Molecular insights into the biochemical functions and signalling mechanisms of plant NLRs

Abstract

Plant intracellular immune receptors known as NLR (nucleotide-binding leucine-rich repeat) proteins confer immunity and cause cell death. Plant NLR proteins that directly or indirectly recognize pathogen effector proteins to initiate immune signalling are regarded as sensor NLRs. Some NLR protein families function downstream of sensor NLRs to transduce immune signalling and are known as helper NLRs. Recent breakthrough studies on plant NLR protein structures and biochemical functions greatly advanced our understanding of NLR biology. Comprehensive and detailed knowledge on NLR biology requires future efforts to solve more NLR protein structures and investigate the signalling events between sensor and helper NLRs, and downstream of helper NLRs.

Keywords: cell death; immune receptors; pathogen effector; plant immunity; signal transduction.

FIGURE 1

Schematic diagram depicting the domain structures of sensor NLRs and helper NLRs and their interdependence for function. TIR, toll/interleukin‐1 receptor; CC, coiled‐coil; CC^R, (Resistance to Powdery Mildew 8)‐like CC; NB, nucleotide‐binding; LRR, leucine‐rich repeat; ID, integrated decoy

FIGURE 2

Current structure knowledge on NLR recognition of effectors. (a) Cryoelectron microscopy (cryo‐EM) structure of RPP1 LRR and C‐JID domains in a complex with effector ATR1 (PDB: 7CRB). (b) Cryo‐EM structure of ROQ1 LRR and C‐JID domains in a complex with effector XopQ (PDB: 7JLU). (c) Crystal structure of RRS1 WRKY domain in a complex with effector AvrRPS4 (PDB: 7P8K). (d) Crystal structure of Pik HMA domain in a complex with effector AvrPik (PDB: 5A6W). (e) Super‐imposition of the ADP‐bound ZAR1 structure in the resting state (6J5W) and the ADP‐free ZAR1 structure in the intermediate state (6J5V). Binding of effector‐produced PBL2^UMP causes RKS1 conformational changes (highlighted in purple) that clash with and push away the NB domain. The NB domain is shown in two colours (cyan and pink) to highlight the close‐to‐open switch that allows allosteric ADP release

FIGURE 3

Plant TIR dimerization interfaces and higher oligomer formation. (a) Crystal structure of the RUN1 TIR domain highlighting filament formation through continuation of alternating AE and DE interfaces. (b) Structure of RPP1 tetrameric resistosome highlighting the TIR tetramer including two symmetrical AE dimerization interfaces and two asymmetric interfaces as NADase catalytic sites

FIGURE 4

Conserved four‐helical‐bundle (4HB) fold structure of plant NLR CC and CC^R domains, and the dynamics of ZAR1 resistosome formation. (a) Superimposition of AtZAR1 CC in green (PDB: 6J5W), Sr33 CC in teal (PDB: 2NCG), AtNRG1.1 CC^R in yellow (PDB: 7L7W), and mouse MLKL 4HB in pink (PDB: 4BTF). (b) Structure of AtZAR1 (PDB: 6J5W) in the resting state. The four helices of the CC domain are coloured in purple, orange, green, and blue. The loop region connecting α4A and NB is shown as a curved dashed line in red, which becomes a helix termed α4B in the active state. (c) Structure of active AtZAR1 (PDB: 6J5T) shown as a monomer from the pentameric resistosome highlighting the α1 helix being flipped and penetrating the plasma membrane (PM). The α4A in the resting state becomes a potentially flexible loop region not resolved in the electron density map and is indicated as a curved dashed line in blue. The newly formed helix α4B is shown in red. (d) Structure of AtZAR1 resistosome (PDB: 6J5T) highlighting the channel pore with a diameter of about 1 nm