Genome wide transcriptome profiling of Fusarium oxysporum f sp. ciceris conidial germination reveals new insights into infection-related genes

Small RNA sequencing analysis provides novel insights into microRNA-mediated regulation of defense responses in chickpea against Fusarium wilt infection

MAIN CONCLUSION

Small RNA sequencing analysis in two chickpea genotypes, JG 62 (Fusarium wilt-susceptible) and WR 315 (Fusarium wilt-resistant), under Fusarium wilt stress led to identification of 544 miRNAs which included 406 known and 138 novel miRNAs. A total of 115 miRNAs showed differential expression in both the genotypes across different combinations. A miRNA, Car-miR398 targeted copper chaperone for superoxide dismutase (CCS) that, in turn, regulated superoxide dismutase (SOD) activity during chickpea-Foc interaction. Fusarium wilt (FW) of chickpea (Cicer arietinum L.) caused by Fusarium oxysporum f. sp. ciceris (Foc) is a destructive soil-borne disease that severely reduces the chickpea yield and quality globally. In the present study, we have investigated microRNAs and the microRNA/target gene crosstalk involved in chickpea resistance to FW. The control and stress samples from two genotypes, JG 62 (FW-susceptible) and WR 315 (FW-resistant), collected at 10 days post-inoculation (dpi), were selected for small RNA sequencing. A total of 12 libraries were constructed and sequenced using Illumina HiSeq 2500 platform. The sequencing and in silico analyses revealed the identification of 544 miRNAs which included 406 known and 138 novel miRNAs. A total of 50 miRNAs were physically co-localized with Foc-resistance QTLs present on chromosome 2 (also known as Foc hotspot). A total of 115 miRNAs showed differential expression in both the genotypes across different combinations. Prediction and functional annotation of miRNA targets revealed their role in transcription regulation, disease resistance, defense response, metabolism, etc. Ten miRNAs and their targets were validated using poly(A)-based qRT-PCR in two genotypes grown under lab and field conditions. Many miRNAs and their targets showed genotype-specific expression. The expression profiling also highlighted, both, similar and different expression patterns for the same sets of miRNA and mRNA at different stages of Foc infection. A high correlation in expression patterns of the miRNAs and their targets in lab- and field-grown plant samples was observed. Interestingly, Car-miR398 targeted copper chaperone for superoxide dismutase (CCS) that, in turn, regulated superoxide dismutase (SOD) activity during chickpea-Foc interaction. The cleavage site in targets was mapped for three miRNAs by analyzing publicly available degradome data for chickpea. The study, for the first time, provides novel insights into microRNA-mediated regulation of resistance and susceptibility mechanisms in chickpea against FW and opens up avenues for the development of the wilt-resistant cultivars in chickpea.

Response of Fusarium oxysporum soil isolate to amphotericin B and fluconazole at the proteomic level

Fusarium oxysporum is a cross-kingdom pathogen that infects humans, animals, and plants. The primary concern regarding this genus revolves around its resistance profile to multiple classes of antifungals, particularly azoles. However, the resistance mechanism employed by Fusarium spp. is not fully understood, thus necessitating further studies to enhance our understanding and to guide future research towards identifying new drug targets. Here, we employed an untargeted proteomic approach to assess the differentially expressed proteins in a soil isolate of Fusarium oxysporum URM7401 cultivated in the presence of amphotericin B and fluconazole. In response to antifungals, URM7401 activated diverse interconnected pathways, such as proteins involved in oxidative stress response, proteolysis, and lipid metabolism. Efflux proteins, antioxidative enzymes and M35 metallopeptidase were highly expressed under amphotericin B exposure. Antioxidant proteins acting on toxic lipids, along with proteins involved in lipid metabolism, were expressed during fluconazole exposure. In summary, this work describes the protein profile of a resistant Fusarium oxysporum soil isolate exposed to medical antifungals, paving the way for further targeted research and discovering new drug targets.

RNA-seq-based transcriptome profiling of early fruit development in Chieh-qua and analysis of related transcription factors

Chieh-qua (Benincasa hispida Cogn. var. Chieh-qua How.) fruit development starts post pollination. With the continuous expansion of the fruit, the soluble solid content of the fruit decreases. Because there are no reports on the early development of Chieh-qua fruit, this study compared fruit transcriptomes at 0-, 3-, and 7 day post pollination (dpp). 104,747 unigenes were assembled from clean reads and compared using six public databases for similarity searching. Compared with those of 0 dpp (C), there were differences in the expression of 12,982 and 6541 genes in the fruit tissue at 3 dpp and 7 dpp, respectively. Compared with 3 dpp (B), there were 14,314 differentially expressed genes in the fruit at 7 dpp (A). Based on the analysis of transcription factors, 213 nucleotides in the MYB superfamily were identified; among them, 94 unigenes of the MYB superfamily were differentially expressed at the three stages. In the pairwise comparison of differential expression, eight unigenes (Gene_id: TRINITY_DN32880_c1_g2, TRINITY_DN35142_c2_g2, TRINITY_DN32454_c11_g6, TRINITY_DN34105_c2_g7, TRINITY_DN32758_c3_g3, TRINITY_DN33604_c4_g10, TRINITY_DN34466_c3_g1, TRINITY_DN35924_c3_g2) were homologous to those of MYB59, MYB-GT3b, MYB18, MYB4, MYB108, MYB306, MYB340, and MYB-bHLH13. These unigenes differed significantly among the three stages. Furthermore, MYB59 and MYB18 exhibited higher expression at 7 dpp. MYB4, MYB-GT3b, MYB108, and MYB306 showed the highest expression levels in fruits at 3 dpp. In addition, MYB340 and MYB-bHLH13 showed higher expression levels during the unpollinated stage. MYB59, MYB-GT3b, MYB18, MYB4, MYB108, MYB306, MYB340, and MYB-bHLH13 may play crucial roles in Chieh-qua fruit development, defense, and blossoming. This study provides a basis for further investigation of MYB superfamily genes involved in early fruit expansion in chieh-qua.

Shoot transcriptome revealed widespread differential expression and potential molecular mechanisms of chickpea (Cicer arietinum L.) against Fusarium wilt

Introduction

The yield of chickpea is severely hampered by infection wilt caused by several races of Fusarium oxysporum f. sp. ciceris (Foc).

Methods

To understand the underlying molecular mechanisms of resistance against Foc4 Fusarium wilt, RNA sequencing-based shoot transcriptome data of two contrasting chickpea genotypes, namely KWR 108 (resistant) and GL 13001 (susceptible), were generated and analyzed.

Results and Discussion

The shoot transcriptome data showed 1,103 and 1,221 significant DEGs in chickpea genotypes KWR 108 and GL 13001, respectively. Among these, 495 and 608 genes were significantly down and up-regulated in genotypes KWR 108, and 427 and 794 genes were significantly down and up-regulated in genotype GL 13001. The gene ontology (GO) analysis of significant DEGs was performed and the GO of the top 50 DEGs in two contrasting chickpea genotypes showed the highest cellular components as membrane and nucleus, and molecular functions including nucleotide binding, metal ion binding, transferase, kinase, and oxidoreductase activity involved in biological processes such as phosphorylation, oxidation-reduction, cell redox homeostasis process, and DNA repair. Compared to the susceptible genotype which showed significant up-regulation of genes involved in processes like DNA repair, the significantly up-regulated DEGs of the resistant genotypes were involved in processes like energy metabolism and environmental adaptation, particularly host-pathogen interaction. This indicates an efficient utilization of environmental adaptation pathways, energy homeostasis, and stable DNA molecules as the strategy to cope with Fusarium wilt infection in chickpea. The findings of the study will be useful in targeting the genes in designing gene-based markers for association mapping with the traits of interest in chickpea under Fusarium wilt which could be efficiently utilized in marker-assisted breeding of chickpea, particularly against Foc4 Fusarium wilt.

Mousa K, Istanbuli T, Abd El-Maksoud MM, Tawkaz S, Hamwieh A. Unveiling the genetic basis of Fusarium wilt resistance in chickpea using GWAS analysis and characterization of candidate genes

Introduction: Chickpea is a legume crop that thrives in regions with semi-arid or temperate climates. Its seeds are an excellent source of proteins, carbohydrates, and minerals, especially high-quality proteins. Chickpea cultivation faces several challenges including Fusarium wilt (FW), a major fungal disease that significantly reduces productivity. Methods: In this study, a Genome-wide Association Analysis (GWAS) was conducted to identify multiple genomic loci associated with FW resistance in chickpea. We conducted a comprehensive evaluation of 180 chickpea genotypes for FW resistance across three distinct locations (Ethiopia, Tunisia, and Lebanon) during the 2-year span from 2015 to 2016. Disease infection measurements were recorded, and the wilt incidence of each genotype was calculated. We employed a set of 11,979 single nucleotide polymorphisms (SNPs) markers distributed across the entire chickpea genome for SNP genotyping. Population structure analysis was conducted to determine the genetic structure of the genotypes. Results and Discussion: The population structure unveiled that the analyzed chickpea germplasm could be categorized into four sub-populations. Notably, these sub-populations displayed diverse geographic origins. The GWAS identified 11 SNPs associated with FW resistance, dispersed across the genome. Certain SNPs were consistent across trials, while others were specific to particular environments. Chromosome CA2 harbored five SNP markers, CA5 featured two, and CA4, CA6, CA7, and CA8 each had one representative marker. Four SNPs demonstrated an association with FW resistance, consistently observed across a minimum of three distinct environments. These SNPs included SNP5826041, SNP5825086, SNP11063413, SNP5825195, which located in CaFeSOD, CaS13like, CaNTAQ1, and CaAARS genes, respectively. Further investigations were conducted to gain insights into the functions of these genes and their role in FW resistance. This progress holds promise for reducing the negative impact of the disease on chickpea production.

Systematic assessment of the antifungal mechanism of soil fumigant methyl isothiocyanate against Fusarium oxysporum

Fusarium oxysporum is an important phytopathogenic fungus, it can be controlled by the soil fumigant methyl isothiocyanate (MITC). However, the antimicrobial mechanism of MITC against F. oxysporum, especially at the transcriptional level, is still unclear. In this experiment, the antimicrobial mechanism of MITC against F. oxysporum was investigated. Our results indicated that when F. oxysporum was exposed to 6 mg/L MITC for 12 h, the inhibitory rate of MITC on F. oxysporum was 80%. Transmission electron microscopes showed that the cell wall and membrane of F. oxysporum had shrunk and folded, vacuoles increased, and mitochondria swelled and deformed. In addition, the enzyme activity of F. oxysporum treated with MITC showed a decrease of 32.50%, 8.28% and 74.04% in catalase, peroxidase and superoxide dismutase, respectively. Transcriptome sequencing of F. oxysporum was performed and the results showed that 1478 differentially expressed genes (DEGs) were produced in response to MITC exposure. GO and KEGG analysis showed that the DEGs identified were involved in substance and energy metabolism, signal transduction, transport and catalysis. MITC disrupted cell homeostasis by influencing the expression of some key genes involved in chitin synthase and detoxification enzymes production, but F. oxysporum also protected itself by up-regulating genes involved in energy synthesis (such as upregulating acnA, CS and LSC2 in TCA). qRT-PCR data validated the reliability of transcriptome data. Our research used biochemical and genetic techniques to identify molecular lesions in the mycelia of F. oxysporum exposed to MITC, and provide valuable insights into the toxic mechanism of pathogenic fungi mediated by MITC. These techniques are also likely to be useful for rapidly screening and identifying new, environmentally-friendly soil fumigants that are efficacious against fungal pathogens.

Evolutionary history of the cytochrome P450s from Colletotrichum species and prediction of their putative functional roles during host-pathogen interactions

The genomes of species belonging to the genus Colletotrichum harbor a substantial number of cytochrome P450 monooxygenases (CYPs) encoded by a broad diversity of gene families. However, the biological role of their CYP complement (CYPome) has not been elucidated. Here, we investigated the putative evolutionary scenarios that occurred during the evolution of the CYPome belonging to the Colletotrichum Graminicola species complex (s.c.) and their biological implications. The study revealed that most of the CYPome gene families belonging to the Graminicola s.c. experienced gene contractions. The reductive evolution resulted in species restricted CYPs are predominant in each CYPome of members from the Graminicola s.c., whereas only 18 families are absolutely conserved among these species. However, members of CYP families displayed a notably different phylogenetic relationship at the tertiary structure level, suggesting a putative convergent evolution scenario. Most of the CYP enzymes of the Graminicola s.c. share redundant functions in secondary metabolite biosynthesis and xenobiotic metabolism. Hence, this current work suggests that the presence of a broad CYPome in the genus Colletotrichum plays a critical role in the optimization of the colonization capability and virulence.

Iron necessity for chlamydospore germination in Fusarium oxysporum f. sp. cubense TR4

Fusarium wilt disease of banana, caused by the notorious soil-borne pathogen Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), is extremely difficult to manage. Manipulation of soil pH or application of synthetic iron chelators can suppress the disease through iron starvation, which inhibits the germination of pathogen propagules called chlamydospores. However, the effect of iron starvation on chlamydospore germination is largely unknown. In this study, scanning electron microscopy was used to assemble the developmental sequence of chlamydospore germination and to assess the effect of iron starvation and pH in vitro. Germination occurs in three distinct phenotypic transitions (swelling, polarized growth, outgrowth). Outgrowth, characterized by formation of a single protrusion (germ tube), occurred at 2 to 3 h, and a maximum value of 69.3% to 76.7% outgrowth was observed at 8 to 10 h after germination induction. Germination exhibited plasticity with pH as over 60% of the chlamydospores formed a germ tube between pH 3 and pH 11. Iron-starved chlamydospores exhibited polarized-growth arrest, characterized by the inability to form a germ tube. Gene expression analysis of rnr1 and rnr2, which encode the iron-dependent enzyme ribonucleotide reductase, showed that rnr2 was upregulated (p < 0.0001) in iron-starved chlamydospores compared to the control. Collectively, these findings suggest that iron and extracellular pH are crucial for chlamydospore germination in Foc TR4. Moreover, inhibition of germination by iron starvation may be linked to a different mechanism, rather than repression of the function of ribonucleotide reductase, the enzyme that controls growth by regulation of DNA synthesis.

Transcriptome profiling reveals the expression and regulation of genes associated with Fusarium wilt resistance in chickpea (Cicer arietinum L.)

Fusarium wilt (FW) is one of the most significant biotic stresses limiting chickpea production worldwide. To dissect the molecular mechanism of FW resistance in chickpea, comparative transcriptome analyses of contrasting resistance sources of chickpea genotypes under control and Fusarium oxysporum f. sp. ciceris (Foc) inoculated conditions were performed. The high-throughput transcriptome sequencing generated about 1137 million sequencing reads from 24 samples representing two resistant genotypes, two susceptible genotypes, and two near-isogenic lines under control and stress conditions at two-time points (7th- and 12th-day post-inoculation). The analysis identified 5182 differentially expressed genes (DEGs) between different combinations of chickpea genotypes. Functional annotation of these genes indicated their involvement in various biological processes such as defense response, cell wall biogenesis, secondary metabolism, and disease resistance. A significant number (382) of transcription factor encoding genes exhibited differential expression patterns under stress. Further, a considerable number of the identified DEGs (287) co-localized with previously reported quantitative trait locus for FW resistance. Several resistance/susceptibility-related genes, such as SERINE/THREONINE PROTEIN KINASE, DIRIGENT, and MLO exhibiting contrasting expression patterns in resistant and susceptible genotypes upon Foc inoculation, were identified. The results presented in the study provide valuable insights into the transcriptional dynamics associated with FW stress response in chickpea and provide candidate genes for the development of disease-resistant chickpea cultivars.

Diagnosis of Fusarium oxysporum f. sp. ciceris causing Fusarium wilt of chickpea using loop-mediated isothermal amplification (LAMP) and conventional end-point PCR

Fusarium oxysporum (Fo) is ubiquitous in soil and forms a species complex of pathogenic and putatively non-pathogenic strains. Pathogenic strains cause disease in over 150 plant species. Fusarium oxysporum f. sp. ciceris (Foc) is a major fungal pathogen causing Fusarium wilt in chickpeas (Cicer arietinum). In some countries such as Australia, Foc is a high-priority pest of biosecurity concern. Specific, sensitive, robust and rapid diagnostic assays are essential for effective disease management on the farm and serve as an effective biosecurity control measure. We developed and validated a novel and highly specific PCR and a LAMP assay for detecting the Indian Foc race 1 based on a putative effector gene uniquely present in its genome. These assays were assessed against 39 Fo formae speciales and found to be specific, only amplifying the target species, in a portable real-time fluorometer (Genie III) and qPCR machine in under 13 min with an anneal derivative temperature ranging from 87.7 to 88.3 °C. The LAMP assay is sensitive to low levels of target DNA (> 0.009 ng/µl). The expected PCR product size is 143 bp. The LAMP assay developed in this study was simple, fast, sensitive and specific and could be explored for other Foc races due to the uniqueness of this marker to the Foc genome.

A cellular and molecular atlas reveals the basis of chytrid development

The chytrids (phylum Chytridiomycota) are a major fungal lineage of ecological and evolutionary importance. Despite their importance, many fundamental aspects of chytrid developmental and cell biology remain poorly understood. To address these knowledge gaps, we combined quantitative volume electron microscopy and comparative transcriptome profiling to create an 'atlas' of the cellular and molecular basis of the chytrid life cycle, using the model chytrid Rhizoclosmatium globosum. From our developmental atlas, we describe the transition from the transcriptionally inactive free-swimming zoospore to the more biologically complex germling, and show that lipid processing is multifaceted and dynamic throughout the life cycle. We demonstrate that the chytrid apophysis is a compartmentalised site of high intracellular trafficking, linking the feeding/attaching rhizoids to the reproductive zoosporangium, and constituting division of labour in the chytrid cell plan. We provide evidence that during zoosporogenesis, zoospores display amoeboid morphologies and exhibit endocytotic cargo transport from the interstitial maternal cytoplasm. Taken together, our results reveal insights into chytrid developmental biology and provide a basis for future investigations into non-dikaryan fungal cell biology.

The decrotonylase FoSir5 facilitates mitochondrial metabolic state switching in conidial germination of Fusarium oxysporum

Fusarium oxysporum is one of the most important pathogenic fungi with a broad range of plant and animal hosts. The first key step of its infection cycle is conidial germination, but there is limited information available on the molecular events supporting this process. We show here that germination is accompanied by a sharp decrease in expression of FoSir5, an ortholog of the human lysine deacetylase SIRT5. We observe that FoSir5 decrotonylates a subunit of the fungal pyruvate dehydrogenase complex (FoDLAT) at K148, resulting in inhibition of the activity of the complex in mitochondria. Moreover, FoSir5 decrotonylates histone H3K18, leading to a downregulation of transcripts encoding enzymes of aerobic respiration pathways. Thus, the activity of FoSir5 coordinates regulation in different organelles to steer metabolic flux through respiration. As ATP content is positively related to fungal germination, we propose that FoSir5 negatively modulates conidial germination in F. oxysporum through its metabolic impact. These findings provide insights into the multifaceted roles of decrotonylation, catalyzed by FoSir5, that support conidial germination in F. oxysporum.

Comparative transcriptomic analysis of races 1, 2, 5 and 6 of Fusarium oxysporum f.sp. pisi in a susceptible pea host identifies differential pathogenicity profiles

BACKGROUND

The fungal pathogen Fusarium oxysporum f.sp. pisi (Fop) causes Fusarium wilt in peas. There are four races globally: 1, 2, 5 and 6 and all of these races are present in Australia. Molecular infection mechanisms have been studied in a few other F. oxysporum formae speciales; however, there has been no transcriptomic Fop-pea pathosystem study.

RESULTS

A transcriptomic study was carried out to understand the molecular pathogenicity differences between the races. Transcriptome analysis at 20 days post-inoculation revealed differences in the differentially expressed genes (DEGs) in the Fop races potentially involved in fungal pathogenicity variations. Most of the DEGs in all the races were engaged in transportation, metabolism, oxidation-reduction, translation, biosynthetic processes, signal transduction, proteolysis, among others. Race 5 expressed the most virulence-associated genes. Most genes encoding for plant cell wall degrading enzymes, CAZymes and effector-like proteins were expressed in race 2. Race 6 expressed the least number of genes at this time point.

CONCLUSION

Fop races deploy various factors and complex strategies to mitigate host defences to facilitate colonisation. This investigation provides an overview of the putative pathogenicity genes in different Fop races during the necrotrophic stage of infection. These genes need to be functionally characterised to confirm their pathogenicity/virulence roles and the race-specific genes can be further explored for molecular characterisation.

Comparative Whole-Genome Sequence Analyses of Fusarium Wilt Pathogen (Foc R1, STR4 and TR4) Infecting Cavendish (AAA) Bananas in India, with a Special Emphasis on Pathogenicity Mechanisms

Fusarium wilt is caused by the fungus Fusarium oxysporum f. sp. cubense (Foc) and is the most serious disease affecting bananas (Musa spp.). The fungus is classified into Foc race 1 (R1), Foc race 2, and Foc race 4 based on host specificity. As the rate of spread and the ranges of the devastation of the Foc races exceed the centre of the banana’s origin, even in non-targeted cultivars, there is a possibility of variation in virulence-associated genes. Therefore, the present study investigates the genome assembly of Foc races that infect the Cavendish (AAA) banana group in India, specifically those of the vegetative compatibility group (VCG) 0124 (race 1), 0120 (subtropical race 4), and 01213/16 (tropical race 4). While comparing the general features of the genome sequences (e.g., RNAs, GO, SNPs, and InDels), the study also looked at transposable elements, phylogenetic relationships, and virulence-associated effector genes, and sought insights into race-specific molecular mechanisms of infection based on the presence of unique genes. The results of the analyses revealed variations in the organisation of genome assembly and virulence-associated genes, specifically secreted in xylem (SIX) genes, when compared to their respective reference genomes. The findings contributed to a better understanding of Indian Foc genomes, which will aid in the development of effective Fusarium wilt management techniques for various Foc VCGs in India and beyond.

Spore Germination of the Obligate Biotroph Spongospora subterranea: Transcriptome Analysis Reveals Germination Associated Genes

For soilborne pathogens, germination of the resting or dormant propagule that enables persistence within the soil environment is a key point in pathogenesis. Spongospora subterranea is an obligate soilborne protozoan that infects the roots and tubers of potato causing root and powdery scab disease for which there are currently no effective controls. A better understanding of the molecular basis of resting spore germination of S. subterranea could be important for development of novel disease interventions. However, as an obligate biotroph and soil dwelling organism, the application of new omics techniques for the study of the pre-infection process in S. subterranea has been problematic. Here, RNA sequencing was used to analyse the reprogramming of S. subterranea resting spores during the transition to zoospores in an in-vitro model. More than 63 million mean high-quality reads per sample were generated from the resting and germinating spores. By using a combination of reference-based and de novo transcriptome assembly, 6,664 unigenes were identified. The identified unigenes were subsequently annotated based on known proteins using BLAST search. Of 5,448 annotated genes, 570 genes were identified to be differentially expressed during the germination of S. subterranea resting spores, with most of the significant genes belonging to transcription and translation, amino acids biosynthesis, transport, energy metabolic processes, fatty acid metabolism, stress response and DNA repair. The datasets generated in this study provide a basic knowledge of the physiological processes associated with spore germination and will facilitate functional predictions of novel genes in S. subterranea and other plasmodiophorids. We introduce several candidate genes related to the germination of an obligate biotrophic soilborne pathogen which could be applied to the development of antimicrobial agents for soil inoculum management.

Historical genomics reveals the evolutionary mechanisms behind multiple outbreaks of the host-specific coffee wilt pathogen Fusarium xylarioides

BACKGROUND

Nearly 50% of crop yields are lost to pests and disease, with plants and pathogens locked in an amplified co-evolutionary process of disease outbreaks. Coffee wilt disease, caused by Fusarium xylarioides, decimated coffee production in west and central Africa following its initial outbreak in the 1920s. After successful management, it later re-emerged and by the 2000s comprised two separate epidemics on arabica coffee in Ethiopia and robusta coffee in east and central Africa.

RESULTS

Here, we use genome sequencing of six historical culture collection strains spanning 52 years to identify the evolutionary processes behind these repeated outbreaks. Phylogenomic reconstruction using 13,782 single copy orthologs shows that the robusta population arose from the initial outbreak, whilst the arabica population is a divergent sister clade to the other strains. A screen for putative effector genes involved in pathogenesis shows that the populations have diverged in gene content and sequence mainly by vertical processes within lineages. However, 15 putative effector genes show evidence of horizontal acquisition, with close homology to genes from F. oxysporum. Most occupy small regions of homology within wider scaffolds, whereas a cluster of four genes occupy a 20Kb scaffold with strong homology to a region on a mobile pathogenicity chromosome in F. oxysporum that houses known effector genes. Lacking a match to the whole mobile chromosome, we nonetheless found close associations with DNA transposons, especially the miniature impala type previously proposed to facilitate horizontal transfer of pathogenicity genes in F. oxysporum. These findings support a working hypothesis that the arabica and robusta populations partly acquired distinct effector genes via transposition-mediated horizontal transfer from F. oxysporum, which shares coffee as a host and lives on other plants intercropped with coffee.

CONCLUSION

Our results show how historical genomics can help reveal mechanisms that allow fungal pathogens to keep pace with our efforts to resist them. Our list of putative effector genes identifies possible future targets for fungal control. In turn, knowledge of horizontal transfer mechanisms and putative donor taxa might help to design future intercropping strategies that minimize the risk of transfer of effector genes between closely-related Fusarium taxa.

Temperature and Soil Moisture Stress Modulate the Host Defense Response in Chickpea During Dry Root Rot Incidence

Dry root rot caused by the necrotrophic phytopathogenic fungus Rhizoctonia bataticola is an emerging threat to chickpea production in India. In the near future, the expected increase in average temperature and inconsistent rainfall patterns resultant of changing climatic scenarios are strongly believed to exacerbate the disease to epidemic proportions. The present study aims to quantify the collective role of temperature and soil moisture content (SMC) on disease progression in chickpea under controlled environmental conditions. In our study, we could find that both temperature and soil moisture played a decisive role in influencing the dry root rot disease scenario. As per the disease susceptibility index (DSI), a combination of high temperature (35°C) and low SMC (60%) was found to elicit the highest disease susceptibility in chickpea. High pathogen colonization was realized in chickpea root tissue at all time-points irrespective of genotype, temperature, and SMC. Interestingly, this was in contrast to the DSI where no visible symptoms were recorded in the roots or foliage during the initial time-points. For each time-point, the colonization was slightly higher at 35°C than 25°C, while the same did not vary significantly with respect to SMC. Furthermore, the differential expression study revealed the involvement of host defense-related genes like endochitinase and PR-3-type chitinase (CHI III) genes in delaying the dry root rot (DRR) disease progression in chickpea. Such genes were found to be highly active during the early stages of infection especially under low SMC.

Breeding, Genetics, and Genomics Approaches for Improving Fusarium Wilt Resistance in Major Grain Legumes

Fusarium wilt (FW) disease is the key constraint to grain legume production worldwide. The projected climate change is likely to exacerbate the current scenario. Of the various plant protection measures, genetic improvement of the disease resistance of crop cultivars remains the most economic, straightforward and environmental-friendly option to mitigate the risk. We begin with a brief recap of the classical genetic efforts that provided first insights into the genetic determinants controlling plant response to different races of FW pathogen in grain legumes. Subsequent technological breakthroughs like sequencing technologies have enhanced our understanding of the genetic basis of both plant resistance and pathogenicity. We present noteworthy examples of targeted improvement of plant resistance using genomics-assisted approaches. In parallel, modern functional genomic tools like RNA-seq are playing a greater role in illuminating the various aspects of plant-pathogen interaction. Further, proteomics and metabolomics have also been leveraged in recent years to reveal molecular players and various signaling pathways and complex networks participating in host-pathogen interaction. Finally, we present a perspective on the challenges and limitations of high-throughput phenotyping and emerging breeding approaches to expeditiously develop FW-resistant cultivars under the changing climate.

Genome-wide mRNA and miRNA analysis in the early stages of germ tube outgrowth in Coprinopsis cinerea

Coprinopsis cinerea is a model mushroom-forming basidiomycete which produces basidiospores during sexual reproduction. This fungus is widely used to study fruiting body formation and development. Molecular mechanisms controlling its growth from vegetative mycelium to multicellular mature fruiting body have been studied extensively. However, little is known about the underlying biological processes during germ tube outgrowth or the transition from basidiospores to multinucleate hyphae. To better understand sexual spore germination in fungi, here we examined the time-dependent cellular events at resting, germinating and fully germinated basidiospores of C. cinerea by genome-wide transcriptional and post-transcriptional analyses and by carbohydrate composition analysis. Our results revealed a high demand of protein degradation, and biosynthesis of various compounds at the early stage of basidiospore gemination and dynamic changes of carbohydrate metabolism throughout the germination process. Seven microRNA-like RNAs (milRNAs) were identified in the resting basidiospores of C. cinerea, six of which were basidiospore-specific. Glycogen and trehalose were shown to be the carbon sources supporting the initiation of germ tube outgrowth. One basidiospore-specific milRNA, cci-milR-37, was found to be a potential regulator of glycogen metabolic pathways related to vegetative hyphal growth. Our results demonstrated the mRNA and miRNA-mediated regulation on energy production, protein and carbohydrate metabolisms at the early developmental stages of germ tube to form totipotent hyphae. To our knowledge, this is the first study to show the roles of miRNAs in mushroom basidiospore germination and out-growth.

Transcriptome Analysis Reveals the Effects of Chinese Chive (Allium tuberosum R.) Extract on Fusarium oxysporum f. sp. radicis-lycopersici Spore Germination

Fusarium oxysporum f. sp. radicis-lycopersici (Forl) causes Fusarium crown and root rot of tomato, leading to severe yield losses. Chinese chive and the Chinese chive extract reportedly have antifungal effects. In this study, Chinese chive extract treatments inhibited Forl spore germination, with an EC₅₀ of 0.40 g ml^-1 in vitro. Furthermore, the mechanism underlying the fungicidal effects of the Chinese chive extract was analyzed by RNA sequencing. A total of 1252 differentially expressed genes (DEGs) were detected, of which 396 were upregulated and 856 were downregulated. The DEGs were related to starch and sucrose metabolism, amino sugar and nucleotide sugar metabolism, galactose metabolism, fatty acid metabolism, sphingolipid metabolism, glycerophospholipid metabolism, peroxisomes, ribosome biogenesis in eukaryotes, mismatch repair, and the phosphatidylinositol signaling system, implying these pathways contribute to the fungicidal activity of the Chinese chive extract. The qRT-PCR results verified the accuracy of the RNA sequencing data. Thus, the Chinese chive extract can inhibit Forl spore germination by affecting spore nutrient metabolism.

Candidate genes expression profiling during wilting in chickpea caused by Fusarium oxysporum f. sp. ciceris race 5

Chickpea production may be seriously threatened by Fusarium wilt, a disease caused by the soil-borne fungus Fusarium oxysporum f. sp. ciceris. F. oxysporum race 5 is the most important race in the Mediterranean basin. Recently, the region responsible for resistance race 5 has been delimited within a region on chromosome 2 that spans 820 kb. To gain a better understanding of this genomic region, we used a transcriptomic approach based on quantitative real-time PCR to analyze the expression profiles of 22 selected candidate genes. We used a pair of near-isogenic lines (NILs) differing in their sensitivity to Fusarium race 5 (resistant vs susceptible) to monitor the transcriptional changes over a time-course experiment (24, 48, and 72 hours post inoculation, hpi). Qualitative differences occurred during the timing of regulation. A cluster of 12 genes were induced by the resistant NIL at 24 hpi, whereas a second cluster contained 9 genes induced by the susceptible NIL at 48 hpi. Their possible functions in the molecular defence of chickpea is discussed. Our study provides new insight into the molecular defence against Fusarium race 5 and demonstrates that development of NILs is a rich resource to facilitate the detection of candidate genes. The new genes regulated here may be useful against other Fusarium races.

Effects of exogenous salicylic acid and pH on pathogenicity of biotrophy-associated secreted protein 1 (BAS1)-overexpressing strain, Magnaporthe oryzae

Abiotic stress can influence the interactions between a pathogen and its host. In this paper, we analyzed the effects of salicylic acid (SA) and pH on the morphological development and pathogenicity of Magnaporthe oryzae, the pathogen that causes rice (Oryza sativa) blast. A strain of rice blast that overexpresses biotrophy-associated secreted protein 1 (BAS1) and a wild-type (WT) strain were pretreated with different levels of pH and different concentrations of SA to analyze M. oryzae colony growth, sporulation, spore germination, dry weight of hypha, and appressorium formation. Disease incidence and the expression of defense-related genes in infected rice were analyzed after pretreatment with pH 5.00 or pH 8.00 and 200 μM SA. The results showed that both SA and pH had some influence on morphological development, including sporulation and appressorium formation of the BAS1-overexpression strain. In the 200 μM SA pretreatment, there was a lower incidence of disease and higher expression levels of the rice defense-related genes PR1a, PAL, HSP90, and PR5 on leaves inoculated with the BAS1-overexpession strain compared with the WT strain, whereas, LOX2 appeared to be downregulated in the BAS1-overexpession strain compared with the WT. In both pH treatments, disease incidence and expression of HSP90 were higher and the expression of PR1a and PR10a and LOX2 and PAL was lower in leaves inoculated with the BAS1-overexpression strain compared with leaves inoculated with the WT strain. We conclude that SA and pH affect morphological development of the BAS1-overexpression blast strain, but that these factors have little influence on the pathogenicity of the strain, indicating that BAS1-overexpression may have enhanced the tolerance of this rice blast strain to abiotic stressors. This work suggests new molecular mechanisms that exogenous SA and pH affect the interactions between M. oryzae and rice.

Effects of exogenous salicylic acid and pH on pathogenicity of biotrophy-associated secreted protein 1 (BAS1)-overexpressing strain, Magnaporthe oryzae

Abiotic stress can influence the interactions between a pathogen and its host. In this paper, we analyzed the effects of salicylic acid (SA) and pH on the morphological development and pathogenicity of Magnaporthe oryzae, the pathogen that causes rice (Oryza sativa) blast. A strain of rice blast that overexpresses biotrophy-associated secreted protein 1 (BAS1) and a wild-type (WT) strain were pretreated with different levels of pH and different concentrations of SA to analyze M. oryzae colony growth, sporulation, spore germination, dry weight of hypha, and appressorium formation. Disease incidence and the expression of defense-related genes in infected rice were analyzed after pretreatment with pH 5.00 or pH 8.00 and 200 μM SA. The results showed that both SA and pH had some influence on morphological development, including sporulation and appressorium formation of the BAS1-overexpression strain. In the 200 μM SA pretreatment, there was a lower incidence of disease and higher expression levels of the rice defense-related genes PR1a, PAL, HSP90, and PR5 on leaves inoculated with the BAS1-overexpession strain compared with the WT strain, whereas, LOX2 appeared to be downregulated in the BAS1-overexpession strain compared with the WT. In both pH treatments, disease incidence and expression of HSP90 were higher and the expression of PR1a and PR10a and LOX2 and PAL was lower in leaves inoculated with the BAS1-overexpression strain compared with leaves inoculated with the WT strain. We conclude that SA and pH affect morphological development of the BAS1-overexpression blast strain, but that these factors have little influence on the pathogenicity of the strain, indicating that BAS1-overexpression may have enhanced the tolerance of this rice blast strain to abiotic stressors. This work suggests new molecular mechanisms that exogenous SA and pH affect the interactions between M. oryzae and rice.

Genome-wide DNA methylation and transcriptomic profiles in the lifestyle strategies and asexual development of the forest fungal pathogen Heterobasidion parviporum

Heterobasidion parviporum is the most devastating fungal pathogen of conifer forests in Northern Europe. The fungus has dual life strategies, necrotrophy on living trees and saprotrophy on dead woods. DNA cytosine methylation is an important epigenetic modification in eukaryotic organisms. Our presumption is that the lifestyle transition and asexual development in H. parviporum could be driven by epigenetic effects. Involvements of DNA methylation in the regulation of aforementioned processes have never been studied thus far. RNA-seq identified lists of highly induced genes enriched in carbohydrate-active enzymes during necrotrophic interaction with host trees and saprotrophic sawdust growth. It also highlighted signaling- and transcription factor-related genes potentially associated with the transition of saprotrophic to necrotrophic lifestyle and groups of primary cellular activities throughout asexual development. Whole-genome bisulfite sequencing revealed that DNA methylation displayed pronounced preference in CpG dinucleotide context across the genome and mostly targeted transposable element (TE)-rich regions. TE methylation level demonstrated a strong negative correlation with TE expression, reinforcing the protective function of DNA methylation in fungal genome stability. Small groups of genes putatively subject to methylation transcriptional regulation in response to saprotrophic and necrotrophic growth in comparison with free-living mycelia were also explored. Our study reported on the first methylome map of a forest pathogen. Analysis of transcriptome and methylome variations associated with asexual development and different lifestyle strategies provided further understanding of basic biological processes in H. parviporum. More importantly, our work raised additional potential roles of DNA methylation in fungi apart from controlling the proliferation of TEs.

Epitypification of Fusarium oxysporum - clearing the taxonomic chaos

Fusarium oxysporum is the most economically important and commonly encountered species of Fusarium. This soil-borne fungus is known to harbour both pathogenic (plant, animal and human) and non-pathogenic strains. However, in its current concept F. oxysporum is a species complex consisting of numerous cryptic species. Identification and naming these cryptic species is complicated by multiple subspecific classification systems and the lack of living ex-type material to serve as basic reference point for phylogenetic inference. Therefore, to advance and stabilise the taxonomic position of F. oxysporum as a species and allow naming of the multiple cryptic species recognised in this species complex, an epitype is designated for F. oxysporum. Using multi-locus phylogenetic inference and subtle morphological differences with the newly established epitype of F. oxysporum as reference point, 15 cryptic taxa are resolved in this study and described as species.

Exploring Combined Effect of Abiotic (Soil Moisture) and Biotic (Sclerotium rolfsii Sacc.) Stress on Collar Rot Development in Chickpea

Plants being sessile are under constant threat of multiple abiotic and biotic stresses within its natural habitat. A combined stress involving an abiotic and a biotic factor reportedly increases susceptibility of the plants to pathogens. The emerging threat, collar rot disease of chickpea (caused by Sclerotium rolfsii Sacc.) is reported to be influenced by soil moisture condition (SMC). Hence, we studied the influence of differential SMC viz. upper optimum (100%), optimum (80%), lower optimum (60%), and limiting (40%) soil moisture conditions on colonization and collar rot development over the course of infection in two chickpea cultivars, Annigeri (susceptible to collar rot) and ICCV 05530 (moderately resistant to collar rot). Disease incidence was found to be directly proportional to increase in soil moisture (R2 = 0.794). Maximum incidence was observed at 80% SMC, followed by 100 and 60% SMC. Expression of genes (qPCR analysis) associated with host cell wall binding (lectin) and degradation viz. endopolygalacturonase-2, endoglucosidase, and cellobiohydrolase during collar rot development in chickpea were relatively less at limiting soil moisture condition (40%) as compared to optimum soil moisture condition (80%). As compared to individual stress, the expression of defense response genes in chickpea seedlings were highly up-regulated in seedlings challenged with combined stress. Our qPCR results indicated that the expression of defense-related genes in chickpea during interaction with S. rolfsii at low SMC was primarily responsible for delayed disease reaction. Involvement of moisture and biotic stress-related genes in combined stress showed a tailored defense mechanism.

Transcriptome Analysis of Tomato Leaf Spot Pathogen Fusarium proliferatum: De novo Assembly, Expression Profiling, and Identification of Candidate Effectors

Leaf spot disease caused by the fungus Fusarium proliferatum (Matsushima) Nirenberg is a destructive disease of tomato plants in China. Typical symptoms of infected tomato plants are softened and wilted stems and leaves, leading to the eventual death of the entire plant. In this study, we resorted to transcriptional profile analysis to gain insight into the repertoire of effectors involved in F. proliferatum–tomato interactions. A total of 61,544,598 clean reads were de novo assembled to provide a F. proliferatum reference transcriptome. From these, 75,044 unigenes were obtained, with 19.46% of the unigenes being assigned to 276 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, with 22.3% having a homology with genes from F. fujikuroi. A total of 18,075 differentially expressed genes (DEGs) were identified, 720 of which were found to code for secreted proteins. Of these, 184 were identified as candidate effectors, while 79.89% had an upregulated expression. Moreover, 17 genes that were differentially expressed in RNA-seq studies were randomly selected for validation by quantitative real-time polymerase chain reaction (qRT–PCR). The study demonstrates that transcriptome analysis could be an effective method for identifying the repertoire of candidate effectors and may provide an invaluable resource for future functional analyses of F. proliferatum pathogenicity in F. proliferatum and tomato plant–host interactions.