Activation of TIR signalling boosts pattern-triggered immunity

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.

Salicylic acid: The roles in plant immunity and crosstalk with other hormones

Land plants use diverse hormones to coordinate their growth, development and responses against biotic and abiotic stresses. Salicylic acid (SA) is an essential hormone in plant immunity, with its levels and signaling tightly regulated to ensure a balanced immune output. Over the past three decades, molecular genetic analyses performed primarily in Arabidopsis have elucidated the biosynthesis and signal transduction pathways of key plant hormones, including abscisic acid, jasmonic acid, ethylene, auxin, cytokinin, brassinosteroids, and gibberellin. Crosstalk between different hormones has become a major focus in plant biology with the goal of obtaining a full picture of the plant hormone signaling network. This review highlights the roles of SA in plant immunity and summarizes our current understanding of the pairwise interactions of SA with other major plant hormones. The complexity of these interactions is discussed, with the hope of stimulating research to address existing knowledge gaps in hormone crosstalk, particularly in the context of balancing plant growth and defense.

Chemical-sensitized MITOGEN-ACTIVATED PROTEIN KINASE 4 provides insights into its functions in plant growth and immunity

Two mitogen-activated protein kinase (MAPK) cascades with MPK4 and MPK3/MPK6 as the bottommost kinases are key to plant growth/development and immune signaling. Disruption of the MPK4 cascade leads to severe dwarfism and autoimmunity, complicating the study of MPK4 in plant growth/development and immunity. In this study, we successfully rescued the Arabidopsis (Arabidopsis thaliana) mpk4 mutant using a chemical-sensitized MPK4 variant, MPK4YG, creating a conditional activity-null mpk4 mutant named MPK4SR (genotype: PMPK4:MPK4YG mpk4) that could be used to examine the functions of MPK4 in plant growth/development and immunity. We discovered that the duration of the loss of MPK4 activity is important to plant immune responses. Short-term loss of MPK4 activity did not impact flg22-induced ROS burst or resistance against Pseudomonas syringae (Pst). Enhanced Pst resistance was only observed in the MPK4SR plants with stunted growth following prolonged inhibition of MPK4 activity. Transcriptome analyses in plants with short-term loss of MPK4 activity revealed a vital role of MPK4 in regulating several housekeeping processes, including mitosis, transcription initiation, and cell wall macromolecule catabolism. Furthermore, the constitutive weak activation of MPK4GA in the MPK4CA plants (genotype: PMPK4:MPK4GA mpk4) led to early flowering and premature senescence, which was associated with its compromised resistance against Pst. These findings suggest that MPK4 plays important roles in plant growth and development and in maintaining the delicate balance between growth/development and immune adaptation in plants.

A canonical protein complex controls immune homeostasis and multipathogen resistance

The calcium (Ca2+) sensor ROD1 (RESISTANCE OF RICE TO DISEASES1) is a master regulator of immunity in rice. By screening suppressors of rod1 mutants, we show that ROD1 governs immune homeostasis by surveilling the activation of a canonical immune pathway. Mutations in OsTIR (TIR-only protein), OsEDS1 (enhanced disease susceptibility 1), OsPAD4 (phytoalexin deficient 4), and OsADR1 (activated disease resistance 1) all abolish enhanced disease resistance of rod1 plants. OsTIR catalyzes the production of second messengers 2'-(5″-phosphoribosyl)-5'-adenosine monophosphate (pRib-AMP) and diphosphate (pRib-ADP), which trigger formation of an OsEDS1-OsPAD4-OsADR1 (EPA) immune complex. ROD1 interacts with OsTIR and inhibits its enzymatic activity, whereas mutation of ROD1 leads to constitutive activation of the EPA complex. Thus, we unveil an immune network that fine-tunes immune homeostasis and multipathogen resistance in rice.

Activation of a helper NLR by plant and bacterial TIR immune signaling

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain sense pathogen effectors to initiate immune signaling. TIR domains across different kingdoms have NADase activities and can produce phosphoribosyl adenosine monophosphate/diphosphate (pRib-AMP/ADP) or cyclic ADPR (cADPR) isomers. The lipase-like proteins EDS1 and PAD4 transduce immune signals from sensor TIR-NLRs to a helper NLR called ADR1, which executes immune function. We report the structure and function of an Arabidopsis EDS1-PAD4-ADR1 (EPA) heterotrimer in complex with pRib-AMP/ADP activated by plant or bacterial TIR signaling. 2'cADPR can be hydrolyzed into pRib-AMP and thus activate EPA signaling. Bacterial TIR domains producing 2'cADPR also activate EPA function. Our findings suggest that 2'cADPR may be the storage form of the unstable signaling molecule pRib-AMP.

Fine-tuning of the dual-role transcription factor WRKY8 via differential phosphorylation for robust broad-spectrum plant immunity

Plants perceive pathogen-associated molecular patterns (PAMPs) using plasma-membrane-localized pattern recognition receptors (PRRs) to activate broad-spectrum pattern-triggered immunity. However, the regulatory mechanisms that ensure robust broad-spectrum plant immunity remain largely unknown. Here, we reveal that the transcription factor WRKY8 has a dual role in the transcriptional regulation of PRR genes: repressing expression of the nlp20/nlp24 receptor gene RLP23 while promoting that of the chitin receptor gene CERK1. SsNLP1 and SsNLP2, two nlp24-type PAMPs from the destructive fungal pathogen Sclerotinia sclerotiorum, activate two calcium-elicited kinases, CPK4 and CPK11, which phosphorylate WRKY8 and thus release its inhibition on RLP23 to promote accumulation of RLP23 transcripts. Meanwhile, SsNLPs activate the RLCK-type kinase PBL19, which phosphorylates WRKY8 and thus enhances accumulation of CERK1 transcripts. Intriguingly, RLP23 is repressed at later stage by PBL19-mediated phosphorylation of WRKY8, thus avoiding excessive immunity and enabling normal growth. Our findings unveil a plant strategy of "killing two birds with one stone" to elicit robust broad-spectrum immunity. This strategy is based on PAMP-triggered fine-tuning of a dual-role transcription factor to simultaneously amplify two PRRs that recognize PAMPs conserved across a wide range of pathogens. Moreover, our results reveal a novel plant strategy for balancing the trade-off between growth and immunity by fine-tuning the expression of multiple PRR genes.

Protein disulfide isomerase A4 binds to Brucella BtpB and mediates intracellular NAD+/NADH metabolism in RAW264.7 cells

The Toll/interleukin-1 receptor (TIR) signaling domain is distributed widely in mammalian Toll-like receptors and adaptors, plant nucleotide-binding leucine-rich repeat receptors, and specific bacterial virulence proteins. Proteins that possess TIR domain exhibit NADase activity which is distinct from the canonical signaling function of these domains. However, the effects of bacterial TIR domain proteins on host metabolic switches and the underlying mechanism of NADase activity in these proteins remain unclear. Here, we utilized Brucella TIR domain-containing type IV secretion system effector protein, BtpB, to explore the mechanism of NADase activity in host cells. We showed that using ectopic expression BtpB not only generates depletion of NAD+ but also loss of NADH and ATP in RAW264.7 macrophage cells. Moreover, immunoprecipitation-mass spectrometry, co-immunoprecipitation, and confocal microscope assays revealed that BtpB interacted with host protein disulfide isomerase A4 (PDIA4). The Brucella mutant strain deleted the gene for BtpB, significantly decreased PDIA4 expression. Furthermore, our data revealed that PDIA4 played an important role in regulating intracellular NAD+/NADH levels in macrophages, and PDIA4 overexpression restored the decline of intracellular NAD+ and NADH levels induced by Brucella BtpB. The results provide new insights into the metabolic regulatory activity of TIR domain proteins in the critical human and animal pathogen Brucella.

Nucleotides and nucleotide derivatives as signal molecules in plants

In reaction to a stimulus, signaling molecules are made, generate a response, and are then degraded. Nucleotides are classically associated with central metabolism and nucleic acid biosynthesis, but there are a number of nucleotides and nucleotide derivatives in plants to which this simple definition of a signaling molecule applies in whole or at least in part. These include cytokinins and chloroplast guanosine tetraposphate (ppGpp), as well as extracellular canonical nucleotides such as extracellular ATP (eATP) and NAD+ (eNAD+). In addition, there is a whole series of compounds derived from NAD+ such as ADP ribose (ADPR), and ATP-ADPR dinucleotides and their hydrolysis products (e.g. pRib-AMP) together with different variants of cyclic ADPR (cADPR, 2´-cADPR, 3´-cADPR), and also cyclic nucleotides such as 3´,5´-cAMP and 2´,3´-cyclic nucleoside monophosphates. Interestingly, some of these compounds have recently been shown to play a central role in pathogen defense. In this review, we highlight these exciting new developments. We also review nucleotide derivatives that are considered as candidates for signaling molecules, for example purine deoxynucleosides, and discuss more controversial cases.

Overexpression of GmSRC2 confers resistance to Phytophthora sojae in soybean

Phytophthora root and stem rot caused by Phytophthora sojae (P. sojae) is one of the most destructive diseases to affect soybean (Glycine max (L.) Merr) production. GmSRC2 that encodes a C2 domain-containing protein can respond to various stresses, however, the molecular mechanism of GmSRC2 in resistance of soybean to P. sojae is yet to be fully elucidated. In this study, GmSRC2 was found to be significantly up-regulated under P. sojae treatment; GmSRC2-overexpression (OE) transgenic lines and GmSRC2-silencing transient plants were generated via Agrobacterium tumefaciens mediated transformation and virus-induced gene silencing (VIGS) system, respectively. Infected leaves and cotyledons of OE-GmSRC2-1 and OE-GmSRC2-2 lines showed significant decreases in the disease symptoms and P. sojae biomass than those of wild type (WT); the activities of superoxide dismutase (SOD) and peroxidase (POD) confirmed the accumulation of reactive oxygen species (ROS) in overexpressed transgenic lines. Whereas, silencing of GmSRC2 severely increased the disease symptoms and the biomass of P. sojae. Further, we confirmed that GmSRC2 interacted with the effector PsAvh23 of P. sojae, and the C2 domain was crucial for the interaction. Overexpression of GmSRC2 upregulated the ADA2/GCN5 module upon P. sojae. The aforementioned results demonstrated that GmSRC2 played vital roles in regulating soybean resistance to oomycetes.

Shared signals, different fates: Calcium and ROS in plant PRR and NLR immunity

Lacking an adaptive immune system, plants rely on innate immunity comprising two main layers: PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI), both utilizing Ca2+ influx and reactive oxygen species (ROS) for signaling. PTI, mediated by pattern-recognition receptors (PRRs), responds to conserved pathogen- or damage-associated molecular patterns. Some pathogens evade PTI using effectors, triggering plants to activate ETI. At the heart of ETI are nucleotide-binding leucine-rich repeat receptors (NLRs), which detect specific pathogen effectors and initiate a robust immune response. NLRs, equipped with a nucleotide-binding domain and leucine-rich repeats, drive a potent immune reaction starting with pronounced, prolonged cytosolic Ca2+ influx, followed by increased ROS levels. This sequence of events triggers the hypersensitive response-a localized cell death designed to limit pathogen spread. This intricate use of Ca2+ and ROS highlights the crucial role of NLRs in supplementing the absence of an adaptive immune system in plant innate immunity.

Comparative analyses of RNA-seq and phytohormone data of sweetpotatoes inoculated with Dickeya dadantii causing bacterial stem and root rot of sweetpotato

Bacterial stem and root rot (BSRR) in sweetpotato caused by Dickeya dadantii is one of the ten major diseases of sweetpotatoes in China. However, the molecular mechanism underlying the resistance of sweetpotato to D. dadantii remains unclear. This study adopted a resistance identification assay that conformed Guangshu87 (GS87) as BSRR-resistant and Xinxiang (XX) as susceptible. Compared to XX, GS87 effectively prevented the invasion and dissemination of D. dadantii in planta. An RNA sequencing (RNA-seq) analysis identified 54,844 expressed unigenes between GS87 and XX at four different stages. Further, it revealed that GS87 was more able to regulate the expressions of more unigenes after the inoculation with D. dadantii, including resistance (R) and transcription factors (TF) genes. Moreover, content measurements of disease resistance-related phytohormones showed that both jasmonic acids (JAs) and salicylic acids (SAs) accumulated in D. dadantii-inoculated sweetpotatoes, and JAs may negatively regulate sweetpotato resistance against D. dadantii and accumulated faster than SAs. Meanwhile, determinations of ROS production rate and relevant enzymatic/non-enzymatic activity highlighted the vital roles of reactive oxygen species (ROS) and superoxide dismutase (SOD) in confering GS87 resistance against D. dadantii. Additionally, several hub genes with high connectivity were highlighted through Protein-Protein interaction (PPI) network analysis. In summary, the findings in this study contribute to the understanding of the different responses of resistant and susceptible sweetpotato cultivars to D. dadantii infection, and it also provide the first insight into the relevant candidate genes and phytohormones involved in the resistance of sweetpotato to D. dadantii.

Plant pattern recognition receptors: from evolutionary insight to engineering

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

Plant immune resilience to a changing climate: molecular insights and biotechnological roadmaps

Successful resistance to disease-causing pathogens is underpinned by properly regulated immune signalling and defence responses in plants. The plant immune system is controlled at multiple levels of gene and protein regulation-from chromatin-associated epigenetic processes to protein post-translational modifications. Optimal fine-tuning of plant immune signalling and responses is important to prevent plant disease development, which is being exacerbated by a globally changing climate. In this review, we focus on how changing climatic factors mechanistically intercept plant immunity at different levels of regulation (chromatin, transcriptional, post-transcriptional, translational, and post-translational). We specifically highlight recent studies that have provided molecular insights into critically important climate-sensitive nodes and mechanisms of the plant immune system. We then propose several potential future directions to build climate-resilient plant disease resistance using cutting-edge biotechnology. Overall, this conceptual understanding and promising biotechnological advances provide a foundational platform towards novel approaches to engineer plant immune resilience.

Recognition of a Fungal Effector Potentiates Pathogen-Associated Molecular Pattern-Triggered Immunity in Cotton

Plants are equipped with multi-layered immune systems that recognize pathogen-derived elicitors to activate immunity. Verticillium dahliae is a soil-borne fungus that infects a broad range of plants and causes devastating wilt disease. The mechanisms underlying immune recognition between plants and V. dahliae remain elusive. Here, a V. dahliae secretory protein, elicitor of plant defense gene (VdEPD1), acts as an elicitor that triggers defense responses in both Nicotiana benthamiana and cotton plants is identified. Targeted gene deletion of VdEPD1 enhances V. dahliae virulence in plants. Expression of VdEPD1 triggers the accumulation of reactive oxygen species (ROS) and the activation of cell death in cotton plants. Gossypium barbadense EPD1-interacting receptor-like cytoplasmic kinase (GbEIR5A) and GbEIR5D interact with VdEPD1. Silencing of GbEIR5A/D significantly impairs VdEPD1-triggered cell death in cotton plants, indicating the contribution of GbEIR5A/D to VdEPD1-activated effector-triggered immunity (ETI). VdEPD1 stimulates the expression of GbEIR5A and GbEIR5D in cotton plants. Interestingly, cotton plants with silenced GbEIR5A/D genes exhibit compromised pathogen-associated molecular patterns (PAMPs)-triggered ROS accumulation, whereas overexpression of GbEIR5A or GbEIR5D enhances PAMP-induced ROS. These findings indicate that recognition of VdEPD1 potentiates GbEIRs to enhance cotton PAMP-triggered immunity (PTI), uncovering a cooperative interplay of PTI and ETI in cotton.

Pangenome Data Analysis Reveals Characteristics of Resistance Gene Analogs Associated with Sclerotinia sclerotiorum Resistance in Sunflower

The sunflower, an important oilseed crop and food source across the world, is susceptible to several pathogens, which cause severe losses in sunflower production. The utilization of genetic resistance is the most economical, effective measure to prevent infectious diseases. Based on the sunflower pangenome, in this study, we explored the variability of resistance gene analogs (RGAs) within the species. According to a comparative analysis of RGA candidates in the sunflower pangenome using the RGAugury pipeline, a total of 1344 RGAs were identified, comprising 1107 conserved, 199 varied, and 38 rare RGAs. We also identified RGAs associated with resistance against Sclerotinia sclerotiorum (S. sclerotiorum) in sunflower at the quantitative trait locus (QTL). A total of 61 RGAs were found to be located at four quantitative trait loci (QTLs). Through a detailed expression analysis of RGAs in one susceptible and two tolerant sunflower inbred lines (ILs) across various time points post inoculation, we discovered that 348 RGAs exhibited differential expression in response to Sclerotinia head rot (SHR), with 17 of these differentially expressed RGAs being situated within the QTL regions. In addition, 15 RGA candidates had gene introgression. Our data provide a better understanding of RGAs, which facilitate genomics-based improvements in disease resistance in sunflower.

Plant NAC transcription factors in the battle against pathogens

BACKGROUND

The NAC transcription factor family, which is recognized as one of the largest plant-specific transcription factor families, comprises numerous members that are widely distributed among various higher plant species and play crucial regulatory roles in plant immunity.

RESULTS

In this paper, we provided a detailed summary of the roles that NAC transcription factors play in plant immunity via plant hormone pathways and reactive oxygen species pathways. In addition, we conducted in-depth investigations into the interactions between NAC transcription factors and pathogen effectors to summarize the mechanism through which they regulate the expression of defense-related genes and ultimately affect plant disease resistance.

CONCLUSIONS

This paper presented a comprehensive overview of the crucial roles that NAC transcription factors play in regulating plant disease resistance through their involvement in diverse signaling pathways, acting as either positive or negative regulators, and thus provided references for further research on NAC transcription factors.

The roles of Magnaporthe oryzae avirulence effectors involved in blast resistance/susceptibility

Phytopathogens represent an ongoing threat to crop production and a significant impediment to global food security. During the infection process, these pathogens spatiotemporally deploy a large array of effectors to sabotage host defense machinery and/or manipulate cellular pathways, thereby facilitating colonization and infection. However, besides their pivotal roles in pathogenesis, certain effectors, known as avirulence (AVR) effectors, can be directly or indirectly perceived by plant resistance (R) proteins, leading to race-specific resistance. An in-depth understanding of the intricate AVR-R interactions is instrumental for genetic improvement of crops and safeguarding them from diseases. Magnaporthe oryzae (M. oryzae), the causative agent of rice blast disease, is an exceptionally virulent and devastating fungal pathogen that induces blast disease on over 50 monocot plant species, including economically important crops. Rice-M. oryzae pathosystem serves as a prime model for functional dissection of AVR effectors and their interactions with R proteins and other target proteins in rice due to its scientific advantages and economic importance. Significant progress has been made in elucidating the potential roles of AVR effectors in the interaction between rice and M. oryzae over the past two decades. This review comprehensively discusses recent advancements in the field of M. oryzae AVR effectors, with a specific focus on their multifaceted roles through interactions with corresponding R/target proteins in rice during infection. Furthermore, we deliberated on the emerging strategies for engineering R proteins by leveraging the structural insights gained from M. oryzae AVR effectors.

Dynamics of epitranscriptomes uncover translational reprogramming directed by ac4C in rice during pathogen infection

Messenger RNA modifications play pivotal roles in RNA biology, but comprehensive landscape changes of epitranscriptomes remain largely unknown in plant immune response. Here we report translational reprogramming directed by ac4C mRNA modification upon pathogen challenge. We first investigate the dynamics of translatomes and epitranscriptomes and uncover that the change in ac4C at single-base resolution promotes translational reprogramming upon Magnaporthe oryzae infection. Then by characterizing the specific distributions of m1A, 2'O-Nm, ac4C, m5C, m6A and m7G, we find that ac4Cs, unlike other modifications, are enriched at the 3rd position of codons, which stabilizes the Watson-Crick base pairing. Importantly, we demonstrate that upon pathogen infection, the increased expression of the ac4C writer OsNAT10/OsACYR (N-ACETYLTRANSFERASE FOR CYTIDINE IN RNA) promotes translation to facilitate rapid activation of immune responses, including the enhancement of jasmonic acid biosynthesis. Our study provides an atlas of mRNA modifications and insights into ac4C function in plant immunity.

CBP60b clade proteins are prototypical transcription factors mediating immunity

Transcriptional reprogramming is critical for plant immunity. Several calmodulin (CaM)-binding protein 60 (CBP60) family transcription factors (TFs) in Arabidopsis (Arabidopsis thaliana), including CBP60g, systemic acquired resistance deficient 1 (SARD1), CBP60a, and CBP60b, are critical for and show distinct roles in immunity. However, there are additional CBP60 members whose function is unclear. We report here that Arabidopsis CBP60c-f, 4 uncharacterized CBP60 members, play redundant roles with CBP60b in the transcriptional regulation of immunity responses, whose pCBP60b-driven expression compensates the loss of CBP60b. By contrast, neither CBP60g nor SARD1 is interchangeable with CBP60b, suggesting clade-specific functionalization. We further show that the function of CBP60b clade TFs relies on DNA-binding domains (DBDs) and CaM-binding domains, suggesting that they are downstream components of calcium signaling. Importantly, we demonstrate that CBP60s encoded in earliest land plant lineage Physcomitrium patens and Selaginella moellendorffii are functionally homologous to Arabidopsis CBP60b, suggesting that the CBP60b clade contains the prototype TFs of the CBP60 family. Furthermore, tomato and cucumber CBP60b-like genes rescue the defects of Arabidopsis cbp60b and activate the expression of tomato and cucumber SALICYLIC ACID INDUCTION DEFICIIENT2 (SID2) and ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) genes, suggesting that immune response pathways centered on CBP60b are also evolutionarily conserved. Together, these findings suggest that CBP60b clade TFs are functionally conserved in evolution and positively mediate immunity.

Activation of the helper NRC4 immune receptor forms a hexameric resistosome

Innate immune responses to microbial pathogens are regulated by intracellular receptors known as nucleotide-binding leucine-rich repeat receptors (NLRs) in both the plant and animal kingdoms. Across plant innate immune systems, "helper" NLRs (hNLRs) work in coordination with "sensor" NLRs (sNLRs) to modulate disease resistance signaling pathways. Activation mechanisms of hNLRs based on structures are unknown. Our research reveals that the hNLR, known as NLR required for cell death 4 (NRC4), assembles into a hexameric resistosome upon activation by the sNLR Bs2 and the pathogenic effector AvrBs2. This conformational change triggers immune responses by facilitating the influx of calcium ions (Ca2+) into the cytosol. The activation mimic alleles of NRC2, NRC3, or NRC4 alone did not induce Ca2+ influx and cell death in animal cells, suggesting that unknown plant-specific factors regulate NRCs' activation in plants. These findings significantly advance our understanding of the regulatory mechanisms governing plant immune responses.

Proteolysis in plant immunity

Compared with transcription and translation, protein degradation machineries can act faster and be targeted to different subcellular compartments, enabling immediate regulation of signaling events. It is therefore not surprising that proteolysis has been used extensively to control homeostasis of key regulators in different biological processes and pathways. Over the past decades, numerous studies have shown that proteolysis, where proteins are broken down to peptides or amino acids through ubiquitin-mediated degradation systems and proteases, is a key regulatory mechanism to control plant immunity output. Here, we briefly summarize the roles various proteases play during defence activation, focusing on recent findings. We also update the latest progress of ubiquitin-mediated degradation systems in modulating immunity by targeting plant membrane-localized pattern recognition receptors, intracellular nucleotide-binding domain leucine-rich repeat receptors, and downstream signaling components. Additionally, we highlight recent studies showcasing the importance of proteolysis in maintaining broad-spectrum resistance without obvious yield reduction, opening new directions for engineering elite crops that are resistant to a wide range of pathogens with high yield.

Cross-family transfer of the Arabidopsis cell-surface immune receptor LORE to tomato confers sensing of 3-hydroxylated fatty acids and enhanced disease resistance

Plant pathogens pose a high risk of yield losses and threaten food security. Technological and scientific advances have improved our understanding of the molecular processes underlying host-pathogen interactions, which paves the way for new strategies in crop disease management beyond the limits of conventional breeding. Cross-family transfer of immune receptor genes is one such strategy that takes advantage of common plant immune signalling pathways to improve disease resistance in crops. Sensing of microbe- or host damage-associated molecular patterns (MAMPs/DAMPs) by plasma membrane-resident pattern recognition receptors (PRR) activates pattern-triggered immunity (PTI) and restricts the spread of a broad spectrum of pathogens in the host plant. In the model plant Arabidopsis thaliana, the S-domain receptor-like kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (AtLORE, SD1-29) functions as a PRR, which senses medium-chain-length 3-hydroxylated fatty acids (mc-3-OH-FAs), such as 3-OH-C10:0, and 3-hydroxyalkanoates (HAAs) of microbial origin to activate PTI. In this study, we show that ectopic expression of the Brassicaceae-specific PRR AtLORE in the solanaceous crop species Solanum lycopersicum leads to the gain of 3-OH-C10:0 immune sensing without altering plant development. AtLORE-transgenic tomato shows enhanced resistance against Pseudomonas syringae pv. tomato DC3000 and Alternaria solani NL03003. Applying 3-OH-C10:0 to the soil before infection induces resistance against the oomycete pathogen Phytophthora infestans Pi100 and further enhances resistance to A. solani NL03003. Our study proposes a potential application of AtLORE-transgenic crop plants and mc-3-OH-FAs as resistance-inducing biostimulants in disease management.

Xanthomonas oryzae pv. oryzicola effector Tal10a directly activates rice OsHXK5 expression to facilitate pathogenesis

Bacterial leaf streak (BLS), caused by Xanthomonas oryzae pv. oryzicola (Xoc), is a major bacterial disease in rice. Transcription activator-like effectors (TALEs) from Xanthomonas can induce host susceptibility (S) genes and facilitate infection. However, knowledge of the function of Xoc TALEs in promoting bacterial virulence is limited. In this study, we demonstrated the importance of Tal10a for the full virulence of Xoc. Through computational prediction and gene expression analysis, we identified the hexokinase gene OsHXK5 as a host target of Tal10a. Tal10a directly binds to the gene promoter region and activates the expression of OsHXK5. CRISPR/Cas9-mediated gene editing in the effector binding element (EBE) of OsHXK5 significantly increases rice resistance to Xoc, while OsHXK5 overexpression enhances the susceptibility of rice plants and impairs rice defense responses. Moreover, simultaneous editing of the promoters of OsSULTR3;6 and OsHXK5 confers robust resistance to Xoc in rice. Taken together, our findings highlight the role of Tal10a in targeting OsHXK5 to promote infection and suggest that OsHXK5 represents a potential target for engineering rice resistance to Xoc.

Plasmopara viticola effector PvCRN20 represses the import of VvDEG5 into chloroplasts to suppress immunity in grapevine

Chloroplasts play a crucial role in plant defense against pathogens, making them primary targets for pathogen effectors that suppress host immunity. This study characterizes the Plasmopara viticola CRN-like effector, PvCRN20, which interacts with DEG5 in the cytoplasm but not with its interacting protein, DEG8, which is located in the chloroplast. By transiently overexpressing in tobacco leaves, we show that PvCRN20 could inhibit INF1- and Bax-triggered cell death. Constitutive expression of PvCRN20 suppresses the accumulation of reactive oxygen species (ROS) and promotes pathogen colonization. PvCRN20 reduces DEG5 entry into chloroplasts, thereby disrupting DEG5 and DEG8 interactions in chloroplasts. Overexpression of VvDEG5 and VvDEG8 induces ROS accumulation and enhances grapevine resistance to P. viticola, whereas knockout of VvDEG8 represses ROS production and promotes P. viticola colonization. Consistently, ectopic expression of VvDEG5 and VvDEG8 in tobacco promotes chloroplast-derived ROS accumulation, whereas co-expression of PvCRN20 counteracted this promotion by VvDEG5. Therefore, DEG5 is essential for the virulence function of PvCRN20. Although PvCRN20 is located in both the nucleus and cytoplasm, only cytoplasmic PvCRN20 suppresses plant immunity and promotes pathogen infection. Our results reveal that PvCRN20 dampens plant defenses by repressing the chloroplast import of DEG5, thus reducing host ROS accumulation and facilitating pathogen colonization.

PTI-ETI synergistic signal mechanisms in plant immunity

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

From molecule to cell: the expanding frontiers of plant immunity

In recent years, the field of plant immunity has witnessed remarkable breakthroughs. During the co-evolution between plants and pathogens, plants have developed a wealth of intricate defense mechanisms to safeguard their survival. Newly identified immune receptors have added unexpected complexity to the surface and intracellular sensor networks, enriching our understanding of the ongoing plant-pathogen interplay. Deciphering the molecular mechanisms of resistosome shapes our understanding of these mysterious molecules in plant immunity. Moreover, technological innovations are expanding the horizon of the plant-pathogen battlefield into spatial and temporal scales. While the development provides new opportunities for untangling the complex realm of plant immunity, challenges remain in uncovering plant immunity across spatiotemporal dimensions from both molecular and cellular levels.

Melon CmRLCK VII-8 kinase genes CmRLCK27, CmRLCK30 and CmRLCK34 modulate resistance against bacterial and fungal diseases in Arabidopsis

Receptor-like cytoplasmic kinases (RLCKs) represent a distinct class of receptor-like kinases crucial for various aspects of plant biology, including growth, development, and stress responses. This study delves into the characterization of RLCK VII-8 members within cucurbits, particularly in melon, examining both structural features and the phylogenetic relationships of these genes/proteins. The investigation extends to their potential involvement in disease resistance by employing ectopic overexpression in Arabidopsis. The promoters of CmRLCK VII-8 genes harbor multiple phytohormone- and stress-responsive cis-acting elements, with the majority (excluding CmRLCK39) displaying upregulated expression in response to defense hormones and fungal infection. Subcellular localization studies reveal that CmRLCK VII-8 proteins predominantly reside on the plasma membrane, with CmRLCK29 and CmRLCK30 exhibiting additional nuclear distribution. Notably, Arabidopsis plants overexpressing CmRLCK30 manifest dwarfing and delayed flowering phenotypes. Overexpression of CmRLCK27, CmRLCK30, and CmRLCK34 in Arabidopsis imparts enhanced resistance against Botrytis cinerea and Pseudomonas syringae pv. tomato DC3000, concomitant with the strengthened expression of defense genes and reactive oxygen species accumulation. The CmRLCK VII-8 members actively participate in chitin- and flg22-triggered immune responses. Furthermore, CmRLCK30 interacts with CmMAPKKK1 and CmARFGAP, adding a layer of complexity to the regulatory network. In summary, this functional characterization underscores the regulatory roles of CmRLCK27, CmRLCK30, and CmRLCK34 in immune responses by influencing pathogen-induced defense gene expression and ROS accumulation.

SARM1 regulates pro-inflammatory cytokine expression in human monocytes by NADase-dependent and -independent mechanisms

SARM1 is a Toll-IL-1 receptor (TIR) domain-containing protein with roles in innate immunity and neuronal death in diverse organisms. Unlike other innate immune TIR proteins that function as adaptors for Toll-like receptors (TLRs), SARM1 has NADase activity, and this activity regulates murine neuronal cell death. However, whether human SARM1, and its NADase activity, are involved in innate immune regulation remains unclear. Here, we show that human SARM1 regulates proinflammatory cytokine expression in both an NADase-dependent and -independent manner in monocytes. SARM1 negatively regulated TLR4-dependent TNF mRNA induction independently of its NADase activity. In contrast, SARM1 inhibited IL-1β secretion through both NADase-dependent inhibition of pro-IL-1β expression, and NADase-independent suppression of the NLRP3 inflammasome and hence processing of pro-IL-1β to mature IL-1β. Our study reveals multiple mechanisms whereby SARM1 regulates pro-inflammatory cytokines in human monocytes and shows, compared to other mammalian TIR proteins, a distinct NADase-dependent role for SARM1 in innate immunity.

Extracellular niche establishment by plant pathogens

The plant extracellular space, referred to as the apoplast, is inhabited by a variety of microorganisms. Reflecting the crucial nature of this compartment, both plants and microorganisms seek to control, exploit and respond to its composition. Upon sensing the apoplastic environment, pathogens activate virulence programmes, including the delivery of effectors with well-established roles in suppressing plant immunity. We posit that another key and foundational role of effectors is niche establishment - specifically, the manipulation of plant physiological processes to enrich the apoplast in water and nutritive metabolites. Facets of plant immunity counteract niche establishment by restricting water, nutrients and signals for virulence activation. The complex competition to control and, in the case of pathogens, exploit the apoplast provides remarkable insights into the nature of virulence, host susceptibility, host defence and, ultimately, the origin of phytopathogenesis. This novel framework focuses on the ecology of a microbial niche and highlights areas of future research on plant-microorganism interactions.

Long-Term Consequences of PTI Activation and Its Manipulation by Root-Associated Microbiota

In nature, plants are constantly colonized by a massive diversity of microbes engaged in mutualistic, pathogenic or commensal relationships with the host. Molecular patterns present in these microbes activate pattern-triggered immunity (PTI), which detects microbes in the apoplast or at the tissue surface. Whether and how PTI distinguishes among soil-borne pathogens, opportunistic pathogens, and commensal microbes within the soil microbiota remains unclear. PTI is a multimodal series of molecular events initiated by pattern perception, such as Ca2+ influx, reactive oxygen burst, and extensive transcriptional and metabolic reprogramming. These short-term responses may manifest within minutes to hours, while the long-term consequences of chronic PTI activation persist for days to weeks. Chronic activation of PTI is detrimental to plant growth, so plants need to coordinate growth and defense depending on the surrounding biotic and abiotic environments. Recent studies have demonstrated that root-associated commensal microbes can activate or suppress immune responses to variable extents, clearly pointing to the role of PTI in root-microbiota interactions. However, the molecular mechanisms by which root commensals interfere with root immunity and root immunity modulates microbial behavior remain largely elusive. Here, with a focus on the difference between short-term and long-term PTI responses, we summarize what is known about microbial interference with host PTI, especially in the context of root microbiota. We emphasize some missing pieces that remain to be characterized to promote the ultimate understanding of the role of plant immunity in root-microbiota interactions.

Extracellular perception of multiple novel core effectors from the broad host-range pear anthracnose pathogen Colletotrichum fructicola in the nonhost Nicotiana benthamiana

Colletotrichum fructicola is emerging as a devastating pathogenic fungus causing anthracnose in a wide range of horticultural crops, particularly fruits. Exploitation of nonhost resistance (NHR) represents a robust strategy for plant disease management. Perception of core effectors from phytopathogens frequently leads to hypersensitive cell death and resistance in nonhost plants; however, such core effectors in C. fructicola and their signaling components in non-hosts remain elusive. Here, we found a virulent C. fructicola strain isolated from pear exhibits non-adaptation in the model plant Nicotiana benthamiana. Perception of secreted molecules from C. fructicola appears to be a dominant factor in NHR, and four novel core effectors-CfCE4, CfCE25, CfCE61, and CfCE66-detected by N. benthamiana were, accordingly, identified. These core effectors exhibit cell death-inducing activity in N. benthamiana and accumulate in the apoplast. With a series of CRISPR/Cas9-edited mutants or gene-silenced plants, we found the coreceptor BAK1 and helper NLRs including ADR1, NRG1, and NRCs mediate perceptions of these core effectors in N. benthamiana. Concurrently, multiple N. benthamiana genes encoding cell surface immune receptors and intracellular immune receptors were greatly induced by C. fructicola. This work represents the first characterization of the repertoire of C. fructicola core effectors responsible for NHR. Significantly, the novel core effectors and their signaling components unveiled in this study offered insights into a continuum of layered immunity during NHR and will be helpful for anthracnose disease management in diverse horticultural crops.

Modulation of plant immunity and biotic interactions under phosphate deficiency

Phosphorus (P) is an essential macronutrient for plant life and growth. P is primarily acquired in the form of inorganic phosphate (Pi) from soil. To cope with Pi deficiency, plants have evolved an elaborate system to improve Pi acquisition and utilization through an array of developmental and physiological changes, termed Pi starvation response (PSR). Plants also assemble and manage mutualistic microbes to enhance Pi uptake, through integrating PSR and immunity signaling. A trade-off between plant growth and defense favors the notion that plants lower a cellular state of immunity to accommodate host-beneficial microbes for nutrition and growth at the cost of infection risk. However, the existing data indicate that plants selectively activate defense responses against pathogens, but do not or less against non-pathogens, even under nutrient deficiency. In this review, we highlight recent advances in the principles and mechanisms with which plants balance immunity and growth-related processes to optimize their adaptation to Pi deficiency.

The Role of Salicylic Acid in the Expression of RECEPTOR-LIKE PROTEIN 23 and Other Immunity-Related Genes

The hormone salicylic acid (SA) plays a crucial role in plant immunity by activating responses that arrest pathogen ingress. SA accumulation also penalizes growth, a phenomenon visible in mutants that hyperaccumulate SA, resulting in strong growth inhibition. An important question, therefore, is why healthy plants produce basal levels of this hormone when defense responses are not activated. Here, we show that basal SA levels in unchallenged plants are needed for the expression of a number of immunity-related genes and receptors, such as RECEPTOR-LIKE PROTEIN 23 (RLP23). This was shown by depleting basal SA levels in transgenic Arabidopsis lines through the overexpression of the SA-inactivating hydroxylases DOWNY MILDEW-RESISTANT 6 (DMR6) or DMR6-LIKE OXYGENASE 1. RNAseq analysis revealed that the expression of a subset of immune receptor and signaling genes is strongly reduced in the absence of SA. The biological relevance of this was shown for RLP23: In SA-depleted and SA-insensitive plants, responses to the RLP23 ligand, the microbial pattern nlp24, were strongly reduced, whereas responses to flg22 remained unchanged. We hypothesize that low basal SA levels are needed for the expression of a subset of immune system components that enable early pathogen detection and activation of immunity.

Integrated multi-omics and genetic analyses reveal molecular determinants underlying Arabidopsis snap33 mutant phenotype

The secretory pathway is essential for plant immunity, delivering diverse antimicrobial molecules into the extracellular space. Arabidopsis thaliana soluble N-ethylmaleimide-sensitive-factor attachment protein receptor SNAP33 is a key actor of this process. The snap33 mutant displays dwarfism and necrotic lesions, however the molecular determinants of its macroscopic phenotypes remain elusive. Here, we isolated several new snap33 mutants that exhibited constitutive cell death and H2O2 accumulation, further defining snap33 as an autoimmune mutant. We then carried out quantitative transcriptomic and proteomic analyses showing that numerous defense transcripts and proteins were up-regulated in the snap33 mutant, among which genes/proteins involved in defense hormone, pattern-triggered immunity, and nucleotide-binding domain leucine-rich-repeat receptor signaling. qRT-PCR analyses and hormone dosages supported these results. Furthermore, genetic analyses elucidated the diverse contributions of the main defense hormones and some nucleotide-binding domain leucine-rich-repeat receptor signaling actors in the establishment of the snap33 phenotype, emphasizing the preponderant role of salicylic acid over other defense phytohormones. Moreover, the accumulation of pattern-triggered immunity and nucleotide-binding domain leucine-rich-repeat receptor signaling proteins in the snap33 mutant was confirmed by immunoblotting analyses and further shown to be salicylic acid-dependent. Collectively, this study unveiled molecular determinants underlying the Arabidopsis snap33 mutant phenotype and brought new insights into autoimmunity signaling.

The plant immune system: From discovery to deployment

Plant diseases cause famines, drive human migration, and present challenges to agricultural sustainability as pathogen ranges shift under climate change. Plant breeders discovered Mendelian genetic loci conferring disease resistance to specific pathogen isolates over 100 years ago. Subsequent breeding for disease resistance underpins modern agriculture and, along with the emergence and focus on model plants for genetics and genomics research, has provided rich resources for molecular biological exploration over the last 50 years. These studies led to the identification of extracellular and intracellular receptors that convert recognition of extracellular microbe-encoded molecular patterns or intracellular pathogen-delivered virulence effectors into defense activation. These receptor systems, and downstream responses, define plant immune systems that have evolved since the migration of plants to land ∼500 million years ago. Our current understanding of plant immune systems provides the platform for development of rational resistance enhancement to control the many diseases that continue to plague crop production.

Recent advances in understanding the role of two mitogen-activated protein kinase cascades in plant immunity

The mitogen-activated protein kinase (MAPK/MPK) cascade is an important intercellular signaling module that regulates plant growth, development, reproduction, and responses to biotic and abiotic stresses. A MAPK cascade usually consists of a MAPK kinase kinase (MAPKKK/MEKK), a MAPK kinase (MAPKK/MKK/MEK), and a MAPK. The well-characterized MAPK cascades in plant immunity to date are the MEKK1-MKK1/2-MPK4 cascade and the MAPKKK3/4/5-MKK4/5-MPK3/6 cascade. Recently, major breakthroughs have been made in understanding the molecular mechanisms associated with the regulation of immune signaling by both of these MAPK cascades. In this review, we highlight the most recent advances in understanding the role of both MAPK cascades in activating plant defense and in suppressing or fine-tuning immune signaling. We also discuss the molecular mechanisms by which plants stabilize and maintain the activation of MAPK cascades during immune signaling. Based on this review, we reveal the complexity and importance of the MEKK1-MKK1/2-MPK4 cascade and the MAPKKK3/4/5-MKK4/5-MPK3/6 cascade, which are tightly controlled by their interacting partners or substrates, in plant immunity.

A plant NLR receptor employs ABA central regulator PP2C-SnRK2 to activate antiviral immunity

Defence against pathogens relies on intracellular nucleotide-binding, leucine-rich repeat immune receptors (NLRs) in plants. Hormone signaling including abscisic acid (ABA) pathways are activated by NLRs and play pivotal roles in defence against different pathogens. However, little is known about how hormone signaling pathways are activated by plant immune receptors. Here, we report that a plant NLR Sw-5b mimics the behavior of the ABA receptor and directly employs the ABA central regulator PP2C-SnRK2 complex to activate an ABA-dependent defence against viral pathogens. PP2C4 interacts with and constitutively inhibits SnRK2.3/2.4. Behaving in a similar manner as the ABA receptor, pathogen effector ligand recognition triggers the conformational change of Sw-5b NLR that enables binding to PP2C4 via the NB domain. This receptor-PP2C4 binding interferes with the interaction between PP2C4 and SnRK2.3/2.4, thereby releasing SnRK2.3/2.4 from PP2C4 inhibition to activate an ABA-specific antiviral immunity. These findings provide important insights into the activation of hormone signaling pathways by plant immune receptors.

Cytoplasmic calcium influx mediated by plant MLKLs confers TNL-triggered immunity

The plant homolog of vertebrate necroptosis inducer mixed-lineage kinase domain-like (MLKL) contributes to downstream steps in Toll-interleukin-1 receptor domain NLR (TNL)-receptor-triggered immunity. Here, we show that Arabidopsis MLKL1 (AtMLKL1) clusters into puncta at the plasma membrane upon TNL activation and that this sub-cellular reorganization is dependent on the TNL signal transducer, EDS1. We find that AtMLKLs confer TNL-triggered immunity in parallel with RPW8-type HeLo-domain-containing NLRs (RNLs) and that the AtMLKL N-terminal HeLo domain is indispensable for both immunity and clustering. We show that the AtMLKL HeLo domain mediates cytoplasmic Ca2+ ([Ca2+]cyt) influx in plant and human cells, and AtMLKLs are responsible for sustained [Ca2+]cyt influx during TNL-triggered, but not CNL-triggered, immunity. Our study reveals parallel immune signaling functions of plant MLKLs and RNLs as mediators of [Ca2+]cyt influx and a potentially common role of the HeLo domain fold in the Ca2+-signal relay of diverse organisms.

The conceptual foundations of innate immunity: Taking stock 30 years later

While largely neglected over decades during which adaptive immunity captured most of the attention, innate immune mechanisms have now become central to our understanding of immunology. Innate immunity provides the first barrier to infection in vertebrates, and it is the sole mechanism of host defense in invertebrates and plants. Innate immunity also plays a critical role in maintaining homeostasis, shaping the microbiota, and in disease contexts such as cancer, neurodegeneration, metabolic syndromes, and aging. The emergence of the field of innate immunity has led to an expanded view of the immune system, which is no longer restricted to vertebrates and instead concerns all metazoans, plants, and even prokaryotes. The study of innate immunity has given rise to new concepts and language. Here, we review the history and definition of the core concepts of innate immunity, discussing their value and fruitfulness in the long run.

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.

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.

Signal Peptide Peptidase and PI4Kβ1/2 play opposite roles in plant ER stress response and immunity

The Arabidopsis pi4kβ1,2 mutant is mutated in the phosphatidylinositol 4-kinase (PI4K) β1 and PI4Kβ2 enzymes which are involved in the biosynthesis of phosphatidylinositol 4-phosphate (PI4P), a minor membrane lipid with important signaling roles. pi4kβ1,2 plants display autoimmunity and shorter roots. Though the pi4kβ1,2 mutant has been extensively characterized, the source of its autoimmunity remains largely unknown. In this study, through a genetic suppressor screen, we identified multiple partial loss-of-function alleles of signal peptide peptidase (spp) that can suppress all the defects of pi4kβ1,2. SPP is an intramembrane cleaving aspartic protease. Interestingly, pi4kβ1,2 plants display enhanced ER stress response and mutations in SPP can suppress such phenotype. Furthermore, reduced ER stress responses were observed in the spp single mutants. Overall, our study reveals a previously unknown function of PI4Kβ and SPP in ER stress and plant immunity.

Identification of epigenetically regulated genes involved in plant-virus interaction and their role in virus-triggered induced resistance

BACKGROUND

Plant responses to a wide range of stresses are known to be regulated by epigenetic mechanisms. Pathogen-related investigations, particularly against RNA viruses, are however scarce. It has been demonstrated that Arabidopsis thaliana plants defective in some members of the RNA-directed DNA methylation (RdDM) or histone modification pathways presented differential susceptibility to the turnip mosaic virus. In order to identify genes directly targeted by the RdDM-related RNA Polymerase V (POLV) complex and the histone demethylase protein JUMONJI14 (JMJ14) during infection, the transcriptomes of infected mutant and control plants were obtained and integrated with available chromatin occupancy data for various epigenetic proteins and marks.

RESULTS

A comprehensive list of virus-responsive gene candidates to be regulated by the two proteins was obtained. Twelve genes were selected for further characterization, confirming their dynamic regulation during the course of infection. Several epigenetic marks on their promoter sequences were found using in silico data, raising confidence that the identified genes are actually regulated by epigenetic mechanisms. The altered expression of six of these genes in mutants of the methyltransferase gene CURLY LEAF and the histone deacetylase gene HISTONE DEACETYLASE 19 suggests that some virus-responsive genes may be regulated by multiple coordinated epigenetic complexes. A temporally separated multiple plant virus infection experiment in which plants were transiently infected with one virus and then infected by a second one was designed to investigate the possible roles of the identified POLV- and JMJ14-regulated genes in wild-type (WT) plants. Plants that had previously been stimulated with viruses were found to be more resistant to subsequent virus challenge than control plants. Several POLV- and JMJ14-regulated genes were found to be regulated in virus induced resistance in WT plants, with some of them poisoned to be expressed in early infection stages.

CONCLUSIONS

A set of confident candidate genes directly regulated by the POLV and JMJ14 proteins during virus infection was identified, with indications that some of them may be regulated by multiple epigenetic modules. A subset of these genes may also play a role in the tolerance of WT plants to repeated, intermittent virus infections.

New insight into Ca2+ -permeable channel in plant immunity

Calcium ions (Ca2+ ) are crucial intracellular second messengers in eukaryotic cells. Upon pathogen perception, plants generate a transient and rapid increase in cytoplasmic Ca2+ levels, which is subsequently decoded by Ca2+ sensors and effectors to activate downstream immune responses. The elevation of cytosolic Ca2+ is commonly attributed to Ca2+ influx mediated by plasma membrane-localized Ca2+ -permeable channels. However, the contribution of Ca2+ release triggered by intracellular Ca2+ -permeable channels in shaping Ca2+ signaling associated with plant immunity remains poorly understood. This review discusses recent advances in understanding the mechanism underlying the shaping of Ca2+ signatures upon the activation of immune receptors, with particular emphasis on the identification of intracellular immune receptors as non-canonical Ca2+ -permeable channels. We also discuss the involvement of Ca2+ release from the endoplasmic reticulum in generating Ca2+ signaling during plant immunity.

MAPK Cascades in Plant Microbiota Structure and Functioning

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules that coordinate diverse biological processes such as plant innate immunity and development. Recently, MAPK cascades have emerged as pivotal regulators of the plant holobiont, influencing the assembly of normal plant microbiota, essential for maintaining optimal plant growth and health. In this review, we provide an overview of current knowledge on MAPK cascades, from upstream perception of microbial stimuli to downstream host responses. Synthesizing recent findings, we explore the intricate connections between MAPK signaling and the assembly and functioning of plant microbiota. Additionally, the role of MAPK activation in orchestrating dynamic changes in root exudation to shape microbiota composition is discussed. Finally, our review concludes by emphasizing the necessity for more sophisticated techniques to accurately decipher the role of MAPK signaling in establishing the plant holobiont relationship.

Substrate-induced condensation activates plant TIR domain proteins

Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain mediate recognition of strain-specific pathogen effectors, typically via their C-terminal ligand-sensing domains1. Effector binding enables TIR-encoded enzymatic activities that are required for TIR-NLR (TNL)-mediated immunity2,3. Many truncated TNL proteins lack effector-sensing domains but retain similar enzymatic and immune activities4,5. The mechanism underlying the activation of these TIR domain proteins remain unclear. Here we show that binding of the TIR substrates NAD+ and ATP induces phase separation of TIR domain proteins in vitro. A similar condensation occurs with a TIR domain protein expressed via its native promoter in response to pathogen inoculation in planta. The formation of TIR condensates is mediated by conserved self-association interfaces and a predicted intrinsically disordered loop region of TIRs. Mutations that disrupt TIR condensates impair the cell death activity of TIR domain proteins. Our data reveal phase separation as a mechanism for the activation of TIR domain proteins and provide insight into substrate-induced autonomous activation of TIR signalling to confer plant immunity.

HvMPK4 phosphorylates HvWRKY1 to enhance its suppression of barley immunity to powdery mildew fungus

Mitogen-activated protein kinase (MAPK) cascades play important roles in disease resistance in model plant species. However, the functions of MAPK signaling pathways in crop disease resistance are largely unknown. Here we report the function of HvMKK1-HvMPK4-HvWRKY1 module in barley immune system. HvMPK4 is identified to play a negative role in barley immune response against Bgh, as virus-induced gene silencing of HvMPK4 results in enhanced disease resistance whilst stably overexpressing HvMPK4 leads to super-susceptibility to Bgh infection. Furthermore, the barley MAPK kinase HvMKK1 is found to specifically interact with HvMPK4, and the activated HvMKK1DD variant specifically phosphorylates HvMPK4 in vitro. Moreover, the transcription factor HvWRKY1 is identified to be a downstream target of HvMPK4 and phosphorylated by HvMPK4 in vitro in the presence of HvMKK1DD. Phosphorylation assay coupled with mutagenesis analyses identifies S122, T284, and S347 in HvWRKY1 as the major residues phosphorylated by HvMPK4. HvWRKY1 is phosphorylated in barley at the early stages of Bgh infection, which enhances its suppression on barley immunity likely due to enhanced DNA-binding and transcriptional repression activity. Our data suggest that the HvMKK1-HvMPK4 kinase pair acts upstream of HvWRKY1 to negatively regulate barley immunity against powdery mildew.

Arabidopsis PHYTOALEXIN DEFICIENT 4 promotes the maturation and nuclear accumulation of immune-related cysteine protease RD19

Arabidopsis PHYTOALEXIN DEFICIENT 4 (PAD4) has an essential role in pathogen resistance as a heterodimer with ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1). Here we investigated an additional PAD4 role in which it associates with and promotes the maturation of the immune-related cysteine protease RESPONSIVE TO DEHYDRATION 19 (RD19). We found that RD19 and its paralog RD19c promoted EDS1- and PAD4-mediated effector-triggered immunity to an avirulent Pseudomonas syringae strain, DC3000, expressing the effector AvrRps4 and basal immunity against the fungal pathogen Golovinomyces cichoracearum. Overexpression of RD19, but not RD19 protease-inactive catalytic mutants, in Arabidopsis transgenic lines caused EDS1- and PAD4-dependent autoimmunity and enhanced pathogen resistance. In these lines, RD19 maturation to a pro-form required its catalytic residues, suggesting that RD19 undergoes auto-processing. In transient assays, PAD4 interacted preferentially with the RD19 pro-protease and promoted its nuclear accumulation in leaf cells. Our results lead us to propose a model for PAD4-stimulated defense potentiation. PAD4 promotes maturation and nuclear accumulation of processed RD19, and RD19 then stimulates EDS1-PAD4 dimer activity to confer pathogen resistance. This study highlights potentially important additional PAD4 functions that eventually converge on canonical EDS1-PAD4 dimer signaling in plant immunity.