The Structural Basis of Ligand Perception and Signal Activation by Receptor Kinases

N-glycosylation facilitates the activation of a plant cell-surface receptor

Plant receptor kinases (RKs) are critical for transmembrane signalling involved in various biological processes including plant immunity. MALE DISCOVERER1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) is a unique RK that recognizes a family of immunomodulatory peptides called SERINE-RICH ENDOGENOUS PEPTIDEs (SCOOPs) and activates pattern-triggered immunity responses. However, the precise mechanisms underlying SCOOP recognition and activation of MIK2 remain poorly understood. Here we present the cryogenic electron microscopy structure of a ternary complex consisting of the extracellular leucine-rich repeat (LRR) of MIK2 (MIK2LRR), SCOOP12 and the extracellular LRR of the co-receptor BAK1 (BAK1LRR) at a resolution of 3.34 Å. The structure reveals that a DNHH motif in MIK2LRR plays a critical role in specifically recognizing the highly conserved SxS motif of SCOOP12. Furthermore, the structure demonstrates that N-glycans at MIK2LRRAsn410 directly interact with the N-terminal capping region of BAK1LRR. Mutation of the glycosylation site, MIK2LRRN410D, completely abolishes the SCOOP12-independent interaction between MIK2LRR and BAK1LRR and substantially impairs the assembly of the MIK2LRR-SCOOP12-BAK1LRR complex. Supporting the biological relevance of N410-glycosylation, MIK2N410D substantially compromises SCOOP12-triggered immune responses in plants. Collectively, these findings elucidate the mechanism underlying the loose specificity of SCOOP recognition by MIK2 and reveal an unprecedented mechanism by which N-glycosylation modification of LRR-RK promotes receptor activation.

Mechanistic study of SCOOPs recognition by MIK2-BAK1 complex reveals the role of N-glycans in plant ligand-receptor-coreceptor complex formation

Ligand-induced receptor and co-receptor heterodimerization is a common mechanism in receptor kinase (RK) signalling activation. SERINE-RICH ENDOGENOUS PEPTIDEs (SCOOPs) mediate the complex formation of Arabidopsis RK MIK2 and co-receptor BAK1, triggering immune responses. Through structural, biochemical and genetic analyses, we demonstrate that SCOOPs use their SxS motif and adjacent residues to bind MIK2 and the carboxy-terminal GGR residues to link MIK2 to BAK1. While N-glycosylation of plant RKs is typically associated with protein maturation, plasma membrane targeting and conformation maintenance, a surprising revelation emerges from our crystal structural analysis of MIK2-SCOOP-BAK1 complexes. Specific N-glycans on MIK2 directly interact with BAK1 upon SCOOP sensing. The absence of N-glycosylation at the specific site in MIK2 neither affects its subcellular localization and protein accumulation in plant cells nor alters its structural conformation, but markedly reduces its affinity for BAK1, abolishing SCOOP-triggered immune responses. This N-glycan-mediated receptor and co-receptor heterodimerization occurs in both Arabidopsis and Brassica napus. Our findings elucidate the molecular basis of SCOOP perception by the MIK2-BAK1 immune complex and underscore the crucial role of N-glycans in plant receptor-coreceptor interactions and signalling activation, shaping immune responses.

A conserved juxtamembrane motif in plant NFR5 receptors is essential for root nodule symbiosis

Establishment of root nodule symbiosis is initiated by the perception of bacterial Nod factor ligands by the plant LysM receptor kinases NFR1 and NFR5. Receptor signaling initiating the symbiotic pathway depends on the kinase activity of NFR1, while the signaling mechanism of the catalytically inactive NFR5 pseudokinase is unknown. Here, we present the crystal structure of the signaling-competent Lotus japonicus NFR5 intracellular domain, comprising the juxtamembrane region and pseudokinase domain. The juxtamembrane region is structurally well defined and forms two α-helices, αA and αA', which contain an exposed hydrophobic motif. We demonstrate that this "juxtamembrane motif" promotes NFR5-NFR5 and NFR1-NFR5 interactions and is essential for symbiotic signaling. Conservation analysis reveals that the juxtamembrane motif is present throughout NFR5-type receptors and is required for symbiosis signaling from barley RLK10, suggesting a conserved and broader function for this motif in plant-microbe symbioses.

Plant pattern recognition receptors: from evolutionary insight to engineering

The plant immune system relies on germline-encoded pattern recognition receptors (PRRs) that sense foreign and plant-derived molecular patterns, and signal health threats. Genomic and pangenomic data sets provide valuable insights into the evolution of PRRs and their molecular triggers, which is furthering our understanding of plant-pathogen co-evolution and convergent evolution. Moreover, in silico and in vivo methods of PRR identification have accelerated the characterization of receptor-ligand complexes, and advances in protein structure prediction algorithms are revealing novel PRR sensor functions. Harnessing these recent advances to engineer PRRs presents an opportunity to enhance plant disease resistance against a broad spectrum of pathogens, enabling more sustainable agricultural practices. This Review summarizes both established and innovative approaches to leverage genomic data and translate resulting evolutionary insights into engineering PRR recognition specificities.

Regulatory balance between ear rot resistance and grain yield and their breeding applications in maize and other crops

BACKGROUND

Fungi are prevalent pathogens that cause substantial yield losses of major crops. Ear rot (ER), which is primarily induced by Fusarium or Aspergillus species, poses a significant challenge to maize production worldwide. ER resistance is regulated by several small effect quantitative trait loci (QTLs). To date, only a few ER-related genes have been identified that impede molecular breeding efforts to breed ER-resistant maize varieties.

AIM OF REVIEW

Our aim here is to explore the research progress and mine genic resources related to ER resistance, and to propose a regulatory model elucidating the ER-resistant mechanism in maize as well as a trade-off model illustrating how crops balance fungal resistance and grain yield. Key Scientific Concepts of Review: This review presents a comprehensive bibliometric analysis of the research history and current trends in the genetic and molecular regulation underlying ER resistance in maize. Moreover, we analyzed and discovered the genic resources by identifying 162 environmentally stable loci (ESLs) from various independent forward genetics studies as well as 1391 conservatively differentially expressed genes (DEGs) that respond to Fusarium or Aspergillus infection through multi-omics data analysis. Additionally, this review discusses the syntenies found among maize ER, wheat Fusariumhead blight (FHB), and rice Bakanaedisease (RBD) resistance-related loci, along with the significant overlap between fungal resistance loci and reported yield-related loci, thus providing valuable insights into the regulatory mechanisms underlying the trade-offs between yield and defense in crops.

Redox regulation of meristem quiescence: outside/in

Quiescence is an essential property of meristematic cells, which restrains the cell cycle while retaining the capacity to divide. This crucial process not only facilitates life-long tissue homeostasis and regenerative capacity but also provides protection against adverse environmental conditions, enabling cells to conserve the proliferative potency while minimizing DNA damage. As a survival attribute, quiescence is inherently regulated by the products of aerobic life, in particular reactive oxygen species (ROS) and the redox (reduction/oxidation) mechanisms that plant have evolved to channel these into pervasive signals. Adaptive responses allow quiescent cells to compensate for reduced oxygen tension (hypoxia) in a reversible manner, while the regulated production of the superoxide anion (O2·-) facilitates cell division and the maintenance of stem cells. Here we discuss the role of ROS and redox reactions in the control of the quiescent state in plant meristems, and how this process is integrated with cellular energy and hormone biochemistry. We consider the pathways that sense and transmit redox signals with a focus on the central significance of redox regulation in the mitochondria and nucleus, which is a major regulator of quiescence in meristems. We discuss recent studies that suggest that ROS are a critical component of the feedback loops that control stem cell identity and fate, and suggest that the ROS/hypoxia interface is an important 'outside/in' positional cue for plant cells, particularly in meristems.

VfLRR-RLK1 benefiting resistance to Fusarium oxysporum reveals infection and defense mechanisms in tung tree

Fusarium wilt, caused by Fusarium oxysporum f. sp. fordiis in Vernicia fordii, manifests as severe symptoms that significantly reduce global tung oil yield. However, the molecular-mechanisms of the Vernicia-Fusarium interaction are yet to be fully elucidated. Here, we cloned VfLRR-RLK1 from tung tree roots, which contained 1134 bp, encoding 378 AA. To further analyze VfLRR-RLK1 function in resistance to Fusarium wilt, we obtained stable T4-generation transgenic Arabidopsis thaliana and tung tree VfLRR-RLK1 virus-induced gene silencing (VIGS) RNAi plants. A. thaliana plants overexpressing VfLRR-RLK1 exhibited more robust root development and markedly increased Fusarium wilt disease resistance. In response to Fusarium wilt stress, transgenic A. thaliana exhibited increased catalase (CAT) and superoxide dismutase (SOD) enzyme activities, while showing reduced O2 - and hydrogen peroxide (H2O2) accumulation. The findings suggest that VfLRR-RLK1 may diminish plant reactive oxygen species (ROS) levels and foster root development by activating the ROS antioxidant scavenging system during plant Pattern Triggered Immunity responses, enhancing resistance to Fusarium wilt. The study on the function of VfLRR-RLK1 is crucial in breeding programs aimed at developing tung tree resistant to Fusarium wilt, and lays the groundwork for more effective disease management strategies and the cultivation of tung tree varieties with enhanced resistance to this disease.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01512-y.

New kids on the block-cysteine-rich receptor-like kinases in pattern-triggered immunity

Plant-specific receptor-like protein kinases (RLKs) are essential for pathogen recognition during pattern-triggered immunity. Together with coreceptors and associated proteins, they act as bona fide immune receptors, perceiving a variety of microbe-associated molecular patterns or damage-associated molecular patterns. The cysteine-rich receptor-like kinases (CRKs) form one of the biggest subgroups of RLKs, but so far, their ligands have not been identified. It has been shown that CRKs play important roles in plant immunity and defense responses as well as in response to abiotic stimuli and in control of plant development. However, molecular information on how CRKs integrate with the known framework of signaling components controlling early defense responses remains enigmatic.

SERKs serve as co-receptors for SYR1 to trigger systemin-mediated defense responses in tomato

Systemin, the first peptide hormone identified in plants, was initially isolated from tomato (Solanum lycopersicum) leaves. Systemin mediates local and systemic wound-induced defense responses in plants, conferring resistance to necrotrophic fungi and herbivorous insects. Systemin is recognized by the leucine-rich-repeat receptor-like kinase (LRR-RLK) receptor SYSTEMIN RECEPTOR1 (SYR1), but how the systemin recognition signal is transduced to intracellular signaling pathways to trigger defense responses is poorly understood. Here, we demonstrate that SERK family LRR-RLKs function as co-receptors for SYR1 to mediate systemin signal transduction in tomato. By using chemical genetic approaches coupled with engineered receptors, we revealed that the association of the cytoplasmic kinase domains of SYR1 with SERKs leads to their mutual trans-phosphorylation and the activation of SYR1, which in turn induces a wide range of defense responses. Systemin stimulates the association between SYR1 and all tomato SERKs (SlSERK1, SlSERK3A, and SlSERK3B). The resulting SYR1-SlSERK heteromeric complexes trigger the phosphorylation of TOMATO PROTEIN KINASE 1B (TPK1b), a receptor-like cytoplasmic kinase that positively regulates systemin responses. Additionally, upon association with SYR1, SlSERKs are cleaved by the Pseudomonas syringae effector HopB1, further supporting the finding that SlSERKs are activated by systemin-bound SYR1. Finally, genetic analysis using Slserk mutants showed that SlSERKs are essential for systemin-mediated defense responses. Collectively, these findings demonstrate that the systemin-mediated association of SYR1 and SlSERKs activates defense responses against herbivorous insects.

Leveraging coevolutionary insights and AI-based structural modeling to unravel receptor-peptide ligand-binding mechanisms

Secreted signaling peptides are central regulators of growth, development, and stress responses, but specific steps in the evolution of these peptides and their receptors are not well understood. Also, the molecular mechanisms of peptide-receptor binding are only known for a few examples, primarily owing to the limited availability of protein structural determination capabilities to few laboratories worldwide. Plants have evolved a multitude of secreted signaling peptides and corresponding transmembrane receptors. Stress-responsive SERINE RICH ENDOGENOUS PEPTIDES (SCOOPs) were recently identified. Bioactive SCOOPs are proteolytically processed by subtilases and are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) in the model plant Arabidopsis thaliana. How SCOOPs and MIK2 have (co)evolved, and how SCOOPs bind to MIK2 are unknown. Using in silico analysis of 350 plant genomes and subsequent functional testing, we revealed the conservation of MIK2 as SCOOP receptor within the plant order Brassicales. We then leveraged AI-based structural modeling and comparative genomics to identify two conserved putative SCOOP-MIK2 binding pockets across Brassicales MIK2 homologues predicted to interact with the "SxS" motif of otherwise sequence-divergent SCOOPs. Mutagenesis of both predicted binding pockets compromised SCOOP binding to MIK2, SCOOP-induced complex formation between MIK2 and its coreceptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1, and SCOOP-induced reactive oxygen species production, thus, confirming our in silico predictions. Collectively, in addition to revealing the elusive SCOOP-MIK2 binding mechanism, our analytic pipeline combining phylogenomics, AI-based structural predictions, and experimental biochemical and physiological validation provides a blueprint for the elucidation of peptide ligand-receptor perception mechanisms.

Chemical genetics reveals cross-activation of plant developmental signaling by the immune peptide-receptor pathway

Cells sense and integrate multiple signals to coordinate development and defence. A receptor-kinase signaling pathway for plant stomatal development shares components with the immunity pathway. The mechanism ensuring their signal specificities remains unclear. Using chemical genetics, here we report the identification of a small molecule, kC9, that triggers excessive stomatal differentiation by inhibiting the canonical ERECTA receptor-kinase pathway. kC9 binds to and inhibits the downstream MAP kinase MPK6, perturbing its substrate interaction. Strikingly, activation of immune signaling by a bacterial flagellin peptide nullified kC9's effects on stomatal development. This cross-activation of stomatal development by immune signaling depends on the immune receptor FLS2 and occurs even in the absence of kC9 if the ERECTA-family receptor population becomes suboptimal. Furthermore, proliferating stomatal-lineage cells are vulnerable to the immune signal penetration. Our findings suggest that the signal specificity between development and immunity can be ensured by MAP Kinase homeostasis reflecting the availability of upstream receptors, thereby providing a novel view on signal specificity.

Molecular Lesions in BRI1 and Its Orthologs in the Plant Kingdom

Brassinosteroids (BRs) are an essential group of plant hormones regulating numerous aspects of plant growth, development, and stress responses. BRI1, along with its co-receptor BAK1, are involved in brassinosteroid sensing and early events in the BR signal transduction cascade. Mutational analysis of a particular gene is a powerful strategy for investigating its biochemical role. Molecular genetic studies, predominantly in Arabidopsis thaliana, but progressively in numerous other plants, have identified many mutants of the BRI1 gene and its orthologs to gain insight into its structure and function. So far, the plant kingdom has identified up to 40 bri1 alleles in Arabidopsis and up to 30 bri1 orthologs in different plants. These alleles exhibit phenotypes that are identical in terms of development and growth. Here, we have summarized bri1 alleles in Arabidopsis and its orthologs present in various plants including monocots and dicots. We have discussed the possible mechanism responsible for the specific allele. Finally, we have briefly debated the importance of these alleles in the research field and the agronomically valuable traits they offer to improve plant varieties.

Wound-induced small-peptide-mediated signaling cascade, regulated by OsPSKR, dictates balance between growth and defense in rice

Wounding is a general stress in plants that results from various pest and pathogenic infections in addition to environment-induced mechanical damages. Plants have sophisticated molecular mechanisms to recognize and respond to wounding, with those of monocots being distinct from dicots. Here, we show the involvement of two distinct categories of temporally separated, endogenously derived peptides, namely, plant elicitor peptides (PEPs) and phytosulfokine (PSK), mediating wound responses in rice. These peptides trigger a dynamic signal relay in which a receptor kinase involved in PSK perception named OsPSKR plays a major role. Perturbation of OsPSKR expression in rice leads to compromised development and constitutive autoimmune phenotypes. OsPSKR regulates the transitioning of defense to growth signals upon wounding. OsPSKR displays mutual antagonism with the OsPEPR1 receptor involved in PEP perception. Collectively, our work indicates the presence of a stepwise peptide-mediated signal relay that regulates the transition from defense to growth upon wounding in monocots.

Allosteric activation of the co-receptor BAK1 by the EFR receptor kinase initiates immune signaling

Transmembrane signaling by plant receptor kinases (RKs) has long been thought to involve reciprocal trans-phosphorylation of their intracellular kinase domains. The fact that many of these are pseudokinase domains, however, suggests that additional mechanisms must govern RK signaling activation. Non-catalytic signaling mechanisms of protein kinase domains have been described in metazoans, but information is scarce for plants. Recently, a non-catalytic function was reported for the leucine-rich repeat (LRR)-RK subfamily XIIa member EFR (elongation factor Tu receptor) and phosphorylation-dependent conformational changes were proposed to regulate signaling of RKs with non-RD kinase domains. Here, using EFR as a model, we describe a non-catalytic activation mechanism for LRR-RKs with non-RD kinase domains. EFR is an active kinase, but a kinase-dead variant retains the ability to enhance catalytic activity of its co-receptor kinase BAK1/SERK3 (brassinosteroid insensitive 1-associated kinase 1/somatic embryogenesis receptor kinase 3). Applying hydrogen-deuterium exchange mass spectrometry (HDX-MS) analysis and designing homology-based intragenic suppressor mutations, we provide evidence that the EFR kinase domain must adopt its active conformation in order to activate BAK1 allosterically, likely by supporting αC-helix positioning in BAK1. Our results suggest a conformational toggle model for signaling, in which BAK1 first phosphorylates EFR in the activation loop to stabilize its active conformation, allowing EFR in turn to allosterically activate BAK1.

LORE receptor homomerization is required for 3-hydroxydecanoic acid-induced immune signaling and determines the natural variation of immunosensitivity within the Arabidopsis genus

The S-domain-type receptor-like kinase (SD-RLK) LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) from Arabidopsis thaliana is a pattern recognition receptor that senses medium-chain 3-hydroxy fatty acids, such as 3-hydroxydecanoic acid (3-OH-C10:0), to activate pattern-triggered immunity. Here, we show that LORE homomerization is required to activate 3-OH-C10:0-induced immune signaling. Fluorescence lifetime imaging in Nicotiana benthamiana demonstrates that AtLORE homomerizes via the extracellular and transmembrane domains. Co-expression of AtLORE truncations lacking the intracellular domain exerts a dominant negative effect on AtLORE signaling in both N. benthamiana and A. thaliana, highlighting that homomerization is essential for signaling. Screening for 3-OH-C10:0-induced reactive oxygen species production revealed natural variation within the Arabidopsis genus. Arabidopsis lyrata and Arabidopsis halleri do not respond to 3-OH-C10:0, although both possess a putative LORE ortholog. Both LORE orthologs have defective extracellular domains that bind 3-OH-C10:0 to a similar level as AtLORE, but lack the ability to homomerize. Thus, ligand binding is independent of LORE homomerization. Analysis of AtLORE and AlyrLORE chimera suggests that the loss of AlyrLORE homomerization is caused by several amino acid polymorphisms across the extracellular domain. Our findings shed light on the activation mechanism of LORE and the loss of 3-OH-C10:0 perception within the Arabidopsis genus.

Allosteric activation of the co-receptor BAK1 by the EFR receptor kinase initiates immune signaling

Transmembrane signaling by plant receptor kinases (RKs) has long been thought to involve reciprocal trans-phosphorylation of their intracellular kinase domains. The fact that many of these are pseudokinase domains, however, suggests that additional mechanisms must govern RK signaling activation. Non-catalytic (pseudo)kinase signaling mechanisms have been described in metazoans, but information is scarce for plants. Recently, a non-catalytic function was reported for the leucine-rich repeat (LRR)-RK subfamily XIIa member EFR (ELONGATION FACTOR TU RECEPTOR) and phosphorylation-dependent conformational changes were proposed to regulate signaling of RKs with non-RD kinase domains. Here, using EFR as a model, we describe a non-catalytic activation mechanism for LRR-RKs with non-RD kinase domains. EFR is an active kinase, but a kinase-dead variant retains the ability to enhance catalytic activity of its co-receptor kinase BAK1/SERK3 (BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3). Applying hydrogen-deuterium exchange mass spectrometry (HDX-MS) analysis and designing homology-based intragenic suppressor mutations, we provide evidence that the EFR kinase domain must adopt its active conformation in order to activate BAK1 allosterically, likely by supporting αC-helix positioning in BAK1. Our results suggest a conformational toggle model for signaling, in which BAK1 first phosphorylates EFR in the activation loop to stabilize its active conformation, allowing EFR in turn to allosterically activate BAK1.

The LRR receptor-like kinase ALR1 is a plant aluminum ion sensor

Plant survival requires an ability to adapt to differing concentrations of nutrient and toxic soil ions, yet ion sensors and associated signaling pathways are mostly unknown. Aluminum (Al) ions are highly phytotoxic, and cause severe crop yield loss and forest decline on acidic soils which represent ∼30% of land areas worldwide. Here we found an Arabidopsis mutant hypersensitive to Al. The gene encoding a leucine-rich-repeat receptor-like kinase, was named Al Resistance1 (ALR1). Al ions binding to ALR1 cytoplasmic domain recruits BAK1 co-receptor kinase and promotes ALR1-dependent phosphorylation of the NADPH oxidase RbohD, thereby enhancing reactive oxygen species (ROS) generation. ROS in turn oxidatively modify the RAE1 F-box protein to inhibit RAE1-dependent proteolysis of the central regulator STOP1, thus activating organic acid anion secretion to detoxify Al. These findings establish ALR1 as an Al ion receptor that confers resistance through an integrated Al-triggered signaling pathway, providing novel insights into ion-sensing mechanisms in living organisms, and enabling future molecular breeding of acid-soil-tolerant crops and trees, with huge potential for enhancing both global food security and forest restoration.

Curation, nomenclature, and topological classification of receptor-like kinases from 528 plant species for novel domain discovery and functional inference

Receptor-like kinases (RLKs) are the most numerous signal transduction components in plants and play important roles in determining how different plants adapt to their ecological environments. Research on RLKs has focused mainly on a small number of typical RLK members in a few model plants. There is an urgent need to study the composition, distribution, and evolution of RLKs at the holistic level to increase our understanding of how RLKs assist in the ecological adaptations of different plant species. In this study, we collected the genome assemblies of 528 plant species and constructed an RLK dataset. Using this dataset, we identified and characterized 524 948 RLK family members. Each member underwent systematic topological classification and was assigned a gene ID based on a unified nomenclature system. Furthermore, we identified two novel extracellular domains in some RLKs, designated Xiao and Xiang. Evolutionary analysis of the RLK family revealed that the RLCK-XVII and RLCK-XII-2 classes were present exclusively in dicots, suggesting that diversification of RLKs between monocots and dicots may have led to differences in downstream cytoplasmic responses. We also used an interaction proteome to help empower data mining for inference of new RLK functions from a global perspective, with the ultimate goal of understanding how RLKs shape the adaptation of different plants to the environments/ecosystems. The assembled RLK dataset, together with annotations and analytical tools, forms an integrated foundation of multiomics data that is publicly accessible via the metaRLK web portal (http://metaRLK.biocloud.top).

Stomatal development: NRPM proteins in dynamic localization of ERECTA receptor

Dynamic cellular localization of receptors is key to the perception of their peptide ligands and the activation of downstream signaling pathways. A new study identifies NRPMs as novel regulators of ERECTA receptor localization and stomatal formation downstream of the EPF1/EPF2 peptide ligands and upstream of the YDA MAPK cascade.

A plant mechanism of hijacking pathogen virulence factors to trigger innate immunity

Polygalacturonase-inhibiting proteins (PGIPs) interact with pathogen-derived polygalacturonases to inhibit their virulence-associated plant cell wall-degrading activity but stimulate immunity-inducing oligogalacturonide production. Here we show that interaction between Phaseolus vulgaris PGIP2 (PvPGIP2) and Fusarium phyllophilum polygalacturonase (FpPG) enhances substrate binding, resulting in inhibition of the enzyme activity of FpPG. This interaction promotes FpPG-catalyzed production of long-chain immunoactive oligogalacturonides, while diminishing immunosuppressive short oligogalacturonides. PvPGIP2 binding creates a substrate binding site on PvPGIP2-FpPG, forming a new polygalacturonase with boosted substrate binding activity and altered substrate preference. Structure-based engineering converts a putative PGIP that initially lacks FpPG-binding activity into an effective FpPG-interacting protein. These findings unveil a mechanism for plants to transform pathogen virulence activity into a defense trigger and provide proof of principle for engineering PGIPs with broader specificity.

OsLRR-RLP2 Gene Regulates Immunity to Magnaporthe oryzae in Japonica Rice

Rice is an important cereal crop worldwide, the growth of which is affected by rice blast disease, caused by the fungal pathogen Magnaporthe oryzae. As climate change increases the diversity of pathogens, the disease resistance genes (R genes) in plants must be identified. The major blast-resistance genes have been identified in indica rice varieties; therefore, japonica rice varieties with R genes now need to be identified. Because leucine-rich repeat (LRR) domain proteins possess R-gene properties, we used bioinformatics analysis to identify the rice candidate LRR domain receptor-like proteins (OsLRR-RLPs). OsLRR-RLP2, which contains six LRR domains, showed differences in the DNA sequence, containing 43 single-nucleotide polymorphisms (SNPs) in indica and japonica subpopulations. The results of the M. oryzae inoculation analysis indicated that indica varieties with partial deletion of OsLRR-RLP2 showed susceptibility, whereas japonica varieties with intact OsLRR-RLP2 showed resistance. The oslrr-rlp2 mutant, generated using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), showed increased pathogen susceptibility, whereas plants overexpressing this gene showed pathogen resistance. These results indicate that OsLRR-RLP2 confers resistance to rice, and OsLRR-RLP2 may be useful for breeding resistant cultivars.

Evolutionary trajectory of pattern recognition receptors in plants

Cell-surface receptors play pivotal roles in many biological processes, including immunity, development, and reproduction, across diverse organisms. How cell-surface receptors evolve to become specialised in different biological processes remains elusive. To shed light on the immune-specificity of cell-surface receptors, we analyzed more than 200,000 genes encoding cell-surface receptors from 350 genomes and traced the evolutionary origin of immune-specific leucine-rich repeat receptor-like proteins (LRR-RLPs) in plants. Surprisingly, we discovered that the motifs crucial for co-receptor interaction in LRR-RLPs are closely related to those of the LRR-receptor-like kinase (RLK) subgroup Xb, which perceives phytohormones and primarily governs growth and development. Functional characterisation further reveals that LRR-RLPs initiate immune responses through their juxtamembrane and transmembrane regions, while LRR-RLK-Xb members regulate development through their cytosolic kinase domains. Our data suggest that the cell-surface receptors involved in immunity and development share a common origin. After diversification, their ectodomains, juxtamembrane, transmembrane, and cytosolic regions have either diversified or stabilised to recognise diverse ligands and activate differential downstream responses. Our work reveals a mechanism by which plants evolve to perceive diverse signals to activate the appropriate responses in a rapidly changing environment.

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.

Control of lateral root formation by rapamycin-induced dimerization of engineered RGI/BAK1 and by BIR3 chimera

Leucine-rich repeat receptor kinases (LRR-RKs) play a pivotal role in diverse aspects of growth, development, and immunity in plants by sensing extracellular signals. Typically, LRR-RKs are activated through the ligand-induced interaction with a SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) coreceptor, triggering downstream signaling. ROOT MERISTEM GROWTH FACTOR1 (RGF1) INSENSITIVEs (RGIs) LRR-RLK receptors promote primary root meristem activity while inhibiting lateral root (LR) development in response to RGF peptide. In this study, we employed rapamycin-induced dimerization (RiD) and BAK1-INTERACTING RECEPTOR-LIKE KINASE3 (BIR3) chimera approaches to explore the gain-of-function of RGI1, RGI4, and RGI5. Rapamycin induced the association of cytosolic kinase domains (CKDs) of RGI1 and the BAK1 coreceptor, activating both mitogen-activated protein kinase 3 (MPK3) and MPK6. Rapamycin significantly inhibited LR formation in RiD-RGI1/RGI4/RGI5-BAK1 plants. Using transgenic Arabidopsis expressing RGI1CKD fused to the BIR3-LRR chimera under estradiol control, we observed a substantial reduction in LR density upon β-estradiol treatment. Additionally, we identified a decrease in root gravitropism in BIR3 chimera plants. In contrast, RiD-RGI/BAK1 plants did not exhibit defects in root gravitropism, implying the importance of combinatorial interactions between RGIs and SERK coreceptors in the inhibition of root gravitropism. Constitutive activation of RGIs with BAK1 in RiD-RGI/BAK1 plants by rapamycin treatment resulted in the inhibition of primary root growth, resembling the inhibitory effects observed with high concentrations of phytohormones on primary root elongation. Our findings highlight that the interactions between CKDs of RGIs and BAK1, constitutively induced by rapamycin or BIR3 chimera, efficiently control LR development.

Rapid Identification of Peptide-Receptor-Coreceptor Complexes in Protoplasts

Secreted signaling peptides, also called peptide hormones, play crucial roles in regulating plant growth, development, and immunity. Plant peptide hormones are perceived by plasma membrane-localized receptor-like kinases (RLKs) or receptor-like proteins (RLPs) that harbor specific extracellular domains to bind and recognize the corresponding peptide ligands. Binding of a peptide ligand to its receptor usually induces the hetero-dimerization of the cognate receptor and a coreceptor, followed by the phosphorylation and activation of the receptor complex to transduce downstream signaling. Therefore, matching peptide ligands with their respective receptors/coreceptors is crucial for elucidating peptide hormone signaling pathways. In this chapter, using the RGF7 peptide-RGI4/RGI5 receptor-BAK1 coreceptor complex as an example, we describe a rapid method to identify the peptide ligand-receptor-coreceptor complexes via co-immunoprecipitation assays using recombinant proteins transiently expressed in Arabidopsis protoplasts.

PSY-PSYR peptide-receptor pairs control the trade-off between plant growth and stress response

Leucine-rich repeat-receptor kinases (LRR-RKs) perceive various endogenous peptide hormones that control plant growth and development. However, the majority of corresponding ligands and their direct ligand-binding receptors have not been identified yet. A recent study demonstrated that three LRR-RK PLANT PEPTIDE CONTAINING SULFATED TYROSINE RECEPTORS (PSYRs) act as ligand-receptors of the PSY family peptides that mediate the trade-off between the optimal plant growth and stress tolerance responses. The genetic, biochemical, and transcriptome analyses suggested that PSYR1, PSYR2, and PSYR3 function as negative regulators of plant growth in the absence of PSY peptides and induce stress tolerance responses, whereas the PSY family peptides repress PSYR signaling, allowing plant growth. This trade-off mechanism between plant growth and stress responses mediated by the PSY-PSYR signaling module allows plants to survive under ever changing environmental stresses.

Arabidopsis MCTP family member QUIRKY regulates the formation of the STRUBBELIG receptor kinase complex

Intercellular communication plays a central role in organogenesis. Tissue morphogenesis in Arabidopsis (Arabidopsis thaliana) requires signaling mediated by a cell surface complex containing the atypical receptor kinase STRUBBELIG (SUB) and the multiple C2 domains and transmembrane region protein QUIRKY (QKY). QKY is required to stabilize SUB at the plasma membrane. However, it is unclear what the in vivo architecture of the QKY/SUB signaling complex is, how it is controlled, and how it relates to the maintenance of SUB at the cell surface. We addressed these questions using a combination of genetics, yeast 2-hybrid assays, and Förster resonance energy transfer (FRET)/fluorescence lifetime imaging microscopy (FLIM) in epidermal cells of seedling roots. We found that QKY promotes the formation of SUB homooligomers in vivo. Homooligomerization of SUB appeared to involve its extracellular domain. We also showed that QKY and SUB physically interact and form a complex at the cell surface in vivo. In addition, the data showed that the N-terminal C2A-B region of QKY interacts with the intracellular domain of SUB. They further revealed that this interaction is essential to maintain SUB levels at the cell surface. Finally, we provided evidence that QKY forms homomultimers in vivo in a SUB-independent manner. We suggest a model in which the physical interaction of QKY with SUB mediates the oligomerization of SUB and attenuates its internalization, thereby maintaining sufficiently high levels of SUB at the cell surface required for the control of tissue morphogenesis.

Molecular mechanisms underpinning quantitative resistance to Phytophthora sojae in Glycine max using a systems genomics approach

Expression of quantitative disease resistance in many host-pathogen systems is controlled by genes at multiple loci, each contributing a small effect to the overall response. We used a systems genomics approach to study the molecular underpinnings of quantitative disease resistance in the soybean-Phytophthora sojae pathosystem, incorporating expression quantitative trait loci (eQTL) mapping and gene co-expression network analysis to identify the genes putatively regulating transcriptional changes in response to inoculation. These findings were compared to previously mapped phenotypic (phQTL) to identify the molecular mechanisms contributing to the expression of this resistance. A subset of 93 recombinant inbred lines (RILs) from a Conrad × Sloan population were inoculated with P. sojae isolate 1.S.1.1 using the tray-test method; RNA was extracted, sequenced, and the normalized read counts were genetically mapped from tissue collected at the inoculation site 24 h after inoculation from both mock and inoculated samples. In total, more than 100,000 eQTLs were mapped. There was a switch from predominantly cis-eQTLs in the mock treatment to an almost entirely nonoverlapping set of predominantly trans-eQTLs in the inoculated treatment, where greater than 100-fold more eQTLs were mapped relative to mock, indicating vast transcriptional reprogramming due to P. sojae infection occurred. The eQTLs were organized into 36 hotspots, with the four largest hotspots from the inoculated treatment corresponding to more than 70% of the eQTLs, each enriched for genes within plant-pathogen interaction pathways. Genetic regulation of trans-eQTLs in response to the pathogen was predicted to occur through transcription factors and signaling molecules involved in plant-pathogen interactions, plant hormone signal transduction, and MAPK pathways. Network analysis identified three co-expression modules that were correlated with susceptibility to P. sojae and associated with three eQTL hotspots. Among the eQTLs co-localized with phQTLs, two cis-eQTLs with putative functions in the regulation of root architecture or jasmonic acid, as well as the putative master regulators of an eQTL hotspot nearby a phQTL, represent candidates potentially underpinning the molecular control of these phQTLs for resistance.

Asymmetric Evolution of Protein Domains in the Leucine-Rich Repeat Receptor-Like Kinase Family of Plant Signaling Proteins

The coding sequences of developmental genes are expected to be deeply conserved, with cis-regulatory change driving the modulation of gene function. In contrast, proteins with roles in defense are expected to evolve rapidly, in molecular arms races with pathogens. However, some gene families include both developmental and defense genes. In these families, does the tempo and mode of evolution differ between genes with divergent functions, despite shared ancestry and structure? The leucine-rich repeat receptor-like kinase (LRR-RLKs) protein family includes members with roles in plant development and defense, thus providing an ideal system for answering this question. LRR-RLKs are receptors that traverse plasma membranes. LRR domains bind extracellular ligands; RLK domains initiate intracellular signaling cascades in response to ligand binding. In LRR-RLKs with roles in defense, LRR domains evolve faster than RLK domains. To determine whether this asymmetry extends to LRR-RLKs that function primarily in development, we assessed evolutionary rates and tested for selection acting on 11 subfamilies of LRR-RLKs, using deeply sampled protein trees. To assess functional evolution, we performed heterologous complementation assays in Arabidopsis thaliana (Arabidopsis). We found that the LRR domains of all tested LRR-RLK proteins evolved faster than their cognate RLK domains. All tested subfamilies of LRR-RLKs had strikingly similar patterns of molecular evolution, despite divergent functions. Heterologous transformation experiments revealed that multiple mechanisms likely contribute to the evolution of LRR-RLK function, including escape from adaptive conflict. Our results indicate specific and distinct evolutionary pressures acting on LRR versus RLK domains, despite diverse organismal roles for LRR-RLK proteins.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

TPLATE complex-dependent endocytosis attenuates CLAVATA1 signaling for shoot apical meristem maintenance

Endocytosis regulates the turnover of cell surface localized receptors, which are crucial for plants to rapidly respond to stimuli. The evolutionary ancient TPLATE complex (TPC) plays an essential role in endocytosis in Arabidopsis plants. Knockout or knockdown of single TPC subunits causes male sterility and seedling lethality phenotypes, complicating analysis of the roles of TPC during plant development. Partially functional alleles of TPC subunits however only cause mild developmental deviations. Here, we took advantage of the partially functional TPLATE allele, WDXM2, to investigate a role for TPC-dependent endocytosis in receptor-mediated signaling. We discovered that reduced TPC-dependent endocytosis confers a hypersensitivity to very low doses of CLAVATA3 peptide signaling. This hypersensitivity correlated with the abundance of the CLAVATA3 receptor protein kinase CLAVATA1 at the plasma membrane. Genetic and biochemical analysis as well as live-cell imaging revealed that TPC-dependent regulation of CLAVATA3-dependent internalization of CLAVATA1 from the plasma membrane is required for shoot stem cell homeostasis. Our findings provide evidence that TPC-mediated endocytosis and degradation of CLAVATA1 is a mechanism to dampen CLAVATA3-mediated signaling during plant development.

The Leucine-Rich Repeat Receptor-Like Kinase Protein TaSERK1 Positively Regulates High-Temperature Seedling Plant Resistance to Puccinia striiformis f. sp. tritici by Interacting with TaDJA7

Somatic embryogenesis receptor kinases (SERKs) belong to the leucine-rich repeat receptor-like kinase (LRR-RLK) subfamily, and many LRR-RLKs have been proven to play a key role in plant immune signal transmission. However, the functions of SERKs in resistance to stripe rust caused by Puccinia striiformis f. sp. tritici remains unknown. Here, we identified a gene, TaSERK1, from Xiaoyan 6, a wheat cultivar possessing high-temperature seedling-plant (HTSP) resistance to the fungal pathogen P. striiformis f. sp. tritici and expresses its resistance at the seedling stage. The expression level of TaSERK1 was upregulated upon P. striiformis f. sp. tritici inoculation under relatively high temperatures. The transcriptional level of TaSERK1 was significantly increased under exogenous salicylic acid and brassinosteroids treatments. The barley stripe mosaic virus-induced gene silencing assay indicated that TaSERK1 positively regulated the HTSP resistance to stripe rust. The transient expression of TaSERK1 in tobacco leaves confirmed its subcellular localization on the plasma membrane. Furthermore, TaSERK1 interacted with and phosphorylated the chaperone protein TaDJA7, which belongs to the heat shock protein 40 subfamily. Silencing TaDJA7 compromised the HTSP resistance to stripe rust. The results indicated that when the membrane immune receptor TaSERK1 perceives the P. striiformis f. sp. tritici infection under relatively high temperatures, it transmits the signal to TaDJA7 to activate HTSP resistance to the pathogen.

Paradigms of receptor kinase signaling in plants

Plant receptor kinases (RKs) function as key plasma-membrane localized receptors in the perception of molecular ligands regulating development and environmental response. Through the perception of diverse ligands, RKs regulate various aspects throughout the plant life cycle from fertilization to seed set. Thirty years of research on plant RKs has generated a wealth of knowledge on how RKs perceive ligands and activate downstream signaling. In the present review, we synthesize this body of knowledge into five central paradigms of plant RK signaling: (1) RKs are encoded by expanded gene families, largely conserved throughout land plant evolution; (2) RKs perceive many different kinds of ligands through a range of ectodomain architectures; (3) RK complexes are typically activated by co-receptor recruitment; (4) post-translational modifications fulfill central roles in both the activation and attenuation of RK-mediated signaling; and, (5) RKs activate a common set of downstream signaling processes through receptor-like cytoplasmic kinases (RLCKs). For each of these paradigms, we discuss key illustrative examples and also highlight known exceptions. We conclude by presenting five critical gaps in our understanding of RK function.

A Plasmodium membrane receptor platform integrates cues for egress and invasion in blood forms and activation of transmission stages

Critical events in the life cycle of malaria-causing parasites depend on cyclic guanosine monophosphate homeostasis by guanylyl cyclases (GCs) and phosphodiesterases, including merozoite egress or invasion of erythrocytes and gametocyte activation. These processes rely on a single GCα, but in the absence of known signaling receptors, how this pathway integrates distinct triggers is unknown. We show that temperature-dependent epistatic interactions between phosphodiesterases counterbalance GCα basal activity preventing gametocyte activation before mosquito blood feed. GCα interacts with two multipass membrane cofactors in schizonts and gametocytes: UGO (unique GC organizer) and SLF (signaling linking factor). While SLF regulates GCα basal activity, UGO is essential for GCα up-regulation in response to natural signals inducing merozoite egress and gametocyte activation. This work identifies a GC membrane receptor platform that senses signals triggering processes specific to an intracellular parasitic lifestyle, including host cell egress and invasion to ensure intraerythrocytic amplification and transmission to mosquitoes.

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.

The receptor-like kinase EMS1 and BRI1 coordinately regulate stamen elongation via the transcription factors BES1/BZR1 in Arabidopsis

Plants possess a large family of receptor kinase proteins to mediate cell-to-cell and cell-to-environment communication, and these regulations are essential for plant growth and development as well as resistance to biotic or abiotic stresses. EMS1 is a receptor kinase which involved in tapetum cell fate determination during anther development, while brassinosteroid (BR) receptor, BRI1, controls most aspects of plant growth and development. Although EMS1 and BRI1 are known to regulate independent biological processes, they interact with identical components of the downstream signaling pathways. However, the biological processes other than the tapetum development controlled by the EMS1 signal are not clear. Here, we report that EMS1 signaling-related mutants exhibited an insufficient stamen elongation phenotype, similar to BR signaling mutants. Transgenic expression of BRI1 restored the short filament phenotype of ems1. Conversely, co-expression of EMS1 and TPD1 also restored the short filaments of BRI1 mutants, bri1. Genetic experiments confirmed that EMS1 and BRI1 regulate filament elongation through their downstream transcription factors BES1/BZR1. Molecular analysis suggested that the decrease in BR signaling output in filaments of the ems1 mutant caused deficient filament development. Moreover, in vitro and in vivo experiments proved BES1 interacts with filament-specific transcription factor MYB21. Together, we found that the two receptor-like kinases (RLKs) EMS1 and BRI1 are cooperatively involved in the regulation of filament elongation via the transcription factors BES1/BZR1. These results indicated that the biological processes regulated by EMS1 and BRI1 in plants are both independent and interactive, which provides us with insights into multidimensional molecular control of the RLK pathway.

Genome-Wide Analysis of Proline-Rich Extensin-Like Receptor Kinases (PERKs) Gene Family Reveals Their Roles in Plant Development and Stress Conditions in Oryza sativa L

Proline-rich extensin-like receptor kinases (PERKs) play a crucial role in a wide range of biological processes in plants. In model plants like Arabidopsis, the PERK gene family has been well investigated. Conversely, no information available on the PERK gene family and their biological functions largely remained unknown in rice. This study analyzed the basic physicochemical properties, phylogeny, gene structure, cis-acting elements, Gene ontology (GO) annotation and protein-protein interaction of OsPERK gene family members using various bioinformatics tools based on the whole-genome data of O. sativa. Thus, in this work, 8 PERK genes in rice were identified, and their roles in plant development, growth, and response to various stresses were studied. A phylogenetic study revealed that OsPERKs are grouped into seven classes. Chromosomal mapping also displayed that 8 PERK genes were unevenly distributed on 12 chromosomes. Further, the prediction of subcellular localization indicated that OsPERKs were mainly located at the endomembrane system. Gene structure analysis of OsPERKs has shown a distinctive evolutionary path. In addition, synteny analysis exhibited the 40 orthologous gene pairs in Arabidopsis thaliana, Triticum aestivum, Hordeum vulgare and Medicago truncatula. Furthermore, Ka to Ks proportion shows that most OsPERK genes experienced resilient purifying selection during evolutionary processes. The OsPERK promoters contained several cis-acting regulatory, which are crucial for plant development processes, phytohormone signaling, stress, and defense response. Moreover, the expression pattern of OsPERK family members showed differential expression patterns in different tissues and various stress conditions. Taken together, these results provide clear messages for a better understanding the roles of OsPERK genes in various development stages, tissues, and multifactorial stress as well as enriched the related research of OsPERK family members in rice.

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.

Cell Wall Integrity Signaling in Fruit Ripening

Plant cell walls are essential structures for plant growth and development as well as plant adaptation to environmental stresses. Thus, plants have evolved signaling mechanisms to monitor the changes in the cell wall structure, triggering compensatory changes to sustain cell wall integrity (CWI). CWI signaling can be initiated in response to environmental and developmental signals. However, while environmental stress-associated CWI signaling has been extensively studied and reviewed, less attention has been paid to CWI signaling in relation to plant growth and development under normal conditions. Fleshy fruit development and ripening is a unique process in which dramatic alternations occur in cell wall architecture. Emerging evidence suggests that CWI signaling plays a pivotal role in fruit ripening. In this review, we summarize and discuss the CWI signaling in relation to fruit ripening, which will include cell wall fragment signaling, calcium signaling, and NO signaling, as well as Receptor-Like Protein Kinase (RLKs) signaling with an emphasis on the signaling of FERONIA and THESEUS, two members of RLKs that may act as potential CWI sensors in the modulation of hormonal signal origination and transduction in fruit development and ripening.

Functional Expression of the Ectodomain of Plant Receptor Kinases in Plant Suspension Culture

Extracellular signals are usually perceived by membrane-localized receptors that transduce intercellular signals to activate various pathways. In plants, single transmembrane receptor kinases act as receptors for extracellular signals. Endogenous secreted peptide hormones have been recognized as novel signaling molecules, functioning through the formation of ligand-receptor pairs in plants. Recently, research on plant peptide hormone-receptor interactions based on the structural biology approach has greatly improved; however, the dissociation constant of recombinant receptor molecules expressed in insect cells using the baculovirus expression system is relatively low. We introduce here a method for creating a stable and functional homogeneous expression system for plant receptor kinases using tobacco BY-2 cells while maintaining conventional ligand-binding activity. This strategy will help improve our understanding of plant endogenous peptide ligand-receptor interactions.

Emerging roles of protein phosphorylation in regulation of stomatal development

Stomata, tiny epidermal spores, control gas exchange between plants and their external environment, thereby playing essential roles in plant development and physiology. Stomatal development requires rapid regulation of components in signaling pathways to respond flexibly to numerous intrinsic and extrinsic signals. In support of this, reversible phosphorylation, which is particularly suitable for rapid signal transduction, has been implicated in this process. This review highlights the current understanding of the essential roles of reversible phosphorylation in the regulation of stomatal development, most of which comes from the dicot Arabidopsis thaliana. Protein phosphorylation tightly controls the activity of SPEECHLESS (SPCH)-SCREAM (SCRM), the stomatal lineage switch, and the activity of several mitogen-activated protein kinases and receptor kinases upstream of SPCH-SCRM, thereby regulating stomatal cell differentiation and patterning. In addition, protein phosphorylation is involved in the establishment of cell polarity during stomatal asymmetric cell division. Finally, cyclin-dependent kinase-mediated protein phosphorylation plays essential roles in cell cycle control during stomatal development.

An Interactome Assay for Detecting Interactions between Extracellular Domains of Receptor Kinases

Interactions between extracellular domains (ECDs) are crucial for many physiological processes in the cell, most importantly perception of its environment. However, studying these often-transient interactions can be challenging. Here we describe a method that allows for in vitro detection of extracellular domain interactions through an oligomerization-based cell surface interaction (CSI) assay. In a CSI, bait- and prey-tagged proteins are produced and secreted by Drosophila S2 cells to ensure proper folding and post-translational modifications. Subsequently, Bait (FC fragment) and Prey (pentamer domain and alkaline phosphatase) tags allow the detection of interactions in protein A-coated 96 wells plates through a colorimetric readout. Due to the easy detection of interactions this approach can be used for high-throughput screening and mapping of extracellular interaction networks.

Crucial role of Arabidopsis glutaredoxin S17 in heat stress response revealed by transcriptome analysis

Heat stress can have detrimental effects on plant growth and development. However, the mechanisms by which the plant is able to perceive changes in ambient temperature, transmit this information, and initiate a temperature-induced response are not fully understood. Previously, we showed that heterologous expression of an Arabidopsis thaliana L. monothiol glutaredoxin AtGRXS17 enhances thermotolerance in various crops, while disruption of AtGRXS17 expression caused hypersensitivity to permissive temperature. In this study, we extend our investigation into the effect of AtGRXS17 and heat stress on plant growth and development. Although atgrxs17 plants were found to exhibit a slight decrease in hypocotyl elongation, shoot meristem development, and root growth compared to wild-type when grown at 22°C, these growth phenotypic differences became more pronounced when growth temperatures were raised to 28°C. Transcriptome analysis revealed significant changes in genome-wide gene expression in atgrxs17 plants compared to wild-type under conditions of heat stress. The expression of genes related to heat stress factors, auxin response, cellular communication, and abiotic stress were altered in atgrxs17 plants in response to heat stress. Overall, our findings indicate that AtGRXS17 plays a critical role in controlling the transcriptional regulation of plant heat stress response pathways.

Peptides/receptors signaling during plant fertilization

Double fertilization is a unique and particularly complicated process for the generation alternation of angiosperms. Sperm cells of angiosperms lose the motility compared with that of gymnosperms. The sperm cells are passively carried and transported by the pollen tube for a long journey before targeting the ovule. Two sperm cells are released at the cleft between the egg and the central cell and fused with two female gametes to produce a zygote and endosperm, respectively, to accomplish the so-called double fertilization process. In this process, extensive communication and interaction occur between the male (pollen or pollen tube) and the female (ovule). It is suggested that small peptides and receptor kinases play critical roles in orchestrating this cell-cell communication. Here, we illuminate the understanding of phases in the process, such as pollen-stigma recognition, the hydration and germination of pollen grains, the growth, guidance, and rupture of tubes, the release of sperm cells, and the fusion of gametes, by reviewing increasing data recently. The roles of peptides and receptor kinases in signaling mechanisms underlying cell-cell communication were focused on, and directions of future studies were perspected in this review.

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.

ZAR1: Guardian of plant kinases

A key facet of innate immunity in plants entails the recognition of pathogen "effector" virulence proteins by host Nucleotide-Binding Leucine-Rich Repeat Receptors (NLRs). Among characterized NLRs, the broadly conserved ZAR1 NLR is particularly remarkable due to its capacity to recognize at least six distinct families of effectors from at least two bacterial genera. This expanded recognition spectrum is conferred through interactions between ZAR1 and a dynamic network of two families of Receptor-Like Cytoplasmic Kinases (RLCKs): ZED1-Related Kinases (ZRKs) and PBS1-Like Kinases (PBLs). In this review, we survey the history of functional studies on ZAR1, with an emphasis on how the ZAR1-RLCK network functions to trap diverse effectors. We discuss 1) the dynamics of the ZAR1-associated RLCK network; 2) the specificity between ZRKs and PBLs; and 3) the specificity between effectors and the RLCK network. We posit that the shared protein fold of kinases and the switch-like properties of their interactions make them ideal effector sensors, enabling ZAR1 to act as a broad spectrum guardian of host kinases.

Plant receptor-like protein activation by a microbial glycoside hydrolase

Plants rely on cell-surface-localized pattern recognition receptors to detect pathogen- or host-derived danger signals and trigger an immune response^1-6. Receptor-like proteins (RLPs) with a leucine-rich repeat (LRR) ectodomain constitute a subgroup of pattern recognition receptors and play a critical role in plant immunity^1-3. Mechanisms underlying ligand recognition and activation of LRR-RLPs remain elusive. Here we report a crystal structure of the LRR-RLP RXEG1 from Nicotiana benthamiana that recognizes XEG1 xyloglucanase from the pathogen Phytophthora sojae. The structure reveals that specific XEG1 recognition is predominantly mediated by an amino-terminal and a carboxy-terminal loop-out region (RXEG1(ID)) of RXEG1. The two loops bind to the active-site groove of XEG1, inhibiting its enzymatic activity and suppressing Phytophthora infection of N. benthamiana. Binding of XEG1 promotes association of RXEG1(LRR) with the LRR-type co-receptor BAK1 through RXEG1(ID) and the last four conserved LRRs to trigger RXEG1-mediated immune responses. Comparison of the structures of apo-RXEG1(LRR), XEG1-RXEG1(LRR) and XEG1-BAK1-RXEG1(LRR) shows that binding of XEG1 induces conformational changes in the N-terminal region of RXEG1(ID) and enhances structural flexibility of the BAK1-associating regions of RXEG1(LRR). These changes allow fold switching of RXEG1(ID) for recruitment of BAK1(LRR). Our data reveal a conserved mechanism of ligand-induced heterodimerization of an LRR-RLP with BAK1 and suggest a dual function for the LRR-RLP in plant immunity.

The Emerging Role of PP2C Phosphatases in Tomato Immunity

The antagonistic effect of plant immunity on growth likely drove evolution of molecular mechanisms that prevent accidental initiation and prolonged activation of plant immune responses. Signaling networks of pattern-triggered and effector-triggered immunity, the two main layers of plant immunity, are tightly regulated by the activity of protein phosphatases that dephosphorylate their protein substrates and reverse the action of protein kinases. Members of the PP2C class of protein phosphatases have emerged as key negative regulators of plant immunity, primarily from research in the model plant Arabidopsis thaliana, revealing the potential to employ PP2C proteins to enhance plant disease resistance. As a first step towards focusing on the PP2C family for both basic and translational research, we analyzed the tomato genome sequence to ascertain the complement of the tomato PP2C family, identify conserved protein domains and signals in PP2C amino acid sequences, and examine domain combinations in individual proteins. We then identified tomato PP2Cs that are candidate regulators of single or multiple layers of the immune signaling network by in-depth analysis of publicly available RNA-seq datasets. These included expression profiles of plants treated with fungal or bacterial pathogen-associated molecular patterns, with pathogenic, nonpathogenic, and disarmed bacteria, as well as pathogenic fungi and oomycetes. Finally, we discuss the possible use of immunity-associated PP2Cs to better understand the signaling networks of plant immunity and to engineer durable and broad disease resistance in crop plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Extracellular pH sensing by plant cell-surface peptide-receptor complexes

The extracellular pH is a vital regulator of various biological processes in plants. However, how plants perceive extracellular pH remains obscure. Here, we report that plant cell-surface peptide-receptor complexes can function as extracellular pH sensors. We found that pattern-triggered immunity (PTI) dramatically alkalinizes the acidic extracellular pH in root apical meristem (RAM) region, which is essential for root meristem growth factor 1 (RGF1)-mediated RAM growth. The extracellular alkalinization progressively inhibits the acidic-dependent interaction between RGF1 and its receptors (RGFRs) through the pH sensor sulfotyrosine. Conversely, extracellular alkalinization promotes the alkaline-dependent binding of plant elicitor peptides (Peps) to its receptors (PEPRs) through the pH sensor Glu/Asp, thereby promoting immunity. A domain swap between RGFR and PEPR switches the pH dependency of RAM growth. Thus, our results reveal a mechanism of extracellular pH sensing by plant peptide-receptor complexes and provide insights into the extracellular pH-mediated regulation of growth and immunity in the RAM.

Pan-brassinosteroid signaling revealed by functional analysis of NILR1 in land plants

Brassinosteroid (BR) signaling has been identified from the ligand BRs sensed by the receptor Brassinosteroid Insensitive 1 (BRI1) to the final activation of Brassinozole Resistant 1/bri1 EMS-Suppressor 1 through a series of transduction events. Extensive studies have been conducted to characterize the role of BR signaling in various biological processes. Our previous study has shown that Excess Microsporocytes 1 (EMS1) and BRI1 control different aspects of plant growth and development via conserved intracellular signaling. Here, we reveal that another receptor, NILR1, can complement the bri1 mutant in the absence of BRs, indicating a pathway that resembles BR signaling activated by NILR1. Genetic analysis confirms the intracellular domains of NILR1, BRI1 and EMS1 have a common signal output. Furthermore, we demonstrate that NILR1 and BRI1 share the coreceptor BRI1 Associated Kinase 1 and substrate BSKs. Notably, the NILR1-mediated downstream pathway is conserved across land plants. In summary, we provide evidence for the signaling cascade of NILR1, suggesting pan-brassinosteroid signaling initiated by a group of distant receptor-ligand pairs in land plants.