Recognition and response in the plant immune system

Genomic insights into adaptive evolution of the species-rich cosmopolitan plant genus Rhododendron

The species-rich cosmopolitan genus Rhododendron offers a good system for exploring the genomic mechanisms underlying adaptation to diverse habitats. Here, we report high-quality chromosomal-level genome assemblies of nine species, representing all five subgenera, different altitudinal distributions, and all flower color types of this genus. Further comprehensive genomic analyses indicate diverse adaptive strategies employed by Rhododendron, particularly adaptation to alpine and subalpine habitats by expansion/contraction of gene families involved in pathogen defense and oxidative phosphorylation, genomic convergent evolution, and gene copy-number variation. The convergent adaptation to high altitudes is further shown by population genomic analysis of R. nivale from the Himalaya-Hengduan Mountains. Moreover, we identify the genes involved in the biosynthesis of anthocyanins and carotenoids, which play a crucial role in shaping flower color diversity and environmental adaptation. Our study is significant for comprehending plant adaptive evolution and the uneven distribution of species diversity across different geographical regions.

Human genetic errors of immunity illuminate an adaptive arsenal model of rapid defenses

New discoveries in the field of human monogenic immune diseases highlight critical genes and pathways governing immune responses. Here, I describe how the ~500 currently defined human inborn errors of immunity help shape what I propose is an 'adaptive arsenal model of rapid defenses', emphasizing the set of immunological defenses poised for rapid responses in the natural environment. This arsenal blurs the lines between innate and adaptive immunity and is established through molecular relays between cell types, often traversing from sensors (pathogen detection) to intermediates to executioners (pathogen clearance) via soluble factors. Predictions and missing information based on the adaptive arsenal model are discussed, as are emergent and outstanding questions fundamental to advances in the field.

ATL Protein Family: Novel Regulators in Plant Response to Environmental Stresses

Plants actively develop intricate regulatory mechanisms to counteract the harmful effects of environmental stresses. The ubiquitin-proteasome pathway, a crucial mechanism, employs E3 ligases (E3s) to facilitate the conjugation of ubiquitin to specific target substrates, effectively marking them for proteolytic degradation. E3s play critical roles in many biological processes, including phytohormonal signaling and adaptation to environmental stresses. Arabidopsis Toxicosa en Levadura (ATL) proteins, belonging to a subfamily of RING-H2 E3s, actively modulate diverse physiological processes and plant responses to environmental stresses. Despite studies on the functions of certain ATL family members in rice and Arabidopsis, most ATLs still need more comprehensive study. This review presents an overview of the ubiquitin-proteasome system (UPS), specifically focusing on the pivotal role of E3s and associated enzymes in plant development and environmental adaptation. Our study seeks to unveil the active modulation of plant responses to environmental stresses by E3s and ATLs, emphasizing the significance of ATLs within this intricate process. By emphasizing the importance of studying the roles of E3s and ATLs, our review contributes to developing more resilient plant varieties and promoting sustainable agricultural practices while establishing a research roadmap for the future.

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.

Mining of long non-coding RNAs with target genes in response to rust based on full-length transcriptome in Kentucky bluegrass

Kentucky bluegrass (Poa pratensis L.) is an eminent turfgrass species with a complex genome, but it is sensitive to rust (Puccinia striiformis). The molecular mechanisms of Kentucky bluegrass in response to rust still remain unclear. This study aimed to elucidate differentially expressed lncRNAs (DELs) and genes (DEGs) for rust resistance based on the full-length transcriptome. First, we used single-molecule real-time sequencing technology to generate the full-length transcriptome of Kentucky bluegrass. A total of 33,541 unigenes with an average read length of 2,233 bp were obtained, which contained 220 lncRNAs and 1,604 transcription factors. Then, the comparative transcriptome between the mock-inoculated leaves and rust-infected leaves was analyzed using the full-length transcriptome as a reference genome. A total of 105 DELs were identified in response to rust infection. A total of 15,711 DEGs were detected (8,278 upregulated genes, 7,433 downregulated genes) and were enriched in plant hormone signal transduction and plant-pathogen interaction pathways. Additionally, through co-location and expression analysis, it was found that lncRNA56517, lncRNA53468, and lncRNA40596 were highly expressed in infected plants and upregulated the expression of target genes AUX/IAA, RPM1, and RPS2, respectively; meanwhile, lncRNA25980 decreased the expression level of target gene EIN3 after infection. The results suggest that these DEGs and DELs are important candidates for potentially breeding the rust-resistant Kentucky bluegrass.

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Characterization of host-effector transcription dynamics during pathogen infection in engineered late blight resistant potato

Phytophthora infestans, the etiologic agent of late blight, is a threat to potato production in areas with high humidity during the growing season. The oomycete pathogen is hemi-biotrophic, it establishes infection on living plant cells and then spreads, kills, and feeds off the necrotized plant tissue material. The interaction between host and pathogen is complex with dynamic pathogen RXLR effectors and potato NB-LRR resistance proteins actively competing for dominance and survival. Late blight protection was brought to several cultivars of potato through insertion of the wild potato (Solanum venturii) NB-LRR resistance gene Rpi-vnt1.1. We have established that the late blight protection trait, mediated by Rpi-vnt1.1, is effective despite low expression of RNA. The RNA expression dynamics of Rpi-vnt1.1 and the cognate pathogen RXLR effector, Avr-vnt1, were evaluated following spray inoculation with up to five different contemporary late blight isolates from North America and South America. Following inoculations, RXLR effector transcript profiles provided insight into interaction compatibility in relation to markers of the late blight hemi-biotrophic lifecycle.

Disentangling the resistant mechanism of Fusarium wilt TR4 interactions with different cultivars and its elicitor application

Fusarium wilt of banana, especially Tropical Race 4 (TR4) is a major factor restricting banana production. Developing a resistant cultivar and inducing plant defenses by elicitor application are currently two of the best options to control this disease. Isotianil is a monocarboxylic acid amide that has been used as a fungicide to control rice blast and could potentially induce systemic acquired resistance in plants. To determine the control effect of elicitor isotianil on TR4 in different resistant cultivars, a greenhouse pot experiment was conducted and its results showed that isotianil could significantly alleviate the symptoms of TR4, provide enhanced disease control on the cultivars 'Baxi' and 'Yunjiao No.1' with control effect 50.14% and 56.14%, respectively. We compared the infection processes in 'Baxi' (susceptible cultivars) and 'Yunjiao No.1' (resistant cultivars) two cultivars inoculated with pathogen TR4. The results showed that TR4 hyphae could rapidly penetrate the cortex into the root vascular bundle for colonization, and the colonization capacity in 'Baxi' was significantly higher than that in 'Yunjiao No.1'. The accumulation of a large number of starch grains was observed in corms cells, and further analysis showed that the starch content in 'Yunjiao No. 1' as resistant cultivar was significantly higher than that in 'Baxi' as susceptible cultivar, and isotianil application could significantly increase the starch content in 'Baxi'. Besides, a mass of tyloses were observed in the roots and corms and these tyloses increased after application with isotianil. Furthermore, the total starch and tyloses contents and the control effect in the corms of 'Yunjiao No.1' was higher than that in the 'Baxi'. Moreover, the expression levels of key genes for plant resistance induction and starch synthesis were analyzed, and the results suggested that these genes were significantly upregulated at different time points after the application of isotianil. These results suggest that there are significant differences between cultivars in response to TR4 invasion and plant reactions with respect to starch accumulation, tyloses formation and the expression of plant resistance induction and starch synthesis related genes. Results also indicate that isotianil application may contribute to disease control by inducing host plant defense against TR4 infection and could be potentially used together with resistant cultivar as integrated approach to manage this destructive disease. Further research under field conditions should be included in the next phases of study.

Genome-wide analysis of NBS-LRR genes revealed contribution of disease resistance from Saccharum spontaneum to modern sugarcane cultivar

INTRODUCTION

During plant evolution, nucleotide-binding sites (NBS) and leucine-rich repeat (LRR) genes have made significant contributions to plant disease resistance. With many high-quality plant genomes sequenced, identification and comprehensive analyses of NBS-LRR genes at whole genome level are of great importance to understand and utilize them.

METHODS

In this study, we identified the NBS-LRR genes of 23 representative species at whole genome level, and researches on NBS-LRR genes of four monocotyledonous grass species, Saccharum spontaneum, Saccharum officinarum, Sorghum bicolor and Miscanthus sinensis, were focused.

RESULTS AND DISCUSSION

We found that whole genome duplication, gene expansion, and allele loss could be factors affecting the number of NBS-LRR genes in the species, and whole genome duplication is likely to be the main cause of the number of NBS-LRR genes in sugarcane. Meanwhile, we also found a progressive trend of positive selection on NBS-LRR genes. These studies further elucidated the evolutionary pattern of NBS-LRR genes in plants. Transcriptome data from multiple sugarcane diseases revealed that more differentially expressed NBS-LRR genes were derived from S. spontaneum than from S. officinarum in modern sugarcane cultivars, and the proportion was significantly higher than the expected. This finding reveals that S. spontaneum has a greater contribution to disease resistance for modern sugarcane cultivars. In addition, we observed allelespecific expression of seven NBS-LRR genes under leaf scald, and 125 NBS-LRR genes responding to multiple diseases were identified. Finally, we built a plant NBS-LRR gene database to facilitate subsequent analysis and use of NBSLRR genes obtained here. In conclusion, this study complemented and completed the research of plant NBS-LRR genes, and discussed how NBS-LRR genes responding to sugarcane diseases, which provided a guide and genetic resources for further research and utilization of NBS-LRR genes.

A "Candidatus Liberibacter asiaticus"-secreted polypeptide suppresses plant immune responses in Nicotiana benthamiana and Citrus sinensis

Citrus Huanglongbing (HLB), known as the most economically devastating disease in citrus industry, is mainly caused by phloem-restricted Gram-negative bacterium "Candidatus Liberibacter asiaticus" (CLas). To date, CLas is still unculturable in vitro, which has been dramatically delaying the research on its pathogenesis, and only few Sec-dependent effectors (SDEs) have been identified to elucidate the pathogenesis of CLas. Here, we confirmed that a CLas-secreted Sec-dependent polypeptide, namely SECP8 (CLIBASIA_05330), localized in nucleus, cytoplasm and cytoplasmic membrane, and showed remarkably higher transcript abundance in citrus than in psyllids. Potato virus X (PVX)-mediated transient expression assays indicated that mSECP8 (the mature form of SECP8) suppressed pro-apoptotic mouse protein BAX and Phytophthora infestans elicitin INF1-triggered hypersensitive response (HR) associated phenotypes, including cell death, H2O2 accumulation and callose deposition. Intriguingly, mSECP8 also inhibited SDE1 (CLIBASIA_05315)-induced water-soaked and dwarfing symptoms in Nicotiana benthamiana. In addition, mSECP8 can promote the susceptibility of transgenic Wanjincheng orange (Citrus sinensis) to CLas invasion and further HLB symptom development, and it contributes to the proliferation of Xanthomonas citri subsp. citri (Xcc). Moreover, the expression of ten immunity-related genes were significantly down-regulated in mSECP8 transgenic citrus than those in wide-type (WT) plants. Overall, we propose that mSECP8 may serve as a novel broad-spectrum suppressor of plant immunity, and provide the first evidence counteractive effect among CLas effectors. This study will enrich and provide new evidences for elucidating the pathogenic mechanisms of CLas in citrus host.

A prophage-encoded nonclassical secretory protein of "Candidatus Liberibacter asiaticus" induces a strong immune response in Nicotiana benthamiana and citrus

Huanglongbing (HLB), associated with "Candidatus Liberibacter asiaticus" (CLas), is a globally devastating plant disease. The highly reduced genome of CLas encodes a number of secretory proteins. The conserved prophage-encoded protein AGH17470 is herein identified as a nonclassical secretory protein. We confirmed that the N-terminal and C-terminal sequences jointly determine the secretion of AGH17470. The transient expression of AGH17470 protein in Nicotiana benthamiana caused hypersensitive response (HR) cell death in infiltrated leaves and systemically infected leaves as well as the dwarfing of the entire plant, suggesting that AGH17470 is involved in the plant immune response, growth, and development. Overexpression of AGH17470 in N. benthamiana and citrus plants up-regulated the transcription of pathogenesis-related and salicylic acid (SA)-signalling pathway genes and promoted SA accumulation. Furthermore, transient expression of AGH17470 enhanced the resistance of sweet orange to Xanthomonas citri subsp. citri. To our knowledge, AGH17470 is the first prophage-encoded secretory protein demonstrated to elicit an HR and induce a strong plant immune response. The findings have increased our understanding of prophage-encoded secretory protein genes, and the results provide clues as to the plant defence response against CLas.

Prediction and expression analysis of deleterious nonsynonymous SNPs of Arabidopsis ACD11 gene by combining computational algorithms and molecular docking approach

Accelerated cell death 11 (ACD11) is an autoimmune gene that suppresses pathogen infection in plants by preventing plant cells from becoming infected by any pathogen. This gene is widely known for growth inhibition, premature leaf chlorosis, and defense-related programmed cell death (PCD) in seedlings before flowering in Arabidopsis plant. Specific amino acid changes in the ACD11 protein’s highly conserved domains are linked to autoimmune symptoms including constitutive defensive responses and necrosis without pathogen awareness. The molecular aspect of the aberrant activity of the ACD11 protein is difficult to ascertain. The purpose of our study was to find the most deleterious mutation position in the ACD11 protein and correlate them with their abnormal expression pattern. Using several computational methods, we discovered PCD vulnerable single nucleotide polymorphisms (SNPs) in ACD11. We analysed the RNA-Seq data, identified the detrimental nonsynonymous SNPs (nsSNP), built genetically mutated protein structures and used molecular docking to assess the impact of mutation. Our results demonstrated that the A15T and A39D mutations in the GLTP domain were likely to be extremely detrimental mutations that inhibit the expression of the ACD11 protein domain by destabilizing its composition, as well as disrupt its catalytic effectiveness. When compared to the A15T mutant, the A39D mutant was more likely to destabilize the protein structure. In conclusion, these mutants can aid in the better understanding of the vast pool of PCD susceptibilities connected to ACD11 gene GLTP domain activation.

Non synonymous single nucleotide polymorphism (nsSNP) is a process in which amino acid sequence of a protein is altered as a result of single nucleotide alteration in the coding region (mRNA) of any living organism. Therefore, the entire protein structure, interactions and stability are altered, which may have a negative impact on living organisms. Hence, to completely comprehend this biological process, we must first solve the unresolved mutational protein structure and mutated protein interactions. The major goal of our research is to identify the most harmful mutation in our target protein structure and how it interacts within cells. However, it was discovered that only a few alterations in residues had the largest negative impact on the protein’s internal structure and also on the protein-ligand interactions. We show that based on the amino acid sequence of a protein computationally, it is feasible to discover mutational positions in the sequence, generate mutation protein structure and interactions with related ligands. Our findings show that the essential mechanisms underlying protein mutations generated by this process are identical. The capacity to correctly detect mutations from sequence allows the annotation and study of protein-ligand interactions throughout a whole organism, which might aid function prediction and gene expression.

Gene expression analysis of resistant and susceptible rice cultivars to sheath blight after inoculation with Rhizoctonia solani

BACKGROUND

Rice sheath blight, caused by Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris), is one of the most severe diseases in rice (Oryza sativa L.) worldwide. Studies on resistance genes and resistance mechanisms of rice sheath blight have mainly focused on indica rice. Rice sheath blight is a growing threat to rice production with the increasing planting area of japonica rice in Northeast China, and it is therefore essential to explore the mechanism of sheath blight resistance in this rice subspecies.

RESULTS

In this study, RNA-seq technology was used to analyse the gene expression changes of leaf sheath at 12, 24, 36, 48, and 72 h after inoculation of the resistant cultivar 'Shennong 9819' and susceptible cultivar 'Koshihikari' with R. solani. In the early stage of R. solani infection of rice leaf sheaths, the number of differentially expressed genes (DEGs) in the inoculated leaf sheaths of resistant and susceptible cultivars showed different regularity. After inoculation, the number of DEGs in the resistant cultivar fluctuated, while the number of DEGs in the susceptible cultivar increased first and then decreased. In addition, the number of DEGs in the susceptible cultivar was always higher than that in the resistant cultivar. After inoculation with R. solani, the overall transcriptome changes corresponding to multiple biological processes, molecular functions, and cell components were observed in both resistant and susceptible cultivars. These included metabolic process, stimulus response, biological regulation, catalytic activity, binding and membrane, and they were differentially regulated. The phenylalanine metabolic pathway; tropane, piperidine, and pyridine alkaloid biosynthesis pathways; and plant hormone signal transduction were significantly enriched in the early stage of inoculation of the resistant cultivar Shennong 9819, but not in the susceptible cultivar Koshihikari. This indicates that the response of the resistant cultivar Shennong 9819 to pathogen stress was faster than that of the susceptible cultivar. The expression of plant defense response marker PR1b gene, transcription factor OsWRKY30 and OsPAL1 and OsPAL6 genes that induce plant resistance were upregulated in the resistant cultivar. These data suggest that in the early stage of rice infection by R. solani, there is a pathogen-induced defence system in resistant rice cultivars, involving the expression of PR genes, key transcription factors, PAL genes, and the enrichment of defence-related pathways.

CONCLUSION

The transcriptome data revealed the molecular and biochemical differences between resistant and susceptible cultivars of rice after inoculation with R. solani, indicating that resistant cultivars have an immune response mechanism in the early stage of pathogen infection. Disease resistance is related to the overexpression of PR genes, key transcriptome factors, and PAL genes, which are potential targets for crop improvement.

Modulation of plant plasma membrane structure by exogenous fatty acid hydroperoxide is a potential perception mechanism for their eliciting activity

Oxylipins are lipid-derived molecules that are ubiquitous in eukaryotes and whose functions in plant physiology have been widely reported. They appear to play a major role in plant immunity by orchestrating reactive oxygen species (ROS) and hormone-dependent signalling pathways. The present work focuses on the specific case of fatty acid hydroperoxides (HPOs). Although some studies report their potential use as exogenous biocontrol agents for plant protection, evaluation of their efficiency in planta is lacking and no information is available about their mechanism of action. In this study, the potential of 13(S)-hydroperoxy-(9Z, 11E)-octadecadienoic acid (13-HPOD) and 13(S)-hydroperoxy-(9Z, 11E, 15Z)-octadecatrienoic acid (13-HPOT), as plant defence elicitors and the underlying mechanism of action is investigated. Arabidopsis thaliana leaf resistance to Botrytis cinerea was observed after root application with HPOs. They also activate early immunity-related defence responses, like ROS. As previous studies have demonstrated their ability to interact with plant plasma membranes (PPM), we have further investigated the effects of HPOs on biomimetic PPM structure using complementary biophysics tools. Results show that HPO insertion into PPM impacts its global structure without solubilizing it. The relationship between biological assays and biophysical analysis suggests that lipid amphiphilic elicitors that directly act on membrane lipids might trigger early plant defence events.

CabZIP23 Integrates in CabZIP63-CaWRKY40 Cascade and Turns CabZIP63 on Mounting Pepper Immunity against Ralstonia solanacearum via Physical Interaction

CabZIP63 and CaWRKY40 were previously found to be shared in the pepper defense response to high temperature stress (HTS) and to Ralstonia solanacearum inoculation (RSI), forming a transcriptional cascade. However, how they activate the two distinct defense responses is not fully understood. Herein, using a revised genetic approach, we functionally characterized CabZIP23 in the CabZIP63-CaWRKY40 cascade and its context specific pepper immunity activation against RSI by interaction with CabZIP63. CabZIP23 was originally found by immunoprecipitation-mass spectrometry to be an interacting protein of CabZIP63-GFP; it was upregulated by RSI and acted positively in pepper immunity against RSI by virus induced gene silencing in pepper plants, and transient overexpression in Nicotiana benthamiana plants. By chromatin immunoprecipitation (ChIP)-qPCR and electrophoresis mobility shift assay (EMSA), CabZIP23 was found to be directly regulated by CaWRKY40, and CabZIP63 was directly regulated by CabZIP23, forming a positive feedback loop. CabZIP23-CabZIP63 interaction was confirmed by co-immunoprecipitation (CoIP) and bimolecular fluorescent complimentary (BiFC) assays, which promoted CabZIP63 binding immunity related target genes, including CaPR1, CaNPR1 and CaWRKY40, thereby enhancing pepper immunity against RSI, but not affecting the expression of thermotolerance related CaHSP24. All these data appear to show that CabZIP23 integrates in the CabZIP63-CaWRKY40 cascade and the context specifically turns it on mounting pepper immunity against RSI.

Endophytic Bacteria Pseudomonas aeruginosa PM389 Subsists Host’s (Triticum aestivum) Immune Response for Gaining Entry Inside the Host

The present study was designed to compare the defense response of the host plant towards endophytic bacteria Pseudomonas aeruginosa PM389 and pathogenic bacteria Erwinia carotovora and to correlate the level of defense enzymes vis-a-vis bacterial colonization in the host. Wheat seedlings were treated with 107-108 cells ml-1 endophytic and pathogenic bacteria in the separate experimental set-up, and the level of plant defense enzyme was measured at various time intervals. Comparatively reduced level of most defense enzymes was produced in endophytic bacteria treated plants. While the endophytic bacterial population was almost constant after 24 HAI (hour after inoculation), the population of pathogenic bacteria kept fluctuating during the study period from 24 HAI. Unlike pathogenic bacteria, we observed attenuated defense response in challenged host plants towards endophytic bacteria, which helps endophytes establish inside plant. This study would be useful for understanding the mechanism of colonization and strategies of endophytes to fight against the host defense response.

Identification of defense related gene families and their response against powdery and downy mildew infections in Vitis vinifera

Background

Grapevine (Vitis vinifera) productivity has been severely affected by various bacterial, viral and fungal diseases worldwide. When a plant is infected with the pathogen, various defense mechanisms are subsequently activated in plants at various molecular levels. Thus, for substantiating the disease control in an eco-friendly way, it is essential to understand the molecular mechanisms governing pathogen resistance in grapes.

Results

In our study, we performed genome-wide identification of various defensive genes expressed during powdery mildew (PM) and downy mildew (DM) infections in grapevine. Consequently, we identified 6, 21, 2, 5, 3 and 48 genes of Enhanced Disease Susceptibility 1 (EDS1), Non-Race-specific Disease Resistance (NDR1), Phytoalexin deficient 4 (PAD4), Nonexpressor of PR Gene (NPR), Required for Mla-specified resistance (RAR) and Pathogenesis Related (PR), respectively, in the grapevine genome. The phylogenetic study revealed that V. vinifera defensive genes are evolutionarily related to Arabidopsis thaliana. Differential expression analysis resulted in identification of 2, 4, 7, 2, 4, 1 and 7 differentially expressed Nucleotide-binding leucine rich repeat receptor (NLR), EDS1, NDR1, PAD4, NPR, RAR1 and PR respectively against PM infections and 28, 2, 5, 4, 1 and 19 differentially expressed NLR, EDS1, NDR1, NPR, RAR1 and PR respectively against DM infections in V. vinifera. The co-expression study showed the occurrence of closely correlated defensive genes that were expressed during PM and DM stress conditions.

Conclusion

The PM and DM responsive defensive genes found in this study can be characterized in future for impelling studies relaying fungal and oomycete resistance in plants, and the functionally validated genes would then be available for conducting in-planta transgenic gene expression studies for grapes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08081-4.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

CmMLO17 and its partner CmKIC potentially support Alternaria alternata growth in Chrysanthemum morifolium

The Mildew Resistance Locus O (MLO) gene family has been investigated in many species. However, there are few studies on chrysanthemum MLO genes. We report in this study that CmMLO17 in Chrysanthemum morifolium was upregulated after Alternaria alternata infection. Silencing of CmMLO17 by artificial microRNA resulted in reduced susceptibility of chrysanthemum to A. alternata infection. Genes in the abscisic acid (ABA) and Ca²⁺ signaling pathways were upregulated in the CmMLO17-silenced line R20 compared to the wild-type plants. We speculated that CmMLO17-silenced plants had a faster and stronger defense response that was mediated by the ABA and Ca²⁺ signaling pathways, resulting in reduced susceptibility of chrysanthemum to A. alternata infection. In addition, a candidate gene, CmKIC, that may interact with CmMLO17 was discovered by the yeast two-hybrid assay. The interaction between CmMLO17 and CmKIC was confirmed using the yeast two-hybrid assay and bimolecular fluorescence complementation (BiFC) analysis. CmMLO17 and CmKIC were both located on the plasma membrane, and CmKIC was also located on the nucleus. CmKIC overexpression increased the susceptibility of chrysanthemum to A. alternata, whereas CmKIC silencing resulted in reduced susceptibility. Therefore, CmMLO17 and CmKIC may work together in C. morifolium to support the growth of A. alternata. The results of this study will provide insight into the potential function of MLO and improve the understanding of plant defense responses to necrotrophic pathogens.

Isolate-Specific Responses of the Nonhost Grass Brachypodium distachyon to the Fungal Pathogen Zymoseptoria tritici Compared with Wheat

Septoria tritici blotch (STB) is an important foliar disease of wheat that is caused by the fungal pathogen Zymoseptoria tritici. The grass Brachypodium distachyon has been used previously as a model system for cereal-pathogen interactions. In this study, we examined the nonhost resistance (NHR) response of B. distachyon to two different Z. tritici isolates in comparison with wheat. These isolates vary in aggressiveness on wheat cultivar Remus, displaying significant differences in disease and pycnidia coverage. Using microscopy, we found that similar isolate-specific responses were observed for hydrogen peroxide accumulation and cell death in both wheat and B. distachyon. Despite this, induction of isolate-specific patterns of defense gene expression by Z. tritici did differ between B. distachyon and wheat. Our results suggest that expression of the phenylalanine ammonia lyase PAL gene may be important for NHR in B. distachyon, while pathogenesis-related PR genes and expression of genes regulating reactive oxygen species may be important to limit disease in wheat. Future studies of the B. distachyon-Z. tritici interaction may allow identification of conserved plant immunity targets that are responsible for the isolate-specific responses observed in both plant species.

Local Responses and Systemic Induced Resistance Mediated by Ectomycorrhizal Fungi

Ectomycorrhizal fungi (EMF) grow as saprotrophs in soil and interact with plants, forming mutualistic associations with roots of many economically and ecologically important forest tree genera. EMF ensheath the root tips and produce an extensive extramatrical mycelium for nutrient uptake from the soil. In contrast to other mycorrhizal fungal symbioses, EMF do not invade plant cells but form an interface for nutrient exchange adjacent to the cortex cells. The interaction of roots and EMF affects host stress resistance but uncovering the underlying molecular mechanisms is an emerging topic. Here, we focused on local and systemic effects of EMF modulating defenses against insects or pathogens in aboveground tissues in comparison with arbuscular mycorrhizal induced systemic resistance. Molecular studies indicate a role of chitin in defense activation by EMF in local tissues and an immune response that is induced by yet unknown signals in aboveground tissues. Volatile organic compounds may be involved in long-distance communication between below- and aboveground tissues, in addition to metabolite signals in the xylem or phloem. In leaves of EMF-colonized plants, jasmonate signaling is involved in transcriptional re-wiring, leading to metabolic shifts in the secondary and nitrogen-based defense metabolism but cross talk with salicylate-related signaling is likely. Ectomycorrhizal-induced plant immunity shares commonalities with systemic acquired resistance and induced systemic resistance. We highlight novel developments and provide a guide to future research directions in EMF-induced resistance.

Sequencing and analysis of gerbera daisy leaf transcriptomes reveal disease resistance and susceptibility genes differentially expressed and associated with powdery mildew resistance

BACKGROUND

RNA sequencing has been widely used to profile genome-wide gene expression and identify candidate genes controlling disease resistance and other important traits in plants. Gerbera daisy is one of the most important flowers in the global floricultural trade, and powdery mildew (PM) is the most important disease of gerbera. Genetic improvement of gerbera PM resistance has become a crucial goal in gerbera breeding. A better understanding of the genetic control of gerbera resistance to PM can expedite the development of PM-resistant cultivars.

RESULTS

The objectives of this study were to identify gerbera genotypes with contrasting phenotypes in PM resistance and sequence and analyze their leaf transcriptomes to identify disease resistance and susceptibility genes differentially expressed and associated with PM resistance. An additional objective was to identify SNPs and SSRs for use in future genetic studies. We identified two gerbera genotypes, UFGE 4033 and 06-245-03, that were resistant and susceptible to PM, respectively. De novo assembly of their leaf transcriptomes using four complementary pipelines resulted in 145,348 transcripts with a N50 of 1124 bp, of which 67,312 transcripts contained open reading frames and 48,268 were expressed in both genotypes. A total of 494 transcripts were likely involved in disease resistance, and 17 and 24 transcripts were up- and down-regulated, respectively, in UFGE 4033 compared to 06-245-03. These gerbera disease resistance transcripts were most similar to the NBS-LRR class of plant resistance genes conferring resistance to various pathogens in plants. Four disease susceptibility transcripts (MLO-like) were expressed only or highly expressed in 06-245-03, offering excellent candidate targets for gene editing for PM resistance in gerbera. A total of 449,897 SNPs and 19,393 SSRs were revealed in the gerbera transcriptomes, which can be a valuable resource for developing new molecular markers.

CONCLUSION

This study represents the first transcriptomic analysis of gerbera PM resistance, a highly important yet complex trait in a globally important floral crop. The differentially expressed disease resistance and susceptibility transcripts identified provide excellent targets for development of molecular markers and genetic maps, cloning of disease resistance genes, or targeted mutagenesis of disease susceptibility genes for PM resistance in gerbera.

HOS15 is a transcriptional corepressor of NPR1-mediated gene activation of plant immunity

Immune responses protect organisms against biotic challenges but can also produce deleterious effects, such as inflammation and necrosis. This growth-defense trade-off necessitates fine control of immune responses, including the activation of defense gene expression. The transcriptional coactivator NPR1 is a key regulatory hub of immune activation in plant cells. Surprisingly, full activation of NPR1-activated defense genes requires proteasome-mediated degradation of NPR1 induced by a CUL3-based E3 ubiquitin ligase complex. Our work demonstrates that HOS15 is the specificity determinant of a CUL1-based E3 ubiquitin ligase complex that limits defense gene expression by targeting NPR1 for proteasome-mediated degradation. Thus, distinct ubiquitin-based degradation pathways coordinately modulate the timing and amplitude of transcriptional outputs during plant defense.

Transcriptional regulation is a complex and pivotal process in living cells. HOS15 is a transcriptional corepressor. Although transcriptional repressors generally have been associated with inactive genes, increasing evidence indicates that, through poorly understood mechanisms, transcriptional corepressors also associate with actively transcribed genes. Here, we show that HOS15 is the substrate receptor for an SCF/CUL1 E3 ubiquitin ligase complex (SCF^HOS15) that negatively regulates plant immunity by destabilizing transcriptional activation complexes containing NPR1 and associated transcriptional activators. In unchallenged conditions, HOS15 continuously eliminates NPR1 to prevent inappropriate defense gene expression. Upon defense activation, HOS15 preferentially associates with phosphorylated NPR1 to stimulate rapid degradation of transcriptionally active NPR1 and thus limit the extent of defense gene expression. Our findings indicate that HOS15-mediated ubiquitination and elimination of NPR1 produce effects contrary to those of CUL3-containing ubiquitin ligase that coactivate defense gene expression. Thus, HOS15 plays a key role in the dynamic regulation of pre- and postactivation host defense.

StRac1 plays an important role in potato resistance against Phytophthora infestans via regulating H2O2 production

ROP GTPases (Rho-related GTPases from plant), a unique subgroup of the Rho family in plants, is a group of key regulators of different signaling pathways controlling plant growth and development, cell polarity and differentiation, and plant response against biotic and abiotic stresses. The present study determined the potential regulatory mechanism of potato ROP GTPase (StRac1) against Phytophthora infestans (P. infestans) infection. Protein secondary structure analysis indicated that StRAC1 is a Rho GTPase. The expression level of StRac1 was variable in different tissues of potato, with the highest expression in young leaves of both Shepody and Hutou potato varieties. After challenging with P. infestans, the expression level of StRac1was higher in resistance varieties Zihuabai and Longshu 7 than in susceptible varieties Shepody and Desiree. StRAC1 fusion with GFP subcellularly localized at the plasma membrane (PM) in tobacco epidermal cells. The potato with transient or stable over-expression of CA-StRac1 (constitutively active form of StRac1)exhibited a dramatic enhancement of its resistance against P. infestans infections. The increased resistance level in transgenic potato was accompanied with elevated H2O2 levels. Importantly, silencing StRac1 via virus-induced gene silencing (VIGS) in potato resulted in higher susceptibility to P. infestans infection than in control plants. In summary, our data reveal that StRac1 regulates potato resistance against P. infestans via positively modulating the accumulation of H2O2.

Tea Tree Oil Induces Systemic Resistance against Fusarium wilt in Banana and Xanthomonas Infection in Tomato Plants

The essential tea tree oil (TTO) derived from Melaleuca alternifolia plant is widely used as a biopesticide to protect crops from several plant-pathogens. Its activity raised queries regarding its ability to, not only act as a bio-fungicide or bio-bactericide, but also systemically inducing resistance in plants. This was examined by TTO application to banana plants challenged by Fusarium oxysporum f. sp. cubense (Foc, Race 1) causing Fusarium wilt and to tomato plants challenged by Xanthomonas campestris. Parameters to assess resistance induction included: disease development, enzymatic activity, defense genes expression correlated to systemic acquired resistance (SAR) and induced systemic resistance (ISR) and priming effect. Spraying TTO on field-grown banana plants infected with Foc and greenhouse tomato plants infected with Xanthomonas campestris led to resistance induction in both hosts. Several marker genes of salicylic acid, jasmonic acid and ethylene pathways were significantly up-regulated in parallel with symptoms reduction. For tomato plants, we have also recorded a priming effect following TTO treatment. In addition to fungicidal and bactericidal effect, TTO can be applied in more sustainable strategies to control diseases by enhancing the plants ability to defend themselves against pathogens and ultimately diminish chemical pesticides applications.

Overexpression of Arabidopsis microRNA167 induces salicylic acid-dependent defense against Pseudomonas syringae through the regulation of its targets ARF6 and ARF8

microRNAs are powerful regulators of growth, development, and stress responses in plants. The Arabidopsis thaliana microRNA miR167 was previously found to regulate diverse processes including flower development, root development, and response to osmotic stress by controlling the patterns of expression of its target genes AUXIN RESPONSE FACTOR 6 (ARF6), ARF8, and IAA-Ala RESISTANT 3. Here, we report that miR167 also modulates defense against pathogens through ARF6 and ARF8. miR167 is differentially expressed in response to the bacterial pathogen Pseudomonas syringae, and overexpression of miR167 confers very high levels of resistance. This resistance appears to be due to suppression of auxin responses and is partially dependent upon salicylic acid signaling, and also depends upon altered stomatal behavior in these plants. Closure of stomata upon the detection of P. syringae is an important aspect of the basal defense response, as it prevents bacterial cells from entering the leaf interior and causing infection. Plants overexpressing miR167 constitutively maintain small stomatal apertures, resulting in very high resistance when the pathogen is inoculated onto the leaf surface. Additionally, the systemic acquired resistance (SAR) response is severely compromised in plants overexpressing miR167, in agreement with previous work showing that the activation of SAR requires intact auxin signaling responses. This work highlights a new role for miR167, and also emphasizes the importance of hormonal balance in short- and long-term defense and of stomata as an initial barrier to pathogen entry.

Post-flowering biotic and abiotic stresses impact nitrogen use efficiency and seed filling in Arabidopsis thaliana

Nitrogen (N) is an essential nutrient that plants require for the synthesis of amino acids, proteins, and many other important metabolites. Plant metabolism and growth are consequently dependent on the amount of N that is assimilated and distributed from source leaves to developing sinks, such as fruits and seeds. The environmental stresses enhanced by climate change deeply influence seed yield and seed composition, and may disturb N use efficiency (NUE) in pants. We aimed to investigate plant responses to extreme climates with regard to NUE, N remobilization efficiency, and seed composition. By studying a collection of Arabidopsis genotypes showing a range of C:N ratios in seeds, we investigated the impact of different post-flowering growth conditions (control, heat, drought, low nitrate availability, induced senescence, and induced plant defense) on seed yield, N allocation in organs, NUE, and N remobilization efficiency. We analysed how post-flowering stresses could change seed filling and showed that post-flowering stresses change both the range of N and C concentrations and the C:N stoichiometry in seeds. Using a new trait, called delta seed composition, we measured the deviation in C:N stoichiometry of each genotype and revealed the genetic determinism of the C:N stoichiometry. Altogether, the results indicate that extreme climate impacts NUE dramatically in plants and generates different bottlenecks in N fluxes during seed filling.

Identification of QTLs for resistance to leaf spots in cultivated peanut (Arachis hypogaea L.) through GWAS analysis

Two QTLs on ChrB09 significantly associated with both early and late leaf spots were identified by genome-wide association study in the US peanut mini-core collection. Early leaf spot (ELS) and late leaf spot (LLS) are two serious peanut diseases in the USA, causing tens of millions of dollars of annual economic losses. However, the genetic factors underlying resistance to those diseases in peanuts have not been well-studied. We conducted a genome-wide association study for the two peanut diseases using Affymetrix version 2.0 SNP array with 120 genotypes mainly coming from the US peanut mini-core collection. A total of 46 quantitative trait loci (QTLs) were identified with phenotypic variation explained (PVE) from 10.19 to 24.11%, in which eighteen QTLs are for resistance to ELS and 28 QTLs for LLS. Among the 46 QTLs, there were four and two major QTLs with PVE higher than 16.99% for resistance ELS and LLS, respectively. Of the six major QTLs, five were located on the B sub-genome and only one was on the A sub-genome, which suggested that the B sub-genome has more potential resistance genomic regions than the A sub-genome. In addition, two genomic regions on chromosome B09 were found to provide significant resistance to both ELS and LLS. A total of 74 non-redundant genes were identified as resistance genes, among which, twelve candidate genes were in significant genomic regions including two candidate genes for both ELS and LLS, and other ten candidate genes for ELS. The QTLs and candidate genes obtained from this study will be useful to breed peanuts for resistances to the diseases.

Endoplasmic reticulum-mediated unfolded protein response is an integral part of singlet oxygen signalling in plants

Singlet oxygen (¹ O₂ ) is a by-product of photosynthesis that triggers a signalling pathway leading to stress acclimation or to cell death. By analyzing gene expressions in a ¹ O₂ -overproducing Arabidopsis mutant (ch1) under different light regimes, we show here that the ¹ O₂ signalling pathway involves the endoplasmic reticulum (ER)-mediated unfolded protein response (UPR). ch1 plants in low light exhibited a moderate activation of UPR genes, in particular bZIP60, and low concentrations of the UPR-inducer tunicamycin enhanced tolerance to photooxidative stress, together suggesting a role for UPR in plant acclimation to low ¹ O₂ levels. Exposure of ch1 to high light stress ultimately leading to cell death resulted in a marked upregulation of the two UPR branches (bZIP60/IRE1 and bZIP28/bZIP17). Accordingly, mutational suppression of bZIP60 and bZIP28 increased plant phototolerance, and a strong UPR activation by high tunicamycin concentrations promoted high light-induced cell death. Conversely, light acclimation of ch1 to ¹ O₂ stress put a limitation in the high light-induced expression of UPR genes, except for the gene encoding the BIP3 chaperone, which was selectively upregulated. BIP3 deletion enhanced Arabidopsis photosensitivity while plants treated with a chemical chaperone exhibited enhanced phototolerance. In conclusion, ¹ O₂ induces the ER-mediated UPR response that fulfils a dual role in high light stress: a moderate UPR, with selective induction of BIP3, is part of the acclimatory response to ¹ O₂ , and a strong activation of the whole UPR is associated with cell death.

Crossing the kingdom border: Human diseases caused by plant pathogens

Interactions between pathogenic microorganisms and their hosts are varied and complex, encompassing open-field scale interactions to interactions at the molecular level. The capacity of plant pathogenic bacteria and fungi to cause diseases in human and animal systems was, until recently, considered of minor importance. However, recent evidence suggests that animal and human infections caused by plant pathogenic fungi, bacteria and viruses may have critical impacts on human and animal health and safety. This review analyses previous research on plant pathogens as causal factors of animal illness. In addition, a case study involving disruption of type III effector-mediated phagocytosis in a human cell line upon infection with an opportunistic phytopathogen, Pseudomonas syringae pv. tomato, is discussed. Further knowledge regarding the molecular interactions between plant pathogens and human and animal hosts is needed to understand the extent of disease incidence and determine mechanisms for disease prevention.

A novel cysteine-rich receptor-like kinase gene, TaCRK2, contributes to leaf rust resistance in wheat

Leaf rust, caused by Puccinia triticina, is one of the most destructive fungal diseases in wheat production worldwide. The hypersensitive reaction (HR) is an important defence response against P. triticina infection. In this study, the physiological races 165 and 260 of P. triticina were combined with a line derived from the bread wheat cultivar Thatcher with the leaf rust resistance locus Lr26 to form compatible and incompatible combinations, respectively. Based on an RNA-Seq database of the interaction systems, a new wheat cysteine-rich receptor-like kinase gene, TaCRK2, is specifically induced and up-regulated in the incompatible combination. We identified that TaCRK2 was regulated in a Ca²⁺ -dependent manner. Knockdown of TaCRK2 by virus-induced gene silencing and RNAi leads to a dramatic increase in HR area and the number of haustorial mother cells at the single infection site. In addition, urediniospores, a P. triticina-specific pathogenic marker in compatible combinations, were observed on leaf surfaces of silenced plants at approximately 15 days after inoculation in the incompatible combination. Moreover, transcription levels of TaPR1, TaPR2, and TaPR5 were obviously reduced in TaCRK2-silenced plants. TaCRK2 overexpression in Nicotiana benthamiana induced strong HR-like cell death. Finally, transient expression of green fluorescent protein fused with TaCRK2 in N. benthamiana indicated that TaCRK2 localizes in the endoplasmic reticulum. Thus, TaCRK2 plays an important role in the resistance to P. triticina infection and has a positive regulation effect on the HR cell death process induced by P. triticina.

Toxicity of Recombinant Necrosis and Ethylene-Inducing Proteins (NLPs) from Neofusicoccum parvum

Neofusicoccum parvum is a fungal pathogen associated with a wide range of plant hosts. Despite being widely studied, the molecular mechanism of infection of N. parvum is still far from being understood. Analysis of N. parvum genome lead to the identification of six putative genes encoding necrosis and ethylene-inducing proteins (NLPs). The sequence of NLPs genes (NprvNep 1-6) were analyzed and four of the six NLP genes were successfully cloned, expressed in E. coli and purified by affinity chromatography. Pure recombinant proteins were characterized according to their phytotoxic and cytotoxic effects to tomato leaves and to mammalian Vero cells, respectively. These assays revealed that all NprvNeps tested are cytotoxic to Vero cells and also induce cell death in tomato leaves. NprvNep2 was the most toxic to Vero cells, followed by NprvNep1 and 3. NprvNep4 induced weaker, but, nevertheless, still significant toxic effects to Vero cells. A similar trend of toxicity was observed in tomato leaves: the most toxic was NprvNep 2 and the least toxic NprvNep 4. This study describes for the first time an overview of the NLP gene family of N. parvum and provides additional insights into its pathogenicity mechanism.

Why is sea buckthorn (Hippophae rhamnoides L.) so exceptional? A review

Sea buckthorn (Hippophae L.) is a valuable, multipurpose plant extensively grown in Asia, Europe and Canada. In order to use it in the best way for products of human nutrition, it is necessary to recognize its positive aspects and to eliminate the negative ones. The exceptional value of sea buckthorn can be seen in the presence of both lipophilic antioxidants (mainly carotenoids and tocopherols) and hydrophilic antioxidants (flavonoids, tannins, phenolic acids, ascorbic acid) in remarkably high quantities. Some of the main nutrients, especially lipids of advantageous fatty acid composition, contribute to nutritional benefits of sea buckthorn products for a consumer as well. This review article focuses, besides the above mentioned compounds and vitamins, also on other important components, such as sugars, sugar derivatives, fibre, organic acids, proteins, amino acids and mineral elements. The article also deals with the effects of sea buckthorn components on the course of non-enzymatic browning of food and in vivo glycation. In addition, sensory perception of sea buckthorn and its constituents from the consumers point of view is discussed.

A temporal gene expression map of Chrysanthemum leaves infected with Alternaria alternata reveals different stages of defense mechanisms

Chrysanthemum (Chrysanthemum morifolium) black spot disease (CBS) poses a major threat to Chrysanthemum cultivation owing to suitable climate conditions and current lack of resistant cultivars for greenhouse cultivation. In this study, we identified a number of genes that respond to Alternaria alternata infection in resistant and susceptible Chrysanthemum cultivars. Based on RNA sequencing technology and a weighted gene coexpression network analysis (WGCNA), we constructed a model to elucidate the response of Chrysanthemum leaves to A. alternata infection at different stages and compared the mapped response of the resistant cultivar 'Jinba' to that of the susceptible cultivar 'Zaoyihong'. In the early stage of infection, when lesions had not yet formed, abscisic acid (ABA), salicylic acid (SA) and EDS1-mediated resistance played important roles in the Chrysanthemum defense system. With the formation of necrotic lesions, ethylene (ET) metabolism and the Ca2+ signal transduction pathway strongly responded to A. alternata infection. During the late stage, when necrotic lesions continued to expand, members of the multidrug and toxic compound extrusion (MATE) gene family were highly expressed, and their products may be involved in defense against A. alternata invasion by exporting toxins produced by the pathogen, which plays important roles in the pathogenicity of A. alternata. Furthermore, the function of hub genes was verified by qPCR and transgenic assays. The identification of hub genes at different stages, the comparison of hub genes between the two cultivars and the highly expressed genes in the resistant cultivar 'Jinba' provide a theoretical basis for breeding cultivars resistant to CBS.

Cloning, characterization, in vitro and in planta expression of a necrosis-inducing Phytophthora protein 1 gene npp1 from Phytophthora cinnamomi

The soil-borne oomycete Phytophthora cinnamomi is a highly destructive Phytophthora species associated with the decline of forest. This pathogen secretes a novel class of necrosis-inducing proteins known as Nep1-like proteins (NLPs). In this work, we report the sequencing and molecular characterization of one of these proteins, more specifically the necrosis-inducing Phytophthora protein 1 (NPP1). The ORF of the npp1 gene (EMBL database AM403130) has 768 bp encoding a putative peptide of 256 amino acids with a molecular weight of approximately 25 kD. In order to understand its function, in vitro gene expression was studied during growth in different carbon sources (glucose, cellulose, and sawdust), and at different times of infection, in vivo by RT-qPCR. The highest expression of the npp1 gene occurred in glucose medium followed by sawdust. In vivo infection of Castanea sativa roots with P. cinnamomi revealed a decrease in npp1 expression from 12 to 24 h; at 36 h its expression increased suggesting the existence of a complex mechanism of defense/attack interaction between the pathogen and the host. Expression of recombinant npp1 gene was achieved in Pichia pastoris and assessed by SDS-PAGE analysis of the protein secreted into the culture supernatant, revealing the presence of the NPP1 protein.

Molecular Characterization of the Transcription Factors in Susceptible Poplar Infected with Virulent Melampsora larici-populina

Transcription factors (TFs) have been shown to play important roles in determining poplar susceptibility. In this study, the transcript profiles of five resistance-related TF groups at different time points were investigated to study the roles of TFs in the compatible interaction between 'Robusta' (Populus nigra × P. deltoides) and the virulent E4 race of Melampsora larici-populina. The susceptibility test indicated that the parasitic process of E4 could be divided into two representative time periods: the infection phase and the production phase. Bioinformatics analysis showed that in these two phases, E4 infection induced a network of TFs in 'Robusta'. Although some TFs responded rapidly and positively, most TFs did not respond to E4, especially during the infection phase. The ethylene, jasmonic acid, and auxin pathways were downregulated, while a calcium-binding protein was upregulated. No other significantly changed phytohormone-related genes were found, which was consistent with the pathological process in the absence of an immune response, suggesting that the lack of response of most TFs during the infection phase of E4 is related to the susceptibility of 'Robusta'.

Structural and biochemical studies of an NB-ARC domain from a plant NLR immune receptor

Plant NLRs are modular immune receptors that trigger rapid cell death in response to attempted infection by pathogens. A highly conserved nucleotide-binding domain shared with APAF-1, various R-proteins and CED-4 (NB-ARC domain) is proposed to act as a molecular switch, cycling between ADP (repressed) and ATP (active) bound forms. Studies of plant NLR NB-ARC domains have revealed functional similarities to mammalian homologues, and provided insight into potential mechanisms of regulation. However, further advances have been limited by difficulties in obtaining sufficient yields of protein suitable for structural and biochemical techniques. From protein expression screens in Escherichia coli and Sf9 insect cells, we defined suitable conditions to produce the NB-ARC domain from the tomato NLR NRC1. Biophysical analyses of this domain showed it is a folded, soluble protein. Structural studies revealed the NRC1 NB-ARC domain had co-purified with ADP, and confirmed predicted structural similarities between plant NLR NB-ARC domains and their mammalian homologues.

Nitric oxide and ROS mediate autophagy and regulate Alternaria alternata toxin-induced cell death in tobacco BY-2 cells

Synergistic interaction of nitric oxide (NO) and reactive oxygen species (ROS) is essential to initiate cell death mechanisms in plants. Though autophagy is salient in either restricting or promoting hypersensitivity response (HR)-related cell death, the crosstalk between the reactive intermediates and autophagy during hypersensitivity response is paradoxical. In this investigation, the consequences of Alternaria alternata toxin (AaT) in tobacco BY-2 cells were examined. At 3 h, AaT perturbed intracellular ROS homeostasis, altered antioxidant enzyme activities, triggered mitochondrial depolarization and induced autophagy. Suppression of autophagy by 3-Methyladenine caused a decline in cell viability in AaT treated cells, which indicated the vital role of autophagy in cell survival. After 24 h, AaT facilitated Ca2+ influx with an accumulation of reactive oxidant intermediates and NO, to manifest necrotic cell death. Inhibition of NO accumulation by 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) decreased the level of necrotic cell death, and induced autophagy, which suggests NO accumulation represses autophagy and facilitates necrotic cell death at 24 h. Application of N-acetyl-L-cysteine at 3 h, confirmed ROS to be the key initiator of autophagy, and together with cPTIO for 24 h, revealed the combined effects of NO and ROS is required for necrotic HR cell death.

Oxidative stress caused by Basagran® herbicide is altered by salicylic acid treatments in peanut plants

The present work was to study a protective role of salicylic acid (SA) on oxidative stress caused by Basagran® herbicide application. Two peanut cultivars (Arachis hypogaea cv. Giza 5 and Giza 6) with different sensitivities to the herbicide were monitored for their antioxidant responses to Basagran® and/or SA treatments. Two weeks after treatment, Basagran® lowered leaf pigments (Chlorophyll a, Chlorophyll b and total Carotenoids) but increased hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents indicating occurrence of lipid peroxidation and oxidative stress. Salicylic acid applied prior to low dose Basagran® lowered H2O2 and MDA contents in both G5 and G6. Except for SOD which is highly stimulated, POD, CAT and APX activities showed slight changes compared to control in leaves treated with Basagran® ± SA. The extracts tested by DPPH showed increase in total antioxidant activity by 4%-7% in SA + Basagran® treated leaves compared to control. The increased total antioxidant activity was related to the accumulation of amounts of phenolics as a protective action stimulated by SA. Alterations of antioxidant enzymatic system, accumulation of phenolics, increasing the total antioxidant activity by SA provide an evidence of protective action of SA in Basagran® detoxification.

PRRs and NB-LRRs: From Signal Perception to Activation of Plant Innate Immunity

To ward off pathogens and pests, plants use a sophisticated immune system. They use pattern-recognition receptors (PRRs), as well as nucleotide-binding and leucine-rich repeat (NB-LRR) domains, for detecting nonindigenous molecular signatures from pathogens. Plant PRRs induce local and systemic immunity. Plasma-membrane-localized PRRs are the main components of multiprotein complexes having additional transmembrane and cytosolic kinases. Topical research involving proteins and their interactive partners, along with transcriptional and posttranscriptional regulation, has extended our understanding of R-gene-mediated plant immunity. The unique LRR domain conformation helps in the best utilization of a surface area and essentially mediates protein–protein interactions. Genome-wide analyses of inter- and intraspecies PRRs and NB-LRRs offer innovative information about their working and evolution. We reviewed plant immune responses with relevance to PRRs and NB-LRRs. This article focuses on the significant functional diversity, pathogen-recognition mechanisms, and subcellular compartmentalization of plant PRRs and NB-LRRs. We highlight the potential biotechnological application of PRRs and NB-LRRs to enhance broad-spectrum disease resistance in crops.

TIR-NB-LRR immune receptor SOC3 pairs with truncated TIR-NB protein CHS1 or TN2 to monitor the homeostasis of E3 ligase SAUL1

Intracellular nucleotide binding (NB) and leucine-rich repeat (NLR) proteins function as immune receptors to recognize effectors from pathogens. They often guard host proteins that are the direct targets of those effectors. Recent findings have revealed that a typical NLR sometimes cooperates with another atypical NLR for effector recognition. Here, by using the CRISPR/Cas9 gene editing method, knockout analysis and biochemical assays, we uncovered differential pairings of typical Toll Interleukin1 receptor (TIR) type NLR (TNL) receptor SOC3 with atypical truncated TIR-NB (TN) proteins CHS1 or TN2 to guard the homeostasis of the E3 ligase SAUL1. Overaccumulation of SAUL1 is monitored by the SOC3-TN2 pair, while SAUL1's disappearance is guarded by the SOC3-CHS1 pair. SOC3 forms a head-to-head genomic arrangement with CHS1 and TN2, indicative of transcriptional co-regulation. Such intricate cooperative interactions can probably enlarge the recognition spectrum and increase the functional flexibility of NLRs, which can partly explain the overwhelming occurrence of NLR gene clustering in higher plants.

Genome-wide characterization revealed role of NBS-LRR genes during powdery mildew infection in Vitis vinifera

NBS-LRR comprises a large class of disease resistance (R) proteins that play a widespread role in plant protection against pathogens. In grapevine, powdery mildew cause significant losses in its productivity and efforts are being directed towards finding of resistance loci or genes imparting resistance/tolerance against such fungal diseases. In the present study, we performed genome-wide analysis of NBS-LRR genes during PM infection in grapevine. We identified 18, 23, 12, 16, 10, 10, 9, 20 and 14 differentially expressed NBS-LRR genes in response to PM infection in seven partially PM-resistant (DVIT3351.27, Husseine, Karadzhandal, Khalchili, Late vavilov, O34-16, Sochal) and 2 PM-susceptible (Carignan and Thompson seedless) V. vinifera accessions. Further, the identified sequences were characterized based on chromosomal locations, physicochemical properties, gene structure and motif analysis, and functional annotation by Gene Ontology (GO) mapping. The NBS-LRR genes responsive to powdery mildew could potentially be exploited to improve resistance in grapes.

Alterations in plant sugar metabolism: signatory of pathogen attack

This review summarizes the current understanding, future challenges and ongoing quest on sugar metabolic alterations that influence the outcome of plant-pathogen interactions. Intricate cellular and molecular events occur during plant-pathogen interactions. They cause major metabolic perturbations in the host and alterations in sugar metabolism play a pivotal role in governing the outcome of various kinds of plant-pathogen interactions. Sugar metabolizing enzymes and transporters of both host and pathogen origin get differentially regulated during the interactions. Both plant and pathogen compete for utilizing the host sugar metabolic machinery and in turn promote resistant or susceptible responses. However, the kind of sugar metabolism alteration that is beneficial for the host or pathogen is yet to be properly understood. Recently developed tools and methodologies are facilitating research to understand the intricate dynamics of sugar metabolism during the interactions. The present review elaborates current understanding, future challenges and ongoing quest on sugar metabolism, mobilization and regulation during various plant-pathogen interactions.

Comparative Analysis of Impatiens Leaf Transcriptomes Reveal Candidate Genes for Resistance to Downy Mildew Caused by Plasmopara obducens

Impatiens downy mildew (IDM) is a devastating disease to garden impatiens. A good understanding of IDM resistance in New Guinea impatiens is essential for improving garden impatiens resistance to this disease. The present study was conducted to sequence, assemble, annotate and compare the leaf transcriptomes of two impatiens cultivars differing in resistance to IDM, reveal sequence polymorphisms and identify candidate genes for IDM resistance. RNA-Seq was performed on cultivars Super Elfin® XP Pink (SEP) and SunPatiens® Compact Royal Magenta (SPR). De novo assembly of obtained sequence reads resulted in 121,497 unigenes with an average length of 1156 nucleotides and N50 length of 1778 nucleotides. Searching the non-redundant protein and non-redundant nucleotide, Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes and Clusters of Orthologous Groups and Gene Ontology databases, resulted in annotation of 57.7% to 73.6% of the unigenes. Fifteen unigenes were highly similar to disease resistance genes and more abundant in the IDM-resistant cultivar than in the susceptible cultivar. A total of 22,484 simple sequence repeats (SSRs) and 245,936 and 120,073 single nucleotide polymorphisms (SNPs) were identified from SPR and SEP respectively. The assembled transcripts and unigenes, identified disease resistance genes and SSRs and SNPs sites will be a valuable resource for improving impatiens and its IDM resistance.

Partnering With a Pest: Genomes of Hemlock Woolly Adelgid Symbionts Reveal Atypical Nutritional Provisioning Patterns in Dual-Obligate Bacteria

Nutritional bacterial symbionts enhance the diets of sap-feeding insects with amino acids and vitamins missing from their diets. In many lineages, an ancestral senior symbiont is joined by a younger junior symbiont. To date, an emergent pattern is that senior symbionts supply a majority of amino acids, and junior symbionts supply a minority. Similar to other hemipterans, adelgids harbor obligate symbionts, but have higher diversity of bacterial associates, suggesting a history of symbiont turnover. The metabolic roles of dual symbionts in adelgids and their contributions to the consortium are largely unexplored. Here, we investigate the symbionts of Adelges tsugae, the hemlock woolly adelgid (HWA), an invasive species introduced from Japan to the eastern United States, where it kills hemlock trees. The response of hemlocks to HWA feeding has aspects of a defensive reaction against pathogens, and some have speculated that symbionts may be involved. We sequenced the genomes of "Ca. Annandia adelgestsuga" and "Ca. Pseudomonas adelgestsugas" symbionts to detail their metabolic capabilities, infer ages of relationship, and search for effectors of plant defenses. We also tested the relationship of "Ca. Annandia" to symbionts of other insects. We find that both symbionts provide nutrients, but in more balanced proportions than dual symbionts of other hemipterans. The lesser contributions of the senior "Ca. Annandia" support our hypothesis for symbiont replacements in adelgids. Phylogenomic results were ambiguous regarding the position of "Ca. Annandia". We found no obvious effectors of plant defenses related to insect virulence, but hypothetical proteins in symbionts are unknown players.

AtOZF1 Positively Regulates Defense Against Bacterial Pathogens and NPR1-Independent Salicylic Acid Signaling

Plant hormone salicylic acid (SA) plays critical roles in defense signaling against biotrophic pathogens. Pathogen inoculation leads to SA accumulation in plants. SA activates a transactivator protein NPR1, which, in turn, transcriptionally activates many defense response genes. Reports also suggest the presence of NPR1-independent pathways for SA signaling in Arabidopsis. Here, we report the characterization of a zinc-finger protein-coding gene AtOZF1 that positively influences NPR1-independent SA signaling. Mutants of AtOZF1 are compromised, whereas AtOZF1-overexpressing plants are hyperactive for defense against virulent and avirulent pathogens. AtOZF1 expression is SA-inducible. AtOZF1 function is not required for pathogenesis-associated biosynthesis and accumulation of SA. However, it is required for SA responsiveness. By generating atozf1npr1 double mutant, we show that contributions of these two genes are additive in terms of defense. We identified AtOZF1-interacting proteins by a yeast-two-hybrid screening of an Arabidopsis cDNA library. VDAC2 and NHL3 are two AtOZF1-interacting proteins, which are positive regulators of basal defense. AtOZF1 interacts with NHL3 and VDAC2 in plasma membrane and mitochondria, respectively. Our results demonstrate that AtOZF1 coordinates multiple steps of plant-pathogen interaction.

RETRACTED: WRKY71 and TGA1a physically interact and synergistically regulate the activity of a novel promoter isolated from Petunia vein-clearing virus

Caulimoviral promoters have become excellent tools for efficient transgene expression in plants. However, the transcriptional framework controlling their systematic regulation is poorly understood. To understand this regulatory mechanism, we extensively studied a novel caulimoviral promoter, PV8 (-163 to +138, 301 bp), isolated from Petunia vein-clearing virus (PVCV). PVCV was found to be Salicylic acid (SA)-inducible and 2.5-3.0 times stronger than the widely used CaMV35S promoter. In silico analysis of the PV8 sequence revealed a unique clustering of two stress-responsive cis-elements, namely, as-11 and W-box1-2, located within a span of 31 bp (-74 to -47) that bound to the TGA1a and WRKY71 plant transcription factors (TFs), respectively. We found that as-1 (TTACG) and W-box (TGAC) elements occupied both TGA1a and WRKY71 on the PV8 backbone. Mutational studies demonstrated that the combinatorial influence of as-1 (-57) and W-box1-2 (-74 and -47) on the PV8 promoter sequence largely modulated its activity. TGA1a and WRKY71 physically interacted and cooperatively enhanced the transcriptional activity of the PV8 promoter. Biotic stress stimuli induced PV8 promoter activity by ~1.5 times. We also established the possible pathogen-elicitor function of AtWRKY71 and NtabWRKY71 TFs. Altogether, this study elucidates the interplay between TFs, biotic stress and caulimoviral promoter function.