Allelic variation in the Arabidopsis TNL CHS3/CSA1 immune receptor pair reveals two functional cell-death regulatory modes

A comprehensive review of soybean RNL and TIR domain proteins

Both prokaryotic and eukaryotic organisms use the nucleotide-binding domain/leucine-rich repeat (NBD/LRR)-triggered immunity (NLR-triggered immunity) signaling pathway to defend against pathogens. Plant NLRs are intracellular immune receptors that can bind to effector proteins secreted by pathogens. Dicotyledonous plants express a type of NLR known as TIR domain-containing NLRs (TNLs). TIR domains are enzymes that catalyze the production of small molecules that are essential for immune signaling and lead to plant cell death. The activation of downstream TNL signaling components, such as enhanced disease susceptibility 1 (EDS1), phytoalexin deficient 4 (PAD4), and senescence-associated gene 101 (SAG101), is facilitated by these small molecules. Helper NLRs (hNLRs) and the EDS1-PAD4/SAG101 complex associate after activation, causing the hNLRs to oligomerize, translocate to the plasma membrane (PM), and produce cation-selective channels. According to a recent theory, cations enter cells through pores created by oligomeric hNLRs and trigger cell death. Occasionally, TNLs can self-associate to create higher-order oligomers. Here, we categorized soybean TNLs based on the protein domains that they possess. We believe that TNLs may help soybean plants effectively fight pathogens by acting as a source of genetic resistance. In summary, the purpose of this review is to elucidate the range of TNLs that are expressed in soybean.

An NLR paralog Pit2 generated from tandem duplication of Pit1 fine-tunes Pit1 localization and function

NLR family proteins act as intracellular receptors. Gene duplication amplifies the number of NLR genes, and subsequent mutations occasionally provide modifications to the second gene that benefits immunity. However, evolutionary processes after gene duplication and functional relationships between duplicated NLRs remain largely unclear. Here, we report that the rice NLR protein Pit1 is associated with its paralogue Pit2. The two are required for the resistance to rice blast fungus but have different functions: Pit1 induces cell death, while Pit2 competitively suppresses Pit1-mediated cell death. During evolution, the suppression of Pit1 by Pit2 was probably generated through positive selection on two fate-determining residues in the NB-ARC domain of Pit2, which account for functional differences between Pit1 and Pit2. Consequently, Pit2 lost its plasma membrane localization but acquired a new function to interfere with Pit1 in the cytosol. These findings illuminate the evolutionary trajectory of tandemly duplicated NLR genes after gene duplication.

Development of an NLR-ID Toolkit and Identification of Novel Disease-Resistance Genes in Soybean

The recognition of pathogen effectors through the nucleotide-binding leucine-rich repeat receptor (NLR) family is an important component of plant immunity. In addition to typical domains such as TIR, CC, NBS, and LRR, NLR proteins also contain some atypical integrated domains (IDs), the roles of which are rarely investigated. Here, we carefully screened the soybean (Glycine max) genome and identified the IDs that appeared in the soybean TNL-like proteins. Our results show that multiple IDs (36) are widely present in soybean TNL-like proteins. A total of 27 Gm-TNL-ID genes (soybean TNL-like gene encoding ID) were cloned and their antiviral activity towards the soybean mosaic virus (SMV)/tobacco mosaic virus (TMV) was verified. Two resistance (R) genes, SRA2 (SMV resistance gene contains AAA_22 domain) and SRZ4 (SMV resistance gene contains zf-RVT domain), were identified to possess broad-spectrum resistance characteristics towards six viruses including SMV, TMV, plum pox virus (PPV), cabbage leaf curl virus (CaLCuV), barley stripe mosaic virus (BSMV), and tobacco rattle virus (TRV). The effects of Gm-TNL-IDX (the domain of the Gm-TNL-ID gene after the TN domain) on the antiviral activity of a R protein SRC7TN (we previously reported the TN domain of the soybean broad-spectrum resistance gene SRC7) were validated, and most of Gm-TNL-IDX inhibits antiviral activity mediated by SRC7TN, possibly through intramolecular interactions. Yeast-two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that seven Gm-TNL-IDX interacted with SMV-component proteins. Truncation analysis on a broad-spectrum antiviral protein SRZ4 indicated that SRZ4TIR is sufficient to mediate antiviral activity against SMV. Soybean cDNA library screening on SRZ4 identified 48 interacting proteins. In summary, our results indicate that the integration of IDs in soybean is widespread and frequent. The NLR-ID toolkit we provide is expected to be valuable for elucidating the functions of atypical NLR proteins in the plant immune system and lay the foundation for the development of engineering NLR for plant-disease control in the future.

Paired plant immune CHS3-CSA1 receptor alleles form distinct hetero-oligomeric complexes

Plant intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) analyzed to date oligomerize and form resistosomes upon activation to initiate immune responses. Some NLRs are encoded in tightly linked co-regulated head-to-head genes whose products function together as pairs. We uncover the oligomerization requirements for different Arabidopsis paired CHS3-CSA1 alleles. These pairs form resting-state heterodimers that oligomerize into complexes distinct from NLRs analyzed previously. Oligomerization requires both conserved and allele-specific features of the respective CHS3 and CSA1 Toll-like interleukin-1 receptor (TIR) domains. The receptor kinases BAK1 and BIRs inhibit CHS3-CSA1 pair oligomerization to maintain the CHS3-CSA1 heterodimer in an inactive state. Our study reveals that paired NLRs hetero-oligomerize and likely form a distinctive "dimer of heterodimers" and that structural heterogeneity is expected even among alleles of closely related paired NLRs.

Unlocking Nature's Defense: Plant Pattern Recognition Receptors as Guardians Against Pathogenic Threats

Embedded in the plasma membrane of plant cells, receptor kinases (RKs) and receptor proteins (RPs) act as key sentinels, responsible for detecting potential pathogenic invaders. These proteins were originally characterized more than three decades ago as disease resistance (R) proteins, a concept that was formulated based on Harold Flor's gene-for-gene theory. This theory implies genetic interaction between specific plant R proteins and corresponding pathogenic effectors, eliciting effector-triggered immunity (ETI). Over the years, extensive research has unraveled their intricate roles in pathogen sensing and immune response modulation. RKs and RPs recognize molecular patterns from microbes as well as dangers from plant cells in initiating pattern-triggered immunity (PTI) and danger-triggered immunity (DTI), which have intricate connections with ETI. Moreover, these proteins are involved in maintaining immune homeostasis and preventing autoimmunity. This review showcases seminal studies in discovering RKs and RPs as R proteins and discusses the recent advances in understanding their functions in sensing pathogen signals and the plant cell integrity and in preventing autoimmunity, ultimately contributing to a robust and balanced plant defense response. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2024.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Interplay between cell-surface receptor and intracellular NLR-mediated immune responses

The functional link between cell-surface receptors and intracellular NLR immune receptors is a critical aspect of plant immunity. To establish disease, successful pathogens have evolved mechanisms to suppress cell-surface immune signalling. In response, plants have adapted by evolving NLRs that recognize pathogen effectors involved in this suppression, thereby counteracting their immune-suppressing function. This ongoing co-evolutionary struggle has seemingly resulted in intertwined signalling pathways in some plant species, where NLRs form a separate signalling branch downstream of activated cell-surface receptor complexes essential for full immunity. Understanding these interconnected receptor networks could lead to novel strategies for developing durable disease resistance.

NLR receptors in plant immunity: making sense of the alphabet soup

Plants coordinately use cell-surface and intracellular immune receptors to perceive pathogens and mount an immune response. Intracellular events of pathogen recognition are largely mediated by immune receptors of the nucleotide binding and leucine rich-repeat (NLR) classes. Upon pathogen perception, NLRs trigger a potent broad-spectrum immune reaction, usually accompanied by a form of programmed cell death termed the hypersensitive response. Some plant NLRs act as multifunctional singleton receptors which combine pathogen detection and immune signaling. However, NLRs can also function in higher order pairs and networks of functionally specialized interconnected receptors. In this article, we cover the basic aspects of plant NLR biology with an emphasis on NLR networks. We highlight some of the recent advances in NLR structure, function, and activation and discuss emerging topics such as modulator NLRs, pathogen suppression of NLRs, and NLR bioengineering. Multi-disciplinary approaches are required to disentangle how these NLR immune receptor pairs and networks function and evolve. Answering these questions holds the potential to deepen our understanding of the plant immune system and unlock a new era of disease resistance breeding.

BAK-up: the receptor kinase BAK-TO-LIFE 2 enhances immunity when BAK1 is lacking

BRI1-ASSOCIATED KINASE 1 (BAK1/SERK3) and its closest homolog BAK1-LIKE 1 (BKK1/SERK4) are leucine-rich repeat receptor kinases (LRR-RKs) belonging to the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family. They act as co-receptors of various other LRR-RKs and participate in multiple signaling events by complexing and transphosphorylating ligand-binding receptors. Initially identified as the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) co-receptor, BAK1 also functions in plant immunity by interacting with pattern recognition receptors. Mutations in BAK1 and BKK1 cause severely stunted growth and cell death, characterized as autoimmune cell death. Several factors play a role in this type of cell death, including RKs and components of effector-triggered immunity (ETI) signaling pathways, glycosylation factors, ER quality control components, nuclear trafficking components, ion channels, and Nod-like receptors (NLRs). The Shan lab has recently discovered a novel RK BAK-TO-LIFE 2 (BTL2) that interacts with BAK1 and triggers cell death in the absence of BAK1 and BKK1. This RK compensates for the loss of BAK1-mediated pattern-triggered immunity (PTI) by activating phytocytokine-mediated immune and cell death responses.

NLR signaling in plants: from resistosomes to second messengers

Nucleotide binding and leucine-rich repeat-containing receptors (NLRs) have a critical role in plant immunity through direct or indirect recognition of pathogen effectors. Recent studies have demonstrated that such recognition induces formation of large protein complexes called resistosomes to mediate NLR immune signaling. Some NLR resistosomes activate Ca2+ influx by acting as Ca2+-permeable channels, whereas others function as active NADases to catalyze the production of nucleotide-derived second messengers. In this review we summarize these studies on pathogen effector-induced assembly of NLR resistosomes and resistosome-mediated production of the second messengers of Ca2+ and nucleotide derivatives. We also discuss downstream events and regulation of resistosome signaling.

The integrated LIM-peptidase domain of the CSA1-CHS3/DAR4 paired immune receptor detects changes in DA1 peptidase inhibitors in Arabidopsis

White blister rust, caused by the oomycete Albugo candida, is a widespread disease of Brassica crops. The Brassica relative Arabidopsis thaliana uses the paired immune receptor complex CSA1-CHS3/DAR4 to resist Albugo infection. The CHS3/DAR4 sensor NLR, which functions together with its partner, the helper NLR CSA1, carries an integrated domain (ID) with homology to DA1 peptidases. Using domain swaps with several DA1 homologs, we show that the LIM-peptidase domain of the family member CHS3/DAR4 functions as an integrated decoy for the family member DAR3, which interacts with and inhibits the peptidase activities of the three closely related peptidases DA1, DAR1, and DAR2. Albugo infection rapidly lowers DAR3 levels and activates DA1 peptidase activity, thereby promoting endoreduplication of host tissues to support pathogen growth. We propose that the paired immune receptor CSA1-CHS3/DAR4 detects the actions of a putative Albugo effector that reduces DAR3 levels, resulting in defense activation.

Plant immunity in soybean: progress, strategies, and perspectives

Soybean (Glycine max) is one of the most important commercial crops worldwide. Soybean hosts diverse microbes, including pathogens that may cause diseases and symbionts that contribute to nitrogen fixation. Study on soybean-microbe interactions to understand pathogenesis, immunity, and symbiosis represents an important research direction toward plant protection in soybean. In terms of immune mechanisms, current research in soybean lags far behind that in the model plants Arabidopsis and rice. In this review, we summarized the shared and unique mechanisms involved in the two-tiered plant immunity and the virulence function of pathogen effectors between soybean and Arabidopsis, providing a molecular roadmap for future research on soybean immunity. We also discussed disease resistance engineering and future perspectives in soybean.

A phospho-switch constrains BTL2-mediated phytocytokine signaling in plant immunity

Enabling and constraining immune activation is of fundamental importance in maintaining cellular homeostasis. Depleting BAK1 and SERK4, the co-receptors of multiple pattern recognition receptors (PRRs), abolishes pattern-triggered immunity but triggers intracellular NOD-like receptor (NLR)-mediated autoimmunity with an elusive mechanism. By deploying RNAi-based genetic screens in Arabidopsis, we identified BAK-TO-LIFE 2 (BTL2), an uncharacterized receptor kinase, sensing BAK1/SERK4 integrity. BTL2 induces autoimmunity through activating Ca2+ channel CNGC20 in a kinase-dependent manner when BAK1/SERK4 are perturbed. To compensate for BAK1 deficiency, BTL2 complexes with multiple phytocytokine receptors, leading to potent phytocytokine responses mediated by helper NLR ADR1 family immune receptors, suggesting phytocytokine signaling as a molecular link connecting PRR- and NLR-mediated immunity. Remarkably, BAK1 constrains BTL2 activation via specific phosphorylation to maintain cellular integrity. Thus, BTL2 serves as a surveillance rheostat sensing the perturbation of BAK1/SERK4 immune co-receptors in promoting NLR-mediated phytocytokine signaling to ensure plant immunity.

Dual function of the CHS3-CSA1 immune receptor pair

Plant nucleotide-binding leucine-rich repeat (NLR) receptors mediate specific recognition of pathogen effectors to initiate effector-triggered immunity. Recently, studies by Schulze et al., Yang et al., and Gu et al. collectively show that the Arabidopsis (Arabidopsis thaliana) NLR pair CHS3-CSA1 acts through two distinct activation modes to recognize different pathogen effectors, thus revealing the dual function of the CHS3-CSA1 pair in plant disease resistance.

Guarding the central regulator of extracellular perception in plants - A job for two

BAK1 is a central regulator of extracellular receptor proteins, essential in plant development and defense. In this issue of Cell Host & Microbe, dual reports (Schultze et al. and Yang et al.) describe how intracellular NLR immune receptors guard BAK1, with implications for extracellular perception and immune receptor engineering.

The Arabidopsis TIR-NBS-LRR protein CSA1 guards BAK1-BIR3 homeostasis and mediates convergence of pattern- and effector-induced immune responses

Arabidopsis BAK1/SERK3, a co-receptor of leucine-rich repeat pattern recognition receptors (PRRs), mediates pattern-triggered immunity (PTI). Genetic inactivation of BAK1 or BAK1-interacting receptor-like kinases (BIRs) causes cell death, but the direct mechanisms leading to such deregulation remains unclear. Here, we found that the TIR-NBS-LRR protein CONSTITUTIVE SHADE AVOIDANCE 1 (CSA1) physically interacts with BIR3, but not with BAK1. CSA1 mediates cell death in bak1-4 and bak1-4 bir3-2 mutants via components of effector-triggered immunity-(ETI) pathways. Effector HopB1-mediated perturbation of BAK1 also results in CSA1-dependent cell death. Likewise, microbial pattern pg23-induced cell death, but not PTI responses, requires CSA1. Thus, we show that CSA1 guards BIR3 BAK1 homeostasis and integrates pattern- and effector-mediated cell death pathways downstream of BAK1. De-repression of CSA1 in the absence of intact BAK1 and BIR3 triggers ETI cell death. This suggests that PTI and ETI pathways are activated downstream of BAK1 for efficient plant immunity.