The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens

Deciphering the complex roles of leucine-rich repeat receptor kinases (LRR-RKs) in plant signal transduction

Leucine-rich repeat receptor kinases (LRR-RKs) are essential receptor protein kinases in plants that are the key to signal perception and responses and central to regulating plant growth, development, and defense. Despite extensive research on the LRR-RK family, gaps persist in our understanding of their ligand recognition and activation mechanisms, interactions with co-receptor, signal transduction pathways, and biochemical and molecular regulation. Researchers have made significant advances in understanding the critical roles of LRR-RKs in plant growth and development, signal transduction, and stress responses. Here, we first summarized the gene expression levels of LRR-RKs in plants. We then reviewed the conservation and evolutionary relationships of these genes across different species. We also investigated the molecular mechanisms underlying the variations in LRR-RK signaling under different environmental conditions. Finally, we provide a comprehensive summary of how abiotic and biotic stresses modulate LRR-RK signaling pathways in plants.

Regeneration and defense: unveiling the molecular interplay in plants

In both plants and animals, tissue or organ regeneration typically follows wounding, which also activates defense responses against pathogenic microbes and herbivores. Both intrinsic and environmental cues guide the molecular decisions between regeneration and defense. In animal studies, extensive research has highlighted the role of various microbes - including pathogenic, commensal, and beneficial species - in influencing the signaling interplay between immunity and regeneration. Conversely, most plant regeneration studies are conducted under sterile conditions, which leaves a gap in our understanding of how plant innate immunity influences regeneration pathways. Recent findings have begun to elucidate the roles of key defense pathways in modulating plant regeneration and the crosstalk between these two processes. These studies also explore how microbes might influence the molecular choice between defense and regeneration in plants. This review examines the molecular mechanisms governing the balance between plant regeneration and innate immunity, with a focus on the emerging role of aging and microbial interactions in shaping these processes.

AtSERK1 and BAK1/AtSERK3 positively regulate seed germination in response to saline condition in Arabidopsis

Salt stress adversely affects plants by inhibiting seed germination, growth, and development throughout their life cycle. Abscisic acid (ABA) is a primary plant hormone that regulates seed dormancy under environmental stresses. The salt-overly sensitive (SOS) pathway regulates ion homeostasis in seedlings grown under saline conditions. Recently, sos mutants have been reported to exhibit higher seed germination rates than wild-type plants under salt conditions. Previously, BRI1-associated receptor kinase 1 (BAK1) was demonstrated to be involved in ABA-induced stomatal closure under drought conditions. In the present study, we demonstrated that bak1 and atserk1 mutants exhibited stronger inhibition of seed germination on salt-containing media than wild-type plants, and the degree of inhibition was greater in atserk1 mutants. However, seedling growth after germination was not different among the genotypes. RNA-sequencing analysis of salt-treated atserk1 seeds revealed that the number of differentially expressed genes (DEGs) induced by salt stress was lower in atserk1 than in wild-type plants. However, most DEGs in atserek1 were involved in signal transduction with phosphorelay events, suggesting that AtSERKs may function post-transcriptionally to regulate seed germination. Furthermore, bak1/sos2 or atserk1/sos2 double mutant analyses showed that AtSERKs act downstream of SOS2 in the salt-induced inhibition of seed germination. Taken together, these results suggest that both AtSERKs and SOS proteins work together to regulate the balance between seed germination and maintenance of seed dormancy under saline conditions.

Rhizosphere microbes mitigate the shade avoidance responses in Arabidopsis

Shade avoidance responses are defined as the plastic responses of plants to neighboring shading signals through changes in the light spectrum, which limit planting density in modern agricultural practices. Here, we found that shade avoidance responses depend on soil microbes and identified a microbe-root-shoot circuit that bolsters aboveground shade tolerance in Arabidopsis thaliana. Rhizosphere microbes systemically regulate the expression of aboveground shade-responsive genes, which are associated with the altered homeostasis of jasmonic-acid- and salicylic-acid-related metabolites. We further found that the plasma-membrane-localized pattern recognition receptors FLS2/BAK1 and transcription factors MYC2/phytochrome-interacting factors (PIFs)/LONG HYPOCOTYL5 (HY5) are required for rhizosphere-microbe-alleviated shade avoidance. Our study characterized a signaling cascade (FLS2/BAK1-MYC2-PIF4/HY5) and provided a strategy for mitigating aboveground shade responses using rhizosphere microorganisms.

The role and structure of molecular glues in plant signalling networks

Protein-protein interactions are one of the pillars of all life processes. Many signalling molecules work by promoting and stabilizing these interactions. These molecular 'glues' bind simultaneously to two proteins inducing their interaction, which would be otherwise less favourable or non-favourable. Importantly, they can be harnessed for a clinical purpose, but, despite advances in medicine, the wealth of natural molecular glues in plants have only rarely been commercially utilized. These molecular glues may be plant-endogenous or plant-exogenous small molecules or peptides, and they may be involved in many different processes, such as growth promotion or stress response, opening new opportunities for crop protection, along with other applications. In this Review, we analyse the underlying structural motives and molecular interactions in detail, classifying the modes of actions based on their nature (small ligands versus peptides) and receptor classes. We discuss both natural metabolites and mimetics of such compounds, highlighting similarities and differences between signalling pathways and comparing them with relevant mechanisms in mammals.

SnRK2s: Kinases or Substrates?

Throughout their life cycle, plants persistent through environmental adversities that activate sophisticated stress-signaling networks, with protein kinases serving as pivotal regulators of these responses. The sucrose non-fermenting-1-related protein kinase 2 (SnRK2), a plant-specific serine/threonine kinase, orchestrates stress adaptation by phosphorylating downstream targets to modulate gene expression and physiological adjustments. While SnRK2 substrates have been extensively identified, the existing literature lacks a systematic classification of these components and their functional implications. This review synthesizes recent advances in characterizing SnRK2-phosphorylated substrates in Arabidopsis thaliana, providing a mechanistic framework for their roles in stress signaling and developmental regulation. Furthermore, we explore the understudied paradigm of SnRK2 undergoing multilayered post-translational modifications (PTMs), including phosphorylation, ubiquitination, SUMOylation, S-nitrosylation, sulfation (S-sulfination and tyrosine sulfation), and N-glycosylation. These PTMs collectively fine-tune SnRK2 stability, activity, and subcellular dynamics, revealing an intricate feedback system that balances kinase activation and attenuation. By integrating substrate networks with regulatory modifications, this work highlights SnRK2's dual role as both a phosphorylation executor and a PTM-regulated scaffold, offering new perspectives for engineering stress-resilient crops through targeted manipulation of SnRK2 signaling modules.

A Sorghum BAK1/SERK4 Homolog Functions in Pathogen-Associated Molecular Patterns-Triggered Immunity and Cell Death in Response to Colletotrichum sublineola Infection

Sorghum bicolor is the fifth most important cereal crop and expected to gain prominence due to its versatility, low input requirements, and tolerance to hot and dry conditions. In warm and humid environments, the productivity of sorghum is severely limited by the hemibiotrophic fungal pathogen Colletotrichum sublineola, the causal agent of anthracnose. Cultivating anthracnose-resistant accessions is the most effective and environmentally benign way to safeguard yield. A previous genome-wide association study for anthracnose resistance in the Sorghum Association Panel uncovered single-nucleotide polymorphisms on chromosome 5 associated with resistance to anthracnose, including one located within the coding region of gene Sobic.005G182400. In this study, we investigated the molecular function of Sobic.005G182400 in response to C. sublineola infection. Conserved domain, phylogenetic, and structural analyses revealed that the protein encoded by Sobic.005G182400 shares significant structural similarity with the Arabidopsis BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1)/SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE4 (SERK4). Although sequence analysis of four sorghum accessions showed no substantial variation in the coding region, accession SC1330, which carries the resistance allele, exhibited significantly higher expression of Sobic.005G182400 during early infection (≤24 h). Co-expression network analysis identified that the module associated with Sobic.005G182400 was enriched in genes involved in endocytosis, autophagy, and vesicle transport. Gene regulatory network analysis further suggested that Sobic.005G182400 regulates genes required for BAK1/SERK4-mediated cell death via protein glycosylation. Together, these findings indicate that Sobic.005G182400 encodes a protein with similarity to Arabidopsis BAK1/SERK4 that enables pathogen-associated molecular patterns-triggered immunity and regulates cell death.

Nanomediated Stimulation: An Alternative to Brassinolide Hormone Replacement Therapy for Plant Resistance Activation

Facing harsher losses of crop yield due to virus infection, it is critical to reduce yield loss by improving plants' disease resistance. Here, we proposed using nanoparticles to prestimulate Nicotiana benthamiana as a nanomediated brassinolide (BR) hormone replacement therapy to trigger immune responses and subsequently increase plant immunity against viruses. Our results showed the prestimulated leaves of zinc oxide nanoparticles (ZnONPs) exhibit accelerated antiviral capability, and the plant resistance activation was increased with a decrease in the ZnONP size. Transcriptome data and hormone assays revealed that ZnONP stimulation activated the brassinolide hormone signaling pathway and increased the brassinolide concentration. Importantly, the induced activity of ZnONPs on antiviral capability could be eliminated by virus-mediated silencing of key genes of brassinolide in Nicotiana benthamiana. In summary, we showed prestimulated plants with ZnONPs induced systemic resistance to TMV by activating the brassinolide pathways. This simple nanostimulant-based hormone replacement therapy may alleviate pathogen infection in crop plants and reduce the need for pesticides.

Functional and transcriptomic characterization of the receptor-like protein kinase gene GmHSL1b involved in salt stress tolerance in soybean roots

The survival and adaptation of plants to adverse environmental conditions is crucial and is facilitated by receptor-like kinases, which act as cell surface receptors for a variety of signals. In this study, we identified a gene, GmHSL1b, encoding a receptor-like protein kinase that is responsive to abscisic acid (ABA) hormonal signals and is involved in the plant's response to drought and salt stresses. Subcellular localization assays have demonstrated that the GmHSL1b protein is located in the plasma membrane. Overexpression of the GmHSL1b gene in soybean enhanced root growth and development, as well as the plant's tolerance to salt stress, while the gmhsl1b mutant revealed increased sensitivity to salt stress. Comparative transcriptome analysis showed that some genes associated with various biological processes, such as mitogen-activated protein kinase (MAPK) cascade signaling, plant hormone signaling, cell wall remodeling, calcium signaling, and defense response mechanisms are differentially expressed in GmHSL1b overexpressing roots. Our research indicated that GmHSL1b can regulate the expression level of the candidate aquaporin GmPIP2-1, thereby affecting cell water content and the accumulation of reactive oxygen species (ROS) under salt stress. These findings indicate that the GmHSL1b participates in regulating root development and enhancing the tolerance to salt stress, thus offering insights for boosting crop adaptability to environmental stresses.

Unraveling key genes and pathways involved in Verticillium wilt resistance by integrative GWAS and transcriptomic approaches in Upland cotton

Verticillium dahliae Kleb, the cause of Verticillium wilt, is a particularly destructive soil-borne vascular disease that affects cotton, resulting in serious decline in fiber quality and causing significant losses in cotton production worldwide. However, the progress in identification of wilt-resistance loci or genes in cotton has been limited, most probably due to the highly complex genetic nature of the trait. Nevertheless, the molecular mechanism behind the Verticillium wilt resistance remains poorly understood. In the present study, we investigated the phenotypic variations in Verticillium tolerance and conducted a genome wide association study (GWAS) among a natural population containing 383 accessions of upland cotton germplasm and performed transcriptomic analysis of cotton genotypes with differential responses to Verticillium wilt. GWAS detected 70 significant SNPs and 116 genes associated with resistance loci in two peak signals on D02 and D11 in E1. The transcriptome analysis identified a total of 2689 and 13289 differentially expressed genes (DEGs) among the Verticillium wilt-tolerant (J46) and wilt-susceptible (J11) genotypes, respectively. The DEGs were predominantly enriched in metabolism, plant hormone signal transduction, phenylpropanoid pathway, MAPK cascade pathway and plant-pathogen interaction pathway in GO and KEGG analyses. The identified DEGs were found to comprise several transcription factor (TF) gene families, primarily including AP2/ERF, ZF, WRKY, NAC and MYB, in addition to pentatricopeptide repeat (PPR) proteins and Resistance (R) genes. Finally, by integrating the two results, 34 candidate genes were found to overlap between GWAS and RNA-seq analyses, associated with Verticillium-wilt resistance, including WRKY, MYB, CYP and RGA. This work contributes to our knowledge of the molecular processes underlying cotton responses to Verticillium wilt, offering crucial insights for additional research into the genes and pathways implicated in these responses and paving the way for developing Verticillium wilt-resistant cotton varieties through accelerated breeding by providing a plethora of candidate genes.

Transposon-triggered epigenetic chromatin dynamics modulate EFR-related pathogen response

Infectious diseases drive wild plant evolution and impact crop yield. Plants, like animals, sense biotic threats through pattern recognition receptors (PRRs). Overly robust immune responses can harm plants; thus, understanding the tuning of defense response mechanisms is crucial for developing pathogen-resistant crops. In this study, we found that an inverted-repeat transposon (EFR-associated IR, Ea-IR) located between the loci encoding PRRs ELONGATION FACTOR-TU RECEPTOR (EFR) and myosin XI-k (XI-k) in Arabidopsis affects chromatin organization, promoting the formation of a repressive chromatin loop. Upon pathogen infection, chromatin changes around EFR and XI-k correlate with increased EFR transcription. Pathogen-induced chromatin opening causes RNA polymerase II readthrough, producing a longer, Ea-IR-containing XI-k transcript, processed by Dicer-like enzymes into small RNAs, which reset chromatin to a repressive state attenuating the immune response after infection. Arabidopsis accessions lacking Ea-IR have higher basal EFR levels and resistance to pathogens. We show a scenario in which a transposon, chromatin organization and gene expression interact to fine-tune immune responses, during both the course of infection and the course of evolution.

Plant microbiota feedbacks through dose-responsive expression of general non-self response genes

The ability of plants to perceive and react to biotic and abiotic stresses is critical for their health. We recently identified a core set of genes consistently induced by members of the leaf microbiota, termed general non-self response (GNSR) genes. Here we show that GNSR components conversely impact leaf microbiota composition. Specific strains that benefited from this altered assembly triggered strong plant responses, suggesting that the GNSR is a dynamic system that modulates colonization by certain strains. Examination of the GNSR to live and inactivated bacteria revealed that bacterial abundance, cellular composition and exposure time collectively determine the extent of the host response. We link the GNSR to pattern-triggered immunity, as diverse microbe- or danger-associated molecular patterns cause dynamic GNSR gene expression. Our findings suggest that the GNSR is the result of a dose-responsive perception and signalling system that feeds back to the leaf microbiota and contributes to the intricate balance of plant-microbiome interactions.

Taming of the microbial beasts: Plant immunity tethers potentially pathogenic microbiota members

Plants are in intimate association with taxonomically structured microbial communities called the plant microbiota. There is growing evidence that the plant microbiota contributes to the holistic performance and general health of plants, especially under unfavorable situations. Despite the attached benefits, surprisingly, the plant microbiota in nature also includes potentially pathogenic strains, signifying that the plant hosts have tight control over these microbes. Despite the conceivable role of plant immunity in regulating its microbiota, we lack a complete understanding of its role in governing the assembly, maintenance, and function of the plant microbiota. Here, we highlight the recent progress on the mechanistic relevance of host immunity in orchestrating plant-microbiota dialogues and discuss the pluses and perils of these microbial assemblies.

The pathogen-induced peptide CEP14 is perceived by the receptor-like kinase CEPR2 to promote systemic disease resistance in Arabidopsis

Secreted plant peptides that trigger cellular signaling are crucial for plant growth, development, and adaptive responses to environmental stresses. In Arabidopsis (Arabidopsis thaliana), the C-TERMINALLY ENCODED PEPTIDE (CEP) family is a class of secreted signaling peptides that is phylogenetically divided into 2 groups: group I (CEP1-CEP12) and group II (CEP13-CEP15). Several group I CEP peptides regulate root architecture and nitrogen starvation responses, whereas the biological activity and roles of group II CEPs remain unknown. Here, we report that a group II CEP peptide, CEP14, functions as a pathogen-induced elicitor of Arabidopsis immunity. In response to infection by the bacterial pathogen Pseudomonas syringae, CEP14 expression was highly induced via the salicylic acid pathway in Arabidopsis leaves and roots. In the absence of a pathogen attack, treatment of Arabidopsis plants with synthetic CEP14 peptides was sufficient to trigger immune responses. Genetic and biochemical analyses demonstrated that the receptor-like kinase CEP RECEPTOR 2 (CEPR2) perceives CEP14 to trigger plant immunity. The SOMATIC EMBRYOGENESIS RECEPTOR KINASES (SERKs) BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and SERK4 also participated in CEP14 perception by forming CEP14-induced complexes with CEPR2. Overexpression of CEP14 largely enhanced Arabidopsis resistance to P. syringae, while CEP14 or CEPR2 mutation significantly attenuated Arabidopsis systemic resistance to P. syringae. Taken together, our data reveal that the pathogen-induced CEP14 peptide, which is perceived by the CEPR2-BAK1/SERK4 receptor complexes, acts as an endogenous elicitor to promote systemic disease resistance in Arabidopsis.

CEP signaling coordinates plant immunity with nitrogen status

Plant endogenous signaling peptides shape growth, development and adaptations to biotic and abiotic stress. Here, we identify C-TERMINALLY ENCODED PEPTIDEs (CEPs) as immune-modulatory phytocytokines in Arabidopsis thaliana. Our data reveals that CEPs induce immune outputs and are required to mount resistance against the leaf-infecting bacterial pathogen Pseudomonas syringae pv. tomato. We show that effective immunity requires CEP perception by tissue-specific CEP RECEPTOR 1 (CEPR1) and CEPR2. Moreover, we identify the related RECEPTOR-LIKE KINASE 7 (RLK7) as a CEP4-specific CEP receptor contributing to CEP-mediated immunity, suggesting a complex interplay of multiple CEP ligands and receptors in different tissues during biotic stress. CEPs have a known role in the regulation of root growth and systemic nitrogen (N)-demand signaling. We provide evidence that CEPs and their receptors promote immunity in an N status-dependent manner, suggesting a previously unknown molecular crosstalk between plant nutrition and cell surface immunity. We propose that CEPs and their receptors are central regulators for the adaptation of biotic stress responses to plant-available resources.

Nutrient Requirements Shape the Preferential Habitat of Allorhizobium vitis VAR03-1, a Commensal Bacterium, in the Rhizosphere of Arabidopsis thaliana

A diverse range of commensal bacteria inhabit the rhizosphere, influencing host plant growth and responses to biotic and abiotic stresses. While root-released nutrients can define soil microbial habitats, the bacterial factors involved in plant-microbe interactions are not well characterized. In this study, we investigated the colonization patterns of two plant disease biocontrol agents, Allorhizobium vitis VAR03-1 and Pseudomonas protegens Cab57, in the rhizosphere of Arabidopsis thaliana using Murashige and Skoog (MS) agar medium. VAR03-1 formed colonies even at a distance from the roots, preferentially in the upper part, while Cab57 colonized only the root surface. The addition of sucrose to the agar medium resulted in excessive proliferation of VAR03-1, similar to its pattern without sucrose, whereas Cab57 formed colonies only near the root surface. Overgrowth of both bacterial strains upon nutrient supplementation inhibited host growth, independent of plant immune responses. This inhibition was reduced in the VAR03-1 ΔrecA mutant, which exhibited increased biofilm formation, suggesting that some activities associated with the free-living lifestyle rather than the sessile lifestyle may be detrimental to host growth. VAR03-1 grew in liquid MS medium with sucrose alone, while Cab57 required both sucrose and organic acids. Supplementation of sugars and organic acids allowed both bacterial strains to grow near and away from Arabidopsis roots in MS agar. These results suggest that nutrient requirements for bacterial growth may determine their growth habitats in the rhizosphere, with nutrients released in root exudates potentially acting as a limiting factor in harnessing microbiota.

Post-translational modifications control the signal at the crossroads of plant-pathogen interactions

The co-evolution of plants and pathogens has enabled them to 'outsmart' each other by promoting their own defence responses and suppressing those of the other. While plants are reliant on their sophisticated immune signalling pathways, pathogens make use of effector proteins to achieve the objective. This entails rapid regulation of underlying molecular mechanisms for prompt induction of associated signalling events in both plants as well as pathogens. The past decade has witnessed the emergence of post-translational modification (PTM) of proteins as a key a factor in modulating cellular responses. The ability of PTMs to expand the functional diversity of the proteome and induce rapid changes at the appropriate time enables them to play crucial roles in the regulation of plant-pathogen interactions. Therefore, this review will delve into the intricate interplay of five major PTMs involved in plant defence and pathogen countermeasures. We discuss how plants employ PTMs to fortify their immune networks, and how pathogen effectors utilize/target host modification systems to gain entry into plants and cause disease. We also emphasize the need for identification of novel PTMs and propose the use of PTM pathways as potential targets for genome editing approaches.

Ralstonia solanacearum Alters Root Developmental Programmes in Auxin-Dependent and -Independent Manners

Microbial pathogens and other parasites can modify the development of their hosts, either as a target or a side effect of their virulence activities. The plant-pathogenic bacterium Ralstonia solanacearum, causal agent of the devastating bacterial wilt disease, is a soilborne microbe that invades host plants through their roots and later proliferates in xylem vessels. In this work, we studied the early stages of R. solanacearum infection in the model plant Arabidopsis thaliana, using an in vitro infection system. In addition to the previously reported inhibition of primary root length and increase in root hair formation at the root tip, we observed an earlier xylem differentiation during R. solanacearum infection that occurs in a HrpG-dependent manner, suggesting that the pathogen actively promotes the development of the vascular system upon invasion of the root. Moreover, we found that the phytohormone auxin, of which the accumulation is promoted by the bacterial infection, is required for the R. solanacearum-triggered induction of root hair formation but not earlier xylem differentiation. Altogether, our results shed light on the capacity of R. solanacearum to induce alterations of root developmental pathways and on the role of auxin in this process.

A peptide-receptor module links cell wall integrity sensing to pattern-triggered immunity

Plants employ cell-surface receptors to perceive non- or altered-self, including the integrity of their cell wall. Here we identify a specific ligand-receptor module responsive to cell wall damage that potentiates immunity in Arabidopsis. Disruption of cell wall integrity by inhibition of cellulose biosynthesis promotes pattern-triggered immunity transcriptionally in a manner dependent on the receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2). Notably, while MIK2 can perceive peptides of the large SERINE RICH ENDOGENOUS PEPTIDE family, a single member of this family, SCOOP18, is transcriptionally induced upon cell wall damage and is required for subsequent responses such as lignification and immunity potentiation. Collectively, our results identify the SCOOP18-MIK2 ligand-receptor module as an important central hub, connecting plant cell wall integrity sensing with immunity.

The parallel narrative of RGF/GLV/CLEL peptide signalling

Plant peptide families share distinct characteristics, and many members are in homologous signalling pathways controlling development and responses to external signals. The root meristem growth factor (RGF) peptides/GOLVEN (GLV)/CLAVATA3-ESR-related like (CLEL) are a family of short signalling peptides that are derived from a precursor protein and undergo post-translational modifications. Their role in root meristem development is well established and recent efforts have identified subtilase processing pathways and several downstream signalling components. This discovery has enabled the convergence of previously distinct pathways and enhanced our understanding of plant developmental processes. Here, we review the structure-function relationship of RGF peptides, the post-translational modification pathways, and the downstream signalling mechanisms and highlight components of these pathways that are known in non-RGF-mediated pathways.

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.

The leucine-rich repeat receptor kinase QSK1 regulates PRR-RBOHD complexes targeted by the bacterial effector HopF2Pto

Plants detect pathogens using cell-surface pattern recognition receptors (PRRs) such as ELONGATION Factor-TU (EF-TU) RECEPTOR (EFR) and FLAGELLIN SENSING 2 (FLS2), which recognize bacterial EF-Tu and flagellin, respectively. These PRRs belong to the leucine-rich repeat receptor kinase (LRR-RK) family and activate the production of reactive oxygen species via the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD). The PRR-RBOHD complex is tightly regulated to prevent unwarranted or exaggerated immune responses. However, certain pathogen effectors can subvert these regulatory mechanisms, thereby suppressing plant immunity. To elucidate the intricate dynamics of the PRR-RBOHD complex, we conducted a comparative coimmunoprecipitation analysis using EFR, FLS2, and RBOHD in Arabidopsis thaliana. We identified QIAN SHOU KINASE 1 (QSK1), an LRR-RK, as a PRR-RBOHD complex-associated protein. QSK1 downregulated FLS2 and EFR abundance, functioning as a negative regulator of PRR-triggered immunity (PTI). QSK1 was targeted by the bacterial effector HopF2Pto, a mono-ADP ribosyltransferase, reducing FLS2 and EFR levels through both transcriptional and transcription-independent pathways, thereby inhibiting PTI. Furthermore, HopF2Pto transcriptionally downregulated PROSCOOP genes encoding important stress-regulated phytocytokines and their receptor MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2. Importantly, HopF2Pto requires QSK1 for its accumulation and virulence functions within plants. In summary, our results provide insights into the mechanism by which HopF2Pto employs QSK1 to desensitize plants to pathogen attack.

Integrated Biological Experiments and Proteomic Analyses of Nicotiana tabacum Xylem Sap Revealed the Host Response to Tomato Spotted Wilt Orthotospovirus Infection

The plant vascular system is not only a transportation system for delivering nutrients but also a highway transport network for spreading viruses. Tomato spotted wilt orthotospovirus (TSWV) is among the most destructive viruses that cause serious losses in economically important crops worldwide. However, there is minimal information about the long-distance movements of TSWV in the host plant vascular system. In this this study, we confirm that TSWV virions are present in the xylem as observed by transmission electron microscopy (TEM). Further, a quantitative proteomic analysis based on label-free methods was conducted to reveal the uniqueness of protein expression in xylem sap during TSWV infection. Thus, this study identified and quantified 3305 proteins in two groups. Furthermore, TSWV infection induced three viral structural proteins, N, Gn and Gc, and 315 host proteins differentially expressed in xylem (163 up-regulated and 152 down-regulated). GO enrichment analysis showed up-regulated proteins significantly enriched in homeostasis, wounding, defense response, and DNA integration terms, while down-regulated proteins significantly enriched in cell wall biogenesis/xyloglucan metabolic process-related terms. KEGG enrichment analysis showed that the differentially expressed proteins (DEPs) were most strongly associated with plant-pathogen interaction, MAPK signaling pathway, and plant hormone signal transduction. Cluster analysis of DEPs function showed the DEPs can be categorized into cell wall metabolism-related proteins, antioxidant proteins, PCD-related proteins, host defense proteins such as receptor-like kinases (RLKs), salicylic acid binding protein (SABP), pathogenesis related proteins (PR), DNA methylation, and proteinase inhibitor (PI). Finally, parallel reaction monitoring (PRM) validated 20 DEPs, demonstrating that the protein abundances were consistent between label-free and PRM data. Finally, 11 genes were selected for RT-qPCR validation of the DEPs and label-free-based proteomic analysis concordant results. Our results contribute to existing knowledge on the complexity of host plant xylem system response to virus infection and provide a basis for further study of the mechanism underlying TSWV long-distance movement in host plant vascular system.

Dual transcriptional characterization of spinach and Peronospora effusa during resistant and susceptible race-cultivar interactions

BACKGROUND

Spinach downy mildew, caused by the obligate oomycete pathogen, Peronospora effusa remains a major concern for spinach production. Disease control is predominantly based on development of resistant spinach cultivars. However, new races and novel isolates of the pathogen continue to emerge and overcome cultivar resistance. Currently there are 20 known races of P. effusa. Here we characterized the transcriptomes of spinach, Spinacia oleracea, and P. effusa during disease progression using the spinach cultivar Viroflay, the near isogenic lines NIL1 and NIL3, and P. effusa races, R13 and R19, at 24 h post inoculation and 6 days post inoculation. A total of 54 samples were collected and subjected to sequencing and transcriptomic analysis.

RESULTS

Differentially expressed gene (DEG) analysis in resistant spinach interactions of R13-NIL1 and R19-NIL3 revealed spinach DEGs from protein kinase-like and P-loop containing families, which have roles in plant defense. The homologous plant defense genes included but were not limited to, receptor-like protein kinases (Spiol0281C06495, Spiol06Chr21559 and Spiol06Chr24027), a BAK1 homolog (Spiol0223C05961), genes with leucine rich repeat motifs (Spiol04Chr08771, Spiol04Chr01972, Spiol05Chr26812, Spiol04Chr11049, Spiol0084S08137, Spiol03Chr20299) and ABC-transporters (Spiol02Chr28975, Spiol06Chr22112, Spiol06Chr03998 and Spiol04Chr09723). Additionally, analysis of the expression of eight homologous to previously reported downy mildew resistance genes revealed that some are differentially expressed during resistant reactions but not during susceptible reactions. Examination of P. effusa gene expression during infection of susceptible cultivars identified expressed genes present in R19 or R13 including predicted RxLR and Crinkler effector genes that may be responsible for race-specific virulence on NIL1 or NIL3 spinach hosts, respectively.

CONCLUSIONS

These findings deliver foundational insight to gene expression in both spinach and P. effusa during susceptible and resistant interactions and provide a library of candidate genes for further exploration and functional analysis. Such resources will be beneficial to spinach breeding efforts for disease resistance in addition to better understanding the virulence mechanisms of this obligate pathogen.

Natural variations of maize ZmLecRK1 determine its interaction with ZmBAK1 and resistance patterns to multiple pathogens

Maize (Zea mays) is one of the most important crops in the world, but its yield and quality are seriously affected by diverse diseases. Identifying broad-spectrum resistance genes is crucial for developing effective strategies to control the disease in maize. In a genome-wide study in maize, we identified a G-type lectin receptor kinase ZmLecRK1, as a new resistance protein against Pythium aphanidermatum, one of the causal pathogens of stalk rot in maize. Genetic analysis showed that the specific ZmLecRK1 allele can confer resistance to multiple pathogens in maize. The cell death and disease resistance phenotype mediated by the resistant variant of ZmLecRK1 requires the co-receptor ZmBAK1. A naturally occurring A404S variant in the extracellular domain of ZmLecRK1 determines the ZmLecRK1-ZmBAK1 interaction and the formation of ZmLecRK1-related protein complexes. Interestingly, the ZmLecRK1 susceptible variant was found to possess the amino acid S404 that is present in the ancestral variants of ZmLecRK1 and conserved among the majority of grass species, while the resistance variant of ZmLecRK1 with A404 is only present in a few maize inbred lines. Substitution of S by A at position 404 in ZmLecRK1-like proteins of sorghum and rice greatly enhances their ability to induce cell death. Further transcriptomic analysis reveals that ZmLecRK1 likely regulates gene expression related to the pathways in cell wall organization or biogenesis in response to pathogen infection. Taken together, these results suggest that the ZmLecRK1 resistance variant enhances its binding affinity to the co-receptor ZmBAK1, thereby enhancing the formation of active complexes for defense in maize. Our work highlights the biotechnological potential for generating disease-resistant crops by precisely modulating the activity of ZmLecRK1 and its homologs through targeted base editing.

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.

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.

Cutin-derived oligomers induce hallmark plant immune responses

The cuticle constitutes the outermost defensive barrier of most land plants. It comprises a polymeric matrix-cutin, surrounded by soluble waxes. Moreover, the cuticle constitutes the first line of defense against pathogen invasion, while also protecting the plant from many abiotic stresses. Aliphatic monomers in cutin have been suggested to act as immune elicitors in plants. This study analyses the potential of cutin oligomers to activate rapid signaling outputs reminiscent of pattern-triggered immunity in the model plant Arabidopsis. Cutin oligomeric mixtures led to Ca2+ influx and mitogen-activated protein kinase activation. Comparable responses were measured for cutin, which was also able to induce a reactive oxygen species burst. Furthermore, cutin oligomer treatment resulted in a unique transcriptional reprogramming profile, having many archetypal features of pattern-triggered immunity. Targeted spectroscopic and spectrometric analyses of the cutin oligomers suggest that the elicitor compounds consist mostly of two up to three 10,16-dihydroxyhexadecanoic acid monomers linked together through ester bonds. This study demonstrates that cutin breakdown products can act as inducers of early plant immune responses. Further investigation is needed to understand how cutin breakdowns are perceived and to explore their potential use in agriculture.

Genome-wide identification of Brassinosteroid insensitive 1-associated receptor kinase 1 genes and expression analysis in response to pathogen infection in cucumber (Cucumis sativus L.)

BACKGROUND

BAK1 (Brassinosteroid insensitive 1-associated receptor kinase 1) plays an important role in disease resistance in plants. However, the function of BAK1 family in cucumber and the decisive genes for disease-resistance remain elusive.

RESULTS

Here, we identified 27 CsBAK1s in cucumber, and classified them into five subgroups based on phylogenetic analysis and gene structure. CsBAK1s in the same subgroup shared the similar motifs, but different gene structures. Cis-elements analysis revealed that CsBAK1s might respond to various stress and growth regulation. Three segmentally duplicated pairwise genes were identified in cucumber. In addition, Ka/Ks analysis indicated that CsBAK1s were under positive selection during evolution. Tissue expression profile showed that most CsBAK1s in Subgroup II and IV showed constitutive expression, members in other subgroups showed tissue-specific expression. To further explore whether CsBAK1s were involved in the resistance to pathogens, the expression patterns of CsBAK1s to five pathogens (gummy stem blight, powdery mildew, downy mildew, grey mildew, and fusarium wilt) reveled that different CsBAK1s had specific roles in different pathogen infections. The expression of CsBAK1-14 was induced/repressed significantly by five pathogens, CsBAK1-14 might play an important role in disease resistance in cucumber.

CONCLUSIONS

27 BAK1 genes were identified in cucumber from a full perspective, which have important functions in pathogen infection. Our study provided a theoretical basis to further clarify the function of BAK1s to disease resistance in cucumber.

GhMPK9-GhRAF39_1-GhWRKY40a Regulates the GhERF1b- and GhABF2-Mediated Pathways to Increase Cotton Disease Resistance

Mitogen-activated protein kinase (MAPK) cascade is the center of plant signal transduction system that amplify immune signals into cellular responses by phosphorylating diverse substrates. The MAPK cascade consisting of MAPK kinase kinases (MAPKKKs), MAPK kinases (MAPKKs), and MAPKs is well characterized in plants, in which Raf-like kinases are generally regarded as MAPKKKs. However, it is rarely reported that Raf-like MAPKKKs function as middle regulators to link MAPK and its downstream transcription factors in plant immunity. Verticillium wilt, caused by the soil-borne vascular fungus Verticillium dahliae, is a serious disease in many plants, including cotton. The previous studies showed that GhMPK9 (a MAPK) is involved in the response to Verticillium wilt. Here, the Raf-like kinase GhRAF39_1 is reported as helper regulates the phosphorylation of WRKY transcription factor GhWRKY40a by GhMPK9. The phosphorylated GhWRKY40a can further activate the transcription of GhERF1b to up-regulate defense-related genes while inhibit the transcription of GhABF2 to regulate the stomatal opening, thus improving the resistance to Verticillium wilt in cotton. This study reveals a new signaling module of GhMPK9-GhRAF39_1-GhWRKY40a to regulate GhERF1b- and GhABF2-mediated defense responses, which triggers plant defense against Verticillium wilt.

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.

Determination of anthracnose (Colletotrichum fructicola) resistance mechanism using transcriptome analysis of resistant and susceptible pear (Pyrus pyrifolia)

BACKGROUND

Anthracnose, mainly caused by Colletotrichum fructicola, leads to severe losses in pear production. However, there is limited information available regarding the molecular response to anthracnose in pears.

RESULTS

In this study, the anthracnose-resistant variety 'Seli' and susceptible pear cultivar 'Cuiguan' were subjected to transcriptome analysis following C. fructicola inoculation at 6 and 24 h using RNA sequencing. A total of 3186 differentially expressed genes were detected in 'Seli' and 'Cuiguan' using Illumina sequencing technology. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses indicated that the transcriptional response of pears to C. fructicola infection included responses to reactive oxygen species, phytohormone signaling, phenylpropanoid biosynthesis, and secondary metabolite biosynthetic processes. Moreover, the mitogen-activated protein kinase (MAPK) signaling pathway and phenylpropanoid biosynthesis were involved in the defense of 'Seli'. Furthermore, the gene coexpression network data showed that genes related to plant-pathogen interactions were associated with C. fructicola resistance in 'Seli' at the early stage.

CONCLUSION

Our results showed that the activation of specific genes in MAPK, calcium signaling pathways and phenylpropanoid biosynthesis was highly related to C. fructicola resistance in 'Seli' and providing several potential candidate genes for breeding anthracnose-resistant pear varieties.

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.

Plant Immunity: At the Crossroads of Pathogen Perception and Defense Response

Plants are challenged by different microbial pathogens that affect their growth and productivity. However, to defend pathogen attack, plants use diverse immune responses, such as pattern-triggered immunity (PTI), effector-triggered immunity (ETI), RNA silencing and autophagy, which are intricate and regulated by diverse signaling cascades. Pattern-recognition receptors (PRRs) and nucleotide-binding leucine-rich repeat (NLR) receptors are the hallmarks of plant innate immunity because they can detect pathogen or related immunogenic signals and trigger series of immune signaling cascades at different cellular compartments. In plants, most commonly, PRRs are receptor-like kinases (RLKs) and receptor-like proteins (RLPs) that function as a first layer of inducible defense. In this review, we provide an update on how plants sense pathogens, microbe-associated molecular patterns (PAMPs or MAMPs), and effectors as a danger signals and activate different immune responses like PTI and ETI. Further, we discuss the role RNA silencing, autophagy, and systemic acquired resistance as a versatile host defense response against pathogens. We also discuss early biochemical signaling events such as calcium (Ca2+), reactive oxygen species (ROS), and hormones that trigger the activation of different plant immune responses. This review also highlights the impact of climate-driven environmental factors on host-pathogen interactions.

Fusarium sacchari FsNis1 induces plant immunity

Pokkah Boeng disease (PBD), caused by Fusarium sacchari, severely affects sugarcane yield and quality. Necrosis-inducing secreted protein 1 (Nis1) is a fungal secreted effector that induces necrotic lesions in plants. It interacts with host receptor-like kinases and inhibits their kinase activity. FsNis1 contains the Nis1 structure and triggered a pathogen-associated molecular pattern-triggered immune response in Nicotiana benthamiana, as reflected by causing reactive oxygen species production, callose accumulation, and the upregulated expression of defense response genes. Knockout of this gene in F. sacchari revealed a significant reduction in its pathogenicity, whereas the pathogenicity of the complementary mutant recovered to the wild-type levels, making this gene an important virulence factor for F. sacchari. In addition, the signal peptide of FsNis1 was required for the induction of cell death and PTI response in N. benthamiana. Thus, FsNis1 may not only be a key virulence factor for F. sacchari but may also induce defense responses in plants. These findings provide new insights into the function of Nis1 in host-pathogen interactions.

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.

Inhibition of PbeXTH1 and PbeSEOB1 is required for the Valsa canker resistance contributed by Wall-associated kinase gene MbWAK1

Wall-associated kinases (WAKs) have been determined to recognize pathogenic signals and initiate plant immune responses. However, the roles of the family members in host resistance against Valsa canker, a serious fungal disease of apples and pears, are largely unknown. Here, we identified MbWAK1 in Malus baccata, a resistant germplasm differentially expressed during infection by Valsa mali (Vm). Over-expression of MbWAK1 enhanced the Valsa canker resistance of apple and pear fruits and 'Duli-G03' (Pyrus betulifolia) suspension cells. A large number of phloem, cell wall, and lipid metabolic process-related genes were differentially expressed in overexpressed suspension cell lines in response to Valsa pyri (Vp) signals. Among these, the expression of xyloglucan endotransglucosylase/hydrolase (XTH) gene PbeXTH1 and sieve element occlusion B-like (SEOB) gene PbeSEOB1 were significantly inhibited. Transient expression of PbeXTH1 or PbeSEOB1 compromised the expressional induction of MbWAK1 and the resistance contributed by MbWAK1. In addition, PbeXTH1 and PbeSEOB1 suppressed the immune response induced by MbWAK1. Our results enriched the molecular mechanisms for MbWAK1 against Valsa canker and resistant breeding.

Pattern recognition receptors as potential therapeutic targets for developing immunological engineered plants

There is an urgent need to combat pathogen infestations in crop plants to ensure food security worldwide. To counter this, plants have developed innate immunity mediated by Pattern Recognition Receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and damage- associated molecular patterns (DAMPs). PRRs activate Pattern-Triggered Immunity (PTI), a defence mechanism involving intricate cell-surface and intracellular receptors. The diverse ligand-binding ectodomains of PRRs, including leucine-rich repeats (LRRs) and lectin domains, facilitate the recognition of MAMPs and DAMPs. Pathogen resistance is mediated by a variety of PTI responses, including membrane depolarization, ROS production, and the induction of defence genes. An integral part of intracellular immunity is the Nucleotide-binding Oligomerization Domain, Leucine-rich Repeat proteins (NLRs) which recognize and respond to effectors in a potent manner. Enhanced understanding of PRRs, their ligands, and downstream signalling pathways has contributed to the identification of potential targets for genetically modified plants. By transferring PRRs across plant species, it is possible to create broad-spectrum resistance, potentially offering innovative solutions for plant protection and global food security. The purpose of this chapter is to provide an update on PRRs involved in disease resistance, clarify the mechanisms by which PRRs recognize ligands to form active receptor complexes and present various applications of PRRs and PTI in disease resistance management for plants.

The ETI-dependent receptor-like kinase 1 positively regulates effector-triggered immunity by stabilizing NLR-required for cell death 4 in Nicotiana benthamiana

Leucine-rich repeat receptor-like kinases (LRR-RLKs) comprise the largest class of membrane-localized receptor-like kinases in plants. Leucine-rich repeat receptor-like kinases are key immune sectors contributing to pattern-triggered immunity (PTI), but whether LRR-RLK mediates effector-triggered immunity (ETI) in plants remains unclear. In this study, we evaluated the function of LRR-RLKs in regulating ETI by using a virus-induced gene silencing (VIGS)-based reverse genetic screening assay, and identified a LRR-RLK named ETI-dependent receptor-like kinase 1 (EDK1) required for ETI triggered by the avirulence effector AVRblb2 secreted by Phytophthora infestans and its cognate receptor Rpi-blb2. Silencing or knockout of EDK1 compromised immunity mediated by Rpi-blb2 and the cell death triggered by recognition of AVRblb2. NLR-required for cell death 4 (NRC4), a signaling component acts downstream of Rpi-blb2, was identified that interacts with EDK1 using the LC-MS analysis and the interaction was further evaluated by co-immunoprecipitation. EDK1 promotes protein accumulation of NRC4 in a kinase-dependent manner and positively regulates resistance to P. infestans in Nicotiana benthamiana. Our study revealed that EDK1 positively regulates plant ETI through modulating accumulation of the NLR signaling component NRC4, representing a new regulatory role of the membrane-localized LRR-RLKs in plant immunity.

S-nitrosylation of a receptor-like cytoplasmic kinase regulates plant immunity

Perception of pathogen/microbial-associated molecular patterns (P/MAMPs) by plant cell surface receptors leads to a sustained burst of reactive oxygen species (ROS), a key feature of P/MAMP-triggered immunity (PTI). Here we report that P/MAMP recognition leads to a rapid nitrosative burst, initiating the accumulation of nitric oxide (NO), subsequently leading to S-nitrosylation of the receptor-like cytoplasmic kinase (RLCK), botrytis-induced kinase 1 (BIK1), at Cys80. This redox-based, posttranslational modification, promotes the phosphorylation of BIK1, subsequently resulting in BIK1 activation and stabilization. Further, BIK1 S-nitrosylation increases its physical interaction with RBOHD, the source of the apoplastic oxidative burst, promoting ROS formation. Our data identify mechanistic links between rapid NO accumulation and the expression of PTI, providing insights into plant immunity.

Structural and biochemical basis of FLS2-mediated signal activation and transduction in rice

The receptor-like kinase FLAGELLIN-SENSITIVE 2 (FLS2) functions as a bacterial flagellin receptor localized on the cell membrane of plants. In Arabidopsis, the co-receptor BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) cooperates with FLS2 to detect the flagellin epitope flg22, resulting in formation of a signaling complex that triggers plant defense responses. However, the co-receptor responsible for recognizing and signaling the flg22 epitope in rice remains to be determined, and the precise structural mechanism underlying FLS2-mediated signal activation and transduction has not been clarified. This study presents the structural characterization of a kinase-dead mutant of the intracellular kinase domain of OsFLS2 (OsFLS2-KDD1013A) in complex with ATP or ADP, resolved at resolutions of 1.98 Å and 2.09 Å, respectively. Structural analysis revealed that OsFLS2 can adopt an active conformation in the absence of phosphorylation, although it exhibits only weak basal catalytic activity for autophosphorylation. Subsequent investigations demonstrated that OsSERK2 effectively phosphorylates OsFLS2, which reciprocally phosphorylates OsSERK2, leading to complete activation of OsSERK2 and rapid phosphorylation of the downstream substrate receptor-like cytoplasmic kinases OsRLCK176 and OsRLCK185. Through mass spectrometry experiments, we successfully identified critical autophosphorylation sites on OsSERK2, as well as sites transphosphorylated by OsFLS2. Furthermore, we demonstrated the interaction between OsSERK2 and OsFLS2, which is enhanced in the presence of flg22. Genetic evidence suggests that OsRLCK176 and OsRLCK185 may function downstream of the OsFLS2-mediated signaling pathway. Our study reveals the molecular mechanism by which OsFLS2 mediates signal transduction pathways in rice and provides a valuable example for understanding RLK-mediated signaling pathways in plants.

A glycoside hydrolase 30 protein BpXynC of Bacillus paralicheniformis NMSW12 recognized as A MAMP triggers plant immunity response

Bacillus spp. has been widely used as a biocontrol agent to control plant diseases. However, little is known about mechanisms of the protein MAMP secreted by Bacillus spp. Herein, our study reported a glycoside hydrolase family 30 (GH30) protein, BpXynC, produced by the biocontrol bacteria Bacillus paralicheniformis NMSW12, that can induce cell death in several plant species. The results revealed that the recombinant protein triggers cell death in Nicotiana benthamiana in a BAK1-dependent manner and elicits an early defense response, including ROS burst, activation of MAPK cascades, and upregulation of plant immunity marker genes. BpXynC was also found to be a glucuronoxylanase that exhibits hydrolysis activity on xlyan. Two mutants of BpXynC which lost the glucuronoxylanase activity still retained the elicitor activity. The qRT-PCR results of defense-related genes showed that BpXynC induces plant immunity responses via an SA-mediated pathway. BpXynC and its mutants could induce resistance in N. benthamiana against infection by Sclerotinia sclerotiorum and tobacco mosaic virus (TMV). Furthermore, BpXynC-treated tomato fruits exhibited strong resistance to the infection of Phytophthora capsica. Overall, our study revealed that GH30 protein BpXynC can induce plant immunity response as MAMP, which can be further applied as a biopesticide to control plant diseases.

Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

A new spider mite elicitor triggers plant defence and promotes resistance to herbivores

Herbivore-associated elicitors (HAEs) are active molecules produced by herbivorous insects. Recognition of HAEs by plants induces defence that resist herbivore attacks. We previously demonstrated that the tomato red spider mite Tetranychus evansi triggered defence in Nicotiana benthamiana. However, our knowledge of HAEs from T. evansi remains limited. Here, we characterize a novel HAE, Te16, from T. evansi and dissect its function in mite-plant interactions. We investigate the effects of Te16 on spider mites and plants by heterologous expression, virus-induced gene silencing assay, and RNA interference. Te16 induces cell death, reactive oxygen species (ROS) accumulation, callose deposition, and jasmonate (JA)-related responses in N. benthamiana leaves. Te16-mediated cell death requires a calcium signalling pathway, cytoplasmic localization, the plant co-receptor BAK1, and the signalling components SGT1 and HSP90. The active region of Te16-induced cell death is located at amino acids 114-293. Moreover, silencing Te16 gene in T. evansi reduces spider mite survival and hatchability, but expressing Te16 in N. benthamiana leaves enhances plant resistance to herbivores. Finally, Te16 gene is specific to Tetranychidae species and is highly conserved in activating plant immunity. Our findings reveal a novel salivary protein produced by spider mites that elicits plant defence and resistance to insects, providing valuable clues for pest management.

Comprehensive analysis of sesame LRR-RLKs: structure, evolution and dynamic expression profiles under Macrophomina phaseolina stress

Leucine-rich repeat receptor-like kinases (LRR-RLKs) can participate in the regulation of plant growth and development, immunity and signal transduction. Sesamum indicum, one of the most important oil crops, has a significant role in promoting human health. In this study, 175 SiLRR-RLK genes were identified in S. indicum, and they were subdivided into 12 subfamilies by phylogenetic analysis. Gene duplication analysis showed that the expansion of the SiLRR-RLK family members in the sesame was mainly due to segmental duplication. Moreover, the gene expansion of subfamilies IV and III contributed to the perception of stimuli under M. phaseolina stress in the sesame. The collinearity analysis with other plant species revealed that the duplication of SiLRR-RLK genes occurred after the differentiation of dicotyledons and monocotyledons. The expression profile analysis and functional annotation of SiLRR-RLK genes indicated that they play a vital role in biotic stress. Furthermore, the protein-protein interaction and coexpression networks suggested that SiLRR-RLKs contributed to sesame resistance to Macrophomina phaseolina by acting alone or as a polymer with other SiLRR-RLKs. In conclusion, the comprehensive analysis of the SiLRR-RLK gene family provided a framework for further functional studies on SiLRR-RLK genes.

Genomic investigation of duplication, functional conservation, and divergence in the LRR-RLK Family of Saccharum

BACKGROUND

Sugarcane (Saccharum spp.) holds exceptional global significance as a vital crop, serving as a primary source of sucrose, bioenergy, and various by-products. The optimization of sugarcane breeding by fine-tuning essential traits has become crucial for enhancing crop productivity and stress resilience. Leucine-rich repeat receptor-like kinases (LRR-RLK) genes present promising targets for this purpose, as they are involved in various aspects of plant development and defense processes.

RESULTS

Here, we present a detailed overview of phylogeny and expression of 288 (495 alleles) and 312 (1365 alleles) LRR-RLK genes from two founding Saccharum species, respectively. Phylogenetic analysis categorized these genes into 15 subfamilies, revealing considerable expansion or reduction in certain LRR-type subfamilies. Compared to other plant species, both Saccharum species had more significant LRR-RLK genes. Examination of cis-acting elements demonstrated that SsLRR-RLK and SoLRR-RLK genes exhibited no significant difference in the types of elements included, primarily involved in four physiological processes. This suggests a broad conservation of LRR-RLK gene function during Saccharum evolution. Synteny analysis indicated that all LRR-RLK genes in both Saccharum species underwent gene duplication, primarily through whole-genome duplication (WGD) or segmental duplication. We identified 28 LRR-RLK genes exhibiting novel expression patterns in response to different tissues, gradient development leaves, and circadian rhythm in the two Saccharum species. Additionally, SoLRR-RLK104, SoLRR-RLK7, SoLRR-RLK113, and SsLRR-RLK134 were identified as candidate genes for sugarcane disease defense response regulators through transcriptome data analysis of two disease stresses. This suggests LRR-RLK genes of sugarcane involvement in regulating various biological processes, including leaf development, plant morphology, photosynthesis, maintenance of circadian rhythm stability, and defense against sugarcane diseases.

CONCLUSIONS

This investigation into gene duplication, functional conservation, and divergence of LRR-RLK genes in two founding Saccharum species lays the groundwork for a comprehensive genomic analysis of the entire LRR-RLK gene family in Saccharum. The results reveal LRR-RLK gene played a critical role in Saccharum adaptation to diverse conditions, offering valuable insights for targeted breeding and precise phenotypic adjustments.

Additive and Specific Effects of Elicitor Treatments on the Metabolic Profile of Arabidopsis thaliana

Several elicitors of plant defense have been identified and numerous efforts to use them in the field have been made. Exogenous elicitor treatments mimic the in planta activation of pattern-triggered immunity (PTI), which relies on the perception of pathogen-associated molecular patterns (PAMPs) such as bacterial flg22 or fungal chitins. Early transcriptional responses to distinct PAMPs are mostly overlapping, regardless of the elicitor being used. However, it remains poorly known if the same patterns are observed for metabolites and proteins produced later during PTI. In addition, little is known about the impact of a combination of elicitors on PTI and the level of induced resistance to pathogens. Here, we monitored Arabidopsis thaliana resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pto DC3000) following application of flg22 and chitosan elicitors, used individually or in combination. A slight, but not statistically significant increase in induced resistance was observed when the elicitors were applied together when compared with individual treatments. We investigated the effect of these treatments on the metabolome by using an untargeted analysis. We found that the combination of flg22 and chitosan impacted a higher number of metabolites and deregulated specific metabolic pathways compared with the elicitors individually. These results contribute to a better understanding of plant responses to elicitors, which might help better rationalize their use in the field. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The phagocytosis oxidase/Bem1p domain-containing protein PB1CP negatively regulates the NADPH oxidase RBOHD in plant immunity

Perception of pathogen-associated molecular patterns (PAMPs) by surface-localized pattern recognition receptors activates RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD) through direct phosphorylation by BOTRYTIS-INDUCED KINASE 1 (BIK1) and induces the production of reactive oxygen species (ROS). RBOHD activity must be tightly controlled to avoid the detrimental effects of ROS, but little is known about RBOHD downregulation. To understand the regulation of RBOHD, we used co-immunoprecipitation of RBOHD with mass spectrometry analysis and identified PHAGOCYTOSIS OXIDASE/BEM1P (PB1) DOMAIN-CONTAINING PROTEIN (PB1CP). PB1CP negatively regulates RBOHD and the resistance against the fungal pathogen Colletotrichum higginsianum. PB1CP competes with BIK1 for binding to RBOHD in vitro. Furthermore, PAMP treatment enhances the PB1CP-RBOHD interaction, thereby leading to the dissociation of phosphorylated BIK1 from RBOHD in vivo. PB1CP localizes at the cell periphery and PAMP treatment induces relocalization of PB1CP and RBOHD to the same small endomembrane compartments. Additionally, overexpression of PB1CP in Arabidopsis leads to a reduction in the abundance of RBOHD protein, suggesting the possible involvement of PB1CP in RBOHD endocytosis. We found PB1CP, a novel negative regulator of RBOHD, and revealed its possible regulatory mechanisms involving the removal of phosphorylated BIK1 from RBOHD and the promotion of RBOHD endocytosis.

SERKs and NIKs: Coreceptors or signaling hubs in a complex crosstalk between growth and defense?

SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES (SERKs) and NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASES (NIKs) belong to superfamily II of leucine-rich repeat receptor-like kinases, which share cytosolic kinase conservation and a similar ectodomain configuration. SERKs have been extensively demonstrated to function as coreceptors of receptor-like kinases, which sense biotic or developmental signals to initiate specific responses. NIKs, on the other hand, have emerged as downstream components in signaling cascades, not functioning as coreceptors but rather serving as hubs that converge information from both biotic and abiotic signals, resulting in a unified response. Like SERKs, NIKs play a crucial role as information spreaders in plant cells, forming hubs of high centrality. However, unlike SERKs, which function as coreceptors and assemble paired receptor-specific responses, NIKs employ a shared signaling circuit to transduce diverse biotic and abiotic signals into the same physiological response. Therefore, this review highlights the concept of signaling hubs that differ from coreceptors in signaling pathways.

PcLRR-RK3, an LRR receptor kinase is required for growth and in-planta infection processes in Phytophthora capsici

Receptor protein kinases (RPKs) critically provide the basic infrastructure to sense, perceive, and conduct the signalling events at the cell surface of organisms. The importance of LRR-RLKs has been well studied in plants, but much less information has been reported in oomycetes. In this work, we have silenced the PcLRR-RK3 and characterised its functional importance in Phytophthora capsici. PcLRR-RK3 was predicted to encode signal peptides, leucine-rich repeats, transmembrane, and kinase domains. PcLRR-RK3-silenced transformants showed impaired colony growth, decreased deformed sporangia, and reduced zoospores count. The mycelium of silenced transformants did not penetrate within the host tissues and showed defects in the pathogenicity of P. capsici. Interestingly, gene silencing also weakens the ability of zoospores germination and penetration into host tissues and fails to produce necrotic lesions. Furthermore, PcLRR-RK3 localisation was found to be the plasma membrane of the cell. Altogether, our results revealed that PcLRR-RK3 was required for the regulation of vegetative growth, zoospores penetration, and establishment into host leaf tissues.