Divergent recruitment of disease resistance proteins to chloroplasts or pathogen interface

Plant pathogens threaten agricultural productivity and food quality, posing a clear and present danger to our food systems. Filamentous plant pathogens alone account for 10-80% of global crop losses, enough to feed several billion people. Outbreaks caused by plant diseases have increased in frequency due to global trade, climate change, and the propensity of plant pathogens to break down the disease resistance that had been painstakingly bred into crop plants.

Although plants have the genetic toolkit to fight diseases, the capacity of pathogens to adapt and evade plant immunity has constrained traditional resistance breeding. Plants prevent parasitism through a sophisticated immune system that relies on timely detection of invaders through specialized immune sensors encoded by plant disease resistance (R) genes. Yet, some pathogens evade detection by the R proteins, limiting the potency of these immune sensors in agriculture. The leading strategy to genetically improve crop resistance is by transferring R genes from wild crop relatives into elite cultivars. This is primarily due to the exceptional potency of these genes in preventing parasitic infestation. However, limited availability of R genes that operate against newly emerging pathogen races and gaps in our fundamental understanding of R gene function, hinder the success of disease resistance breeding in crop plants. Our long-term goal is to decipher the mechanisms underpinning R gene mediated immunity in order to develop guiding principles for breeding plant disease resistance into crop plants.

Nucleotide-binding domain leucine-rich repeat (NLR) immune receptors are the most abundant class of disease resistance proteins that have been persistently employed in breeding disease resistant crops. Recent breakthroughs revealed that activated NLRs form oligomeric structures which insert into cellular membranes, making microscopic pores to trigger form of programmed cell death as a defense mechanism. Despite these advances, our knowledge in NLR mode of action during infection by relevant pathogens is still limited, which constrains their potential use in agriculture. We recently made an exciting discovery of how NLRs behave during live cell infection by the Irish potato famine pathogen Phytophthora infestans. Our work revealed that an NLR navigates to pathogen invasion sites (plant-pathogen interface) and upon activation further spreads to other cellular membranes, possibly to accelerate the immune response. We now discovered another NLR type of resistance protein that targets the chloroplasts, the Photosynthetic organelles that generates energy within the plant cells.

These finding provide a proof of concept that NLRs are mobile disease detectors that can propagate defense signals to distant cellular compartments and enhance the effectiveness of the immune response.


In this proposal, we aim to understand the molecular mechanisms of divergent trafficking of NLRs towards infection sites or the chloroplasts and investigate how the activated NLR ultimately execute the immune response leading disease resistance. We have collected exciting preliminary data that supports our view that multidirectional NLR trafficking pre-and post-activation during infection is functionally relevant to modulate the strength of the immune response to eliminate infectious agents. By decrypting these mechanisms, we will generate fundamental knowledge that will be helpful to remodel plant immune system towards improved pathogen resistance. This work will have far-reaching implications, as the NLR proteins that we work are key members of disease resistance networks, providing resistance to a diversity of agronomically important pathogens and pests.

Nucleotide-binding domain leucine-rich repeat (NLR) immune receptors have been persistently employed in breeding disease resistant crops due to their exceptional potency in limiting infections. Activated NLRs trigger a cell death response known as the hypersensitive response (HR) that restricts infections. Ground-breaking research revealed that many NLRs work as singletons, in pairs or in networks. In network model, sensor NLRs, specialized to sense pathogens, are coupled to a helper NLR that executes the immune response. The emerging paradigm is that activated helper/singleton NLRs undergo conformational changes by oligomerizing into pentameric structures called the resistosomes, which can insert into plasma membrane to trigger HR. We recently discovered that a helper NLR, NRC4, accumulates at the specialised plant derived membrane-the extrahaustorial membrane (EHM)-that surrounds the pathogen haustorium. Activated NRC4 formed resistosome-like structures inserting not only to the EHM but also to the plasma membrane, revealing that NLRs can propagate immune signals to distinct membranes. We now made an exciting discovery that an activated helper NLR targets the chloroplast outer membrane, revealing that activated NLRs execute immune functions by undertaking different trajectories. However, how, and why NLRs target different membrane interfaces are unknown. Here we propose to dissect the molecular mechanisms and functions of divergent trafficking of the helper NLRs to the EHM versus the chloroplasts during infection with relevant pathogens. We aim to functionally characterize the trafficking pathways and the components that mediate multidirectional NLR trafficking. These will provide novel insights into NLR mode of action during infection and shed light on the process of pathogen accommodation in plant cells. Ultimately, the knowledge and materials generated should help engineering of NLRs and their accessory components towards improved disease resistance.