An EFR-Cf-9 chimera confers enhanced resistance to bacterial pathogens by SOBIR1- and BAK1-dependent recognition of elf18

Overexpression of plant chitin receptors in wheat confers broad-spectrum resistance to fungal diseases

Wheat (Triticum aestivum L.) is a globally staple crop vulnerable to various fungal diseases, significantly impacting its yield. Plant cell surface receptors play a crucial role in recognizing pathogen-associated molecular patterns (PAMPs) and activating PAMP-triggered immunity, boosting resistance against a wide range of plant diseases. Although the role of plant chitin receptor CERK1 in immune recognition and defense has been established in Arabidopsis and rice, its function and potential agricultural applications in enhancing resistance to crop diseases remain largely unexplored. Here, we identify and characterize TaCERK1 in Triticeae crop wheat, uncovering its involvement in chitin recognition, immune regulation, and resistance to fungal diseases. By a comparative analysis of CERK1 homologs in Arabidopsis and monocot crops, we demonstrate that AtCERK1 in Arabidopsis elicits the most robust immune response. Moreover, we show that overexpressing TaCERK1 and AtCERK1 in wheat confers resistance to multiple fungal diseases, including Fusarium head blight, stripe rust, and powdery mildew. Notably, transgenic wheat lines with moderately expressed AtCERK1 display superior disease resistance and heightened immune responses without adversely affecting growth and yield, compared to TaCERK1 overexpression transgenics. Our findings highlight the significance of plant chitin receptors across diverse plant species and suggest potential strategies for bolstering crop resistance against broad-spectrum diseases in agricultural production through the utilization of plant immune receptors.

Immune signaling: receptor-like proteins make the difference

To resist biotic attacks, plants have evolved a sophisticated, receptor-based immune system. Cell-surface immune receptors, which are either receptor-like kinases (RLKs) or receptor-like proteins (RLPs), form the front line of the plant defense machinery. RLPs lack a cytoplasmic kinase domain for downstream immune signaling, and leucine-rich repeat (LRR)-containing RLPs constitutively associate with the RLK SOBIR1. The RLP/SOBIR1 complex was proposed to be the bimolecular equivalent of genuine RLKs. However, it appears that the molecular mechanisms by which RLP/SOBIR1 complexes and RLKs mount immunity show some striking differences. Here, we summarize the differences between RLP/SOBIR1 and RLK signaling, focusing on the way these receptors recruit the BAK1 co-receptor and elaborating on the negative crosstalk taking place between the two signaling networks.

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.

Plant immune receptors interact with hemibiotrophic pathogens to activate plant immunity

Phytopathogens pose a devastating threat to the productivity and yield of crops by causing destructive plant diseases in natural and agricultural environments. Hemibiotrophic pathogens have a variable-length biotrophic phase before turning to necrosis and are among the most invasive plant pathogens. Plant resistance to hemibiotrophic pathogens relies mainly on the activation of innate immune responses. These responses are typically initiated after the plant plasma membrane and various plant immune receptors detect immunogenic signals associated with pathogen infection. Hemibiotrophic pathogens evade pathogen-triggered immunity by masking themselves in an arms race while also enhancing or manipulating other receptors to promote virulence. However, our understanding of plant immune defenses against hemibiotrophic pathogens is highly limited due to the intricate infection mechanisms. In this review, we summarize the strategies that different hemibiotrophic pathogens interact with host immune receptors to activate plant immunity. We also discuss the significant role of the plasma membrane in plant immune responses, as well as the current obstacles and potential future research directions in this field. This will enable a more comprehensive understanding of the pathogenicity of hemibiotrophic pathogens and how distinct plant immune receptors oppose them, delivering valuable data for the prevention and management of plant diseases.

The emerging frontier of plant immunity's core hubs

The ever-growing world population, increasingly frequent extreme weather events and conditions, emergence of novel devastating crop pathogens and the social strive for quality food products represent a huge challenge for current and future agricultural production systems. To address these challenges and find realistic solutions, it is becoming more important by the day to understand the complex interactions between plants and the environment, mainly the associated organisms, but in particular pathogens. In the past several years, research in the fields of plant pathology and plant-microbe interactions has enabled tremendous progress in understanding how certain receptor-based plant innate immune systems function to successfully prevent infections and diseases. In this review, we highlight and discuss some of these new ground-breaking discoveries and point out strategies of how pathogens counteract the function of important core convergence hubs of the plant immune system. For practical reasons, we specifically place emphasis on potential applications that can be detracted by such discoveries and what challenges the future of agriculture has to face, but also how these challenges could be tackled.

Emerging principles in the design of bioengineered made-to-order plant immune receptors

Crop yield and global food security are under constant threat from plant pathogens with the potential to cause epidemics. Traditional breeding for disease resistance can be too slow to counteract these emerging threats, resulting in the need to retool the plant immune system using bioengineered made-to-order immune receptors. Efforts to engineer immune receptors have focused primarily on nucleotide-binding domain and leucine-rich repeat (NLR) immune receptors and proof-of-principles studies. Based upon a near-exhaustive literature search of previously engineered plant immune systems we distil five emerging principles in the design of bioengineered made-to-order plant NLRs and describe approaches based on other components. These emerging principles are anticipated to assist the functional understanding of plant immune receptors, as well as bioengineering novel disease resistance specificities.

Molecular plant immunity against biotrophic, hemibiotrophic, and necrotrophic fungi

Pathogenic fungi use diverse infection strategies to obtain nutrients from plants. Biotrophic fungi feed only on living plant tissue, whereas necrotrophic fungi kill host cells to extract nutrients. To prevent disease, plants need to distinguish between pathogens with different life cycles, as a successful defense against a biotroph, which often involves programmed cell-death around the site of infection, is not an appropriate response to some necrotrophs. Plants utilize a vast collection of extracellular and intracellular receptors to detect the signatures of pathogen attack. In turn, pathogens are under strong selection to mask or avoid certain receptor responses while enhancing or manipulating other receptor responses to promote virulence. In this review, we focus on the plant receptors involved in resistance responses to fungal pathogens and highlight, with examples, how the infection strategy of fungal pathogens can determine if recognition responses are effective at preventing disease.

Future of Bacterial Disease Management in Crop Production

Bacterial diseases are a constant threat to crop production globally. Current management strategies rely on an array of tactics, including improved cultural practices; application of bactericides, plant activators, and biocontrol agents; and use of resistant varieties when available. However, effective management remains a challenge, as the longevity of deployed tactics is threatened by constantly changing bacterial populations. Increased scrutiny of the impact of pesticides on human and environmental health underscores the need for alternative solutions that are durable, sustainable, accessible to farmers, and environmentally friendly. In this review, we discuss the strengths and shortcomings of existing practices and dissect recent advances that may shape the future of bacterial disease management. We conclude that disease resistance through genome modification may be the most effective arsenal against bacterial diseases. Nonetheless, more research is necessary for developing novel bacterial disease management tactics to meet the food demand of a growing global population.

Extracellular vesicles: Their functions in plant-pathogen interactions

Extracellular vesicles (EVs) are rounded vesicles enclosed by a lipid bilayer membrane, released by eukaryotic cells and by bacteria. They carry various types of bioactive substances, including nucleic acids, proteins, and lipids. Depending on their cargo, EVs have a variety of well-studied functions in mammalian systems, including cell-to-cell communication, cancer progression, and pathogenesis. In contrast, EVs in plant cells (which have rigid walls) have received very little research attention for many decades. Increasing evidence during the past decade indicates that both plant cells and plant pathogens are able to produce and secrete EVs, and that such EVs play key roles in plant-pathogen interactions. Plant EVs contains small RNAs (sRNAs) and defence-related proteins, and may be taken up by pathogenic fungi, resulting in reduced virulence. On the other hand, EVs released by gram-negative bacteria contain a wide variety of effectors and small molecules capable of activating plant immune responses via pattern-recognition receptor- and BRI1-ASSOCIATED RECEPTOR KINASE- and SUPPRESSOR OF BIR1-mediated signalling pathways, and salicylic acid-dependent and -independent processes. The roles of EVs in plant-pathogen interactions are summarized in this review, with emphasis on important molecules (sRNAs, proteins) present in plant EVs.

Structural analysis of receptor-like kinase SOBIR1 reveals mechanisms that regulate its phosphorylation-dependent activation

Plant leucine-rich repeat (LRR) receptor-like kinases (RLKs) and LRR receptor-like proteins (RLPs) comprise a large family of cell surface receptors that play critical roles in signal perception and transduction. Both LRR-RLKs and LRR-RLPs rely on regulatory LRR-RLKs to initiate downstream signaling pathways. BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3 (BAK1/SERK3) and SUPPRESSOR OF BIR1-1 (SOBIR1) are important and extensively studied regulatory LRR-RLKs with distinct functions. Although the regulatory mechanism of BAK1 activation has been studied in detail, the activation mechanism of SOBIR1 remains poorly understood. Here, the crystal structures of the catalytically inactive kinase domain of SOBIR1 (SOBIR1-KD) from Arabidopsis thaliana were determined in complexes with AMP-PNP and Mg2+. The results show that SOBIR1-KD contains a uniquely long β3-αC loop and adopts an Src-like inactive conformation with an unusual architecture at the activation segment, which comprises three helices. Biochemical studies revealed that SOBIR1 is transphosphorylated by BAK1 following its autophosphorylation via an intermolecular mechanism, and the phosphorylation of Thr529 in the activation segment and the β3-αC loop are critical for SOBIR1 phosphorylation. Further functional analysis confirmed the importance of Thr529 and the β3-αC loop for the SOBIR1-induced cell death response in Nicotiana benthamiana. Taken together, these findings provide a structural basis for the regulatory mechanism of SOBIR1 and reveal the important elements and phosphorylation events in the special stepwise activation of SOBIR1-KD, the first such processes found in regulatory LRR-RLKs.

WAKsing plant immunity, waning diseases

With the requirement to breed more productive crop plants in order to feed a growing global population, compounded by increasingly widespread resistance to pesticides exhibited by pathogens, plant immunity is becoming an increasingly important area of research. Of the genes that contribute to disease resistance, the wall-associated receptor-like kinases (WAKs) are increasingly shown to play a major role, in addition to their contribution to plant growth and development or tolerance to abiotic stresses. Being transmembrane proteins, WAKs form a central pillar of a plant cell's ability to monitor and interact with the extracellular environment. Found in both dicots and monocots, WAKs have been implicated in defence against pathogens with diverse lifestyles and contribute to plant immunity in a variety of ways. Whilst some act as cell surface-localized immune receptors recognizing either pathogen- or plant-derived invasion molecules (e.g. effectors or damage-associated molecular patterns, respectively), others promote innate immunity through cell wall modification and strengthening, thus limiting pathogen intrusion. The ability of some WAKs to provide both durable resistance against pathogens and other agronomic benefits makes this gene family important targets in the development of future crop ideotypes and important to a greater understanding of the complexity and robustness of plant immunity.

Receptors in the Induction of the Plant Innate Immunity

Plants adjust amplitude and duration of immune responses via different strategies to maintain growth, development, and resistance to pathogens. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) play vital roles. Pattern recognition receptors, comprising a large number of receptor-like protein kinases and receptor-like proteins, recognize related ligands and trigger immunity. PTI is the first layer of the innate immune system, and it recognizes PAMPs at the plasma membrane to prevent infection. However, pathogens exploit effector proteins to bypass or directly inhibit the PTI immune pathway. Consistently, plants have evolved intracellular nucleotide-binding domain and leucine-rich repeat-containing proteins to detect pathogenic effectors and trigger a hypersensitive response to activate ETI. PTI and ETI work together to protect plants from infection by viruses and other pathogens. Diverse receptors and the corresponding ligands, especially several pairs of well-studied receptors and ligands in PTI immunity, are reviewed to illustrate the dynamic process of PTI response here.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Engineering plant disease resistance against biotrophic pathogens

Breeding for disease resistance against microbial pathogens is essential for food security in modern agriculture. Conventional breeding, although widely accepted, is time consuming. An alternative approach is generating crop plants with desirable traits through genetic engineering. The collective efforts of many labs in the past 30 years have led to a comprehensive understanding of how plant immunity is achieved, enabling the application of genetic engineering to enhance disease resistance in crop plants. Here, we briefly review the engineering of disease resistance against biotrophic pathogens using various components of the plant immune system. Recent breakthroughs in immune receptors signaling and systemic acquired resistance (SAR), along with innovations in precise gene editing methods, provide exciting new opportunities for the development of improved environmentally friendly crop varieties that are disease resistant and high-yield.

A receptor-like protein from Nicotiana benthamiana mediates VmE02 PAMP-triggered immunity

Plants use their innate immune system to defend against phytopathogens. As a part of this, pattern triggered-immunity is activated via pattern recognition receptor (PRR) detection of pathogen-associated molecular patterns (PAMPs). Although an increasing number of PAMPs have been identified, the PRRs for their recognition remain largely unknown. In the present study, we report a receptor-like protein RE02 (Response to VmE02) in Nicotiana benthamiana, which mediates the perception of VmE02, a PAMP previously identified from the phytopathogenic fungus Valsa mali, using virus-induced gene silencing (VIGS), co-immunoprecipitation, pull-down and microscale thermophoresis assays. We show that silencing of RE02 markedly attenuated VmE02-triggred cell death and immune responses. RE02 specifically interacted with VmE02 in vivo and in vitro, and it displayed a high affinity for VmE02. Formation of a complex with the receptor-like kinases SOBIR1 and BAK1 was essential for RE02 to perceive VmE02. Moreover, RE02-silenced plants exhibited enhanced susceptibility to both the oomycete Phytophthora capsici and the fungus Sclerotinia sclerotiorum, while overexpression of RE02 increased plant resistance to these pathogens. Together, our results indicate that the PAMP VmE02 and the receptor-like protein RE02 represent a new ligand-receptor pair in plant immunity, and that RE02 represents a promising target for engineering disease resistance.

Protein Phosphorylation Changes During Systemic Acquired Resistance in Arabidopsis thaliana

Systemic acquired resistance (SAR) in plants is a defense response that provides resistance against a wide range of pathogens at the whole-plant level following primary infection. Although the molecular mechanisms of SAR have been extensively studied in recent years, the role of phosphorylation that occurs in systemic leaves of SAR-induced plants is poorly understood. We used a data-independent acquisition (DIA) phosphoproteomics platform based on high-resolution mass spectrometry in an Arabidopsis thaliana model to identify phosphoproteins related to SAR establishment. A total of 8011 phosphorylation sites from 3234 proteins were identified in systemic leaves of Pseudomonas syringae pv. maculicola ES4326 (Psm ES4326) and mock locally inoculated plants. A total of 859 significantly changed phosphoproteins from 1119 significantly changed phosphopeptides were detected in systemic leaves of Psm ES4326 locally inoculated plants, including numerous transcription factors and kinases. A variety of defense response-related proteins were found to be differentially phosphorylated in systemic leaves of Psm ES4326 locally inoculated leaves, suggesting that these proteins may be functionally involved in SAR through phosphorylation or dephosphorylation. Significantly changed phosphoproteins were enriched mainly in categories related to response to abscisic acid, regulation of stomatal movement, plant-pathogen interaction, MAPK signaling pathway, purine metabolism, photosynthesis-antenna proteins, and flavonoid biosynthesis. A total of 28 proteins were regulated at both protein and phosphorylation levels during SAR. RT-qPCR analysis revealed that changes in phosphorylation levels of proteins during SAR did not result from changes in transcript abundance. This study provides comprehensive details of key phosphoproteins associated with SAR, which will facilitate further research on the molecular mechanisms of SAR.

Molecular Characterization of Differences between the Tomato Immune Receptors Flagellin Sensing 3 and Flagellin Sensing 2

Plants mount defense responses by recognizing indicators of pathogen invasion, including microbe-associated molecular patterns (MAMPs). Flagellin, from the bacterial pathogen Pseudomonas syringae pv. tomato (Pst), contains two MAMPs, flg22 and flgII-28, that are recognized by tomato (Solanum lycopersicum) receptors Flagellin sensing2 (Fls2) and Fls3, respectively, but to what degree each receptor contributes to immunity and whether they promote immune responses using the same molecular mechanisms are unknown. Here, we characterized CRISPR/Cas9-generated Fls2 and Fls3 tomato mutants and found that the two receptors contribute equally to disease resistance both on the leaf surface and in the apoplast. However, we observed striking differences in certain host responses mediated by the two receptors. Compared to Fls2, Fls3 mediated a more sustained production of reactive oxygen species and an increase in transcript abundance of 44 tomato genes, with two genes serving as specific reporters for the Fls3 pathway. Fls3 had greater in vitro kinase activity than Fls2 and could transphosphorylate a substrate. Using chimeric Fls2/Fls3 proteins, we found no evidence that a single receptor domain is responsible for the Fls3-sustained reactive oxygen species, suggesting involvement of multiple structural features or a nullified function of the chimeric construct. This work reveals differences in certain immunity outputs between Fls2 and Fls3, suggesting that they might use distinct molecular mechanisms to activate pattern-triggered immunity in response to flagellin-derived MAMPs.

Diversity, Function and Regulation of Cell Surface and Intracellular Immune Receptors in Solanaceae

The first layer of the plant immune system comprises plasma membrane-localized receptor proteins and intracellular receptors of the nucleotide-binding leucine-rich repeat protein superfamily. Together, these immune receptors act as a network of surveillance machines in recognizing extracellular and intracellular pathogen invasion-derived molecules, ranging from conserved structural epitopes to virulence-promoting effectors. Successful pathogen recognition leads to physiological and molecular changes in the host plants, which are critical for counteracting and defending against biotic attack. A breadth of significant insights and conceptual advances have been derived from decades of research in various model plant species regarding the structural complexity, functional diversity, and regulatory mechanisms of these plant immune receptors. In this article, we review the current state-of-the-art of how these host surveillance proteins function and how they are regulated. We will focus on the latest progress made in plant species belonging to the Solanaceae family, because of their tremendous importance as model organisms and agriculturally valuable crops.