Rice is one of the world's most important agricultural crops, with 741.5 million tonnes produced in 2014. A large proportion of the global population relies on rice as a staple food, particularly in Asia and Africa. However, harvests are threatened by rice blast disease, caused by the fungus Magnaporthe oryzae, which destroys enough rice to feed around 200 million people every year. Rice and the rice blast fungus are involved in a co-evolutionary arms race, fighting for the upper hand. As the fungus relies on effector proteins to help it infect and reproduce within rice plants, rice has evolved immune receptors that allow it to detect and prevent the spread of the fungus. However, the rice blast fungus has evolved stealthy effector proteins that remain undetected by the rice immune system but can still promote disease. In work recently published in the Journal of Biological Chemistry, an international team of scientists has investigated how one stealthy effector protein might maintain its disease-promoting activity but evade immune detection. This research has an ultimate aim of engineering a receptor that would allow rice plants to better defend themselves.
We're familiar with images of the rice paddies of Asia, but this impressive sight represents an irresistible target for the rice blast fungus, Magnaporthe oryzae. Unable to run away from pests and pathogens, plants have evolved immune systems to detect and defend against attack. However, huge swathes planted with the same variety creates an evolutionary pressure for pests and pathogens; a feast is at hand if they can evade those defences.
One way that pathogens try and gain an advantage is through the use of effector proteins. These proteins can suppress the plant's immune system and manipulate the plant's own systems to help the pathogen infect and replicate. However, the mechanisms they employ to do so are not fully understood.
In collaboration with scientists from Japan and Thailand, researchers at the UK's John Innes Centre and The Sainsbury Laboratory have been investigating the interaction between rice plants and the rice blast fungus, with the ultimate goal of engineering new genetic resources that will help rice fight this damaging disease.
One of the effector proteins deployed by the rice blast fungus is called AVR-Pik. AVR-Pik targets host proteins containing heavy-metal associated domains (HMAs), presumably to promote disease. Intriguingly, HMA domains are also found in a subset of plant intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors. NLRs are conceptually similar to human antibodies as they can bind pathogen proteins to initiate an immune response to prevent disease. A specific HMA-containing NLR receptor called Pik-1/2 recognises AVR-Pik. In this work, researchers investigated how the AVR-Pik effector from the rice blast fungus binds to the rice HMA protein OsHIPP19. They found that each of the six AVR-Pik variants (A-F) identified to date all bound to the rice HMA protein, including AVR-PikC and AVR-PikF, which are known as stealthy effectors as they evade detection by the rice immune system.
For a critical part of this research, the team used Macromolecular Crystallography (MX) on the I03 beamline to determine the structure of the stealthy AVR-PikF variant in complex with the HMA domain of the rice protein OsHIPP19, revealing previously unseen contacts between AVR-Pik effectors and HMA domains. This result, in combination with the rest of the study, provides a foundation for engineering the HMA domain of the Pik-1/2 immune receptor to bind to stealthy AVR-Pik variants and engineer rice resistance against a wider range of disease-causing fungal strains.
Professor Mark Banfield from the John Innes Centre, explains:
We're part of a consortium, with the Universities of Essex and East Anglia, with BAG access to Diamond, which means we can send samples for MX on a regular basis.”. Prof. Banfield further noted, “The beamline staff are an essential part of making the whole process happen, from the background work to prepare the beamline, to solving problems during the experiment. They make it possible for us to get the results we need.
While this research is a proof-of-concept study, it has the potential for real-world applications. Rice is essential on every scale, from providing a livelihood to subsistence farmers to being a crucial part of national economies. Although this work was specific to rice, different fungal strains can cause blast disease in other essential food crops, including wheat, barley and millet. We hope to use approaches such as those suggested in this study to engineer defences against those as well.
Maidment JHR et al. Multiple variants of the fungal effector AVR-Pik bind the HMA domain of the rice protein OsHIPP19, providing a foundation to engineer plant defence. Journal of Biological Chemistry (2021): 100371. DOI:10.1016/j.jbc.2021.100371.
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