One of the most important challenges in plant breeding is increasing plant resistance to biotic stresses. Pathogens and pests threaten global food security by limiting crop production. To fend off invading organisms, plants have evolved complex multi-layered immune systems, comprised of cell surface pattern recognition receptors (PRRs) and intracellular immune receptors, largely members of the nucleotide binding and leucine-rich repeat containing (NLR) family. The emerging paradigm is that these receptors can function in networks to detect invading pathogens and activate immunity. Understanding how plant immune receptor networks function and how they have evolved to counteract pathogens and pests will enable the development of novel strategies for disease resistance breeding.
To date, most cloned plant disease resistance genes encode for NLR proteins. NLRs perceive pathogen secreted molecules, termed effectors, and initiate a robust immune response, known as NLR-triggered immunity (NTI). In the Solanaceae plant family, a major phylogenetic clade of NLRs form a complex immunoreceptor network that mediates immunity to diverse pathogens (i.e viruses, bacteria, oomycetes, nematodes, and insects). In this network, NLRs are functionally specialised as sensors that detect pathogen effectors, or helpers, known as NLR-Required for Cell death (NRCs), which trigger downstream immune responses. NRCs form redundant central nodes within the network and display distinct and overlapping sensor specificities. The NRC network emerged ~100 million years ago and includes up to half of all NLRs in some plant species.
We aim to functionally characterise the NRC network and determine the molecular basis of NLR network mediated immunity. Mapping NLR network architectures will boost our capability to breed for disease resistance against multiple diverse pathogens. We will apply our findings to enhance resistance to economically important Solanaceous crops (e.g. potato) against pathogens that infect them (e.g. potato cyst nematode).
How do sensor and helper NLRs function together?
NRCs are core signalling hubs within the NLR network. They function in a partially redundant manner with a large number of sensor NLRs to mediate resistance against diverse pathogens. The determinants of sensor – helper specificity in NLR networks are unknown. We will carry out comparative genetic and biochemical studies of orthologous sensor and orthologous helper NLRs that carry distinct specificity spectrums. We will use this knowledge to decipher the molecular mechanisms that underpin sensor – helper interactions and determine how these NLRs function together to confer disease resistance.
How do NLR networks contribute to immunity in roots?
Roots are major plant organs, they encounter the highest density of microorganisms and represent important entryways for soilborne pathogens. Despite this, our knowledge of how the root immune system functions to initiate defence responses is lacking. We aim to determine how the NLR network mediates immunity to root infecting pathogens by studying the role of root-specific helper NLR hubs. These helper hubs are highly and often exclusively expressed in plant root tissue, however, their role in mediating immunity to root infecting pathogens are unknown.
How do pathogens interfere with NLR network mediated immunity?
Pathogens secrete effectors that modulate plant processes in order to facilitate host infection and colonisation. Some of these effectors inadvertently activate the plant immune system by being perceived by NLRs, resulting in NTI. A subset of effectors can function as suppressors of immunity and promote virulence leading to host susceptibility. Our recent work revealed that pathogens can target the NRC network by suppressing NTI (Derevnina et al 2021). These effectors proved to be fantastic molecular probes and provided insights into how the NLR network functions. Studying effectors with immunosuppression activity can, therefore, help improve our understanding of how pathogens successfully overcome NLR network mediated immunity and lead to the identification of novel components and processes involved in the plant immune system. We will decrypt the biochemical activities of effector suppressors to improve our understanding of the functional principles and evolutionary dynamics that underpin plant immune receptor networks. This knowledge can then be leveraged to guide new approaches for disease resistance breeding to maximise crop protection, for example, by engineering NLRs that evade pathogen suppression.
