Recognition, Immune Activation and Signal Regulation of Plant NLR Immune Receptor

doi:10.11983/CBB21159

Abstract

Abstract: A large area of membrane surface and intracellular immune receptors have been evolved in higher plants to sense various pathogen signals and prevent pathogen invasion. Among them, pattern recognition receptors on the cell surface activate basic immune response after sensing pattern molecules, while nucleotide-bounding leucine-rich repeat proteins (NLRs) activate specific immune response by sensing effector proteins secreted by pathogenic microorganisms, resulting in hypersensitivity and cell death. In this review, the latest research progress of plant immunity is mainly reviewed from the aspects of NLRs on the recognition of effector proteins, plant immune activation and downstream signal regulation.

Key words: plant immunity, NLRs, receptor recognition, signal regulation

Lei Qin, Zhihong Peng, Shitou Xia. Recognition, Immune Activation and Signal Regulation of Plant NLR Immune Receptor[J]. Chinese Bulletin of Botany, 2022, 57(1): 12-23.

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URL: https://www.chinbullbotany.com/EN/10.11983/CBB21159

https://www.chinbullbotany.com/EN/Y2022/V57/I1/12

Figures/Tables 3

Figure 1 Structural composition of NLRs and its recognition pattern to effector proteins (modified from Duxbury et al., 2021) (A) The domains of plant NLRs are divided into three categories, including a central nucleotide-binding (NB) domain, a C-terminal leucine-rich repeats (LRR) region and N-terminal TIR, CC, or RPW8-like CC domain; (B) Different patterns of effector recognition by plant NLRs: Some plant NLRs directly bind to the corresponding effector proteins or indirectly detect the pathogen effector through the guardee or decoy proteins; Some plant NLRs have special integrated domains (ID) to mediate effector recognition.

Figure 2 Activation modes of two NLRs (modified from Duxbury et al., 2021) (A) Schematic diagram of CNL (ZAR1) resistosome formation. The Xanthomonas effector AvrAC uridylates the Arabidopsis thaliana kinase PBL2. Uridylated PBL2 (PBL2UMP) associates with the intracellular pre-formed ZAR1-RKS1 dimer. This leads to a conformational change of ZAR1 and replacement of adenosine diphosphate (ADP) by adenosine triphosphate or deoxyadenosine triphosphate ((d)ATP) in the nucleotide-binding site of the NBD of ZAR1. Ultimately, this results in the formation of a pentameric wheel-like ZAR1 resistosome, which is composed of five ZAR1-RKS1-PBL2UMPprotomers. (B) Schematic diagram of TNL (RPP1) resistosome formation. Direct recognition of a pathogen avirulence effector by the leucine-rich repeat (LRR) and carboxy-terminal domains (C-JID) of a canonical Toll/interleukin-1 receptor (TIR) domain-containing intracellular nucleotide- binding domain (NBD)-like receptor (TIR-type NLR) leads to the formation of a tetrameric structure with nicotinamide adenine dinucleotide glycohydrolase (NADase) activity.

Figure 3 Working models of activation of NLRs-mediated immunity in higher plants (modified from Liu et al., 2021) Upon an infection of a plant cell by a pathogen, some pathogens can secrete effectors to break through the immune defense line of plants. In the process of long-term evolution, plants have evolved many intracellular receptors to recognize these effectors, so as to promote resistance to pathogens. CNLs triggers pentamerization and resistosome formation on the plasma membrane (PM) through sensing effector (The example here depicts a ZAR1 resistosome which indirect recognition effector assembly). The pore formed by the N-terminal CC α1 helices serves as a Ca2+ influx channel, mediating increase of cytosolic Ca2+ concentration and turning on cell death and defense responses. TNLs, upon perception of effectors (the model here depicts an example of direct effector-receptor recognition as with RPP1), formation of the TNL resistosome leads to activation of TIR NADase activity, triggering assembly of oligomeric complexes presumably containing EDS1-PAD4-ADR1s or EDS1-SAG101-NRG1s. The oligomerization of the helper NLRs enables a similar pore formation as CNLs, serving as Ca2+ influx channels to mediate downstream immunity and cell death.The red arrows indicate CNL signal, and the blue arrows indicate TNL and RNL signals.

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