Animal NLRs provide structural insights into plant NLR function

Since the characterisation of the gene-for-gene concept in plant disease resistance by Flor in the 1940s, resistance (R) genes have played a central role in breeding resistant crops. Gene-for-gene resistance is now known to be the result of the effector-triggered immunity response in plants. Molecular characterisation of R proteins reveals that they function analogously to the intracellular NOD-like receptors (NLRs) of the animal innate immune system, and are therefore typically referred to as plant NLR proteins.

Model for signalling by cooperative assembly formation (SCAF) of plant NLRs.
Model for signalling by cooperative assembly formation (SCAF) of plant NLRs. When unchallenged by pathogen effectors, plant NLRs exist in equilibrium between a closed inactive conformation (stabilized by ADP binding) and an open activated conformation, with the equilibrium strongly skewed towards the former. Both ATP and effector (or effector-induced elicitor in the case of indirect effector recognition) binding stabilize the active conformation, but only when both ATP and effector are bound, the equilibrium shifts sufficiently towards the active conformation to cause downstream events to take place. The active conformer presents new interfaces supporting oligomerization, and the NLRs are able to oligomerize. In analogy with NAIP/NLRC4, a small proportion of active NLRs can seed the conformational transition of further inactive NLRs to the active conformation and allow them to participate in the oligomerization, leading to a cooperative assembly of the resistosome. The downstream adaptors and ‘effector enzymes’ have not been identified in plant systems at this stage.

Bentham et al. compare the structural and functional characteristics of plant and animal NLRs, and draw upon the more extensive structural information available for animal NLRs to help reconcile the current structural and biochemical knowledge available for plant NLRs.

This paper is part of the Annals of Botany Special Issue on Plant Immunity. It will be free access till June 2017 and after April 2018.

Reference List

Bentham, A., Burdett, H., Anderson, P. A., Williams, S. J., & Kobe, B. (2016). Animal NLRs provide structural insights into plant NLR function. Annals of Botany, mcw171. https://doi.org/10.1093/aob/mcw171