Arabidopsis RIN4 is a target of the type III virulence effector AvrRpt2 and modulates RPS2-mediated resistance

Construction of the super pan-genome for the genus Actinidia reveals structural variations linked to phenotypic diversity

Kiwifruits, belonging to the genus Actinidia, are acknowledged as one of the most successfully domesticated fruits in the twentieth century. Despite the rich wild resources and diverse phenotypes within this genus, insights into the genomic changes are still limited. Here, we conducted whole-genome sequencing on seven representative materials from highly diversified sections of Actinidia, leading to the assembly and annotation of 14 haplotype genomes with sizes spanning from 602.0 to 699.6 Mb. By compiling these haplotype genomes, we constructed a super pan-genome for the genus. We identified numerous structural variations (SVs, including variations in gene copy number) and highly diverged regions in these genomes. Notably, significant SV variability was observed within the intronic regions of the MED25 and TTG1 genes across different materials, suggesting their potential roles in influencing fruit size and trichome formation. Intriguingly, our findings indicated a high genetic divergence between two haplotype genomes, with one individual, tentatively named Actinidia × leiocacarpae, from sect. Leiocacarpae. This likely hybrid with a heterozygous genome exhibited notable genetic adaptations related to resistance against bacterial canker, particularly through the upregulation of the RPM1 gene, which contains a specific SV, after infection by Pseudomonas syringae pv. actinidiae. In addition, we also discussed the interlineage hybridizations and taxonomic treatments of the genus Actinidia. Overall, the comprehensive pan-genome constructed here, along with our findings, lays a foundation for examining genetic compositions and markers, particularly those related to SVs, to facilitate hybrid breeding aimed at developing desired phenotypes in kiwifruits.

Subcellular targets and recognition mechanism of Ralstonia solanacearum effector RipE1

Some plant NLRs carry unusual integrated protein domains (IDs) that mimic host targets of pathogen effectors. RipE1 is a core Ralstonia solanacearum Type III effector with a predicted cysteine protease activity that activates defense responses in resistant plants. In this study, we used a library of NLR-IDs as an investigative tool to screen for potential host-cell targets of RipE1. Based on these findings and the effector's localization, we identified two plant membrane trafficking components as RipE1's subcellular targets. Depending on its protease activity, RipE1 promotes the degradation of both exocyst complex subunit Exo70B1 and its known interactor RPM1-interacting protein-4 (RIN4), a known plant immunity regulator. RipE1 protease activity is recognized by the RIN4-guarding NLR Pseudomonas tomato race 1 (Ptr1) in Nicotiana benthamiana. Overall, the data presented here, along with the existing literature, suggest a possible link between RipE1 activity upon the host secretion machinery and its NLR-mediated recognition.

Monkeys at Rigged Typewriters: A Population and Network View of Plant Immune System Incompatibility

Immune system incompatibilities between naturally occurring genomic variants underlie many hybrid defects in plants and present a barrier for crop improvement. In this review, we approach immune system incompatibilities from pan-genomic and network perspectives. Pan-genomes offer insights into how natural variation shapes the evolutionary landscape of immune system incompatibilities, and through it, selection, polymorphisms, and recombination resistance emerge as common features that synergistically drive these incompatibilities. By contextualizing incompatibilities within the immune network, immune receptor promiscuity, complex dysregulation, and single-point failure appear to be recurrent themes of immune system defects. As geneticists break genes to investigate their function, so can we investigate broken immune systems to enrich our understanding of plant immune systems and work toward improving them.

Structure and Functions of NDR1/HIN1-Like (NHL) Proteins in Plant Development and Response to Environmental Stresses

The NON-RACE-SPECIFIC DISEASE RESISTANCE 1/harpin-induced 1-LIKE (NHL) gene family plays pivotal roles, including pathogen resistance, abiotic stress tolerance, and developmental regulation, underscoring their functional versatility in developmental and physiological processes of plants. NHL proteins often localize to the plasma membrane and contain conserved motifs, including the LEA2 and transmembrane domains, enabling dynamic interactions with signalling molecules and transcription factors. The ability of NHL proteins to dimerize and oligomerize further enhances their regulatory potential in signalling pathways. This review explores the structural and functional diversity of NHL proteins including their localizations, interacting proteins, and responses to abiotic and biotic stresses, ion transportation, seed germination, and responses to phytohormones. Future research integrating phylogenetics, and advanced tools including artificial intelligence will unlock the full potential of this gene family for breeding climate-resilient crops and agricultural sustainability.

Transcriptomic Profiling Reveals Regulatory Pathways of Tomato in Resistance to Verticillium Wilt Triggered by VdR3e

Tomatoes are important horticultural crops worldwide. Verticillium wilt is a disease caused by Verticillium dahliae that causes serious tomato yield losses. V. dahliae can be classified into three distinct races in tomatoes. We identified the specific VdR3e gene of V. dahliae race 3 and found that VdR3e triggered immune responses in the resistant tomato cultivar IVF6384. We confirmed that VdR3e triggers immune responses in the parents of IVF6384 plants and conducted transcriptome sequencing between male and female IVF6384 plants after VdR3e infiltration to analyze the potential regulatory network response to VdR3e. We found that both parents had a series of detoxification and stress resistance responses to VdR3e, but those of the male IVF6384 parent were concentrated in disease resistance-related signaling pathways. Moreover, several vital differentially expressed genes involved in functional annotation related to plant-pathogen interactions and plant hormone signaling stimulated immune responses in Nicotiana benthamiana. This study provides a new and comprehensive perspective on tomato resistance to Verticillium wilt.

Molecular Dialog of Ralstonia solanacearum and Plant Hosts with Highlights on Type III Effectors

Ralstonia solanacearum is a highly destructive soil-borne bacterium that causes bacterial wilt disease in more than 310 plant species worldwide. The pathogenicity of the bacteria is closely associated with type III effectors (T3Es), a class of virulence factors that are delivered to host plant cells by the type III secretion system. In spite of the complex evolutionary history and genetic diversity of the R. solanacearum species complex (RSSC), more than 100 different T3Es have been identified from the genomes of various strains. Based on the available functional studies, certain T3Es interact with host plant proteins and suppress host cell immunity, whereas other T3Es are recognized by the host plant to trigger specific resistance mechanisms. This review summarizes the mechanisms by which T3Es interfere with plant immune responses and the activation of the plant defense system upon T3E recognition. This in-depth review of the molecular interactions between R. solanacearum and its host plants offers insights into the complexity of plant-pathogen interactions and provides a scientific rationale and theoretical support for the future breeding of resistant crops.

A Ralstonia solanacearum effector regulates plant cell death by disrupting the homeostasis of the BPA1-ACD11 complex

Effectors secreted by phytopathogenic bacteria can suppress ETI responses induced by avirulence effectors, thereby overcoming crop resistance. However, the detailed mechanisms remain largely unknown. We report that the effector RipD from Ralstonia solanacearum regulates plant cell death in a protein abundance-dependent manner. RipD targets Arabidopsis BPA1, which directly interacts with the key cell death negative regulator ACD11. RipD competes with ACD11 for binding to BPA1, leading to the selective degradation of BPA1 via autophagy, sparing ACD11. A lower dose of RipD promotes BPA1 degradation but leads to ACD11 accumulation, thereby inhibiting RipAA-induced cell death. Conversely, higher levels of RipD degrade both BPA1 and ACD11, resulting in autophagy-dependent cell death. Visualization of RipD delivery by R. solanacearum indicated that it reaches levels sufficient to promote ACD11 accumulation and inhibit cell death. Our study reveals a novel mechanism by which an effector inhibits ETI and, for the first time, highlights the critical role of protein abundance in its function.IMPORTANCER. solanacearum infects major economic crops, notably tomato, potato, and tobacco, leading to substantial yield reductions and economic losses. This pathogen utilizes various type III effectors to suppress host resistance, often resulting in weakened or lost resistance. However, the underlying mechanisms remain largely unknown. Here, we reveal a novel mechanism by which RipD targets the BPA1-ACD11 complex, which is involved in host immunity and cell death. RipD regulates ACD11 protein homeostasis in a dose-dependent manner by competitively binding and activating autophagy, thereby modulating plant cell death. Importantly, visualization analysis revealed that the amount of RipD secreted by R. solanacearum into host cells is sufficient to inhibit Avr effector-induced cell death. Our study highlights for the first time the critical role of effector dosage, deepening the understanding of how R. solanacearum suppresses host ETI-related cell death and providing guidance and resources for breeding bacterial wilt resistance.

A core Plasmopara viticola effector attenuates the DNA-binding activity of bZIP transcription factor to compromise plant immunity

Grapevine (Vitis vinifera L.) frequently faces challenges from various pathogens, among which Plasmopara viticola is the most devastating one hindering grape production. During infection, P. viticola secretes a series of effectors into host cells to manipulate plant immune responses. Here, an RXLR effector of P. viticola, PvRXLR13, was identified as one that could disrupt immune processes and thus promote pathogen colonization. PvRXLR13 contained a functional signal peptide and was highly conserved across different destructive oomycetes. PvRXLR13 was significantly induced during P. viticola infection and could suppress elicitor chitin-induced reactive oxygen species (ROS), callose deposition, and INF1-triggered cell death. Furthermore, PvRXLR13 could also inhibit P. viticola- and P. capsici-triggered H2O2 accumulation and promote pathogen colonization in both grapevine and Nicotiana benthamiana, respectively. VvHY5, a basic leucine zipper (bZIP) transcription factor, was found to be the host target of PvRXLR13. Further analysis revealed that overexpression of VvHY5 enhanced grapevine resistance to P. viticola and P. viticola-triggered H2O2 accumulation. Furthermore, we found that VvHY5 directly bound to the promoter of the positive immune factor VvEDS1 and activated its expression, whereas PvRXLR13 attenuated the DNA-binding activity of VvHY5 during P. viticola infection. Further analysis revealed that other members of grape bZIPs, VvbZIP6/9/21/32/34/37, were also involved in the defense response against P. viticola invasion. Just like HY5/HYH, all these bZIP family members were targeted by the effector PvRXLR13. Collectively, our findings suggest that P. viticola secretes a key effector PvRXLR13 to compromise the function in immune regulation of bZIP transcription factors to promote infection in grapevine.

Bacterial XopR subverts RIN4 complex-mediated plant immunity via plasma membrane-associated percolation

Phytobacteria release type 3 effectors (T3Es) abundant in intrinsically disordered regions (IDRs) to undermine plant defenses. How flexible IDRs contribute to T3Es' function in subverting plant immunity remains unclear. Here, we identify a plant plasma membrane (PM)-associated macromolecular condensation mechanism that governs the sophisticated interplay between T3E XopR and the plant's Resistance to Pseudomonas syringae pv. maculicola 1 (RPM1)-interacting protein 4 (RIN4) immune complex. Upon deployment into plants, XopR undergoes PM association, percolation clustering, and spanning networking on the PM, ranging from subnanomolar to tens of nanomolar. This spatiotemporal building of the XopR network enables an efficient manipulation of plant surface immune regulators, including a coiled-coil nucleotide-binding leucine-rich repeat receptor (CNL)-guardee complex with highly disordered RIN4. When XopR hijacks and fluidizes the RIN4-RPM1 condensates, Arabidopsis shows reduced RIN4 phosphorylation and diminished RPM1-activated defense in vivo, consistent with XopR-impaired RIN4 phosphorylation by RPM1-interacting protein kinase (RIPK). Our research illuminates the mechanism underlying the dynamic interplay between bacterial T3Es and plant receptor complex condensates during infection.

Phylogenetic, Structural, and Evolutionary Insights into Pepper NBS-LRR Resistance Genes

The comprehensive analysis of NBS-LRR resistance genes in the pepper (Capsicum annuum L.) genome reveals their structural diversity, evolutionary history, and functional importance in plant immunity. A total of 252 NBS-LRR genes were identified, distributed unevenly across all chromosomes, with 54% forming 47 gene clusters. These clusters, driven by tandem duplications and genomic rearrangements, underscore the dynamic evolution of resistance genes. Phylogenetic analysis demonstrated the dominance of the nTNL subfamily over the TNL subfamily, reflecting lineage-specific adaptations and evolutionary pressures. Structural analyses identified six conserved motifs (P-loop, RNBS-A, kinase-2, RNBS-B, RNBS-C, and GLPL) essential for ATP/GTP binding and resistance signaling. Subfamily-specific differences in motif composition and sequence similarity highlight their functional divergence and specialization. Comparative analyses across species further revealed a greater prevalence of nTNL genes in angiosperms, with significant losses of TNL genes in monocots. This study enhances our understanding of the evolution and diversification of plant-resistance genes and provides a foundation for developing disease-resistant crops through targeted breeding strategies.

Exploring the Tomato Root Protein Network Exploited by Core Type 3 Effectors from the Ralstonia solanacearum Species Complex

Proteomics has become a powerful approach for the identification and characterization of type III effectors (T3Es). Members of the Ralstonia solanacearum species complex (RSSC) deploy T3Es to manipulate host cells and to promote root infection of, among others, a wide range of solanaceous plants such as tomato, potato, and tobacco. Here, we used TurboID-mediated proximity labeling (PL) in tomato hairy root cultures to explore the proxeomes of the core RSSC T3Es RipU, RipD, and RipB. The RipU proxeome was enriched for multiple protein kinases, suggesting a potential impact on the two branches of the plant immune surveillance system, being the membrane-localized PAMP-triggered immunity (PTI) and the RIN4-dependent effector-triggered immunity (ETI) complexes. In agreement, a transcriptomics analysis in tomato revealed the potential involvement of RipU in modulating reactive oxygen species (ROS) signaling. The proxeome of RipB was putatively enriched for mitochondrial and chloroplast proteins and that of RipD for proteins potentially involved in the endomembrane system. Together, our results demonstrate that TurboID-PL in tomato hairy roots represents a promising tool to study Ralstonia T3E targets and functioning and that it can unravel potential host processes that can be hijacked by the bacterial pathogen.

The role of Exo70s in plant defense against pathogens and insect pests and their application for crop breeding

Plant diseases caused by pathogens and pests lead to crop losses, posing a threat to global food security. The secretory pathway is an integral component of plant defense. The exocyst complex regulates the final step of the secretory pathway and is thus essential for secretory defense. In the last decades, several subunits of the exocyst complex have been reported to be involved in plant defense, especially Exo70s. This comprehensive review focuses on the functions of the exocyst Exo70s in plant immunity, particularly in recognizing pathogen and pest signatures. We discussed Exo70's interactions with immune receptors and other immune-related proteins, its symbiotic relationships with microbes, and its role in non-host resistance. Finally, we discussed the future engineering breeding of crops with resistance to pathogens and pests based on our current understanding of Exo70s.

CsNAC17 enhances resistance to Colletotrichum gloeosporioides by interacting with CsbHLH62 in Camellia sinensis

The pathogen Colletotrichum gloeosporioides causes anthracnose, a serious threat to tea trees around the world, particularly in warm and humid regions. RNA-Seq data have previously indicated NAC transcription factors are involved in anthracnose resistance, but underlying mechanisms remain unclear. The BiFC, Split-LUC, and Co-IP assays validated the interaction between CsbHLH62 and CsNAC17 identified through yeast two-hybrid (Y2H) screening. CsNAC17 or CsbHLH62 overexpression enhanced anthracnose resistance, as well as enhanced levels of H2O2, hypersensitivity, and cell death in Nicotiana benthamiana. The NBS-LRR gene CsRPM1 is regulated by CsNAC17 by binding directly to its promoter (i.e. CACG, CATGTG), while CsbHLH62 facilitates CsNAC17's binding and increases transcriptional activity of CsRPM1. Additionally, transient silencing of CsNAC17 and CsbHLH62 in tea plant leaves using the virus-induced gene silencing (VIGS) system resulted in decreased resistance to anthracnose. Conversely, transient overexpression of CsNAC17 and CsbHLH62 in tea leaves significantly enhanced the resistance against anthracnose. Based on these results, it appears that CsbHLH62 facilitates the activity of CsNAC17 on CsRPM1, contributing to increased anthracnose resistance.

Mannose-binding lectin 1.1A interacts with hypersensitive-induced response 4 to promote hypersensitive cell death and defense responses in cotton upon Verticillium dahliae infection

Increasing evidence indicates that mannose-binding lectins (MBLs) act as a lectin-receptor-like protein in plant immune responses, yet the functional basis remains elusive. In this study, we dissected the functional mechanism of GbMBL1.1A in defense against Verticillium dahliae infection in sea island cotton (Gossypium barbadense). GbMBL1.1A expressed preferentially in cotton roots and significantly upregulated upon V. dahliae infection. Transgenic expression of GbMBL1.1A in upland cotton (Gossypium hirsutum) and Arabidopsis remarkably improved the disease resistance of the plants, while silencing of GbMBL1.1A resulted in an increased susceptibility of cotton plants in response to V. dahliae attack. Protein interaction assays revealed that GbMBL1.1A interacted with the cotton hypersensitive-induced response protein 4 (GbHIR4, a scaffold protein for immune signaling in the plasma membrane microdomain) through PAN domain. GbHIR4 expression was upregulated in response to V. dahliae invasion, and silencing of GbHIR4 seriously attenuated the disease tolerance of cotton plants. GbMBL1.1A enhanced the cell death phenotype induced by the transient expression of GbHIR4, and meanwhile promoted HR-PCD and GbHIR4-dependent resistance upon V. dahliae infection. The results suggested that GbMBL1.1A employed GbHIR4 as a downstream component to trigger the hypersensitive responses and therefore contributed to cotton resistance against V. dahliae. In addition, we observed that GbMBL1.1A overexpression could alter the phytohormone-mediated defense and growth signaling under both normal and V. dahliae infection conditions. Collectively, these results demonstrated that the lectin receptor-like protein GbMBL1.1A interacts with GbHIR4 in cotton immunity to induce the hypersensitive response, which is associated with phytohormone-mediated defense and growth signaling.

RIN4 immunity regulators mediate recognition of the core effector RipE1 of Ralstonia solanacearum by the receptor Ptr1

Ralstonia solanacearum causes lethal bacterial wilt diseases in numerous crops, resulting in considerable yield losses. Harnessing genetic resistance is desirable for safeguarding plants against phytopathogens. However, genetic resources resistant to bacterial wilt are limited in crops. RipE1, a conserved type Ⅲ effector with cysteine protease activity, is recognized in Nicotiana benthamiana and Arabidopsis (Arabidopsis thaliana). Here, using a virus-induced gene silencing approach, we identified the gene encoding N. benthamiana homolog of Ptr1 (NbPtr1a), a coiled-coil nucleotide-binding leucine-rich repeat receptor (NLR) recognizing RipE1. Silencing or editing NbPtr1a completely abolished RipE1-induced cell death, indicating recognition of RipE1 by NbPtr1a. Genetic complementation confirmed this recognition, which is conserved across multiple solanaceous plants. Expression of RipE1 in planta or within pathogenic bacteria promoted pathogen colonization of Nbptr1a mutant plants, demonstrating its virulence function independent of NLR recognition. Silencing NbRIN4 enhanced RipE1-induced cell death, while expressing NbRIN4 inhibited it, suggesting that NbRIN4 is involved in recognition of NbPtr1a-RipE1. Furthermore, RipE1 associated with and cleaved NbRIN4, AtRIN4, and tomato (Solanum lycopersicum) SlRIN4 proteins through its cysteine protease activity. Silencing NbRIN4 in Nbptr1a mutants did not prevent RipE1 from promoting pathogen colonization, suggesting that NbRIN4 is not the primary target for RipE1-mediated virulence. Additionally, NbRIN4 suppressed self-association of the coiled-coil domain of NbPtr1a, which is critical for NbPtr1a-mediated cell death and resistance. Finally, we demonstrated that activation of NbPtr1a requires RipE1-mediated elimination of NbRIN4. Given the conserved nature of RipE1, Ptr1 holds great potential for protecting crops from diverse R. solanacearum strains and other distinct pathogens.

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.

NbPTR1 confers resistance against Pseudomonas syringae pv. actinidiae in kiwifruit

Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) causes a devastating canker disease in yellow-fleshed kiwifruit (Actinidia chinensis). The effector HopZ5, which is present in all isolates of Psa3 causing global outbreaks of pandemic kiwifruit canker disease, triggers immunity in Nicotiana benthamiana and is not recognised in susceptible A. chinensis cultivars. In a search for N. benthamiana nonhost resistance genes against HopZ5, we found that the nucleotide-binding leucine-rich repeat receptor NbPTR1 recognised HopZ5. RPM1-interacting protein 4 orthologues from N. benthamiana and A. chinensis formed a complex with NbPTR1 and HopZ5 activity was able to disrupt this interaction. No functional orthologues of NbPTR1 were found in A. chinensis. NbPTR1 transformed into Psa3-susceptible A. chinensis var. chinensis 'Hort16A' plants introduced HopZ5-specific resistance against Psa3. Altogether, this study suggested that expressing NbPTR1 in Psa3-susceptible kiwifruit is a viable approach to acquiring resistance to Psa3 and it provides valuable information for engineering resistance in otherwise susceptible kiwifruit genotypes.

Cotton Bollworm (H. armigera) Effector PPI5 Targets FKBP17-2 to Inhibit ER Immunity and JA/SA Responses, Enhancing Insect Feeding

The cotton bollworm causes severe mechanical damage to plants during feeding and leaves oral secretions (OSs) at the mechanical wounds. The role these OSs play in the invasion of plants is still largely unknown. Here, a novel H. armigera effector peptidyl prolyl trans-isomerase 5 (PPI5) was isolated and characterized. PPI5 induces the programmed cell death (PCD) due to the unfolded protein response (UPR) in tobacco leaf. We reveal that PPI5 is important for the growth and development of cotton bollworm on plants, as it renders plants more susceptible to feeding. The GhFKBP17-2, was identified as a host target for PPI5 with peptidyl-prolyl isomerase (PPIase) activity. CRISPR/Cas9 knock-out cotton mutant (CR-GhFKBP17-1/3), VIGS (TRV: GhFKBP17-2) and overexpression lines (OE-GhFKBP17-1/3) were created and the data indicate that GhFKBP17-2 positively regulates endoplasmic reticulum (ER) stress-mediated plant immunity in response to cotton bollworm infestation. We further confirm that PPI5 represses JA and SA levels by downregulating the expression of JA- and SA-associated genes, including JAZ3/9, MYC2/3, JAR4, PR4, LSD1, PAD4, ICS1 and PR1/5. Taken together, our results reveal that PPI5 reduces plant defense responses and makes plants more susceptible to cotton bollworm infection by targeting and suppressing GhFKBP17-2 -mediated plant immunity.

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.

A NAC triad modulates plant immunity by negatively regulating N-hydroxy pipecolic acid biosynthesis

N-hydroxy pipecolic acid (NHP) plays an important role in plant immunity. In contrast to its biosynthesis, our current knowledge with respect to the transcriptional regulation of the NHP pathway is limited. This study commences with the engineering of Arabidopsis plants that constitutively produce high NHP levels and display enhanced immunity. Label-free proteomics reveals a NAC-type transcription factor (NAC90) that is strongly induced in these plants. We find that NAC90 is a target gene of SAR DEFICIENT 1 (SARD1) and induced by pathogen, salicylic acid (SA), and NHP. NAC90 knockout mutants exhibit constitutive immune activation, earlier senescence, higher levels of NHP and SA, as well as increased expression of NHP and SA biosynthetic genes. In contrast, NAC90 overexpression lines are compromised in disease resistance and accumulated reduced levels of NHP and SA. NAC90 could interact with NAC61 and NAC36 which are also induced by pathogen, SA, and NHP. We next discover that this protein triad directly represses expression of the NHP and SA biosynthetic genes AGD2-LIKE DEFENSE RESPONSE PROTEIN 1 (ALD1), FLAVIN MONOOXYGENASE 1 (FMO1), and ISOCHORISMATE SYNTHASE 1 (ICS1). Constitutive immune response in nac90 is abolished once blocking NHP biosynthesis in the fmo1 background, signifying that NAC90 negative regulation of immunity is mediated via NHP biosynthesis. Our findings expand the currently documented NHP regulatory network suggesting a model that together with NHP glycosylation, NAC repressors take part in a 'gas-and-brake' transcriptional mechanism to control NHP production and the plant growth and defense trade-off.

Decomposition of dynamic transcriptomic responses during effector-triggered immunity reveals conserved responses in two distinct plant cell populations

Rapid plant immune responses in the appropriate cells are needed for effective defense against pathogens. Although transcriptome analysis is often used to describe overall immune responses, collection of transcriptome data with sufficient resolution in both space and time is challenging. We reanalyzed public Arabidopsis time-course transcriptome data obtained after low-dose inoculation with a Pseudomonas syringae strain expressing the effector AvrRpt2, which induces effector-triggered immunity in Arabidopsis. Double-peak time-course patterns are prevalent among thousands of upregulated genes. We implemented a multi-compartment modeling approach to decompose the double-peak pattern into two single-peak patterns for each gene. The decomposed peaks reveal an "echoing" pattern: the peak times of the first and second peaks correlate well across most upregulated genes. We demonstrated that the two peaks likely represent responses of two distinct cell populations that respond either cell autonomously or indirectly to AvrRpt2. Thus, the peak decomposition has extracted spatial information from the time-course data. The echoing pattern also indicates a conserved transcriptome response with different initiation times between the two cell populations despite different elicitor types. A gene set highly overlapping with the conserved gene set is also upregulated with similar kinetics during pattern-triggered immunity. Activation of a WRKY network via different entry-point WRKYs can explain the similar but not identical transcriptome responses elicited by different elicitor types. We discuss potential benefits of the properties of the WRKY activation network as an immune signaling network in light of pressure from rapidly evolving pathogens.

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.

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.

Pathogen perception and signaling in plant immunity

Plant diseases are a constant and serious threat to agriculture and ecological biodiversity. Plants possess a sophisticated innate immunity system capable of detecting and responding to pathogen infection to prevent disease. Our understanding of this system has grown enormously over the past century. Early genetic descriptions of plant disease resistance and pathogen virulence were embodied in the gene-for-gene hypothesis, while physiological studies identified pathogen-derived elicitors that could trigger defense responses in plant cells and tissues. Molecular studies of these phenomena have now coalesced into an integrated model of plant immunity involving cell surface and intracellular detection of specific pathogen-derived molecules and proteins culminating in the induction of various cellular responses. Extracellular and intracellular receptors engage distinct signaling processes but converge on many similar outputs with substantial evidence now for integration of these pathways into interdependent networks controlling disease outcomes. Many of the molecular details of pathogen recognition and signaling processes are now known, providing opportunities for bioengineering to enhance plant protection from disease. Here we provide an overview of the current understanding of the main principles of plant immunity, with an emphasis on the key scientific milestones leading to these insights.

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.

A Putative Apoplastic Effector of Clavibacter capsici, ChpGCc as Hypersensitive Response and Virulence (Hrv) Protein in Plants

Clavibacter bacteria use secreted apoplastic effectors, such as putative serine proteases, for virulence in host plants and for hypersensitive response (HR) induction in nonhost plants. Previously, we have shown that Clavibacter capsici ChpGCc is important for the necrosis development in pepper (Capsicum annuum) leaves. Here, we determine the function of ChpGCc, along with three paralogous proteins, for HR induction in the apoplastic space of a nonhost plant, Nicotiana tabacum. The full-length and signal peptide-deleted (ΔSP) mature forms of all proteins fused with the tobacco PR1b signal sequence were generated. The full-length and ΔSP forms of ChpGCc and only the ΔSP forms of ChpECc and Pat-1Cc, but none of the ChpCCc, triggered HR. Based on the predicted protein structures, ChpGCc carries amino acids for a catalytic triad and a disulfide bridge in positions like Pat-1Cm. Substituting these amino acids of ChpGCc with alanine abolished or reduced HR-inducing activity. To determine whether these residues are important for necrosis development in pepper, alanine-substituted chpGCc genes were transformed into the C. capsici PF008ΔpCM1 strain, which lacks the intact chpGCc gene. The strain with any variants failed to restore the necrosis-causing ability. These results suggest that ChpGCc has a dual function as a virulence factor in host plants and an HR elicitor in nonhost plants. Based on our findings and previous results, we propose Clavibacter apoplastic effectors, such as ChpGCc, Pat-1Cm, Chp-7Cs, and ChpGCm, as hypersensitive response and virulence (Hrv) proteins that display phenotypic similarities to the hypersensitive response and pathogenicity (Hrp) proteins found in gram-negative bacteria. [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.

Pseudomonas effector AvrB is a glycosyltransferase that rhamnosylates plant guardee protein RIN4

The plant pathogen Pseudomonas syringae encodes a type III secretion system avirulence effector protein, AvrB, that induces a form of programmed cell death called the hypersensitive response in plants as a defense mechanism against systemic infection. Despite the well-documented catalytic activities observed in other Fido (Fic, Doc, and AvrB) proteins, the enzymatic activity and target substrates of AvrB have remained elusive. Here, we show that AvrB is an unprecedented glycosyltransferase that transfers rhamnose from UDP-rhamnose to a threonine residue of the Arabidopsis guardee protein RIN4. We report structures of various enzymatic states of the AvrB-catalyzed rhamnosylation reaction of RIN4, which reveal the structural and mechanistic basis for rhamnosylation by a Fido protein. Collectively, our results uncover an unexpected reaction performed by a prototypical member of the Fido superfamily while providing important insights into the plant hypersensitive response pathway and foreshadowing more diverse chemistry used by Fido proteins and their substrates.

The RIN4-like/NOI proteins NOI10 and NOI11 modulate the response to biotic stresses mediated by RIN4 in Arabidopsis

KEY MESSAGE

NOI10 and NOI11 are two RIN4-like/NOI proteins that participate in the immune response of the Arabidopsis plant and affect the RIN4-regulated mechanisms involving the R-proteins RPM1 and RPS2. The immune response in plants depends on the regulation of signaling pathways triggered by pathogens and herbivores. RIN4, a protein of the RIN4-like/NOI family, is considered to be a central immune signal in the interactions of plants and pathogens. In Arabidopsis thaliana, four of the 15 members of the RIN4-like/NOI family (NOI3, NOI5, NOI10, and NOI11) were induced in response to the plant herbivore Tetranychus urticae. While overexpressing NOI10 and NOI11 plants did not affect mite performance, opposite callose accumulation patterns were observed when compared to RIN4 overexpressing plants. In vitro and in vivo analyses demonstrated the interaction of NOI10 and NOI11 with the RIN4 interactors RPM1, RPS2, and RIPK, suggesting a role in the context of the RIN4-regulated immune response. Transient expression experiments in Nicotiana benthamiana evidenced that NOI10 and NOI11 differed from RIN4 in their functionality. Furthermore, overexpressing NOI10 and NOI11 plants had significant differences in susceptibility with WT and overexpressing RIN4 plants when challenged with Pseudomonas syringae bacteria expressing the AvrRpt2 or the AvrRpm1 effectors. These results demonstrate the participation of NOI10 and NOI11 in the RIN4-mediated pathway. Whereas RIN4 is considered a guardee protein, NOI10 and NOI11 could act as decoys to modulate the concerted activity of effectors and R-proteins.

Barley MLA3 recognizes the host-specificity effector Pwl2 from Magnaporthe oryzae

Plant nucleotide-binding leucine-rich repeat (NLRs) immune receptors directly or indirectly recognize pathogen-secreted effector molecules to initiate plant defense. Recognition of multiple pathogens by a single NLR is rare and usually occurs via monitoring for changes to host proteins; few characterized NLRs have been shown to recognize multiple effectors. The barley (Hordeum vulgare) NLR gene Mildew locus a (Mla) has undergone functional diversification, and the proteins encoded by different Mla alleles recognize host-adapted isolates of barley powdery mildew (Blumeria graminis f. sp. hordei [Bgh]). Here, we show that Mla3 also confers resistance to the rice blast fungus Magnaporthe oryzae in a dosage-dependent manner. Using a forward genetic screen, we discovered that the recognized effector from M. oryzae is Pathogenicity toward Weeping Lovegrass 2 (Pwl2), a host range determinant factor that prevents M. oryzae from infecting weeping lovegrass (Eragrostis curvula). Mla3 has therefore convergently evolved the capacity to recognize effectors from diverse pathogens.

Plant U-box E3 ligases PUB20 and PUB21 negatively regulate pattern-triggered immunity in Arabidopsis

KEY MESSAGE

Plant U-box E3 ligases PUB20 and PUB21 are flg22-triggered signaling components and negatively regulate immune responses. Plant U-box proteins (PUBs) constitute a class of E3 ligases that are associated with various stress responses. Among the class IV PUBs featuring C-terminal Armadillo (ARM) repeats, PUB20 and PUB21 are closely related homologs. Here, we show that both PUB20 and PUB21 negatively regulate innate immunity in plants. Loss of PUB20 and PUB21 function leads to enhanced resistance to surface inoculation with the virulent bacterium Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). However, the resistance levels remain unaffected after infiltration inoculation, suggesting that PUB20 and PUB21 primarily function during the early defense stages. The enhanced resistance to Pst DC3000 in PUB mutant plants (pub20-1, pub21-1, and pub20-1/pub21-1) correlates with extensive flg22-triggered reactive oxygen production, strong MPK3 activation, and enhanced transcriptional activation of early immune response genes. Additionally, PUB mutant plants (except pub21-1) exhibit constitutive stomatal closure after Pst DC3000 inoculation, implying the significant role of PUB20 in stomatal immunity. Comparative analyses of flg22 responses between PUB mutants and wild-type plants reveals that the robust activation of the pattern-induced immune responses may enhance resistance against Pst DC3000. Notably, the hypersensitivity responses triggered by RPM1/avrRpm1 and RPS2/avrRpt2 are independent of PUB20 and PUB21. These results suggest that PUB20 and PUB21 knockout mutations affect bacterial invasion, likely during the early stages, acting as negative regulators of plant immunity.

The exocyst complex is an evolutionary battleground in plant-microbe interactions

Exocytosis is a conserved trafficking pathway that transports secretory vesicles to the extracellular space, replenishes the plasma membrane and is essential for establishing cell polarity. Its spatiotemporal regulation is mediated by an evolutionary conserved octameric tethering complex, the exocyst. In plants, certain subunits of this complex have diversified and acquired multiple functions, including a central role in defense against pathogens and pests. Here, I review the latest evidence suggesting the dramatic expansion and functional diversification of the exocyst subunit Exo70 is likely driven by a coevolutionary arms race, in which Exo70 proteins are repeatedly targeted by effectors from multiple pathogens and, in turn, are monitored by plant immune receptors for pathogen perception.

The amino acid region from 448-517 of CAMTA3 transcription factor containing a part of the TIG domain represses the N-terminal repression module function

CAMTA3, a Ca2+-regulated transcription factor, is a repressor of plant immune responses. A truncated version of CAMTA3; CAMTA3334 called N-terminal repression module (NRM), and its extended version (CAMTA447), which include the DNA binding domain, were previously reported to complement the camta3/2 mutant phenotype. Here, we generated a series of CAMTA3 truncated versions [the N-terminus (aa 1-517), C-terminus (aa 517-1032), R1 (aa 1-173), R2 (aa 174-345), R3 (aa 346-517), R4 (aa 517-689), R5 (aa 690-861) and R6 (aa 862-1032)], expressed in camta3 mutant and analyzed the phenotypes of the transgenic lines. Interestingly, unlike CAMTA447, extending the N-terminal region to 517 aa did not complement the camta3 phenotype, suggesting that the amino acid region from 448-517 (70 aa), which includes a part of the TIG domain suppresses the NRM activity. The C-terminus and other truncated versions (R1-R6) also failed to complement the camta3 mutant. Expressing the full length or NRM of CAMTA3 in camta3 plants suppressed the activation of immune-responsive genes and increased the expression of cold-induced genes. In contrast, the transgenic lines expressing the N- or C-terminus or R1-R6 of CAMTA3 showed expression patterns like those of the camta3 with enhanced expression of the defense genes and suppressed expression of the cold response genes. Furthermore, like camta3, the transgenic lines expressing the N- or C-terminus, or R1-R6 of CAMTA3 exhibited higher levels of H2O2 and increased resistance to a Pst DC3000 as compared to WT, NRM, or FL-CAMTA3 transgenic plants. Our studies identified a novel regulatory region in CAMTA3 that suppresses the NRM activity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01401-w.

MHP1 and MHL generate odd-chain fatty acids from 2-hydroxy fatty acids in sphingolipids and are related to immunity in Arabidopsis thaliana

In plants, the 2-hydroxy fatty acids (HFAs) of sphingolipids are important for plant growth and stress responses. Although the synthetic pathway of HFAs is well understood, their degradation has not yet been elucidated. In Saccharomyces cerevisiae, Mpo1 has been identified as a dioxygenase that degrades HFAs. This study examined the functions of two homologs of yeast Mpo1, MHP1 and MHL, in Arabidopsis thaliana. The mhp1 and mhp1mhl mutants showed a dwarf phenotype compared to that of the wild type. Lipid analysis of the mutants revealed the involvement of MHP1 and MHL in synthesizing odd-chain fatty acids (OCFAs), possibly by the degradation of HFAs. OCFAs are present in trace amounts in plants; however, their physiological significance is largely unknown. RNA sequence analysis of the mhp1mhl mutant revealed that growth-related genes decreased, whereas genes involved in stress response increased. Additionally, the mhp1mhl mutant had increased expression of defense-related genes and increased resistance to infection by Pseudomonas syringae pv. tomato DC3000 (Pto), and Pto carrying the effector AvrRpt2. Phytohormone analysis demonstrated that jasmonic acid in mhp1mhl was higher than that in the wild type. These results indicate that MHP1 and MHL are involved in synthesizing OCFAs and immunity in Arabidopsis.

Regulation of plant immunity via small RNA-mediated control of NLR expression

Plants use different receptors to detect potential pathogens: membrane-anchored pattern recognition receptors (PRRs) activated upon perception of pathogen-associated molecular patterns (PAMPs) that elicit pattern-triggered immunity (PTI); and intracellular nucleotide-binding leucine-rich repeat proteins (NLRs) activated by detection of pathogen-derived effectors, activating effector-triggered immunity (ETI). The interconnections between PTI and ETI responses have been increasingly reported. Elevated NLR levels may cause autoimmunity, with symptoms ranging from fitness cost to developmental arrest, sometimes combined with run-away cell death, making accurate control of NLR dosage key for plant survival. Small RNA-mediated gene regulation has emerged as a major mechanism of control of NLR dosage. Twenty-two nucleotide miRNAs with the unique ability to trigger secondary siRNA production from target transcripts are particularly prevalent in NLR regulation. They enhance repression of the primary NLR target, but also bring about repression of NLRs only complementary to secondary siRNAs. We summarize current knowledge on miRNAs and siRNAs in the regulation of NLR expression with an emphasis on 22 nt miRNAs and propose that miRNA and siRNA regulation of NLR levels provides additional links between PTI and NLR defense pathways to increase plant responsiveness against a broad spectrum of pathogens and control an efficient deployment of defenses.

Suppression of NLR-mediated plant immune detection by bacterial pathogens

The plant immune system is constituted of two functionally interdependent branches that provide the plant with an effective defense against microbial pathogens. They can be considered separate since one detects extracellular pathogen-associated molecular patterns by means of receptors on the plant surface, while the other detects pathogen-secreted virulence effectors via intracellular receptors. Plant defense depending on both branches can be effectively suppressed by host-adapted microbial pathogens. In this review we focus on bacterially driven suppression of the latter, known as effector-triggered immunity (ETI) and dependent on diverse NOD-like receptors (NLRs). We examine how some effectors secreted by pathogenic bacteria carrying type III secretion systems can be subject to specific NLR-mediated detection, which can be evaded by the action of additional co-secreted effectors (suppressors), implying that virulence depends on the coordinated action of the whole repertoire of effectors of any given bacterium and their complex epistatic interactions within the plant. We consider how ETI activation can be avoided by using suppressors to directly alter compromised co-secreted effectors, modify plant defense-associated proteins, or occasionally both. We also comment on the potential assembly within the plant cell of multi-protein complexes comprising both bacterial effectors and defense protein targets.

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

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

The Ralstonia pseudosolanacearum effector RipE1 is recognized at the plasma membrane by NbPtr1, the Nicotiana benthamiana homologue of Pseudomonas tomato race 1

The bacterial wilt disease caused by soilborne bacteria of the Ralstonia solanacearum species complex (RSSC) threatens important crops worldwide. Only a few immune receptors conferring resistance to this devastating disease are known so far. Individual RSSC strains deliver around 70 different type III secretion system effectors into host cells to manipulate the plant physiology. RipE1 is an effector conserved across the RSSC and triggers immune responses in the model solanaceous plant Nicotiana benthamiana. Here, we used multiplexed virus-induced gene silencing of the nucleotide-binding and leucine-rich repeat receptor family to identify the genetic basis of RipE1 recognition. Specific silencing of the N. benthamiana homologue of Solanum lycopersicoides Ptr1 (confers resistance to Pseudomonas syringae pv. tomato race 1) gene (NbPtr1) completely abolished RipE1-induced hypersensitive response and immunity to Ralstonia pseudosolanacearum. The expression of the native NbPtr1 coding sequence was sufficient to restore RipE1 recognition in Nb-ptr1 knockout plants. Interestingly, RipE1 association with the host cell plasma membrane was necessary for NbPtr1-dependent recognition. Furthermore, NbPtr1-dependent recognition of RipE1 natural variants is polymorphic, providing additional evidence for the indirect mode of activation of NbPtr1. Altogether, this work supports NbPtr1 relevance for resistance to bacterial wilt disease in Solanaceae.

Traffic Control: Subversion of Plant Membrane Trafficking by Pathogens

Membrane trafficking pathways play a prominent role in plant immunity. The endomembrane transport system coordinates membrane-bound cellular organelles to ensure that immunological components are utilized effectively during pathogen resistance. Adapted pathogens and pests have evolved to interfere with aspects of membrane transport systems to subvert plant immunity. To do this, they secrete virulence factors known as effectors, many of which converge on host membrane trafficking routes. The emerging paradigm is that effectors redundantly target every step of membrane trafficking from vesicle budding to trafficking and membrane fusion. In this review, we focus on the mechanisms adopted by plant pathogens to reprogram host plant vesicle trafficking, providing examples of effector-targeted transport pathways and highlighting key questions for the field to answer moving forward.

Signaling and Resistosome Formation in Plant Innate Immunity to Viruses: Is There a Common Mechanism of Antiviral Resistance Conserved across Kingdoms?

Virus-specific proteins, including coat proteins, movement proteins, replication proteins, and suppressors of RNA interference are capable of triggering the hypersensitive response (HR), which is a type of cell death in plants. The main cell death signaling pathway involves direct interaction of HR-inducing proteins with nucleotide-binding leucine-rich repeats (NLR) proteins encoded by plant resistance genes. Singleton NLR proteins act as both sensor and helper. In other cases, NLR proteins form an activation network leading to their oligomerization and formation of membrane-associated resistosomes, similar to metazoan inflammasomes and apoptosomes. In resistosomes, coiled-coil domains of NLR proteins form Ca2+ channels, while toll-like/interleukin-1 receptor-type (TIR) domains form oligomers that display NAD+ glycohydrolase (NADase) activity. This review is intended to highlight the current knowledge on plant innate antiviral defense signaling pathways in an attempt to define common features of antiviral resistance across the kingdoms of life.

Ptr1 and ZAR1 immune receptors confer overlapping and distinct bacterial pathogen effector specificities

Some nucleotide-binding and leucine-rich repeat receptors (NLRs) indirectly detect pathogen effectors by monitoring their host targets. In Arabidopsis thaliana, RIN4 is targeted by multiple sequence-unrelated effectors and activates immune responses mediated by RPM1 and RPS2. These effectors trigger cell death in Nicotiana benthamiana, but the corresponding NLRs have yet not been identified. To identify N. benthamiana NLRs (NbNLRs) that recognize Arabidopsis RIN4-targeting effectors, we conducted a rapid reverse genetic screen using an NbNLR VIGS library. We identified that the N. benthamiana homolog of Ptr1 (Pseudomonas tomato race 1) recognizes the Pseudomonas effectors AvrRpt2, AvrRpm1, and AvrB. We demonstrated that recognition of the Xanthomonas effector AvrBsT and the Pseudomonas effector HopZ5 is conferred independently by the N. benthamiana homolog of Ptr1 and ZAR1. Interestingly, the recognition of HopZ5 and AvrBsT is contributed unequally by Ptr1 and ZAR1 in N. benthamiana and Capsicum annuum. In addition, we showed that the RLCK XII family protein JIM2 is required for the NbZAR1-dependent recognition of AvrBsT and HopZ5. The recognition of sequence-unrelated effectors by NbPtr1 and NbZAR1 provides an additional example of convergently evolved effector recognition. Identification of key components involved in Ptr1 and ZAR1-mediated immunity could reveal unique mechanisms of expanded effector recognition.

The wheels of destruction: Plant NLR immune receptors are mobile and structurally dynamic disease resistance proteins

Nucleotide-binding leucine-rich repeat (NLR) proteins are intracellular immune receptors that restrict plant invasion by pathogens. Most NLRs operate in intricate networks to detect pathogen effectors in a robust and efficient manner. NLRs are not static sensors; rather, they exhibit remarkable mobility and structural plasticity during the innate immune response. Inactive NLRs localize to diverse subcellular compartments where they are poised to sense pathogen effectors. During pathogen attack, some NLRs relocate toward the plant-pathogen interface, possibly to ensure their timely activation. Activated NLRs reorganize into wheel-shaped oligomers, some of which then form plasma membrane pores that promote calcium influx and programmed cell death. The emerging paradigm is that this variable and dynamic nature underpins effective NLR-mediated immunity.

Diversified host target families mediate convergently evolved effector recognition across plant species

Recognition of pathogen effectors is a crucial step for triggering plant immunity. Resistance (R) genes often encode for nucleotide-binding leucine-rich repeat receptors (NLRs), and NLRs detect effectors from pathogens to trigger effector-triggered immunity (ETI). NLR recognition of effectors is observed in diverse forms where NLRs directly interact with effectors or indirectly detect effectors by monitoring host guardees/decoys (HGDs). HGDs undergo different biochemical modifications by diverse effectors and expand the effector recognition spectrum of NLRs, contributing robustness to plant immunity. Interestingly, in many cases of the indirect recognition of effectors, HGD families targeted by effectors are conserved across the plant species while NLRs are not. Notably, a family of diversified HGDs can activate multiple non-orthologous NLRs across plant species. Further investigation on HGDs would reveal the mechanistic basis of how the diversification of HGDs confers novel effector recognition by NLRs.

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.

Immune receptor mimicking hormone receptors: a new guarding strategy

Plant intracellular nucleotide-binding domain leucine-rich repeat (NLR) receptors play crucial roles in immune responses against pathogens. How diverse NLRs recognize different pathogen effectors remains a significant question. A recent study published in Nature uncovered how pepper NLR Tsw detects phytohormone receptors' interference caused by tomato spotted wilt virus (TSWV) effector, triggering a robust immune response, showcasing a new manner of NLR guarding.

Contrasting effector profiles between bacterial colonisers of kiwifruit reveal redundant roles converging on PTI-suppression and RIN4

Testing effector knockout strains of the Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) for reduced in planta growth in their native kiwifruit host revealed a number of nonredundant effectors that contribute to Psa3 virulence. Conversely, complementation in the weak kiwifruit pathogen P. syringae pv. actinidifoliorum (Pfm) for increased growth identified redundant Psa3 effectors. Psa3 effectors hopAZ1a and HopS2b and the entire exchangeable effector locus (ΔEEL; 10 effectors) were significant contributors to bacterial colonisation of the host and were additive in their effects on virulence. Four of the EEL effectors (HopD1a, AvrB2b, HopAW1a and HopD2a) redundantly contribute to virulence through suppression of pattern-triggered immunity (PTI). Important Psa3 effectors include several redundantly required effectors early in the infection process (HopZ5a, HopH1a, AvrPto1b, AvrRpm1a and HopF1e). These largely target the plant immunity hub, RIN4. This comprehensive effector profiling revealed that Psa3 carries robust effector redundancy for a large portion of its effectors, covering a few functions critical to disease.

Mechanisms controlling plant proteases and their substrates

In plants, proteolysis is emerging as an important field of study due to a growing understanding of the critical involvement of proteases in plant cell death, disease and development. Because proteases irreversibly modify the structure and function of their target substrates, proteolytic activities are stringently regulated at multiple levels. Most proteases are produced as dormant isoforms and only activated in specific conditions such as altered ion fluxes or by post-translational modifications. Some of the regulatory mechanisms initiating and modulating proteolytic activities are restricted in time and space, thereby ensuring precision activity, and minimizing unwanted side effects. Currently, the activation mechanisms and the substrates of only a few plant proteases have been studied in detail. Most studies focus on the role of proteases in pathogen perception and subsequent modulation of the plant reactions, including the hypersensitive response (HR). Proteases are also required for the maturation of coexpressed peptide hormones that lead essential processes within the immune response and development. Here, we review the known mechanisms for the activation of plant proteases, including post-translational modifications, together with the effects of proteinaceous inhibitors.

Plant translational reprogramming for stress resilience

Organisms regulate gene expression to produce essential proteins for numerous biological processes, from growth and development to stress responses. Transcription and translation are the major processes of gene expression. Plants evolved various transcription factors and transcriptome reprogramming mechanisms to dramatically modulate transcription in response to environmental cues. However, even the genome-wide modulation of a gene's transcripts will not have a meaningful effect if the transcripts are not properly biosynthesized into proteins. Therefore, protein translation must also be carefully controlled. Biotic and abiotic stresses threaten global crop production, and these stresses are seriously deteriorating due to climate change. Several studies have demonstrated improved plant resistance to various stresses through modulation of protein translation regulation, which requires a deep understanding of translational control in response to environmental stresses. Here, we highlight the translation mechanisms modulated by biotic, hypoxia, heat, and drought stresses, which are becoming more serious due to climate change. This review provides a strategy to improve stress tolerance in crops by modulating translational regulation.

Ger1 is a secreted aspartic acid protease essential for spore germination in Ustilago maydis

Ustilago maydis expresses a number of proteases during its pathogenic lifecycle. Some of the proteases including both intracellular and extracellular ones have previously been shown to influence the virulence of the pathogen. However, any role of secreted proteases in the sporulation process of U. maydis have not been explored earlier. In this study we have investigated the biological function of one such secreted protease, Ger1 belonging to aspartic protease A1 family. An assessment of the real time expression of ger1 revealed an infection specific expression of the protein especially during late phases of infection. We also evaluated any contribution of the protein in the pathogenicity of the fungus. Our data revealed an involvement of Ger1 in the sporulation and spore germination processes of U. maydis. Ger1 also showed positive influence on the pathogenicity of the fungus and accordingly the ger1 deletion mutant exhibited reduced pathogenicity. The study also demonstrated the protease activity associated with Ger1 to be essential for its biological function. Fluorescence microscopy of maize plants infected with U. maydis cells expressing Ger1-mcherry-HA also revealed that Ger1 is efficiently secreted within maize apoplast.