenic fungal effectors, could perhaps also bind to the other fungal effectors.
The researchers introduced different combinations of rice NLRs and fungal effectors into tobacco plants (a model system for studying plant immunity) and used rice plants to show if any unusual pairs could come together and elicit immune responses.
An AVR-Pik-binding rice NLR called Pikp triggered cell-death in response to AVR-Pik as expected, but, surprisingly, the experiments showed that plants expressing this NLR also partially reacted to AVR-Pia.
The authors investigated the unexpected pairing using X-ray crystallography and noticed that the rice NLR possessed two separate docking sites for AVR-Pia and AVR-Pik. With this particular NLR performing a dual role by triggering immune reactions in response to two separate fungal proteins.
“In its current form, Pikp causes meager immune reactions after binding AVR-Pia, however, the receptor’s DNA could be modified to improve its affinity for mismatched effectors”, said Professor Banfield.
“If we can find a way to harness that capability, we could produce a super NLR that’s able to bind multiple pathogen effectors,” he added.
The genes that encode this receptor could become a template for engineering new receptors that can each detect multiple fungal proteins, and thereby improve disease resistance in rice crops.
Ultimately gene-editing technologies could be used to insert enhanced versions of NLRs—like Pikp—into plants, said Professor Banfield, which could tip the scale in favor of rice crops in the face of rice blast disease.
Read the full story on the John Innes Centre’s news pages.