Engineering CC-HMA-NLR immune receptors for disease resistance in crops (ERiC)

Every year, significant yields of our key global food crops are lost to pre-harvest plant disease. These diseases are caused by pathogenic micro-organisms such as fungi, oomycetes, bacteria, and pests. These yield losses are set against the world's increasing demands for food, which continue to rise as the world's population grows and there are changes in dietary habits.

Plants have an immune system that helps defend them against disease. However, unlike humans and other mammals, they don't have antibodies and their first line of defence are disease resistance genes. Many of these genes encode immune receptors, which are proteins that function to detect pathogens and pests, and activate the immune response. Some plant immune receptors act as sensors - they carry specific regions that serve as baits for detecting pathogen molecules. The identity of the bait determines the capacity of the sensor to detect a particular type of a parasite. Pathogens and pests are often adapted to cause disease on a particular host, so plant/parasite co-evolution has led to a diverse set of immune receptors encoded in various plants with different mechanisms for parasite detection.

In previous work by our laboratories, we determined the molecular details by which some plant receptors sense and bait pathogen molecules. We succeeded in imaging the contact points between the plant and pathogen proteins at the molecular level. We also went on to discover that the strength with which the plant sensor binds the pathogen molecule correlates with the strength of the plant's immune response. This work opened up new avenues for engineering better plant responses against pathogens by building sensors with increased strength of binding to pathogen proteins, and therefore conferring enhanced resistance to disease.

This project will further build on these studies and will develop plant receptor sensors with new domains that bait different pathogen molecules. It has the potential to expand the usefulness of the receptors in agriculture. We will focus our work on the blast fungus Magnaporthe oryzae, a pathogen that threatens staple cereal food crops like rice, wheat and barley and is a major contributor to food insecurity around the world. Recent new epidemics of the disease caused by this organism on crops in Asia and Africa have highlighted the potential for the pathogen spreading to new areas, and the critical need for us to understand how we can help plants fight back.

Once we understand how to engineer new plant sensors to detect invading pathogens, we will be able to help protect rice, wheat and other important food crops from disease.

Engineering the plant immune system provides opportunities to develop novel genetic approaches to disease resistance in key crops that feed the world. In previous work, we determined the molecular details of how integrated HMA domains in the rice paired NLRs Pik-1/Pik-2 recognise different alleles of the rice blast pathogen effector AVR-Pik. This work demonstrated the potential of conferring enhanced resistance to plant diseases by engineering increased binding strength between NLR-IDs and pathogen effectors. This proof-of-principle study has paved the way for the work proposed here, to generate chimeric CC-HMA-NLRs with bespoke domains that detect divergent, widely distributed effectors from across host-specific pathogen lineages. This is important as different M. oryzae lineages infect different cereal crops. The research performed here will inform approaches to address epidemics of diseases such as wheat blast, which has recently emerged in Asia and Africa.

To deliver our objectives we propose a multi-disciplinary approach combining biochemistry, structural biology, genetics and plant pathology. Building on our preliminary data, we will first determine the extent to which cereal HMA proteins bind variants of the M. oryzae PWL effector family to understand specificity and identify targets for CC-HMA-NLR engineering. We will then use in vitro and in planta assays to optimise the binding of these HMA domains to PWL effectors, including incorporating into the CC-HMA-NLR scaffold (the latter will also allow monitoring of immune responses). Further, we will develop the Pik-1/Pik-2 system outside the HMA domain to deliver optimal immune responses. Finally, we will transform rice, barley and wheat cultivars with engineered CC-HMA-NLRs and test for resistance against both Lab strains of M. oryzae with different PWL effector complements and natural isolates collected from disease outbreaks.