The HSP90-SGT1 chaperone complex for NLR immune sensors

Proxitome profiling reveals a conserved SGT1-NSL1 signaling module that activates NLR-mediated immunity

Suppressor of G2 allele of skp1 (SGT1) is a highly conserved eukaryotic protein that plays a vital role in growth, development, and immunity in both animals and plants. Although some SGT1 interactors have been identified, the molecular regulatory network of SGT1 remains unclear. SGT1 serves as a co-chaperone to stabilize protein complexes such as the nucleotide-binding leucine-rich repeat (NLR) class of immune receptors, thereby positively regulating plant immunity. SGT1 has also been found to be associated with the SKP1-Cullin-F-box (SCF) E3 ubiquitin ligase complex. However, whether SGT1 targets immune repressors to coordinate plant immune activation remains elusive. In this study, we constructed a toolbox for TurboID- and split-TurboID-based proximity labeling (PL) assays in Nicotiana benthamiana and used the PL toolbox to explore the SGT1 interactome during pre- and post-immune activation. The comprehensive SGT1 interactome network we identified highlights a dynamic shift from proteins associated with plant development to those linked with plant immune responses. We found that SGT1 interacts with Necrotic Spotted Lesion 1 (NSL1), which negatively regulates salicylic acid-mediated defense by interfering with the nucleocytoplasmic trafficking of non-expressor of pathogenesis-related genes 1 (NPR1) during N NLR-mediated response to tobacco mosaic virus. SGT1 promotes the SCF-dependent degradation of NSL1 to facilitate immune activation, while salicylate-induced protein kinase-mediated phosphorylation of SGT1 further potentiates this process. Besides N NLR, NSL1 also functions in several other NLR-mediated immunity. Collectively, our study unveils the regulatory landscape of SGT1 and reveals a novel SGT1-NSL1 signaling module that orchestrates plant innate immunity.

Identification and application of a candidate gene AhAftr1 for aflatoxin production resistance in peanut seed (Arachis hypogaea L.)

INTRODUCTION

Peanut is susceptible to infection of Aspergillus fungi and conducive to aflatoxin contamination, hence developing aflatoxin-resistant variety is highly meaningful. Identifying functional genes or loci conferring aflatoxin resistance and molecular diagnostic marker are crucial for peanut breeding.

OBJECTIVES

This work aims to (1) identify candidate gene for aflatoxin production resistance, (2) reveal the related resistance mechanism, and (3) develop diagnostic marker for resistance breeding program.

METHODS

Resistance to aflatoxin production in a recombined inbred line (RIL) population derived from a high-yielding variety Xuhua13 crossed with an aflatoxin-resistant genotype Zhonghua 6 was evaluated under artificial inoculation for three consecutive years. Both genetic linkage analysis and QTL-seq were conducted for QTL mapping. The candidate gene was further fine-mapped using a secondary segregation mapping population and validated by transgenic experiments. RNA-Seq analysis among resistant and susceptible RILs was used to reveal the resistance pathway for the candidate genes.

RESULTS

The major effect QTL qAFTRA07.1 for aflatoxin production resistance was mapped to a 1.98 Mbp interval. A gene, AhAftr1 (Arachis hypogaea Aflatoxin resistance 1), was detected structure variation (SV) in leucine rich repeat (LRR) domain of its production, and involved in disease resistance response through the effector-triggered immunity (ETI) pathway. Transgenic plants with overexpression of AhAftr1(ZH6) exhibited 57.3% aflatoxin reduction compared to that of AhAftr1(XH13). A molecular diagnostic marker AFTR.Del.A07 was developed based on the SV. Thirty-six lines, with aflatoxin content decrease by over 77.67% compared to the susceptible control Zhonghua12 (ZH12), were identified from a panel of peanut germplasm accessions and breeding lines through using AFTR.Del.A07.

CONCLUSION

Our findings would provide insights of aflatoxin production resistance mechanisms and laid meaningful foundation for further breeding programs.

Tilletia horrida glycoside hydrolase family 128 protein, designated ThGhd_7, modulates plant immunity by blocking reactive oxygen species production

Tilletia horrida is an important soilborne fungal pathogen that causes rice kernel smut worldwide. We found a glycoside hydrolase family 128 protein, designated ThGhd_7, caused cell death in Nicotiana benthamiana leaves. The predicted signal peptide (SP) of ThGhd_7 targets it for secretion. However, loss of the SP did not affect its ability to induce cell death. The 23-201 amino acid sequence of ThGhd_7 was sufficient to trigger cell death in N. benthamiana. ThGhd_7 expression was induced and upregulated during T. horrida infection. ThGhd_7 localised to both the cytoplasm and nucleus of plant cells, and nuclear localisation was required to induce cell death. The ability of ThGhd_7 to trigger cell death in N. benthamiana depends on RAR1 (required for Mla12 resistance), SGT1 (suppressor of G2 allele of Skp1), and BAK1/SERK3 (somatic embryogenesis receptor-like kinase 3). Heterologous overexpression of ThGhd_7 in rice reduced reactive oxygen species (ROS) production and enhanced susceptibility to T. horrida. Further research revealed that ThGhd_7 interacted with and destabilised OsSGT1, which is required for ROS production and is a positive regulator of rice resistance to T. horrida. Taken together, these findings suggest that T. horrida employs ThGhd_7 to disrupt ROS production and thereby promote infection.

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.

A fungal core effector exploits the OsPUX8B.2-OsCDC48-6 module to suppress plant immunity

Proteins containing a ubiquitin regulatory X (UBX) domain are cofactors of Cell Division Cycle 48 (CDC48) and function in protein quality control. However, whether and how UBX-containing proteins participate in host-microbe interactions remain unclear. Here we show that MoNLE1, an effector from the fungal pathogen Magnaporthe oryzae, is a core virulence factor that suppresses rice immunity by specifically interfering with OsPUX8B.2. The UBX domain of OsPUX8B.2 is required for its binding to OsATG8 and OsCDC48-6 and controls its 26 S proteasome-dependent stability. OsPUX8B.2 and OsCDC48-6 positively regulate plant immunity against blast fungus, while the high-temperature tolerance heat-shock protein OsBHT, a putative cytoplasmic substrate of OsPUX8B.2-OsCDC48-6, negatively regulates defense against blast infection. MoNLE1 promotes the nuclear migration and degradation of OsPUX8B.2 and disturbs its association with OsBHT. Given the high conservation of MoNLE1 among fungal isolates, plants with broad and durable blast resistance might be generated by engineering intracellular proteins resistant to MoNLE1.

HSP90β controls NLRP3 autoactivation

Gain-of-function mutations in NLRP3 are linked to cryopyrin-associated periodic syndromes (CAPS). Although NLRP3 autoinflammasome assembly triggers inflammatory cytokine release, its activation mechanisms are not fully understood. Our study used a functional genetic approach to identify regulators of NLRP3 inflammasome formation. We identified the HSP90β-SGT1 chaperone complex as crucial for autoinflammasome activation in CAPS. A deficiency in HSP90β, but not in HSP90α, impaired the formation of ASC specks without affecting the priming and expression of inflammasome components. Conversely, activating NLRP3 with stimuli such as nigericin or alum bypassed the need for SGT1 and HSP90β, suggesting the existence of alternative inflammasome assembly pathways. The role of HSP90β was further demonstrated in PBMCs derived from CAPS patients. In these samples, the pathological constitutive secretion of IL-1β could be suppressed using a pharmacological inhibitor of HSP90β. This finding underscores the potential of SGT1-HSP90β modulation as a therapeutic strategy in CAPS while preserving NLRP3's physiological functions.

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

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

Mapping phenotypic performance and novel SNPs for cold tolerance in tomato (Solanum lycopersicum) genotypes through GWAS and population genetics

The cold stress susceptibility of tomato (Solanum lycopersicum) curtails its cultivation, with significant impact in temperate regions and on cropping seasons. To unravel genomic regions responsible for cold stress resilience, a diverse set of fifty genotypes encompassing cultivated, wild species, and landraces were genotyped using genotyping-by-sequencing. Over two years and six trials employing both early and late sowing, these lines were evaluated. Illumina-based next-generation sequencing produced up to 3 million reads per sample from individually sequenced library pools. The Tassel pipeline yielded 10,802 variants, subsequently filtered to 3,854 SNPs for genome-wide association analysis (GWAS). Employing clustering methods (population structure) via TASSEL, SNPhylo, and Kinship matrix, the fifty genotypes clustered into four distinct gene pools. The GWAS for cold tolerance in tomato integrated key traits including yield. Using six independent phenotypic datasets representing various environments, the study identified 4,517 significant marker-trait associations for cold tolerance traits. Notably, pivotal variations (> 10%) in cold stress tolerance, particularly proline content, were linked to marker-trait associations. Additionally, 5,727 significant marker-trait associations for yield and yield-related traits were unveiled, shedding light on fruit yield and directly associated attributes. The investigation pinpointed 685 candidate genes across all examined traits, including 60 genes associated with biological processes within these genomic regions. Remarkably, 7 out of the 60 genes were directly linked to abiotic stress tolerance, functioning as stress-responsive genes either directly or indirectly. The identified genes, particularly those associated with stress response, could hold the key to enhancing cold tolerance and overall crop productivity in tomato cultivation.

Enhanced Susceptibility to Tomato Chlorosis Virus (ToCV) in Hsp90- and Sgt1-Silenced Plants: Insights from Gene Expression Dynamics

The emerging whitefly-transmitted crinivirus tomato chlorosis virus (ToCV) causes substantial economic losses by inducing yellow leaf disorder in tomato crops. This study explores potential resistance mechanisms by examining early-stage molecular responses to ToCV. A time-course transcriptome analysis compared naïve, mock, and ToCV-infected plants at 2, 7, and 14 days post-infection (dpi). Gene expression changes were most notable at 2 and 14 dpi, likely corresponding to whitefly feeding and viral infection. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed key genes and pathways associated with ToCV infection, including those related to plant immunity, flavonoid and steroid biosynthesis, photosynthesis, and hormone signaling. Additionally, virus-derived small interfering RNAs (vsRNAs) originating from ToCV predominantly came from RNA2 and were 22 nucleotides in length. Furthermore, two genes involved in plant immunity, Hsp90 (heat shock protein 90) and its co-chaperone Sgt1 (suppressor of the G2 allele of Skp1) were targeted through viral-induced gene silencing (VIGS), showing a potential contribution to basal resistance against viral infections since their reduction correlated with increased ToCV accumulation. This study provides insights into tomato plant responses to ToCV, with potential implications for developing effective disease control strategies.

A plasma membrane nucleotide-binding leucine-rich repeat receptor mediates the recognition of the Ralstonia pseudosolanacearum effector RipY in Nicotiana benthamiana

Bacterial wilt disease caused by several Ralstonia species is one of the most destructive diseases in Solanaceae crops. Only a few functional resistance genes against bacterial wilt have been cloned to date. Here, we show that the broadly conserved type III secreted effector RipY is recognized by the Nicotiana benthamiana immune system, leading to cell death induction, induction of defense-related gene expression, and restriction of bacterial pathogen growth. Using a multiplexed virus-induced gene-silencing-based N. benthamiana nucleotide-binding and leucine-rich repeat receptor (NbNLR) library, we identified a coiled-coil (CC) nucleotide-binding and leucine-rich repeat receptor (CNL) required for recognition of RipY, which we named RESISTANCE TO RALSTONIA SOLANACEARUM RIPY (RRS-Y). Genetic complementation assays in RRS-Y-silenced plants and stable rrs-y knockout mutants demonstrated that RRS-Y is sufficient to activate RipY-induced cell death and RipY-induced immunity to Ralstonia pseudosolanacearum. RRS-Y function is dependent on the phosphate-binding loop motif of the nucleotide-binding domain but independent of the characterized signaling components ENHANCED DISEASE SUSCEPTIBILITY 1, ACTIVATED DISEASE RESISTANCE 1, and N REQUIREMENT GENE 1 and the NLR helpers NB-LRR REQUIRED FOR HR-ASSOCIATED CELL DEATH-2, -3, and -4 in N. benthamiana. We further show that RRS-Y localization at the plasma membrane is mediated by two cysteine residues in the CC domain and is required for RipY recognition. RRS-Y also broadly recognizes RipY homologs across Ralstonia species. Lastly, we show that the C-terminal region of RipY is indispensable for RRS-Y activation. Together, our findings provide an additional effector/receptor pair system to deepen our understanding of CNL activation in plants.

Combination of transcriptomic, proteomic, and degradomic profiling reveals common and distinct patterns of pathogen-induced cell death in maize

Regulated cell death (RCD) is crucial for plant development, as well as in decision-making in plant-microbe interactions. Previous studies revealed components of the molecular network controlling RCD, including different proteases. However, the identity, the proteolytic network as well as molecular components involved in the initiation and execution of distinct plant RCD processes, still remain largely elusive. In this study, we analyzed the transcriptome, proteome, and N-terminome of Zea mays leaves treated with the Xanthomonas effector avrRxo1, the mycotoxin Fumonisin B1 (FB1), or the phytohormone salicylic acid (SA) to dissect plant cellular processes related to cell death and plant immunity. We found highly distinct and time-dependent biological processes being activated on transcriptional and proteome levels in response to avrRxo1, FB1, and SA. Correlation analysis of the transcriptome and proteome identified general, as well as trigger-specific markers for cell death in Zea mays. We found that proteases, particularly papain-like cysteine proteases, are specifically regulated during RCD. Collectively, this study characterizes distinct RCD responses in Z. mays and provides a framework for the mechanistic exploration of components involved in the initiation and execution of cell death.

Uncovering genomic and transcriptional variations facilitates utilization of wild resources in cotton disease resistance improvement

BACKGROUND

Upland cotton wild/landraces represent a valuable resource for disease resistance alleles. Genetic differentiation between genotypes, as well as variation in Verticillium wilt (VW) resistance, has been poorly characterized for upland cotton accessions on the domestication spectrum (from wild/landraces to elite lines).

RESULTS

To illustrate the effects of modern breeding on VW resistance in upland cotton, 37 wild/landraces were resequenced and phenotyped for VW resistance. Genomic patterns of differentiation were identified between wild/landraces and improved upland cotton, and a significant decline in VW resistance was observed in association with improvement. Four genotypes representing different degrees of improvement were used in a full-length transcriptome analysis to study the genetic basis of VW resistance. ROS signaling was highly conserved at the transcriptional level, likely providing the basis for VW resistance in upland cotton. ASN biosynthesis and HSP90-mediated resistance moderated the response to VW in wild/landraces, and loss of induction activity of these genes resulted in VW susceptibility. The observed genomic differentiation contributed to the loss of induction of some important VW resistance genes such as HSP90.4 and PR16.

CONCLUSIONS

Besides providing new insights into the evolution of upland cotton VW resistance, this study also identifies important resistance pathways and genes for both fundamental research and cotton breeding.

Comparative analysis of soybean transcriptional profiles reveals defense mechanisms involved in resistance against Diaporthe caulivora

Soybean stem canker (SSC) caused by the fungal pathogen Diaporthe caulivora is an important disease affecting soybean production worldwide. However, limited information related to the molecular mechanisms underlying soybean resistance to Diaporthe species is available. In the present work, we analyzed the defense responses to D. caulivora in the soybean genotypes Williams and Génesis 5601. The results showed that compared to Williams, Génesis 5601 is more resistant to fungal infection evidenced by significantly smaller lesion length, reduced disease severity and pathogen biomass. Transcriptional profiling was performed in untreated plants and in D. caulivora-inoculated and control-treated tissues at 8 and 48 h post inoculation (hpi). In total, 2.322 and 1.855 genes were differentially expressed in Génesis 5601 and Williams, respectively. Interestingly, Génesis 5601 exhibited a significantly higher number of upregulated genes compared to Williams at 8 hpi, 1.028 versus 434 genes. Resistance to D. caulivora was associated with defense activation through transcriptional reprogramming mediating perception of the pathogen by receptors, biosynthesis of phenylpropanoids, hormone signaling, small heat shock proteins and pathogenesis related (PR) genes. These findings provide novel insights into soybean defense mechanisms leading to host resistance against D. caulivora, and generate a foundation for the development of resistant SSC varieties within soybean breeding programs.

The Great Game between Plants and Viruses: A Focus on Protein Homeostasis

Plant viruses are tiny pathogenic obligate parasites that cause significant damage to global crop production. They exploit and manipulate the cellular components of host plants to ensure their own survival. In response, plants activate multiple defense signaling pathways, such as gene silencing and plant hormone signaling, to hinder virus propagation. Growing evidence suggests that the regulation of protein homeostasis plays a vital role in the ongoing battle between plants and viruses. The ubiquitin-proteasome-degradation system (UPS) and autophagy, as two major protein-degradation pathways, are widely utilized by plants and viruses in their arms race. One the one hand, these pathways act as essential components of plant's antiviral defense system by facilitating the degradation of viral proteins; on the other hand, viruses exploit the UPS and autophagy to create a favorable intracellular environment for viral infection. This review aims to provide a comprehensive summary of the events involved in protein homeostasis regulation during viral infection in plants. Gaining knowledge in this area will enhance our understanding of the complex interplay between plants and viruses.

The battle within: How pathogen effectors suppress NLR-mediated immunity

To successfully colonise plants, pathogens must circumvent the plant immune system. Intracellular immune receptors of the nucleotide-binding leucine-rich repeat (NLR) class of proteins are major components of the plant immune system. NLRs function as disease resistance genes by recognising effectors secreted by diverse pathogens, triggering a localised form of programmed cell death known as the hypersensitive response. To evade detection, effectors have evolved to suppress NLR-mediated immunity by targeting NLRs either directly or indirectly. Here, we compile the latest discoveries related to NLR-suppressing effectors and categorise these effectors based on their mode of action. We discuss the diverse strategies pathogens use to perturb NLR-mediated immunity, and how we can use our understanding of effector activity to help guide new approaches for disease resistance breeding.

Single-cell transcriptomic analyses reveal cellular and molecular patterns of rubber tree response to early powdery mildew infection

As a perennial woody plant, the rubber tree (Hevea brasiliensis) must adapt to various environmental challenges through gene expression in multiple cell types. It is still unclear how genes in this species are expressed at the cellular level and the precise mechanisms by which cells respond transcriptionally to environmental stimuli, especially in the case of pathogen infection. Here, we characterized the transcriptomes in Hevea leaves during early powdery mildew infection using single-cell RNA sequencing. We identified 10 cell types and constructed the first single-cell atlas of Hevea leaves. Distinct gene expression patterns of the cell clusters were observed under powdery mildew infection, which was especially significant in the epidermal cells. Most of the genes involved in host-pathogen interactions in epidermal cells exhibited a pattern of dramatically increased expression with increasing pseudotime. Interestingly, we found that the HbCNL2 gene, encoding a nucleotide-binding leucine-rich repeat protein, positively modulated the defence of rubber leaves against powdery mildew. Overexpression of the HbCNL2 gene triggered a typical cell death phenotype in tobacco leaves and a higher level of reactive oxygen species in the protoplasts of Hevea leaves. The HbCNL2 protein was located in the cytomembrane and nucleus, and its leucine-rich repeat domain interacted with the histidine kinase-like ATPase domain of the molecular chaperone HbHSP90 in the nucleus. Collectively, our results provide the first observation of the cellular and molecular responses of Hevea leaves to biotrophic pathogen infection and can guide the identification of disease-resistance genes in this important tree species.

Horizontal gene transfer of Cccyt contributes to virulence of mycoparasite Calcarisporium cordycipiticola by interacting with a host heat shock protein

Horizontal gene transfer (HGT) is an important driving force for virulence evolution of pathogens, however, functions of these transferred genes are still not fully investigated. Here, an HGT effector, CcCYT was reported to contribute to virulence of a mycoparasite, Calcarisporium cordycipiticola to the host Cordyceps militaris, an important mushroom. Cccyt was predicted to be horizontally transferred from Actinobacteria ancestor by phylogenetic, synteny, GC content and codon usage pattern analyses. The transcript of Cccyt was sharply up-regulated at the early stage of infecting C. militaris. This effector was localized to the cell wall and contributed to the virulence of C. cordycipiticola without affecting its morphology, mycelial growth, conidiation, and resistance to abiotic stress. CcCYT can firstly bind the septa, and finally cytoplasm of the deformed hyphal cells of C. militaris. Pull-down assay coupled mass spectrometry revealed that proteins with which CcCYT interacted were related to protein process, folding and degradation. GST-Pull down assay confirmed that C. cordycipiticola effector CcCYT can interact with host protein CmHSP90 to inhibit the immune response of host. The results provided functional evidence that HGT is an important driving force for the virulence evolution and will be helpful for revealing the interaction between mycoparasite and mushroom host.

A single region of the Phytophthora infestans avirulence effector Avr3b functions in both cell death induction and plant immunity suppression

As a destructive plant pathogen, Phytophthora infestans secretes diverse host-entering RxLR effectors to facilitate infection. One critical RxLR effector, PiAvr3b, not only induces effector-triggered immunity (ETI), which is associated with the potato resistance protein StR3b, but also suppresses pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). To date, the molecular basis underlying such dual activities remains unknown. Based on phylogenetic analysis of global P. infestans isolates, we found two PiAvr3b isoforms that differ by three amino acids. Despite this sequence variation, the two isoforms retain the same properties in activating the StR3b-mediated hypersensitive response (HR) and inhibiting necrosis induced by three PAMPs (PiNpp, PiINF1, and PsXeg1) and an RxLR effector (Pi10232). Using a combined mutagenesis approach, we found that the dual activities of PiAvr3b were tightly linked and determined by 88 amino acids at the C-terminus. We further determined that either the W60 or the E134 residue of PiAvr3b was essential for triggering StR3b-associated HR and inhibiting PiNpp- and Pi10232-associated necrosis, while the S99 residue partially contributed to PTI suppression. Additionally, nuclear localization of PiAvr3b was required to stimulate HR and suppress PTI, but not to inhibit Pi10232-associated cell death. Our study revealed that PiAvr3b suppresses the plant immune response at different subcellular locations and provides an example in which a single amino acid of an RxLR effector links ETI induction and cell death suppression.

Cloning and functional analysis of the root-knot nematode resistance gene NtRk1 in tobacco

Tobacco (Nicotiana tabacum L.) is an economically important crop worldwide. Root-knot nematodes (RKNs) are responsible for yield losses in tobacco and other crops, such as tomato, potato, peanut, and soybean. Therefore, screening for resistance genes that can prevent RKN infestation and the associated damage is crucial. However, there is no report of cloning tobacco RKN resistance genes to date. Here, we cloned the tobacco RKN resistance gene NtRk1 from the resistant variety TI706, using rapid amplification of cDNA ends. NtRk1 has high homology with other RKN resistance genes (CaMi in pepper, Mi-1.1 and Mi-1.2 in tomato). Under normal conditions, NtRk1 was barely expressed in the roots; however, following RKN infection, its expression level rapidly increased. Overexpression of NtRk1 in the susceptible cultivar "Changbohuang" enhanced its resistance to Meloidogyne incognita, while RNA interference of NtRk1 in the resistant cultivar K326 resulted in its susceptibility to M. incognita. Moreover, compared with resistant variety K326, we found the salicylic acid and jasmonic acid contents of RNAi plants decreased after inoculation with M. incognita, and confirmed that the function of NtRk1 is related to these phytohormones. These findings indicate that NtRk1 is an RKN resistance gene, which is abundantly expressed in response to RKN infection and may enhance host defense responses by elevating salicylic acid and jasmonic acid levels.

A conserved protein disulfide isomerase enhances plant resistance against herbivores

Herbivore-associated molecular patterns (HAMPs) enable plants to recognize herbivores and may help plants adjust their defense responses. Here, we report on herbivore-induced changes in a protein disulfide isomerase (PDI) widely distributed across arthropods. PDI from the spider mite Tetranychus evansi (TePDI), a mesophyll-feeding agricultural pest worldwide, triggered immunity in multiple Solanaceae plants. TePDI-mediated cell death in Nicotiana benthamiana required the plant signaling proteins SGT1 (suppressor of the G2 allele of skp1) and HSP90 (heat shock protein 90), but was suppressed by spider mite effectors Te28 and Te84. Moreover, PDIs from phylogenetically distinct herbivorous and nonherbivorous arthropods triggered plant immunity. Finally, although PDI-induced plant defenses impaired the performance of spider mites on plants, RNAi experiments revealed that PDI genes are essential for the survival of mites and whiteflies. Our findings indicate that plants recognize evolutionarily conserved HAMPs to activate plant defense and resist pest damage, pointing to opportunities for broad-spectrum pest management.

NLRP3 leucine-rich repeats control induced and spontaneous inflammasome activation in cryopyrin-associated periodic syndrome

BACKGROUND

The cryopyrin-associated periodic syndromes (CAPS) comprise a group of rare autoinflammatory diseases caused by gain-of-function mutations in the NLRP3 gene. NLRP3 contains a leucine-rich repeats (LRR) domain with a highly conserved exonic organization that is subjected to extensive alternative splicing. Aberrant NLRP3 inflammasome assembly in CAPS causes chronic inflammation; however, the mechanisms regulating inflammasome function remain unclear.

OBJECTIVE

We aimed to elucidate the mechanisms regulating NLRP3-mediated autoinflammation in human disease, characterizing the role of LRR in inflammasome activation.

METHODS

We analyzed sequence read archive data to characterize the pattern of NLRP3 splicing in human monocytes and investigated the role of each LRR-coding exon in inflammasome regulation in genetically modified U937 cells representing CAPS and healthy conditions.

RESULTS

We detected a range of NLRP3 splice variants in human primary cells and monocytic cell lines, including 2 yet-undescribed splice variants. We observe that lipopolysaccharides affect the abundance of certain splice variants, suggesting that they may regulate NLRP3 activation by affecting alternative splicing. We showed that exons 4, 5, 7, and 9 are essential for inflammasome function, both in the context of wild-type NLRP3 activation by the agonist molecule nigericin and in a model of CAPS-mediated NLRP3 inflammasome assembly. Moreover, the SGT1-NLRP3 interaction is decreased in nonfunctional variants, suggesting that alternative splicing may regulate the recruitment of proteins that facilitate inflammasome assembly.

CONCLUSION

These findings demonstrate the contribution of the LRR domain in inflammasome function and suggest that navigating LRR exon usage within NLRP3 is sufficient to dampen inflammasome assembly in CAPS.

A stripe rust fungal effector PstSIE1 targets TaSGT1 to facilitate pathogen infection

Puccinia striiformis f. sp. tritici (Pst), the causal agent of stripe rust, is a destructive pathogen of Triticum aestivum (wheat), threatening wheat production worldwide. Pst delivers hundreds of effectors to manipulate processes in its hosts during infection. The SGT1 (suppressor of the G2 allele of skp1), RAR1 (required for Mla12 resistance) and HSP90 (heat-shock protein 90) proteins form a chaperone complex that acts as a core modulator in plant immunity. However, little is known about how Pst effectors target this immune component to suppress plant immunity. Here, we identified a Pst effector PstSIE1 that interacts with TaSGT1 in wheat and is upregulated during the early infection stage. Transient expression of PstSIE1 suppressed cell death in Nicotiana benthamiana induced by VmE02 and PcNLP2. Transgenic expression of PstSIE1-RNAi constructs in wheat significantly reduced the virulence of Pst. Overexpression of PstSIE1 in wheat increased the number of rust pustules and reduced the accumulation of reactive oxygen species (ROS), indicating that PstSIE1 functions as an important pathogenicity factor in Pst. PstSIE1 was found to compete with TaRAR1 to bind TaSGT1, thus disrupting the formation of the TaRAR1-TaSGT1 subcomplex. Taken together, PstSIE1 is an important Pst effector targeting the immune component TaSGT1 and involved in suppressing wheat defense.

Functional Analyses of a Small Secreted Cysteine-Rich Protein ThSCSP_14 in Tilletia horrida

Tilletia horrida is a biotrophic basidiomycete fungus that causes rice kernel smut, one of the most significant diseases in hybrid rice-growing areas worldwide. Little is known about the pathogenic mechanisms and functions of effectors in T. horrida. Here, we performed functional studies of the effectors in T. horrida and found that, of six putative effectors tested, only ThSCSP_14 caused the cell death phenotype in epidermal cells of Nicotiana benthamiana leaves. ThSCSP_14 was upregulated early on during the infection process, and the encoded protein was secreted. The predicted signal peptide (SP) of ThSCSP_14 was required for its ability to induce the necrosis phenotype. Furthermore, the ability of ThSCSP_14 to trigger cell death in N. benthamiana depended on suppressing the G2 allele of Skp1 (SGT1), required for Mla12 resistance (RAR1), heat-shock protein 90 (HSP90), and somatic embryogenesis receptor-like kinase (SERK3). It is important to note that ThSCSP_14 induced a plant defense response in N. benthamiana leaves. Hence, these results demonstrate that ThSCSP_14 is a possible effector that plays an essential role in T. horrida-host interactions.

ThSCSP_12: Novel Effector in Tilletia horrida That Induces Cell Death and Defense Responses in Non-Host Plants

The basidiomycete fungus Tilletia horrida causes rice kernel smut (RKS), a crucial disease afflicting hybrid-rice-growing areas worldwide, which results in significant economic losses. However, few studies have investigated the pathogenic mechanisms and functions of effectors in T. horrida. In this study, we found that the candidate effector ThSCSP_12 caused cell necrosis in the leaves of Nicotiana benthamiana. The predicted signal peptide (SP) of this protein has a secreting function, which is required for ThSCSP_12 to induce cell death. The 1- 189 amino acid (aa) sequences of ThSCSP_12 are sufficient to confer it the ability to trigger cell death in N. benthamiana. The expression of ThSCSP_12 was induced and up-regulated during T. horrida infection. In addition, we also found that ThSCSP_12 localized in both the cytoplasm and nucleus of plant cells and that nuclear localization of this protein is required to induce cell death. Furthermore, the ability of ThSCSP_12 to trigger cell death in N. benthamiana depends on the (RAR1) protein required for Mla12 resistance but not on the suppressor of the G2 allele of Skp1 (SGT1), heat shock protein 90 (HSP90), or somatic embryogenesis receptor-like kinase (SERK3). Crucially, however, ThSCSP_12 induced a defense response in N. benthamiana leaves; yet, the expression of multiple defense-related genes was suppressed in response to heterologous expression in host plants. To sum up, these results strongly suggest that ThSCSP_12 operates as an effector in T. horrida-host interactions.

SnRK1A-mediated phosphorylation of a cytosolic ATPase positively regulates rice innate immunity and is inhibited by Ustilaginoidea virens effector SCRE1

Rice false smut caused by Ustilaginoidea virens is becoming one of the most recalcitrant rice diseases worldwide. However, the molecular mechanisms underlying rice immunity against U. virens remain unknown. Using genetic, biochemical and disease resistance assays, we demonstrated that the xb24 knockout lines generated in non-Xa21 rice background exhibit an enhanced susceptibility to the fungal pathogens U. virens and Magnaporthe oryzae. Consistently, flg22- and chitin-induced oxidative burst and expression of pathogenesis-related genes in the xb24 knockout lines were greatly attenuated. As a central mediator of energy signaling, SnRK1A interacts with and phosphorylates XB24 at Thr83 residue to promote ATPase activity. SnRK1A is activated by pathogen-associated molecular patterns and positively regulates plant immune responses and disease resistance. Furthermore, the virulence effector SCRE1 in U. virens targets host ATPase XB24. The interaction inhibits ATPase activity of XB24 by blocking ATP binding to XB24. Meanwhile, SCRE1 outcompetes SnRK1A for XB24 binding, and thereby suppresses SnRK1A-mediated phosphorylation and ATPase activity of XB24. Our results indicate that the conserved SnRK1A-XB24 module in multiple crop plants positively contributes to plant immunity and uncover an unidentified molecular strategy to promote infection in U. virens and a novel host target in fungal pathogenesis.

A secreted fungal subtilase interferes with rice immunity via degradation of SUPPRESSOR OF G2 ALLELE OF skp1

Serine protease subtilase, found widely in both eukaryotes and prokaryotes, participates in various biological processes. However, how fungal subtilase regulates plant immunity is a major concern. Here, we identified a secreted fungal subtilase, UvPr1a, from the rice false smut (RFS) fungus Ustilaginoidea virens. We characterized UvPr1a as a virulence effector localized to the plant cytoplasm that inhibits plant cell death induced by Bax. Heterologous expression of UvPr1a in rice (Oryza sativa) enhanced plant susceptibility to rice pathogens. UvPr1a interacted with the important rice protein SUPPRESSOR OF G2 ALLELE OF skp1 (OsSGT1), a positive regulator of innate immunity against multiple rice pathogens, degrading OsSGT1 in a protease activity-dependent manner. Furthermore, host-induced gene silencing of UvPr1a compromised disease resistance of rice plants. Our work reveals a previously uncharacterized fungal virulence strategy in which a fungal pathogen secretes a subtilase to interfere with rice immunity through degradation of OsSGT1, thereby promoting infection. These genetic resources provide tools for introducing RFS resistance and further our understanding of plant-pathogen interactions.

Regulation and physiological function of proteins for heat tolerance in cowpea (Vigna unguiculata) genotypes under controlled and field conditions

The expression of heat shock proteins is considered a central adaptive mechanism to heat stress. This study investigated the expression of heat shock proteins (HSPs) and other stress-protective proteins against heat stress in cowpea genotypes under field (IT-96D-610 and IT-16) and controlled (IT-96D-610) conditions. Heat stress response analysis of proteins at 72 h in the controlled environment showed 270 differentially regulated proteins identified using label-free quantitative proteomics in IT-96D-610 plants. These plants expressed HSPs and chaperones [BAG family molecular chaperone 6 (BAG6), Multiprotein bridging factor1c (MBF1C) and cold shock domain protein 1 (CSDP1) in the controlled environment]. However, IT-96D-610 plants expressed a wider variety of small HSPs and more HSPs in the field. IT-96D-610 plants also responded to heat stress by exclusively expressing chaperones [DnaJ chaperones, universal stress protein and heat shock binding protein (HSBP)] and non-HSP proteins (Deg1, EGY3, ROS protective proteins, temperature-induced lipocalin and succinic dehydrogenase). Photosynthesis recovery and induction of proteins related to photosynthesis were better in IT-96D-610 because of the concurrent induction of heat stress response proteins for chaperone functions, protein degradation for repair and ROS scavenging proteins and PSII operating efficiency (Fq'/Fm') than IT-16. This study contributes to identification of thermotolerance mechanisms in cowpea that can be useful in knowledge-based crop improvement.

Genome-wide identification and expression analysis of the Hsp20, Hsp70 and Hsp90 gene family in Dendrobium officinale

Dendrobium officinale, an important orchid plant with great horticultural and medicinal values, frequently suffers from abiotic or biotic stresses in the wild, which may influence its well-growth. Heat shock proteins (Hsps) play essential roles in the abiotic stress response of plants. However, they have not been systematically investigated in D. officinale. Here, we identified 37 Hsp20 genes (DenHsp20s), 43 Hsp70 genes (DenHsp70s) and 4 Hsp90 genes (DenHsp90s) in D. officinale genome. These genes were classified into 8, 4 and 2 subfamilies based on phylogenetic analysis and subcellular predication, respectively. Sequence analysis showed that the same subfamily members have relatively conserved gene structures and similar protein motifs. Moreover, we identified 33 pairs of paralogs containing 30 pairs of tandem duplicates and 3 pairs of segmental duplicates among these genes. There were 7 pairs in DenHsp70s under positive selection, which may have important functions in helping cells withstand extreme stress. Numerous gene promoter sequences contained stress and hormone response cis-elements, especially light and MeJA response elements. Under MeJA stress, DenHsp20s, DenHsp70s and DenHsp90s responded to varying degrees, among which DenHsp20-5,6,7,16 extremely up-regulated, which may have a strong stress resistance. Therefore, these findings could provide useful information for evolutional and functional investigations of Hsp20, Hsp70 and Hsp90 genes in D. officinale.

Identification of HbHSP90 gene family and characterization HbHSP90.1 as a candidate gene for stress response in rubber tree

Heat shock protein 90 (HSP90), an essential molecular chaperone, is triggered in response to stress situations in plants. However, the roles of HSP90 gene family members in rubber tree have not been totally specified. In this study, 7 HbHSP90 genes were identified from rubber tree genome. Classification of HbHSP90 family genes into three groups, namely A, B, and C was based on phylogenetic analysis. The structural and motif analyses showed similar structural features in the same group of HbHSP90 members, but differences between groups. Analysis of cis-regulatory element sequences of HbHSP90 genes indicates that the HbHSP90 gene promoter is rich in drought, temperature, and hormone elements. qRT-PCR analysis showed that the 7 HbHSP90 genes responded in different degrees to temperature, drought and powdery mildew infection, and in particularly, HbHSP90.1 was differentially expressed under both abiotic and biotic stresses. Meanwhile, HbHSP90.1 gene was significantly expressed under the treatment of different phytohormone and H₂O₂ (Hydrogen Peroxide) treatments, which means that HbHSP90.1 gene performs an essential part in the growth and development of rubber trees. Furthermore, the protein interaction results showed that HbHSP90.1 interacted with HbSGT1b. Subcellular localization showed that both HbHSP90.1 and HbSGT1b located in the nucleus. Taken together, we speculate that HbHSP90.1 interacts with HbSGT1b in the nucleus to respond to rubber tree stress processes. The results of this study provide a solid foundation for further studies on the mechanism of HbHSP90 family genes in the stress resistance response of rubber tree.

HSP90 Contributes to chs3-2D-Mediated Autoimmunity

Plants employ multi-layered immune system to fight against pathogen infections. Different receptors are able to detect the invasion activities of pathogens, transduce signals to downstream components, and activate defense responses. Among those receptors, nucleotide-binding domain leucine-rich repeat containing proteins (NLRs) are the major intracellular ones. CHILLING SENSITIVE 3 (CHS3) is an Arabidopsis NLR with an additional Lin-11, Isl-1 and Mec-3 (LIM) domain at its C terminus. The gain-of-function mutant, chs3-2D, exhibiting severe dwarfism and constitutively activated defense responses, was selected as a genetic background in this study for a forward genetic screen. A mutant allele of hsp90.2 was isolated as a partial suppressor of chs3-2D, suggesting that HSP90 is required for CHS3-mediated defense signaling. In addition, HSP90 is also required for the autoimmunity of the Dominant Negative (DN)-SNIPER1 and gain-of-function ADR1-L2 D484V transgenic lines, suggesting a broad role for HSP90 in NLR-mediated defense. Overall, our work indicates a larger contribution of HSP90 not only at the sensor, but also the helper NLR levels.

An interolog-based barley interactome as an integration framework for immune signaling

The barley MLA nucleotide-binding leucine-rich-repeat (NLR) receptor and its orthologs confer recognition specificity to many fungal diseases, including powdery mildew, stem-, and stripe rust. We used interolog inference to construct a barley protein interactome (Hordeum vulgare predicted interactome, HvInt) comprising 66,133 edges and 7,181 nodes, as a foundation to explore signaling networks associated with MLA. HvInt was compared with the experimentally validated Arabidopsis interactome of 11,253 proteins and 73,960 interactions, verifying that the 2 networks share scale-free properties, including a power-law distribution and small-world network. Then, by successive layering of defense-specific "omics" datasets, HvInt was customized to model cellular response to powdery mildew infection. Integration of HvInt with expression quantitative trait loci (eQTL) enabled us to infer disease modules and responses associated with fungal penetration and haustorial development. Next, using HvInt and infection-time-course RNA sequencing of immune signaling mutants, we assembled resistant and susceptible subnetworks. The resulting differentially coexpressed (resistant - susceptible) interactome is essential to barley immunity, facilitates the flow of signaling pathways and is linked to mildew resistance locus a (Mla) through trans eQTL associations. Lastly, we anchored HvInt with new and previously identified interactors of the MLA coiled coli + nucleotide-binding domains and extended these to additional MLA alleles, orthologs, and NLR outgroups to predict receptor localization and conservation of signaling response. These results link genomic, transcriptomic, and physical interactions during MLA-specified immunity.

Thirty years of resistance: Zig-zag through the plant immune system

Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 . Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.

Regulation of heat shock proteins 70 and their role in plant immunity

Heat shock proteins 70 (HSP70s) are steadily gaining more attention in the field of plant biotic interactions. Though their regulation and activity in plants are much less well characterized than are those of their counterparts in mammals, accumulating evidence indicates that the role of HSP70-mediated defense mechanisms in plant cells is indispensable. In this review, we summarize current knowledge of HSP70 post-translational control in plants. We comment on the phytohormonal regulation of HSP70 expression and protein abundance, and identify a prominent role for cytokinin in HSP70 control. We outline HSP70s' subcellular localizations, chaperone activity, and chaperone-mediated protein degradation. We focus on the role of HSP70s in plant pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and discuss the contribution of different HSP70 subfamilies to plant defense against pathogens.

Near-Isogenic Barley Lines Show Enhanced Susceptibility to Powdery Mildew Infection Following High-Temperature Stress

Barley cultivation is adversely affected by high-temperature stress, which may modulate plant defense responses to pathogens such as barley powdery mildew (Blumeria graminis f. sp. hordei, Bgh). Earlier research focused mainly on the influence of short-term heat stress (heat shock) of barley on Bgh infection. In this study, our aim was to investigate the effects of both short- and long-term heat stress (35 °C from 30 s to 5 days) on Bgh infection in the barley cultivar Ingrid and its near-isogenic lines containing different powdery mildew resistance genes (Mla12, Mlg, and mlo5) by analyzing symptom severity and Bgh biomass with RT-qPCR. The expression of selected barley defense genes (BAX inhibitor-1, Pathogenesis- related protein-1b, Respiratory burst oxidase homologue F2, and Heat shock protein 90-1) was also monitored in plants previously exposed to heat stress followed by inoculation with Bgh. We demonstrated that pre-exposure to short- and long-term heat stress negatively affects the resistance of all resistant lines manifested by the appearance of powdery mildew symptoms and increased Bgh biomass. Furthermore, prolonged heat stress (48 and 120 h) enhanced both Bgh symptoms and biomass in susceptible wild-type Ingrid. Heat stress suppressed and delayed early defense gene activation in resistant lines, which is a possible reason why resistant barley became partially susceptible to Bgh.

SGT1-Specific Domain Mutations Impair Interactions with the Barley MLA6 Immune Receptor in Association with Loss of NLR Protein

The Mla (Mildew resistance locus a) of barley (Hordeum vulgare L.) is an effective model for cereal immunity against fungal pathogens. Like many resistance proteins, variants of the MLA coiled-coil nucleotide-binding leucine-rich repeat (CC-NLR) receptor often require the HRS complex (HSP90, RAR1, and SGT1) to function. However, functional analysis of Sgt1 has been particularly difficult, as deletions are often lethal. Recently, we identified rar3 (required for Mla6 resistance 3), an in-frame Sgt1_ΔKL308-309 mutation in the SGT1-specific domain, that alters resistance conferred by MLA but without lethality. Here, we use autoactive MLA6 and recombinant yeast-two-hybrid strains with stably integrated HvRar1 and HvHsp90 to determine that this mutation weakens but does not entirely disrupt the interaction between SGT1 and MLA. This causes a concomitant reduction in MLA6 protein accumulation below the apparent threshold required for effective resistance. The ΔKL308-309 deletion had a lesser effect on intramolecular interactions than alanine or arginine substitutions, and MLA variants that display diminished interactions with SGT1 appear to be disproportionately affected by the SGT1_ΔKL308-309 mutation. We hypothesize that those dimeric plant CC-NLRs that appear unaffected by Sgt1 silencing are those with the strongest intermolecular interactions with it. Combining our data with recent work in CC-NLRs, we propose a cyclical model of the MLA-HRS resistosome interactions.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

The hybrid lethality of interspecific F1 hybrids of Nicotiana: a clue to understanding hybrid inviability-a major obstacle to wide hybridization and introgression breeding of plants

Reproductive isolation poses a major obstacle to wide hybridization and introgression breeding of plants. Hybrid inviability in the postzygotic isolation barrier inevitably reduces hybrid fitness, consequently causing hindrances in the establishment of novel genotypes from the hybrids among genetically divergent parents. The idea that the plant immune system is involved in the hybrid problem is applicable to the intra- and/or interspecific hybrids of many different taxa. The lethality characteristics and expression profile of genes associated with the hypersensitive response of the hybrids, along with the suppression of causative genes, support the deleterious epistatic interaction of parental NB-LRR protein genes, resulting in aberrant hyper-immunity reactions in the hybrid. Moreover, the cellular, physiological, and biochemical reactions observed in hybrid cells also corroborate this hypothesis. However, the difference in genetic backgrounds of the respective hybrids may contribute to variations in lethality phenotypes among the parental species combinations. The mixed state in parental components of the chaperone complex (HSP90-SGT1-RAR1) in the hybrid may also affect the hybrid inviability. This review article discusses the facts and hypothesis regarding hybrid inviability, alongside the findings of studies on the hybrid lethality of interspecific hybrids of the genus Nicotiana. A possible solution for averting the hybrid problem has also been scrutinized with the aim of improving the wide hybridization and introgression breeding program in plants.

Electrochromogenic Detection of Live Bacteria Using Soluble and Insoluble Prussian Blue

Microbial detection is crucial for the control and prevention of infectious diseases, being one of the leading causes of mortality worldwide. Among the techniques developed for bacterial detection, those based on metabolic indicators are progressively gaining interest due to their simplicity, adaptability, and, most importantly, their capacity to differentiate between live and dead bacteria. Prussian blue (PB) may act as a metabolic indicator, being reduced by bacterial metabolism, producing a visible color change from blue to colorless. This molecule can be present in two main forms, namely, the soluble and the insoluble, having different properties and structures. In the current work, the bacterial-sensing capacity of soluble and insoluble PB will be tested and compared both in suspensions as PB-NPs and after deposition on transparent indium tin oxide-poly(ethylene terephthalate) (ITO-PET) electrodes. In the presence of live bacteria, PB-NPs are metabolized and completely reduced to the Prussian white state in less than 10 h for soluble and insoluble forms. However, when electrodeposited on ITO-PET substrates, less than 1 h of incubation with bacteria is required for both forms, although the soluble one presents faster metabolic reduction kinetics. This study paves the way to the use of Prussian blue as a metabolic indicator for the early detection of bacterial infection in fields like microbial diagnostics, surface sterilization, food and beverage contamination, and environmental pollution, among others.

A karyopherin constrains nuclear activity of the NLR protein SNC1 and is essential to prevent autoimmunity in Arabidopsis

The nucleotide-binding and leucine-rich repeat (NLR) proteins comprise a major class of intracellular immune receptors that are capable of detecting pathogen-derived molecules and activating immunity and cell death in plants. The activity of some NLRs, particularly the Toll-like/interleukin-1 receptor (TIR) type, is highly correlated with their nucleocytoplasmic distribution. However, whether and how the nucleocytoplasmic homeostasis of NLRs is coordinated through a bidirectional nuclear shuttling mechanism remains unclear. Here, we identified a nuclear transport receptor, KA120, which is capable of affecting the nucleocytoplasmic distribution of an NLR protein and is essential in preventing its autoactivation. We showed that the ka120 mutant displays an autoimmune phenotype and NLR-induced transcriptome features. Through a targeted genetic screen using an artificial NLR microRNA library, we identified the TIR-NLR gene SNC1 as a genetic interactor of KA120. Loss-of-function snc1 mutations as well as compromising SNC1 protein activities all substantially suppressed ka120-induced autoimmune activation, and the enhanced SNC1 activity upon loss of KA120 functionappeared to occur at the protein level. Overexpression of KA120 efficiently repressed SNC1 activity and led to a nearly complete suppression of the autoimmune phenotype caused by the gain-of-function snc1-1 mutation or SNC1 overexpression in transgenic plants. Further florescence imaging analysis indicated that SNC1 undergoes altered nucleocytoplasmic distribution with significantly reduced nuclear signal when KA120 is constitutively expressed, supporting a role of KA120 in coordinating SNC1 nuclear abundance and activity. Consistently, compromising the SNC1 nuclear level by disrupting the nuclear pore complex could also partially rescue ka120-induced autoimmunity. Collectively, our study demonstrates that KA120 is essential to avoid autoimmune activation in the absence of pathogens and is required to constrain the nuclear activity of SNC1, possibly through coordinating SNC1 nucleocytoplasmic homeostasis as a potential mechanism.

The Sw-5b NLR nucleotide-binding domain plays a role in oligomerization, and its self-association is important for activation of cell death signaling

Plant and animal intracellular nucleotide-binding and leucine-rich repeat (NLR) receptors play important roles in sensing pathogens and activating defense signaling. However, the molecular mechanisms underlying the activation of host defense signaling by NLR proteins remain largely unknown. Many studies have determined that the coil-coil (CC) or Toll and interleukin-1 receptor/resistance protein (TIR) domain of NLR proteins and their dimerization/oligomerization are critical for activating downstream defense signaling. In this study, we demonstrated that, in tomato, the nucleotide-binding (NB) domain Sw-5b NLR alone can activate downstream defense signaling, leading to elicitor-independent cell death. Sw-5b NB domains can self-associate, and this self-association is crucial for activating cell death signaling. The self-association was strongly compromised after the introduction of a K568R mutation into the P-loop of the NB domain. Consequently, the NBK568R mutant induced cell death very weakly. The NBCΔ20 mutant lacking the C-terminal 20 amino acids can self-associate but cannot activate cell death signaling. The NBCΔ20 mutant also interfered with wild-type NB domain self-association, leading to compromised cell death induction. By contrast, the NBK568R mutant did not interfere with wild-type NB domain self-association and its ability to induce cell death. Structural modeling of Sw-5b suggests that NB domains associate with one another and likely participate in oligomerization. As Sw-5b-triggered cell death is dependent on helper NLR proteins, we propose that the Sw-5b NB domain acts as a nucleation point for the assembly of an oligomeric resistosome, probably by recruiting downstream helper partners, to trigger defense signaling.

Genome-Wide Identification and Characterization of HSP90-RAR1-SGT1-Complex Members From Arachis Genomes and Their Responses to Biotic and Abiotic Stresses

The molecular chaperone complex HSP90-RAR1-SGT1 (HRS) plays important roles in both biotic and abiotic stress responses in plants. A previous study showed that wild peanut Arachis diogoi SGT1 (AdSGT1) could enhance disease resistance in transgenic tobacco and peanut. However, no systematic analysis of the HRS complex in Arachis has been conducted to date. In this study, a comprehensive analysis of the HRS complex were performed in Arachis. Nineteen HSP90, two RAR1 and six SGT1 genes were identified from the allotetraploid peanut Arachis hypogaea, a number close to the sum of those from the two wild diploid peanut species Arachis duranensis and Arachis ipaensis. According to phylogenetic and chromosomal location analyses, thirteen orthologous gene pairs from Arachis were identified, all of which except AhHSP90-A8, AhHSP90-B9, AdHSP90-9, and AiHSP90-9 were localized on the syntenic locus, and they shared similar exon-intron structures, conserved motifs and expression patterns. Phylogenetic analysis showed that HSP90 and RAR1 from dicot and monocot plants diverged into different clusters throughout their evolution. Chromosomal location analysis indicated that AdSGT1 (the orthologous gene of AhSGT1-B3 in this study) might provide resistance to leaf late spot disease dependent on the orthologous genes of AhHSP90-B10 and AhRAR1-B in the wild peanut A. diogoi. Several HRS genes exhibited tissue-specific expression patterns, which may reflect the sites where they perform functions. By exploring published RNA-seq data, we found that several HSP90 genes play major roles in both biotic and abiotic stress responses, especially salt and drought responses. Autoactivation assays showed that AhSGT1-B1 could not be used as bait for yeast two-hybrid (Y2H) library screening. AhRAR1 and AhSGT1 could strongly interact with each other and interact with AhHSP90-B8. The present study represents the first systematic analysis of HRS complex genes in Arachis and provides valuable information for functional analyses of HRS complex genes. This study also offers potential stress-resistant genes for peanut improvement.

A Novel, Small Cysteine-Rich Effector, RsSCR10 in Rhizoctonia solani Is Sufficient to Trigger Plant Cell Death

The necrotrophic phytopathogen Rhizoctonia solani (R. solani) is a fungus that causes disease in a wide range of plant species. Fungal genomes encode abundant, small cysteine-rich (SCR) secreted proteins, and the probable importance of these to pathogenesis has been highlighted in various pathogens. However, there are currently no reports of an R. solani SCR-secreted protein with evidential elicitor activity. In this study, the molecular function of 10 SCR-secreted protein genes from R. solani was explored by agroinfiltration into Nicotiana benthamiana (N. benthamiana) leaves, and a novel SCR protein RsSCR10 was identified that triggered cell death and oxidative burst in tobacco. RsSCR10 comprises 84 amino acids, including a signal peptide (SP) of 19 amino acids that is necessary for RsSCR10 to induce tobacco cell death. Elicitation of cell death by RsSCR10 was dependent on Hsp90 but not on RAR1, proving its effector activity. Two cysteine residues have important effects on the function of RsSCR10 in inducing cell death. Furthermore, RsSCR10 showed cross-interaction with five rice molecules, and the inferred functions of these rice proteins suggest they are instrumental in how the host copes with adversity. Overall, this study demonstrates that RsSCR10 is a potential effector that has a critical role in R. solani AG1 IA-host interactions.

Disruption of barley immunity to powdery mildew by an in-frame Lys-Leu deletion in the essential protein SGT1

Barley (Hordeum vulgare L.) Mla (Mildew resistance locus a) and its nucleotide-binding, leucine-rich-repeat receptor (NLR) orthologs protect many cereal crops from diseases caused by fungal pathogens. However, large segments of the Mla pathway and its mechanisms remain unknown. To further characterize the molecular interactions required for NLR-based immunity, we used fast-neutron mutagenesis to screen for plants compromised in MLA-mediated response to the powdery mildew fungus, Blumeria graminis f. sp. hordei. One variant, m11526, contained a novel mutation, designated rar3 (required for Mla6 resistance3), that abolishes race-specific resistance conditioned by the Mla6, Mla7, and Mla12 alleles, but does not compromise immunity mediated by Mla1, Mla9, Mla10, and Mla13. This is analogous to, but unique from, the differential requirement of Mla alleles for the co-chaperone Rar1 (required for Mla12 resistance1). We used bulked-segregant-exome capture and fine mapping to delineate the causal mutation to an in-frame Lys-Leu deletion within the SGS domain of SGT1 (Suppressor of G-two allele of Skp1, Sgt1ΔKL308-309), the structural region that interacts with MLA proteins. In nature, mutations to Sgt1 usually cause lethal phenotypes, but here we pinpoint a unique modification that delineates its requirement for some disease resistances, while unaffecting others as well as normal cell processes. Moreover, the data indicate that the requirement of SGT1 for resistance signaling by NLRs can be delimited to single sites on the protein. Further study could distinguish the regions by which pathogen effectors and host proteins interact with SGT1, facilitating precise editing of effector incompatible variants.

Autophagy-related genes serve as heat shock protein 90 co-chaperones in disease resistance against cassava bacterial blight

Heat shock protein 90 (HSP90) is involved in plant growth and various stress responses via regulating protein homeostasis. Autophagy keeps cellular homeostasis by recycling the components of cellular cytoplasmic constituents. Although they have similar effects on cellular protein homeostasis, the direct association between HSP90 and autophagy signaling remains unclear in plants, especially in tropical crops. In this study, the correlation between HSP90 and autophagy signaling was systematically analyzed by protein-protein interaction in cassava, one of the most important economy fruit in tropic. In addition, their effects on plant disease response and underlying mechanisms in cassava were investigated by functional genomics and genetic phenotype assay. The potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex interacts with MeATGs and subsequently triggers autophagy signaling, conferring improved disease resistance to cassava bacterial blight (CBB). On the contrary, HSP90 inhibitor and autophagy inhibitor decreased disease resistance against CBB in cassava, and autophagy may be involved in the potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex-mediated multiple immune responses. This study highlights the precise modulation of autophagy signaling by potential MeHSP90.9-MeSGT1-MeRAR1 chaperone complex in autophagy-mediated disease resistance to CBB.

Characterization of the Roles of SGT1/RAR1, EDS1/NDR1, NPR1, and NRC/ADR1/NRG1 in Sw-5b-Mediated Resistance to Tomato Spotted Wilt Virus

The tomato Sw-5b gene confers resistance to tomato spotted wilt virus (TSWV) and encodes a nucleotide-binding leucine-rich repeat (NLR) protein with an N-terminal Solanaceae-specific domain (SD). Although our understanding of how Sw-5b recognizes the viral NSm elicitor has increased significantly, the process by which Sw-5b activates downstream defense signaling remains to be elucidated. In this study, we used a tobacco rattle virus (TRV)-based virus-induced gene silencing (VIGS) system to investigate the roles of the SGT1/RAR1, EDS1/NDR1, NPR1, and NRC/ADR1/NRG1 genes in the Sw-5b-mediated signaling pathway. We found that chaperone SGT1 was required for Sw-5b function, but co-chaperone RAR1 was not. Sw-5b-mediated immune signaling was independent of both EDS1 and NDR1. Silencing NPR1, which is a central component in SA signaling, did not result in TSWV systemic infection in Sw-5b-transgenic N. benthamiana plants. Helper NLR NRCs (NLRs required for cell death) were required for Sw-5b-mediated systemic resistance to TSWV infection. Suppression of NRC2/3/4 compromised the Sw-5b resistance. However, the helper NLRs ADR1 and NRG1 may not participate in the Sw-5b signaling pathway. Silencing ADR1, NRG1, or both genes did not affect Sw-5b-mediated resistance to TSWV. Our findings provide new insight into the requirement for conserved key components in Sw-5b-mediated signaling pathways.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

The Ubiquitin Proteasome System as a Double Agent in Plant-Virus Interactions

The ubiquitin proteasome is a rapid, adaptive mechanism for selective protein degradation, crucial for proper plant growth and development. The ubiquitin proteasome system (UPS) has also been shown to be an integral part of plant responses to stresses, including plant defence against pathogens. Recently, significant progress has been made in the understanding of the involvement of the UPS in the signalling and regulation of the interaction between plants and viruses. This review aims to discuss the current knowledge about the response of plant viral infection by the UPS and how the viruses counteract this system, or even use it for their own benefit.

The CC-NB-LRR OsRLR1 mediates rice disease resistance through interaction with OsWRKY19

Nucleotide-binding site-leucine-rich repeat (NB-LRR) resistance proteins are critical for plant resistance to pathogens; however, their mechanism of activation and signal transduction is still not well understood. We identified a mutation in an as yet uncharacterized rice coiled-coil (CC)-NB-LRR, Oryza sativa RPM1-like resistance gene 1 (OsRLR1), which leads to hypersensitive response (HR)-like lesions on the leaf blade and broad-range resistance to the fungal pathogen Pyricularia oryzae (syn. Magnaporthe oryzae) and the bacterial pathogen Xanthomonas oryzae pv. oryzae, together with strong growth reduction. Consistently, OsRLR1-overexpression lines showed enhanced resistance to both pathogens. Moreover, we found that OsRLR1 mediates the defence response through direct interaction in the nucleus with the transcription factor OsWRKY19. Down-regulation of OsWRKY19 in the rlr1 mutant compromised the HR-like phenotype and resistance response, and largely restored plant growth. OsWRKY19 binds to the promoter of OsPR10 to activate the defence response. Taken together, our data highlight the role of a new residue involved in the NB-LRR activation mechanism, allowing identification of a new NB-LRR downstream signalling pathway.

Genome-Wide Association Study of Local Thai Indica Rice Seedlings Exposed to Excessive Iron

Excess soluble iron in acidic soil is an unfavorable environment that can reduce rice production. To better understand the tolerance mechanism and identify genetic loci associated with iron toxicity (FT) tolerance in a highly diverse indica Thai rice population, a genome-wide association study (GWAS) was performed using genotyping by sequencing and six phenotypic data (leaf bronzing score (LBS), chlorophyll content, shoot height, root length, shoot biomass, and root dry weight) under both normal and FT conditions. LBS showed a high negative correlation with the ratio of chlorophyll content and shoot biomass, indicating the FT-tolerant accessions can regulate cellular homeostasis when encountering stress. Sixteen significant single nucleotide polymorphisms (SNPs) were identified by association mapping. Validation of candidate SNP using other FT-tolerant accessions revealed that SNP:2_21262165 might be associated with tolerance to FT; therefore, it could be used for SNP marker development. Among the candidate genes controlling FT tolerance, RAR1 encodes an innate immune responsive protein that links to cellular redox homeostasis via interacting with abiotic stress-responsive Hsp90. Future research may apply the knowledge obtained from this study in the molecular breeding program to develop FT-tolerant rice varieties.

The role of chaperone complex HSP90-SGT1-RAR1 as the associated machinery for hybrid inviability between Nicotiana gossei Domin and N. tabacum L

Two cultured cell lines (GTH4 and GTH4S) of a Nicotiana interspecific F1 hybrid (N. gossei × N. tabacum) were comparatively analyzed to find genetic factors related to hybrid inviability. Both cell lines proliferated at 37 °C, but after shifting to 26 °C, GTH4 started to die similar to the F1 hybrid seedlings, whereas GTH4S survived. As cell death requires de novo expression of genes and proteins, we compared expressed protein profiles between the two cell lines, and found that NgSGT1, a cochaperone of the chaperone complex (HSP90-SGT1-RAR1), was expressed in GTH4 but not in GTH4S. Agrobacterium-mediated transient expression of NgSGT1, but not NtSGT1, induced cell death in leaves of N. tabacum, suggesting its possible role in hybrid inviability. Cell death in N. tabacum was also induced by transient expression of NgRAR1, but not NtRAR1. In contrast, transient expression of any parental combinations of three components revealed that NgRAR1 promoted cell death, whereas NtRAR1 suppressed it in N. tabacum. A specific inhibitor of HSP90, geldanamycin, inhibited the progression of hypersensitive response-like cell death in GTH4 and leaf tissue after agroinfiltration. The present study suggested that components of the chaperone complex are involved in the inviability of Nicotiana interspecific hybrid.