Broad taxonomic characterization of Verticillium wilt resistance genes reveals an ancient origin of the tomato Ve1 immune receptor

Distribution of Three Verticillium dahliae Races in Coastal California and Evaluation of Resistance in Lettuce

Verticillium wilt, caused by Verticillium dahliae, is one of the most devastating soilborne diseases of lettuce (Lactuca sativa L.). There are three races of V. dahliae, and each race has been characterized by markers representing race-specific effectors. Race 1 is differentiated by the presence of the functional secretory Ave1 effector. Similarly, races 2 and 3 are differentiated by effectors VdR2e and VdR3e, respectively. Although the presence of race 1 in coastal California was well established, the presence of effector-based races 2 and 3 was uncertain. This study therefore focused on characterizing 727 isolates collected from 142 ranches of symptomatic lettuce and other crops from coastal California. Based on this evaluation, 523 isolates were designated as race 1, 20 isolates as race 2, 23 isolates as race 3, and 17 as race undefined. Isolates representing other Verticillium species totaled 110, and 34 were non-Verticillium fungal species. Because the use of resistant cultivars is a key strategy to manage this disease, we evaluated 48 lettuce germplasm lines and 1 endive (Cichorium endivia L.) line, comprising commercial cultivars and breeding lines, including the race 1-resistant heirloom cultivar La Brillante and the susceptible cultivar Salinas as controls. Resistance against races 1, 2, and 3 along with VdLs17, a virulent isolate of V. dahliae from lettuce that is currently not assigned to a race, was evaluated in replicated greenhouse experiments. Two crisphead lettuce lines, HL28 and HL29, exhibited resistance against race 1 and a partial resistance against race 2, whereas all other lines were highly susceptible to races 1 and 2 and VdLs17. The majority of lines exhibited higher resistance to race 3 relative to the other two races. This study documents the current distribution of the different races in coastal California. In addition, the sources of resistance currently being developed should be effective or partially effective against these races for targeted deployment as soon as they are available.

Pinus thunbergii Parl. Somatic Plants' Resistance to Bursaphelenchus xylophilus Depends on Pathogen-Induced Differential Transcriptomic Responses

Pinus thunbergii Parl. is an economically and medicinally important plant, as well as a world-renowned horticultural species of the Pinus genus. Pine wilt disease is a dangerous condition that affects P. thunbergii. However, understanding of the genetics underlying resistance to this disease is poor. Our findings reveal that P. thunbergii's resistance mechanism is based on differential transcriptome responses generated by the early presence of the pathogen Bursaphelenchus xylophilus, also known as the pine wood nematode. A transcriptome analysis (RNA-seq) was performed to examine gene expression in shoot tissues from resistant and susceptible P. thunbergii trees. RNA samples were collected from the shoots of inoculated pines throughout the infection phases by the virulent Bursaphelenchus xylophilus AMA3 strain. The photosynthesis and plant-pathogen interaction pathways were significantly enriched in the first and third days after infection. Flavonoid biosynthesis was induced in response to late infestation (7 and 14 days post-infestation). Calmodulin, RBOH, HLC protein, RPS, PR1, and genes implicated in phytohormone crosstalk (e.g., SGT1, MYC2, PP2C, and ERF1) showed significant alterations between resistant and susceptible trees. Furthermore, salicylic acid was found to aid pine wood nematodes tolerate adverse conditions and boost reproduction, which may be significant for pine wood nematode colonization within pines. These findings provide new insights into how host defenses overcame pine wood nematode infection in the early stage, which could potentially contribute to the development of novel strategies for the control of pine wilt disease.

In silico search for and analysis of R gene variation in primitive cultivated potato species

Pathogen recognition receptors encoded by R genes play a key role in plant protection. Nowadays, R genes are a basis for breeding many crops, including potato. Many potato R genes have been discovered and found suitable for breeding thanks to the studies of a wide variety of wild potato species. The use of primitive cultivated potato species (PCPS) as representatives of the primary gene pool can also be promising in this respect. PCPS are the closest to the early domesticated forms of potato; therefore, their investigation could help understand the evolution of R genes. The present study was aimed at identifying and analyzing R genes in PCPS listed in the open database of NCBI and Solomics DB. In total, the study involved 27 accessions belonging to three species: Solanum phureja Juz. & Bukasov, S. stenotomum Juz. & Bukasov and S. goniocalyx Juz. & Bukasov Materials for the analysis were the sequencing data for the said three species from the PRJNA394943 and PRJCA006011 projects. An in silico search was carried out for sequences homologous to 26 R genes identified in potato species differing in phylogenetic distance from PCPS, namely nightshade (S. americanum), North- (S. bulbocastanum, S. demissum) and South-American (S. venturii, S. berthaultii) wild potato species, as well as the cultivated potato species S. tuberosum and S. andigenum. Homologs of all investigated protein-coding sequences were discovered in PCPS with a relatively high degree of similarity (85-100 %). Homologs of the Rpi-R3b, Rpi-amr3 and Rpi-ber1 genes have been identified in PCPS for the first time. An analysis of polymorphism of nucleotide and amino acid sequences has been carried out for 15 R genes. The differences in frequencies of substitutions in PCPS have been demonstrated by analysis of R genes, the reference sequences of which have been identified in different species. For all the studied NBS-LRR genes, the proportion of substituted amino acids in the LRR domain exceeds this figure for the NBS domain. The potential prospects of using PCPS as sources of resistance to Verticillium wilt have been shown.

Interplay between cell-surface receptor and intracellular NLR-mediated immune responses

The functional link between cell-surface receptors and intracellular NLR immune receptors is a critical aspect of plant immunity. To establish disease, successful pathogens have evolved mechanisms to suppress cell-surface immune signalling. In response, plants have adapted by evolving NLRs that recognize pathogen effectors involved in this suppression, thereby counteracting their immune-suppressing function. This ongoing co-evolutionary struggle has seemingly resulted in intertwined signalling pathways in some plant species, where NLRs form a separate signalling branch downstream of activated cell-surface receptor complexes essential for full immunity. Understanding these interconnected receptor networks could lead to novel strategies for developing durable disease resistance.

Functional Characterization of Verticillium dahliae Race 3-Specific Gene VdR3e in Virulence and Elicitation of Plant Immune Responses

Verticillium dahliae is a soilborne fungal pathogen that causes disease on many economically important crops. Based on the resistance or susceptibility of differential cultivars in tomato, isolates of V. dahliae are divided into three races. Avirulence (avr) genes within the genomes of the three races have also been identified. However, the functional role of the avr gene in race 3 isolates of V. dahliae has not been characterized. In this study, bioinformatics analysis showed that VdR3e, a cysteine-rich secreted protein encoded by the gene characterizing race 3 in V. dahliae, was likely obtained by horizontal gene transfer from the fungal genus Bipolaris. We demonstrate that VdR3e causes cell death by triggering multiple defense responses. In addition, VdR3e localized at the periphery of the plant cell and triggered immunity depending on its subcellular localization and the cell membrane receptor BAK1. Furthermore, VdR3e is a virulence factor and shows differential pathogenicity in race 3-resistant and -susceptible hosts. These results suggest that VdR3e is a virulence factor that can also interact with BAK1 as a pathogen-associated molecular pattern (PAMP) to trigger immune responses. IMPORTANCE Based on the gene-for-gene model, research on the function of avirulence genes and resistance genes has had an unparalleled impact on breeding for resistance in most crops against individual pathogens. The soilborne fungal pathogen, Verticillium dahliae, is a major pathogen on many economically important crops. Currently, avr genes of the three races in V. dahliae have been identified, but the function of avr gene representing race 3 has not been described. We investigated the characteristics of VdR3e-mediated immunity and demonstrated that VdR3e acts as a PAMP to activate a variety of plant defense responses and induce plant cell death. We also demonstrated that the role of VdR3e in pathogenicity was host dependent. This is the first study to describe the immune and virulence functions of the avr gene from race 3 in V. dahliae, and we provide support for the identification of genes mediating resistance against race 3.

Revealing the contribution of GbPR10.5D1 to resistance against Verticillium dahliae and its regulation for structural defense and immune signaling

As an important family of pathogenesis-related (PR) proteins, the functional diversification and roles of PR10s in biotic stress have been well documented. However, the molecular basis of PR10s in plant defense responses against pathogens remains to be further understood. In the present study, we analyzed the phylogenetic relationship and function of a novel PR10 named GbPR10.5D1 in Sea-Island (or Pima or Egyptian) cotton (Gossypium barbadense L.), which has been identified as a Verticillium dahliae Kleb.-induced protein in a previous proteomics study. Phylogenetic analysis revealed that GbPR10.5D1, located on chromosome 2, is a unique member of GbPR10. The expression of GbPR10.5D1 was preferably in the root and induced upon V. dahliae infection. GbPR10.5D1 proteins were distributed in both nucleus and cytoplasm. GbPR10.5D1-virus-induced gene-silencing (VIGS) cotton plants were more susceptible to infection by V. dahliae, whereas overexpression (OE) of GbPR10.5D1 in cotton enhanced the resistance. By comparative transcriptome analysis between GbPR10.5D1-OE and wild-type (WT) plants and quantitative real-time polymerase chain reaction (qRT-PCR) verification, we found transcriptional activation of genes involved in cutin, suberine, and wax biosynthesis and mitogen-activated protein kinase (MAPK) signaling under normal conditions. Upon pathogen infection, defense signaling, fatty acid degradation, and glycerolipid metabolism were specifically activated in GbPR10.5D1-OE plants; biological processes (BPs), including glycolysis and gluconeogenesis, DNA replication, and cell wall organization, were specifically repressed in WT plants. Collectively, we proposed that GbPR10.5D1 possibly mediated lipid metabolism pathway to strengthen structural defense and activate defense signaling, which largely released the repression of cell growth caused by V. dahliae infection.

Chromosome-level genome assembly of Mentha longifolia L. reveals gene organization underlying disease resistance and essential oil traits

Mentha longifolia (L.) Huds., a wild, diploid mint species, has been developed as a model for mint genetic and genomic research to aid breeding efforts that target Verticillium wilt disease resistance and essential oil monoterpene composition. Here, we present a near-complete, chromosome-scale mint genome assembly for M. longifolia USDA accession CMEN 585. This new assembly is an update of a previously published genome draft, with dramatic improvements. A total of 42,107 protein-coding genes were annotated and placed on 12 chromosomal scaffolds. One hundred fifty-three genes contained conserved sequence domains consistent with nucleotide binding site-leucine-rich-repeat plant disease resistance genes. Homologs of genes implicated in Verticillium wilt resistance in other plant species were also identified. Multiple paralogs of genes putatively involved in p-menthane monoterpenoid biosynthesis were identified and several cases of gene clustering documented. Heterologous expression of candidate genes, purification of recombinant target proteins, and subsequent enzyme assays allowed us to identify the genes underlying the pathway that leads to the most abundant monoterpenoid volatiles. The bioinformatic and functional analyses presented here are laying the groundwork for using marker-assisted selection in improving disease resistance and essential oil traits in mints.

Dynamic Tissue—Specific Transcriptome Changes in Response to Verticillium dahliae in Wild Mint Species Mentha longifolia

Mentha longifolia is a wild mint species being used as a model to study the genetics of resistance to the fungal wilt pathogen Verticillium dahliae. We used high-throughput Illumina sequencing to study gene expression in response to V. dahliae inoculation in two M. longifolia USDA accessions with contrasting phenotypes: wilt-resistant CMEN 585 and wilt-susceptible CMEN 584. Roots and stems were sampled at two early post-inoculation time points, four hours and twenty-four hours, and again at ten days and twenty days post-inoculation. Overall, many more genes were differentially-regulated in wilt-resistant CMEN 585 than in wilt-susceptible CMEN 584. The greatest numbers of differentially expressed genes were found in the roots of CMEN 585 at the early time points. Specific genes exhibiting early, strong upregulation in roots of CMEN 585 but not in CMEN 584 included homologs of known plant defense response genes as well as genes involved in monoterpene biosynthesis. These genes were also upregulated in stems at the later time points. This study provides a comprehensive view of transcription reprogramming in Verticillium wilt-resistant mint, which will be the basis for further study and for molecular marker development.

Tomato Ve-resistance locus: resilience in the face of adversity?

The Ve-resistance locus in tomato acts as a resilience gene by affecting both the stress/defense cascade and growth, constituting a signaling intercept with a competitive regulatory mechanism. For decades, the tomato Ve-gene has been recognized as a classical resistance R-gene, inherited as a dominant Mendelian trait and encoding a receptor protein that binds with a fungal effector to provide defense against Verticillium dahliae and V. albo-atrum. However, recent molecular studies suggest that the function and role(s) of the Ve-locus and the two proteins that it encodes are more complex than previously understood. This review summarizes both the background and recent molecular evidence and provides a reinterpretation of the function and role(s) of the Ve1- and Ve2-genes and proteins that better accommodates existing data. It is proposed that these two plasma membrane proteins interact to form a signaling intercept that directly links defense and growth. The induction of Ve1 by infection or wounding promotes growth but also downregulates Ve2 signaling, resulting in a decreased biosynthesis of PR proteins. In this context, the Ve1 R-gene acts as a Resilience gene rather than a Resistance gene, promoting taller more robust tomato plants with reduced symptoms (biotic and abiotic) and Verticillium concentration.

Identification and Characterization of Verticillium nonalfalfae-Responsive MicroRNAs in the Roots of Resistant and Susceptible Hop Cultivars

MicroRNAs are 21- to 24-nucleotide-long, non-coding RNA molecules that regulate gene expression at the post-transcriptional level. They can modulate various biological processes, including plant response and resistance to fungal pathogens. Hops are grown for use in the brewing industry and, recently, also for the pharmaceutical industry. Severe Verticillium wilt caused by the phytopathogenic fungus Verticillium nonalfalfae, is the main factor in yield loss in many crops, including hops (Humulus lupulus L.). In our study, we identified 56 known and 43 novel miRNAs and their expression patterns in the roots of susceptible and resistant hop cultivars after inoculation with V. nonalfalfae. In response to inoculation with V. nonalfalfae, we found five known and two novel miRNAs that are differentially expressed in the susceptible cultivar and six known miRNAs in the resistant cultivar. Differentially expressed miRNAs target 49 transcripts involved in protein localization and pigment synthesis in the susceptible cultivar, whereas they are involved in transcription factor regulation and hormone signalling in the resistant cultivar. The results of our study suggest that the susceptible and resistant hop cultivars respond differently to V. nonalfalfae inoculation at the miRNA level and that miRNAs may contribute to the successful defence of the resistant cultivar.

Torres D, Usami T, Schouten HJ, Bai Y, Seidl MF, Thomma BPHJ. Comparative genomics reveals the in planta-secreted Verticillium dahliae Av2 effector protein recognized in tomato plants that carry the V2 resistance locus

Plant pathogens secrete effector molecules during host invasion to promote colonization. However, some of these effectors become recognized by host receptors to mount a defence response and establish immunity. Recently, a novel resistance was identified in wild tomato, mediated by the single dominant V2 locus, to control strains of the soil-borne vascular wilt fungus Verticillium dahliae that belong to race 2. With comparative genomics of race 2 strains and resistance-breaking race 3 strains, we identified the avirulence effector that activates V2 resistance, termed Av2. We identified 277 kb of race 2-specific sequence comprising only two genes encoding predicted secreted proteins that are expressed during tomato colonization. Subsequent functional analysis based on genetic complementation into race 3 isolates and targeted deletion from the race 1 isolate JR2 and race 2 isolate TO22 confirmed that one of the two candidates encodes the avirulence effector Av2 that is recognized in V2 tomato plants. Two Av2 allelic variants were identified that encode Av2 variants that differ by a single acid. Thus far, a role in virulence could not be demonstrated for either of the two variants.

Identification of Tomato Ve1 Homologous Proteins in Flax and Assessment for Race-Specific Resistance in Two Fiber FlaxCultivars against Verticillium dahliae Race 1

In the last decade, the soil borne fungal pathogen Verticillium dahliae has had an increasingly strong effect on fiber flax (Linum usitatissimum L.), thus causing important yield losses in Normandy, France. Race-specific resistance against V. dahliae race 1 is determined by tomato Ve1, a leucine-rich repeat (LRR) receptor-like protein (RLP). Furthermore, homologous proteins have been found in various plant families. Herein, four homologs of tomato Ve1 were identified in the flax proteome database. The selected proteins were named LuVe11, LuVe12, LuVe13 and LuVe14 and were compared to other Ve1. Sequence alignments and phylogenic analysis were conducted and detected a high similarity in the content of amino acids and that of the Verticillium spp. race 1 resistance protein cluster. Annotations on the primary structure of these homologs reveal several features of tomato Ve1, including numerous copies of a 28 amino acids consensus motif [XXIXNLXXLXXLXLSXNXLSGXIP] in the LRR domain. An in vivo assay was performed using V. dahliae race 1 on susceptible and tolerant fiber flax cultivars. Despite the presence of homologous genes and the stronger expression of LuVe11 compared to controls, both cultivars exhibited symptoms and the pathogen was observed within the stem. Amino acid substitutions within the segments of the LRR domain could likely affect the ligand binding and thus the race-specific resistance. The results of this study indicate that complex approaches including pathogenicity tests, microscopic observations and gene expression should be implemented for assessing race-specific resistance mediated by Ve1 within the large collection of flax genotypes.

Regulatory Network of Cotton Genes in Response to Salt, Drought and Wilt Diseases (Verticillium and Fusarium): Progress and Perspective

In environmental conditions, crop plants are extremely affected by multiple abiotic stresses including salinity, drought, heat, and cold, as well as several biotic stresses such as pests and pathogens. However, salinity, drought, and wilt diseases (e.g., Fusarium and Verticillium) are considered the most destructive environmental stresses to cotton plants. These cause severe growth interruption and yield loss of cotton. Since cotton crops are central contributors to total worldwide fiber production, and also important for oilseed crops, it is essential to improve stress tolerant cultivars to secure future sustainable crop production under adverse environments. Plants have evolved complex mechanisms to respond and acclimate to adverse stress conditions at both physiological and molecular levels. Recent progresses in molecular genetics have delivered new insights into the regulatory network system of plant genes, which generally includes defense of cell membranes and proteins, signaling cascades and transcriptional control, and ion uptake and transport and their relevant biochemical pathways and signal factors. In this review, we mainly summarize recent progress concerning several resistance-related genes of cotton plants in response to abiotic (salt and drought) and biotic (Fusarium and Verticillium wilt) stresses and classify them according to their molecular functions to better understand the genetic network. Moreover, this review proposes that studies of stress related genes will advance the security of cotton yield and production under a changing climate and that these genes should be incorporated in the development of cotton tolerant to salt, drought, and fungal wilt diseases (Verticillium and Fusarium).

Comparative Genome Analyses of 18 Verticillium dahliae Tomato Isolates Reveals Phylogenetic and Race Specific Signatures

Host resistance is one of the few strategies available to combat the soil borne pathogenic fungus Verticillium dahliae. Understanding pathogen diversity in populations is key to successfully deploying host resistance. In this study the genomes of 18 V. dahliae isolates of races 1 (n = 2), 2 (n = 4), and 3 (n = 12) from Japan, California, and North Carolina were sequenced and mapped to the reference genome of JR2 (from tomato). The genomes were analyzed for phylogenetic and pathogen specific signatures to classify specific strains or genes for future research. Four highly clonal lineages/groups were discovered, including a lineage unique to North Carolina isolates, which had the rare MAT1-1 mating type. No evidence for recombination between isolates of different mating types was observed, even in isolates of different mating types discovered in the same field. By mapping these 18 isolates genomes to the JR2 reference genome, 193 unique candidate effectors were found using SignalP and EffectorP. Within these effectors, 144 highly conserved effectors, 42 mutable effectors (truncated or present in some isolates but absent in others), and 7 effectors present in highly variable regions of the chromosomes were discovered. Of the 144 core effectors, 21 were highly conserved in V. alfalfae and V. longisporum, 7 of which have no known function. Within the non-core effectors 30 contained large numbers of non-synonymous mutations, while 15 of them contained indels, frameshift mutations, or were present on highly variable regions of the chromosome. Two of these highly variable region effectors (HVREs) were only present in race 2 isolates, but not in race 3 isolates. The race 1 effector Ave1 was also present in a highly variable region. These data may suggest that these highly variable regions are enriched in race determinant genes, consistent with the two-speed genome hypothesis.

Hop Polyphenols in Relation to Verticillium Wilt Resistance and Their Antifungal Activity

(1) Background: Verticillium wilt (VW) of hop is a devastating disease caused by the soil-borne fungi Verticillium nonalfalfae and Verticillium dahliae. As suggested by quantitative trait locus (QTL) mapping and RNA-Seq analyses, the underlying molecular mechanisms of resistance in hop are complex, consisting of preformed and induced defense responses, including the synthesis of various phenolic compounds. (2) Methods: We determined the total polyphenolic content at two phenological stages in roots and stems of 14 hop varieties differing in VW resistance, examined the changes in the total polyphenols of VW resistant variety Wye Target (WT) and susceptible Celeia (CE) on infection with V. nonalfalfae, and assessed the antifungal activity of six commercial phenolic compounds and total polyphenolic extracts from roots and stems of VW resistant WT and susceptible CE on the growth of two different V. nonalfalfae hop pathotypes. (3) Results: Generally, total polyphenols were higher in roots than stems and increased with maturation of the hop. Before flowering, the majority of VW resistant varieties had a significantly higher content of total polyphenols in stems than susceptible varieties. At the symptomatic stage of VW disease, total polyphenols decreased in VW resistant WT and susceptible CE plants in both roots and stems. The antifungal activity of total polyphenolic extracts against V. nonalfalfae was higher in hop extracts from stems than those from roots. Among the tested phenolic compounds, only p-coumaric acid and tyrosol markedly restricted fungal growth. (4) Conclusions: Although the correlation between VW resistance and total polyphenols content is not straightforward, higher levels of total polyphenols in the stems of the majority of VW resistant hop varieties at early phenological stages probably contribute to fast and efficient activation of signaling pathways, leading to successful defense against V. nonalfalfae infection.

Overview of Biotic Stresses in Pepper (Capsicum spp.): Sources of Genetic Resistance, Molecular Breeding and Genomics

Pepper (Capsicum spp.) is one of the major vegetable crops grown worldwide largely appreciated for its economic importance and nutritional value. This crop belongs to the large Solanaceae family, which, among more than 90 genera and 2500 species of flowering plants, includes commercially important vegetables such as tomato and eggplant. The genus includes over 30 species, five of which (C. annuum, C. frutescens, C. chinense, C. baccatum, and C. pubescens) are domesticated and mainly grown for consumption as food and for non-food purposes (e.g., cosmetics). The main challenges for vegetable crop improvement are linked to the sustainable development of agriculture, food security, the growing consumers' demand for food. Furthermore, demographic trends and changes to climate require more efficient use of plant genetic resources in breeding programs. Increases in pepper consumption have been observed in the past 20 years, and for maintaining this trend, the development of new resistant and high yielding varieties is demanded. The range of pathogens afflicting peppers is very broad and includes fungi, viruses, bacteria, and insects. In this context, the large number of accessions of domesticated and wild species stored in the world seed banks represents a valuable resource for breeding in order to transfer traits related to resistance mechanisms to various biotic stresses. In the present review, we report comprehensive information on sources of resistance to a broad range of pathogens in pepper, revisiting the classical genetic studies and showing the contribution of genomics for the understanding of the molecular basis of resistance.

An Overview of the Molecular Genetics of Plant Resistance to the Verticillium Wilt Pathogen Verticillium dahliae

Verticillium dahliae is a soil-borne hemibiotrophic fungus that can lead to plant vascular disease and significant economic loss worldwide. Its hosts include over 400 dicotyledon plant species, such as annual herbs, perennials, and woody plants. The average yield loss of cotton crop caused by Verticillium wilt is approximately 10–35%. As the control of this disease is an urgent task for many countries, further understanding of the interaction between plants and V. dahliae is essential. Fungi can promote or inhibit plant growth, which is important; however, the most important relationship between plants and fungi is the host–pathogen relationship. Plants can become resistant to V. dahliae through diverse mechanisms such as cell wall modifications, extracellular enzymes, pattern recognition receptors, transcription factors, and salicylic acid (SA)/jasmonic acid (JA)/ethylene (ET)-related signal transduction pathways. Over the last decade, several studies on the physiological and molecular mechanisms of plant resistance to V. dahliae have been undertaken. In this review, many resistance-related genes are summarised to provide a theoretical basis for better understanding of the molecular genetic mechanisms of plant resistance to V. dahliae. Moreover, it is intended to serve as a resource for research focused on the development of genetic resistance mechanisms to combat Verticillium wilt.

Molecular breeding approaches for production of disease-resilient commercially important tobacco

Tobacco is one of the most widely cultivated nonfood cash crops, a source of income, model organism for plant molecular research, a natural pesticide and of pharmaceutical importance. First domesticated in South Americas, the modern-day tobacco (Nicotiana tabacum) is now cultivated in more than 125 countries to generate revenues worth billions of dollars each year. However, the production of this crop is highly threatened by the global presence of devastating infectious agents, which cause huge fiscal loss. These threats have been battled through breeding for acquiring disease resilience in tobacco plants, first, via conventional and now with the use of modern molecular breeding approaches. For efficacy and precision, the characterization of the genetic components underlying disease resistance is the key tool in tobacco for resistance breeding programs. The past few decades have witnessed significant progress in resilience breeding through advanced molecular techniques. The current review discusses history of tobacco breeding since its time of origin till date, highlighting the most widely used techniques and recent advances in molecular research and strategies for resistance breeding. In addition, we narrate the budding possibilities for the future. This review will provide a comprehensive and valuable information for the tobacco growers and researchers to deal with the destructive infectious diseases.

Temporal and spatial assessment of defence responses in resistant and susceptible hop cultivars during infection with Verticillium nonalfalfae

Hop (Humulus lupulus L.) is an important industrial plant providing ingredients for brewing and pharmaceutical industry worldwide. Its intensive production is challenged by numerous diseases. One of the most lethal and difficult to control is verticillium wilt, a vascular disease caused by the fungal pathogen Verticillium nonalfalfae. The disease can be successfully controlled by the host resistance. Despite various studies that already researched resistance mechanisms of hops, only limited number of resistance genes and markers that could be utilized for efficient resistance breeding has been identified. In this study we aimed to follow fungus colonization pattern and the differential expression of selected genes during pre-symptomatic period of susceptible (Celeia) and resistant (Wye Target) hop cultivars. Results of gene expressions and fungal colonisation of compatible and incompatible interactions with V. nonalfalfae suggest that the hop plant is challenged already at the very early fungal colonisation stages. In total, nine out of 17 gene targets investigated in our study resulted in differential expression between inoculated and control plants of susceptible and resistant cultivars. The difference was the most evident in stems at an early stage of colonisation (6 dpi), showing relatively stronger changes in targeted gene expression to infection in the resistant cultivar than in the susceptible one. Analysed gene targets are involved in the overall defence response processes of nucleic acid binding, signalling, protein ubiquitination, cell oxidative burst, hydroxylation, peroxidation, alternative splicing, and metabolite biosynthesis. The up-regulation of some genes (e.g. glycine-rich RNA-binding family protein, protein phosphatase, cysteine-rich receptor-like protein kinase, zinc finger CCCH domain-containing protein 40, cinnamic acid 4-hydroxylase, class III peroxidase, putative MAPK2, peroxiredoxin-2F) upon infection in incompatible interactions might reflect defence activation, restriction of disease spreading throughout the plant and successful response of resistant genotype.

The LsVe1L allele provides a molecular marker for resistance to Verticillium dahliae race 1 in lettuce

BACKGROUND

Verticillium wilt caused by the fungus Verticillium dahliae race 1 is among the top disease concerns for lettuce in the Salinas and Pajaro Valleys of coastal central California. Resistance of lettuce against V. dahliae race 1 was previously mapped to the single dominant Verticillium resistance 1 (Vr1) locus. Lines of tomato resistant to race 1 are known to contain the closely linked Ve1 and Ve2 genes that encode receptor-like proteins with extracellular leucine-rich repeats; the Ve1 and Ve2 proteins act antagonistically to provide resistance against V. dahliae race 1. The Vr1 locus in lettuce contains a cluster of several genes with sequence similarity to the tomato Ve genes. We used genome sequencing and/or PCR screening along with pathogenicity assays of 152 accessions of lettuce to investigate allelic diversity and its relationship to race 1 resistance in lettuce.

RESULTS

This approach identified a total of four Ve genes: LsVe1, LsVe2, LsVe3, and LsVe4. The majority of accessions, however, contained a combination of only three of these LsVe genes clustered on chromosomal linkage group 9 (within ~ 25 kb in the resistant cultivar La Brillante and within ~ 127 kb in the susceptible cultivar Salinas).

CONCLUSIONS

A single allele, LsVe1L, was present in all resistant accessions and absent in all susceptible accessions. This allele can be used as a molecular marker for V. dahliae race 1 resistance in lettuce. A PCR assay for rapid detection of race 1 resistance in lettuce was designed based on nucleotide polymorphisms. Application of this assay allows identification of resistant genotypes in early stages of plant development or at seed-level without time- and labor-intensive testing in the field.

Genetic Diversity of Verticillium dahliae Populations From Olive and Potato in Lebanon

Verticillium dahliae is widely distributed in potato and olive fields in Lebanon, causing serious economic losses. However, little is known about the inoculum source, population structure, and genetic diversity of the pathogen or the mechanisms of dissemination within Lebanon. To understand the population structure, a total of 203 isolates sampled from olive (n = 78) and potato (n = 125) were characterized for species, mating type, and race, and the genetic relationships were delineated using 13 microsatellite markers. All isolates except one from potato were V. dahliae, with 55.1 and 12.1% race 1, and 43.6 and 83.1% race 2 in olive and potato, respectively. The genetic structure of the studied population was best described by two large and two small clusters. Membership in the two large clusters was determined by the presence or absence of the effector gene Ave1. Furthermore, genetic structure was moderately associated with the host of origin but was weakly associated with the geographic origin. All but four isolates represented by three multilocus haploid genotypes were MAT1-2. This study identified a clear lack of gene flow between virulence genotypes of V. dahliae despite the proximity of these cropping systems and the wide distribution of genetic diversity among hosts and geographic regions in Lebanon.

The dynamic transcriptome and metabolomics profiling in Verticillium dahliae inoculated Arabidopsis thaliana

Verticillium wilt caused by the soil-borne fungus Verticillium dahliae is a common, devastating plant vascular disease notorious for causing economic losses. Despite considerable research on plant resistance genes, there has been little progress in modeling the effects of this fungus owing to its complicated pathogenesis. Here, we analyzed the transcriptional and metabolic responses of Arabidopsis thaliana to V. dahliae inoculation by Illumina-based RNA sequencing (RNA-seq) and nuclear magnetic resonance (NMR) spectroscopy. We identified 13,916 differentially expressed genes (DEGs) in infected compared with mock-treated plants. Gene ontology analysis yielded 11,055 annotated DEGs, including 2,308 for response to stress and 2,234 for response to abiotic or biotic stimulus. Pathway classification revealed involvement of the metabolic, biosynthesis of secondary metabolites, plant–pathogen interaction, and plant hormone signal transduction pathways. In addition, 401 transcription factors, mainly in the MYB, bHLH, AP2-EREBP, NAC, and WRKY families, were up- or downregulated. NMR analysis found decreased tyrosine, asparagine, glutamate, glutamine, and arginine and increased alanine and threonine levels following inoculation, along with a significant increase in the glucosinolate sinigrin and a decrease in the flavonoid quercetin glycoside. Our data reveal corresponding changes in the global transcriptomic and metabolic profiles that provide insights into the complex gene-regulatory networks mediating the plant’s response to V. dahliae infection.

The island cotton NBS-LRR gene GbaNA1 confers resistance to the non-race 1 Verticillium dahliae isolate Vd991

Wilt caused by Verticillium dahliae significantly reduces cotton yields, as host resistance in commercially cultivated Gossypium species is lacking. Understanding the molecular basis of disease resistance in non-commercial Gossypium species could galvanize the development of Verticillium wilt resistance in cultivated species. Nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins play a central role in plant defence against pathogens. In this study, we focused on the relationship between a locus enriched with eight NBS-LRR genes and Verticillium wilt resistance in G. barbadense. Independent virus-induced gene silencing of each of the eight NBS-LRR genes in G. barbadense cultivar Hai 7124 revealed that silencing of GbaNA1 alone compromised the resistance of G. barbadense to V. dahliae isolate Vd991. In cultivar Hai 7124, GbaNA1 could be induced by V. dahliae isolate Vd991 and by ethylene, jasmonic acid and salicylic acid. Nuclear protein localization of GbaNA1 was demonstrated by transient expression. Sequencing of the GbaNA1 orthologue in nine G. hirsutum accessions revealed that all carried a non-functional allele, caused by a premature peptide truncation. In addition, all 10 G. barbadense and nine G. hirsutum accessions tested carried a full-length (∼1140 amino acids) homologue of the V. dahliae race 1 resistance gene Gbve1, although some sequence polymorphisms were observed. Verticillium dahliae Vd991 is a non-race 1 isolate that lacks the Ave1 gene. Thus, the resistance imparted by GbaNA1 appears to be mediated by a mechanism distinct from recognition of the fungal effector Ave1.

Vegetative compatibility groups partition variation in the virulence of Verticillium dahliae on strawberry

Verticillium dahliae infection of strawberry (Fragaria x ananassa) is a major cause of disease-induced wilting in soil-grown strawberries across the world. To understand what components of the pathogen are affecting disease expression, the presence of the known effector VdAve1 was screened in a sample of Verticillium dahliae isolates. Isolates from strawberry were found to contain VdAve1 and were divided into two major clades, based upon their vegetative compatibility groups (VCG); no UK strawberry isolates contained VdAve1. VC clade was strongly related to their virulence levels. VdAve1-containing isolates pathogenic on strawberry were found in both clades, in contrast to some recently published findings. On strawberry, VdAve1-containing isolates had significantly higher virulence during early infection, which diminished in significance as the infection progressed. Transformation of a virulent non-VdAve1 containing isolate, with VdAve1 was found neither to increase nor decrease virulence when inoculated on a susceptible strawberry cultivar. There are therefore virulence factors that are epistatic to VdAve1 and potentially multiple independent routes to high virulence on strawberry in V. dahliae lineages. Genome sequencing a subset of isolates across the two VCGs revealed that isolates were differentiated at the whole genome level and contained multiple changes in putative effector content, indicating that different clonal VCGs may have evolved different strategies for infecting strawberry, leading to different virulence levels in pathogenicity tests. It is therefore important to consider both clonal lineage and effector complement as the adaptive potential of each lineage will differ, even if they contain the same race determining effector.

Transfer of tomato immune receptor Ve1 confers Ave1-dependent Verticillium resistance in tobacco and cotton

Verticillium wilts caused by soilborne fungal species of the Verticillium genus are economically important plant diseases that affect a wide range of host plants and are notoriously difficult to combat. Perception of pathogen(-induced) ligands by plant immune receptors is a key component of plant innate immunity. In tomato, race-specific resistance to Verticillium wilt is governed by the cell surface-localized immune receptor Ve1 through recognition of the effector protein Ave1 that is secreted by race 1 strains of Verticillium spp. It was previously demonstrated that transgenic expression of tomato Ve1 in the model plant Arabidopsis thaliana leads to Verticillium wilt resistance. Here, we investigated whether tomato Ve1 can confer Verticillium resistance when expressed in the crop species tobacco (Nicotiana tabcum) and cotton (Gossypium hirsutum). We show that transgenic tobacco and cotton plants constitutively expressing tomato Ve1 exhibit enhanced resistance against Verticillium wilt in an Ave1-dependent manner. Thus, we demonstrate that the functionality of tomato Ve1 in Verticillium wilt resistance through recognition of the Verticillium effector Ave1 is retained after transfer to tobacco and cotton, implying that the Ve1-mediated immune signalling pathway is evolutionary conserved across these plant species. Moreover, our results suggest that transfer of tomato Ve1 across sexually incompatible plant species can be exploited in breeding programmes to engineer Verticillium wilt resistance.

Host-induced gene silencing compromises Verticillium wilt in tomato and Arabidopsis

Verticillium wilt, caused by soil-borne fungi of the genus Verticillium, is an economically important disease that affects a wide range of host plants. Unfortunately, host resistance against Verticillium wilts is not available for many plant species, and the disease is notoriously difficult to combat. Host-induced gene silencing (HIGS) is an RNA interference (RNAi)-based process in which small RNAs are produced by the host plant to target parasite transcripts. HIGS has emerged as a promising strategy for the improvement of plant resistance against pathogens by silencing genes that are essential for these pathogens. Here, we assessed whether HIGS can be utilized to suppress Verticillium wilt disease by silencing three previously identified virulence genes of V. dahliae (encoding Ave1, Sge1 and NLP1) through the host plants tomato and Arabidopsis. In transient assays, tomato plants were agroinfiltrated with Tobacco rattle virus (TRV) constructs to target V. dahliae transcripts. Subsequent V. dahliae inoculation revealed the suppression of Verticillium wilt disease on treatment with only one of the three TRV constructs. Next, expression of RNAi constructs targeting transcripts of the same three V. dahliae virulence genes was pursued in stable transgenic Arabidopsis thaliana plants. In this host, V. dahliae inoculation revealed reduced Verticillium wilt disease in two of the three targets. Thus, our study suggests that, depending on the target gene chosen, HIGS against V. dahliae is operational in tomato and A. thaliana plants and may be exploited to engineer resistance in Verticillium wilt-susceptible crops.

Tolerance of olive (Olea europaea) cv Frantoio to Verticillium dahliae relies on both basal and pathogen-induced differential transcriptomic responses

Verticillium wilt of olive (VWO) is one of the most serious biotic constraints for this tree crop. Our knowledge of the genetics of the tolerance/resistance to this disease is very limited. Here we show that tolerance of the cv Frantoio relies on both basal and early pathogen-induced differential transcriptomic responses. A comparative transcriptomic analysis (RNA-seq) was conducted in root tissues of cvs Frantoio (VWO-tolerant) and Picual (VWO-susceptible). RNA samples originated from roots of inoculated olive plants during the early infection stages by Verticillium dahliae (highly virulent, defoliating pathotype). A huge number of differentially expressed genes (DEGs) were found between 'Frantoio' and 'Picual' (27 312 unigenes) in the absence of the pathogen. Upon infection with V. dahliae, 'Picual' and 'Frantoio' plants responded differently too. In the early infection stages, four clusters of DEGs could be identified according to their time-course expression patterns. Among others, a pathogenesis-related protein of the Bet v I family and a dirigent-like protein involved in lignification, and several BAK1, NHL1, reactive oxygen species stress response and BAM unigenes showed noticeable differences between cultivars. Tolerance of 'Frantoio' plants to VWO is a consequence of a complex and multifaceted process which involves many plant traits.

Comparative transcriptional analysis of hop responses to infection with Verticillium nonalfalfae

KEY MESSAGE

Dynamic transcriptome profiling revealed excessive, yet ineffective, immune response to V. nonalfalfae infection in susceptible hop, global gene downregulation in shoots of resistant hop and only a few infection-associated genes in roots. Hop (Humulus lupulus L.) production is hampered by Verticillium wilt, a disease predominantly caused by the soil-borne fungus Verticillium nonalfalfae. Only a few hop cultivars exhibit resistance towards it and mechanisms of this resistance have not been discovered. In this study, we compared global transcriptional responses in roots and shoots of resistant and susceptible hop plants infected by a lethal strain of V. nonalfalfae. Time-series differential gene expression profiles between infected and mock inoculated plants were determined and subjected to network-based analysis of functional enrichment. In the resistant hop cultivar, a remarkably low number of genes were differentially expressed in roots in response to V. nonalfalfae infection, while the majority of differentially expressed genes were down-regulated in shoots. The most significantly affected genes were related to cutin biosynthesis, cell wall biogenesis, lateral root development and terpenoid biosynthesis. On the other hand, susceptible hop exhibited a strong defence response in shoots and roots, including increased expression of genes associated with plant responses, such as innate immunity, wounding, jasmonic acid pathway and chitinase activity. Strong induction of defence-associated genes in susceptible hop and a low number of infection-responsive genes in the roots of resistant hop are consistent with previous findings, confirming the pattern of excessive response of the susceptible cultivar, which ultimately fails to protect the plant from V. nonalfalfae. This research offers a multifaceted overview of transcriptional responses of susceptible and resistant hop cultivars to V. nonalfalfae infection and represents a valuable resource in the study of this plant-pathogen interaction.

Verticillium dahliae LysM effectors differentially contribute to virulence on plant hosts

Chitin-binding lysin motif (LysM) effectors contribute to the virulence of various plant-pathogenic fungi that are causal agents of foliar diseases. Here, we report the LysM effectors of the soil-borne fungal vascular wilt pathogen Verticillium dahliae. Comparative genomics revealed three core LysM effectors that are conserved in a collection of V. dahliae strains. Remarkably, and in contrast with the previously studied LysM effectors of other plant pathogens, no expression of core LysM effectors was monitored in planta in a taxonomically diverse panel of host plants. Moreover, targeted deletion of the individual LysM effector genes in V. dahliae strain JR2 did not compromise virulence in infections on Arabidopsis, tomato or Nicotiana benthamiana. Interestingly, an additional lineage-specific LysM effector is encoded in the genome of V. dahliae strain VdLs17, but not in any other V. dahliae strain sequenced to date. Remarkably, this lineage-specific effector is expressed in planta and contributes to the virulence of V. dahliae strain VdLs17 on tomato, but not on Arabidopsis or N. benthamiana. Functional analysis revealed that this LysM effector binds chitin, is able to suppress chitin-induced immune responses and protects fungal hyphae against hydrolysis by plant hydrolytic enzymes. Thus, in contrast with the core LysM effectors of V. dahliae, this lineage-specific LysM effector of strain VdLs17 contributes to virulence in planta.

Large-scale transcriptome comparison of sunflower genes responsive to Verticillium dahliae

BACKGROUND

Sunflower Verticillium wilt (SVW) is a vascular disease caused by root infection with Verticillium dahliae (V. dahlia). It is a serious threat to the yield and quality of sunflower. However, chemical and agronomic measures for controlling this disease are not effective. The selection of more resistant genotypes is a desirable strategy to reduce contamination. A deeper knowledge of the molecular mechanisms and genetic basis underlying sunflower Verticillium wilt is necessary to accelerate breeding progress.

RESULTS

An RNA-Seq approach was used to perform global transcriptome profiling on the roots of resistant (S18) and susceptible (P77) sunflower genotypes infected with V. dahlia. Different pairwise transcriptome comparisons were examined over a time course (6, 12 and 24 h, and 2, 3, 5 and 10 d post inoculation). In RD, SD and D datasets, 1231 genes were associated with SVW resistance in a genotype-common transcriptional pattern. Moreover, 759 and 511 genes were directly related to SVW resistance in the resistant and susceptible genotypes, respectively, in a genotype-specific transcriptional pattern. Most of the genes were demonstrated to participate in plant defense responses; these genes included peroxidase (POD), glutathione peroxidase, aquaporin PIP, chitinase, L-ascorbate oxidase, and LRR receptors. For the up-regulated genotype-specific differentially expressed genes (DEGs) in the resistant genotype, higher average fold-changes were observed in the resistant genotype compared to those in the susceptible genotype. An inverse effect was observed in the down-regulated genotype-specific DEGs in the resistant genotype. KEGG analyses showed that 98, 112 and 52 genes were classified into plant hormone signal transduction, plant-pathogen interaction and flavonoid biosynthesis categories, respectively. Many of these genes, such as CNGC, RBOH, FLS2, JAZ, MYC2 NPR1 and TGA, regulate crucial points in defense-related pathway and may contribute to V. dahliae resistance in sunflower.

CONCLUSIONS

The transcriptome profiling results provided a clearer understanding of the transcripts associated with the crosstalk between sunflower and V. dahliae. The results identified several differentially expressed unigenes involved in the hyper sensitive response (HR) and the salicylic acid (SA)/jasmonic acid (JA)-mediated signal transduction pathway for resistance against V. dahliae. These results are useful for screening resistant sunflower genotypes.

Transposons passively and actively contribute to evolution of the two-speed genome of a fungal pathogen

Genomic plasticity enables adaptation to changing environments, which is especially relevant for pathogens that engage in “arms races” with their hosts. In many pathogens, genes mediating virulence cluster in highly variable, transposon-rich, physically distinct genomic compartments. However, understanding of the evolution of these compartments, and the role of transposons therein, remains limited. Here, we show that transposons are the major driving force for adaptive genome evolution in the fungal plant pathogen Verticillium dahliae. We show that highly variable lineage-specific (LS) regions evolved by genomic rearrangements that are mediated by erroneous double-strand repair, often utilizing transposons. We furthermore show that recent genetic duplications are enhanced in LS regions, against an older episode of duplication events. Finally, LS regions are enriched in active transposons, which contribute to local genome plasticity. Thus, we provide evidence for genome shaping by transposons, both in an active and passive manner, which impacts the evolution of pathogen virulence.