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Title: Mechanisms of NB-LRR disease resistance protein function.
The goals of this work are to understand the mechanism of activation and sub-cellular localization of the NB-LRR proteins RPM1 and ADR1, and the novel TIR domain only protein RBA1.
Project Summary: The long-term goal of this project is to understand the mechanisms by which the intracellular innate immune receptors are activated and how they function, once active, to initiate successful disease resistance responses. We focus on the intracellular receptors of the NLR protein superfamily which are critical for pathogen detection in the innate immune systems of both plants and animals. NLRs typically contain one of several N-terminal signaling domains, a central nucleotide-binding domain with ATPase and/or GTPase function, and C-terminal leucine-rich repeats (LRRs). Originally discovered in plants in the mid-1990s, NLRs are the basis for disease resistance in plants and have been manipulated, unknowingly at first, by crop breeders as ‘disease resistance genes’ for over a century. NLRs were subsequently discovered in animals, where they play a major role in regulating innate immune signaling in infectious and autoimmune disease. Once activated, NLRs direct a complex output response that ultimately restricts pathogen growth.
Despite the central role of NLRs in innate immunity in plants and animals, there is to date no generalizable model describing how NLRs transition from an inactive resting state to an active signaling state after recognition of microbial signals. Recent demonstrations that not all NLR proteins are activated in, or function in, the same manner suggests mechanistic differences. This necessitates a broad comparative experimental approach. Filling this gap is the critical unresolved research issue pertaining to NLR biology in both plant and animal innate immune research, and is the focus of our proposal.
Thus, our project is aimed at understanding particular mechanistic exemplars that span the wide gamut of NLR modularity in order to illuminate the core principles underlying NLR activation, specifically: (1) as a naturally occurring effector-activated minimal TIR only domain (RBA1); (2) as a paradigmatic guardee and microbial sensor NLR (RIN4 and RPM1); and (3) as a helper NLR that uses a canonical mechanism to control cell death but a non-canonical mechanism to enhance sensor NLR function and contribute to the control of salicylic acid levels (ADR1-L2).
Broader impacts: Plants are fertile sources of nutrients for a variety of microbes. Many of these reduce plant fitness and productivity, and hence are pathogens. Plant pathogens devastate crops, particularly in developing areas where expensive (and often unsustainable) fungicides and pesticides are beyond the economic reach of most farmers. Yield losses due to plant disease are also ‘water losses’, since that resource is often invested before disease decimates a crop. Hence, successfully combating plant diseases through rational deployment of the plant immune system will contribute directly to human and environmental health, and save lots of water.
A mechanistic understanding of NLR function is a prerequisite for rational deployment of the plant immune system in crops and for the development of treatments for various human diseases. This requires the combined efforts of plant and animal innate immunologists who seek to understand how the NLR proteins of each kingdom function. Plant NLR receptors are used by essentially all land plants to sense and respond to pathogen attack. Similarly, the animal NLR receptors respond to microbial signals in hosts as diverse as sea urchins and humans. Researchers investigating NLR experimental systems in either kingdom recognize that it is now vital to understand how signal competent receptors are organized before infection, the precise mechanisms by which they are activated, and how this activation is translated in an appropriate output response. Hence, the broadest impacts of the proposed research project will significantly inform translation to crop species and to human health.