Wheat Resistances to Fusarium Root Rot and Head Blight Are Both Associated with Deoxynivalenol- and Jasmonate-Related Gene Expression

Knockout of MITOGEN-ACTIVATED PROTEIN KINASE 3 causes barley root resistance against Fusarium graminearum

The roles of mitogen-activated protein kinases (MAPKs) in plant-fungal pathogenic interactions are poorly understood in crops. Here, microscopic, phenotypic, proteomic, and biochemical analyses revealed that roots of independent transcription activator-like effector nuclease (TALEN)-based knockout lines of barley (Hordeum vulgare L.) MAPK 3 (HvMPK3 KO) were resistant against Fusarium graminearum infection. When co-cultured with roots of the HvMPK3 KO lines, F. graminearum hyphae were excluded to the extracellular space, the growth pattern of extracellular hyphae was considerably deregulated, mycelia development was less efficient, and number of appressoria-like structures and their penetration potential were substantially reduced. Intracellular penetration of hyphae was preceded by the massive production of reactive oxygen species (ROS) in attacked cells of the wild-type (WT), but ROS production was mitigated in the HvMPK3 KO lines. Suppression of ROS production in these lines coincided with elevated abundance of catalase (CAT) and ascorbate peroxidase (APX). Moreover, differential proteomic analysis revealed downregulation of several defense-related proteins in WT, and the upregulation of pathogenesis-related protein 1 (PR-1) and cysteine proteases in HvMPK3 KO lines. Proteins involved in suberin formation, such as peroxidases, lipid transfer proteins (LTPs), and the GDSL esterase/lipase (containing "GDSL" aminosequence motif) were differentially regulated in HvMPK3 KO lines after F. graminearum inoculation. Consistent with proteomic analysis, microscopic observations showed enhanced suberin accumulation in roots of HvMPK3 KO lines, most likely contributing to the arrested infection by F. graminearum. These results suggest that TALEN-based knockout of HvMPK3 leads to barley root resistance against Fusarium root rot.

A chitinase CsChi23 promoter polymorphism underlies cucumber resistance against Fusarium oxysporum f. sp. cucumerinum

Fusarium wilt disease, caused by Fusarium oxysporum f. sp. cucumerinum (Foc), leads to widespread yield loss and quality decline in cucumber. However, the molecular mechanisms underlying Foc resistance remain poorly understood. We report the mapping and functional characterisation of CsChi23, encoding a cucumber class I chitinase with antifungal properties. We assessed sequence variations at CsChi23 and the associated defence response against Foc. We functionally characterised CsChi23 using transgenic assay and expression analysis. The mechanism regulating CsChi23 expression was assessed by genetic and molecular approaches. CsChi23 was induced by Foc infection, which led to rapid upregulation in resistant cucumber lines. Overexpressing CsChi23 enhanced fusarium wilt resistance and reduced fungal biomass accumulation, whereas silencing CsChi23 causes loss of resistance. CsHB15, a homeodomain leucine zipper (HD-Zip) III transcription factor, was found to bind to the CsChi23 promoter region and activate its expression. Furthermore, silencing of CsHB15 reduces CsChi23 expression. A single-nucleotide polymorphism variation -400 bp upstream of CsChi23 abolished the HD-Zip III binding site in a susceptible cucumber line. Collectively, our study indicates that CsChi23 is sufficient to enhance fusarium wilt resistance and reveals a novel function of an HD-Zip III transcription factor in regulating chitinase expression in cucumber defence against fusarium wilt.

Tissue-specific transcriptome responses to Fusarium head blight and Fusarium root rot

Fusarium head blight (FHB) and Fusarium root rot (FRR) are important diseases of small-grain cereals caused by Fusarium species. While host response to FHB has been subject to extensive study, very little is known about response to FRR and the transcriptome responses of FHB and FRR have not been thoroughly compared. Brachypodium distachyon (Bd) is an effective model for investigating host responses to both FHB and FRR. In this study the transcriptome response of Bd to F. graminearum (Fg) infection of heads and roots was investigated. An RNA-seq analysis was performed on both Bd FHB and FRR during the early infection. Additionally, an RNA-seq analysis was performed on in vitro samples of Fg for comparison with Fg gene expression in planta. Differential gene expression and gene-list enrichment analyses were used to compare FHB and FRR transcriptome responses in both Bd and Fg. Differential expression of selected genes was confirmed using RT-qPCR. Most genes associated with receptor signalling, cell-wall modification, oxidative stress metabolism, and cytokinin and auxin biosynthesis and signalling genes were generally upregulated in FHB or were downregulated in FRR. In contrast, Bd genes involved in jasmonic acid and ethylene biosynthesis and signalling, and antimicrobial production were similarly differentially expressed in both tissues in response to infection. A transcriptome analysis of predicted Fg effectors with the same infected material revealed elevated expression of core tissue-independent genes including cell-wall degradation enzymes and the gene cluster for DON production but also several tissue-dependent genes including those for aurofusarin production and cutin degradation. This evidence suggests that Fg modulates its transcriptome to different tissues of the same host.

Transcriptome analysis reveals infection strategies employed by Fusarium graminearum as a root pathogen

The fungal pathogen Fusarium graminearum (Fg) infects both heads and roots of cereal crops causing several economically important diseases such as head blight, seedling blight, crown rot and root rot. Trichothecene mycotoxins such as deoxynivalenol (DON), a well-known virulence factor, produced by Fg during disease development is also an important health concern. Although how Fg infects above-ground tissues is relatively well studied, very little is known about molecular processes employed by the pathogen during below-ground infection. Also unknown is the role of DON during root infection. In the present study, we analyzed the transcriptome of Fg during root infection of the model cereal Brachypodium distachyon (Bd). We also compared our Fg transcriptome data obtained during Bd root infection with those reported during wheat head infection. These analyses suggested that both shared and unique infection strategies were employed by the pathogen during colonization of different host tissues. Several metabolite biosynthesis genes induced in Fg during root infection could be linked to phytohormone production, implying that the pathogen likely interferes with root specific defenses. In addition, to understand the role of DON in Fg root infection, we analyzed the transcriptome of the DON deficient Tri5 mutant. These analyses showed that the absence of DON had a significant effect on fungal transcriptional responses. Although DON was produced in infected roots, this mycotoxin did not act as a Fg virulence factor during root infection. Our results reveal new mechanistic insights into the below-ground strategies employed by Fg that may benefit the development of new genetic tools to combat this important cereal pathogen.

Speaking the language of lipids: the cross-talk between plants and pathogens in defence and disease

Lipids and fatty acids play crucial roles in plant immunity, which have been highlighted over the past few decades. An increasing number of studies have shown that these molecules are pivotal in the interactions between plants and their diverse pathogens. The roles played by plant lipids fit in a wide spectrum ranging from the first physical barrier encountered by the pathogens, the cuticle, to the signalling pathways that trigger different immune responses and expression of defence-related genes, mediated by several lipid molecules. Moreover, lipids have been arising as candidate biomarkers of resistance or susceptibility to different pathogens. Studies on the apoplast and extracellular vesicles have been highlighting the possible role of lipids in the intercellular communication and the establishment of systemic acquired resistance during plant-pathogen interactions. From the pathogen perspective, lipid metabolism and specific lipid molecules play pivotal roles in the pathogen's life cycle completion, being crucial during recognition by the plant and evasion from the host immune system, therefore potentiating infection. Studies conducted in the last years have contributed to a better understanding of the language of lipids during the cross-talk between plants and pathogens. However, it is essential to continue exploring the knowledge brought up to light by transcriptomics and proteomics studies towards the elucidation of lipid signalling processes during defence and disease. In this review, we present an updated overview on lipids associated to plant-pathogen interactions, exploiting their roles from the two sides of this battle.

Overexpression of a Cytochrome P450 Monooxygenase Involved in Orobanchol Biosynthesis Increases Susceptibility to Fusarium Head Blight

Fusarium Head Blight (FHB) is a cereal disease caused primarily by the ascomycete fungus Fusarium graminearum with public health issues due to the production of mycotoxins including deoxynivalenol (DON). Genetic resistance is an efficient protection means and numerous quantitative trait loci have been identified, some of them related to the production of resistance metabolites. In this study, we have functionally characterized the Brachypodium distachyon BdCYP711A29 gene encoding a cytochrome P450 monooxygenase (CYP). We showed that BdCYP711A29 belongs to an oligogenic family of five members. However, following infection by F. graminearum, BdCYP711A29 is the only copy strongly transcriptionally induced in a DON-dependent manner. The BdCYP711A29 protein is homologous to the Arabidopsis thaliana MAX1 and Oryza sativa MAX1-like CYPs representing key components of the strigolactone biosynthesis. We show that BdCYP711A29 is likely involved in orobanchol biosynthesis. Alteration of the BdCYP711A29 sequence or expression alone does not modify plant architecture, most likely because of functional redundancy with the other copies. B. distachyon lines overexpressing BdCYP711A29 exhibit an increased susceptibility to F. graminearum, although no significant changes in defense gene expression were detected. We demonstrate that both orobanchol and exudates of Bd711A29 overexpressing lines stimulate the germination of F. graminearum macroconidia. We therefore hypothesize that orobanchol is a susceptibility factor to FHB.

Exploring the Potential of Meyerozyma guilliermondii on Physiological Performances and Defense Response against Fusarium Crown Rot on Durum Wheat

Coating seeds with bio-control agents is a potentially effective approach to reduce the usage of pesticides and fertilizers applied and protect the natural environment. This study evaluated the effect of seed coating with Meyerozyma guilliermondii, strain INAT (MT731365), on seed germination, plant growth and photosynthesis, and plant resistance against Fusarium culmorum, in durum wheat under controlled conditions. Compared to control plants, seed coating with M. guilliermondii promoted the wheat growth (shoot and roots length and biomass), and photosynthesis and transpiration traits (chlorophyll, ɸPSII, rates of photosynthesis and transpiration, etc.) together with higher nitrogen balance index (NBI) and lower flavonols and anthocyanins. At 21 days post infection with Fusarium, M. guilliermondii was found to reduce the disease incidence and the severity, with reduction rates reaching up to 31.2% and 30.4%, respectively, as well as to alleviate the disease damaging impact on photosynthesis and plant growth. This was associated with lower ABA, flavonols and anthocyanins, compared to infected control. A pivotal function of M. guilliermondii as an antagonist of F. culmorum and a growth promoter is discussed.

WFhb1-1 plays an important role in resistance against Fusarium head blight in wheat

Fusarium head blight (FHB) is a severe disease of wheat (Triticum aestivum L.). Qfhb1 is the most important quantitative trait locus (QTL) for FHB resistance. We previously identified wheat gene WFhb1-1 (aka WFhb1-c1) as a candidate for FHB resistance gene. Here we report that WFhb1-1 has been cloned. The gene (GenBank # KU304333.1) consists of a single exon, encoding a putative membrane protein of 127 amino acids. WFhb1-1 protein produced in Pichia pastoris inhibits growth of both F. graminearum and P. pastoris in culture. Western Blotting with anti- WFhb1-1 antibody revealed a significant decrease (p < 0.01) in WFhb1-1 accumulation, 12 hours post Fusarium inoculation in non-Qfhb1-carrier wheat but not in Qfhb1-carrier wheat. Overexpressing WFhb1-1 in non-Qfhb1-carrier wheat led to a significant decrease (p < 0.01) in Fusarium-damaged rachis rate, Fusarium-diseased kernel rate and DON content in harvested kernels, while silencing WFhb1-1 in Qfhb1-carrier wheat resulted in a significant increase (p < 0.01) in FHB severity. Therefore, WFhb1-1 is an important FHB resistance gene with a potential antifungal function and probably a key functional component of Qfhb1 in wheat. A model regarding how WFhb1-1 functions in FHB resistance/susceptibility is hypothesized and discussed.

Regulators of nitric oxide signaling triggered by host perception in a plant pathogen

The rhizosphere interaction between plant roots or pathogenic microbes is initiated by mutual exchange of signals. However, how soil pathogens sense host signals is largely unknown. Here, we studied early molecular events associated with host recognition in Fusarium graminearum, an economically important fungal pathogen that can infect both roots and heads of cereal crops. We found that host sensing prior to physical contact with plant roots radically alters the transcriptome and triggers nitric oxide (NO) production in F. graminearum We identified an ankyrin-repeat domain containing protein (FgANK1) required for host-mediated NO production and virulence in F. graminearum In the absence of host plant, FgANK1 resides in the cytoplasm. In response to host signals, FgANK1 translocates to the nucleus and interacts with a zinc finger transcription factor (FgZC1), also required for specific binding to the nitrate reductase (NR) promoter, NO production, and virulence in F. graminearum Our results reveal mechanistic insights into host-recognition strategies employed by soil pathogens.

Trichothecenes in Cereal Grains - An Update

Trichothecenes are sesquiterpenoid mycotoxins produced by fungi from the order Hypocreales, including members of the Fusarium genus that infect cereal grain crops. Different trichothecene-producing Fusarium species and strains have different trichothecene chemotypes belonging to the Type A and B class. These fungi cause a disease of small grain cereals, called Fusarium head blight, and their toxins contaminate host tissues. As potent inhibitors of eukaryotic protein synthesis, trichothecenes pose a health risk to human and animal consumers of infected cereal grains. In 2009, Foroud and Eudes published a review of trichothecenes in cereal grains for human consumption. As an update to this review, the work herein provides a comprehensive and multi-disciplinary review of the Fusarium trichothecenes covering topics in chemistry and biochemistry, pathogen biology, trichothecene toxicity, molecular mechanisms of resistance or detoxification, genetics of resistance and breeding strategies to reduce their contamination of wheat and barley.

Selection of an Endophytic Streptomyces sp. Strain DEF09 From Wheat Roots as a Biocontrol Agent Against Fusarium graminearum

Selection of biological control agents (BCA) profits from an integrated study of the tripartite interactions occurring among the BCA, the plant and the pathogen. The environment plays a crucial role in the efficacy of BCA, therefore, the selection process shall utmost mimic naturally occurring conditions. To identify effective biocontrol strains against Fusarium graminearum, the major cause of Fusarium head blight (FHB) in wheat and deoxynivalenol (DON) accumulation in grains, a workflow consisting of in vitro and in vivo assays was set up. Twenty-one Streptomyces strains, 16 of which were endophytes of different plants, were analyzed. In vitro and in vivo tests characterized their plant growth promoting (PGP) traits. Biocontrol activity against F. graminearum was firstly assessed with a dual culture assay. An in vivo germination blotter assay measured Fusarium foot rot and root rot symptoms (FFR-FRR) reduction as well as growth parameters of the plant treated with the Streptomyces strains. A selected subset of Streptomyces spp. strains was then assessed in a growth chamber measuring FFR symptoms and growth parameters of the wheat plant. The approach led to the identification of an effective Streptomyces sp. strain, DEF09, able to inhibit FHB on wheat in controlled conditions by blocking the spread of the pathogen at the infection site. The results were further confirmed in field conditions on both bread and durum wheat, where DEF09 decreased disease severity up to 60%. This work confirms that FRR and FFR pathosystems can be used to identify BCA effective against FHB.

Chemical Activation of the Ethylene Signaling Pathway Promotes Fusarium graminearum Resistance in Detached Wheat Heads

Plant signaling hormones such as ethylene have been shown to affect the host response to various pathogens. Often, the resistance responses to necrotrophic fungi are mediated through synergistic interactions of ethylene (ET) with the jasmonate signaling pathway. On the other hand, ET is also an inducer of senescence and cell death, which could be beneficial for some invading necrotrophic pathogens. Fusarium graminearum, a causative agent in Fusarium head blight of wheat, is a hemibiotrophic pathogen, meaning it has both biotrophic and necrotrophic phases during the course of infection. However, the role of ET signaling in the host response to Fusarium spp. is unclear; some studies indicate that ET mediates resistance, while others have shown that it is associated with susceptibility. These discrepancies could be related to various aspects of different experimental designs, and suggest that the role of ET signaling in the host response to FHB is potentially dependent on interactions with some undetermined factors. To investigate whether wheat genotype can influence the ET-mediated response to FHB, the effect of chemical treatments affecting the ET pathway was studied in six wheat genotypes in detached-head assays. ET-inhibitor treatments broke down resistance to both initial infection and disease spread in three resistant wheat genotypes, whereas ET-enhancer treatments resulted in reduced susceptibility in three susceptible genotypes. The results presented here show that the ET signaling can mediate FHB resistance to F. graminearum in different wheat backgrounds.

Deoxynivalenol Detoxification in Transgenic Wheat Confers Resistance to Fusarium Head Blight and Crown Rot Diseases

Fusarium diseases, including Fusarium head blight (FHB) and Fusarium crown rot (FCR), reduce crop yield and grain quality and are major agricultural problems worldwide. These diseases also affect food safety through fungal production of hazardous mycotoxins. Among these, deoxynivalenol (DON) acts as a virulence factor during pathogenesis on wheat. The principal mechanism underlying plant tolerance to DON is glycosylation by specific uridine diphosphate-dependent glucosyltransferases (UGTs), through which DON-3-β-d-glucoside (D3G) is produced. In this work, we tested whether DON detoxification by UGT could confer to wheat a broad-spectrum resistance against Fusarium graminearum and F. culmorum. These widespread Fusarium species affect different plant organs and developmental stages in the course of FHB and FCR. To assess DON-detoxification potential, we produced transgenic durum wheat plants constitutively expressing the barley HvUGT13248 and bread wheat plants expressing the same transgene in flower tissues. When challenged with F. graminearum, FHB symptoms were reduced in both types of transgenic plants, particularly during early to mid-infection stages of the infection progress. The transgenic durum wheat displayed much greater DON-to-D3G conversion ability and a considerable decrease of total DON+D3G content in flour extracts. The transgenic bread wheat exhibited a UGT dose-dependent efficacy of DON detoxification. In addition, we showed, for the first time, that DON detoxification limits FCR caused by F. culmorum. FCR symptoms were reduced throughout the experiment by nearly 50% in seedlings of transgenic plants constitutively expressing HvUGT13248. Our results demonstrate that limiting the effect of the virulence factor DON via in planta glycosylation restrains FHB and FCR development. Therefore, ability for DON detoxification can be a trait of interest for wheat breeding targeting FHB and FCR resistance.

Fusarium graminearum KP4-like proteins possess root growth-inhibiting activity against wheat and potentially contribute to fungal virulence in seedling rot

The virally encoded KP4 killer toxin protein was first identified from Ustilago maydis (Um), and its homologues are present in diverse fungi and in one species of moss. No KP4-like (KP4L) proteins have been functionally characterized. Here, we report the identification and functional analysis of four KP4L proteins from Fusarium graminearum (Fg), the primary causal pathogen of Fusarium head blight (FHB), which is also known to associate with seedling rot of wheat. The four FgKP4L proteins (FgKP4L-1, -2, -3 and -4) are encoded by small open reading frames (378-825 bp) located on chromosome 1 with the FgKP4L-1, -2 and -3 genes clustering together. Sequence analysis indicated that FgKP4L proteins have conserved domains predicted to form a three-dimensional alpha/beta-sandwich structure as first reported for UmKP4, with FgKP4L-4 featuring double Kp4 domains. Further analyses revealed that the FgKP4L genes are expressed in vitro under certain stress conditions, and all up-regulated during FHB and/or seedling rot development, the recombinant FgKP4L-2 protein does not induce cell death in wheat leaves or spikelets, but inhibits root growth of young seedlings, and the elimination of the FgKP4L-1/-2/-3 gene cluster from the fungal genome results in reduced virulence in seedling rot but not in FHB. Database searches revealed KP4L proteins from ∼80 fungal species with more than half from human/animal pathogens. Phylogenetic analysis suggested that UmKP4 and the moss KP4L proteins are closely related to those from a zygromycete and Aspergillus, respectively, implying cross-kingdom horizontal gene transfer.

Histology-guided high-resolution AP-SMALDI mass spectrometry imaging of wheat-Fusarium graminearum interaction at the root-shoot junction

BACKGROUND

Fungal pathogens like Fusarium graminearum can cause severe yield losses and mycotoxin contamination of food and feed worldwide. We recently showed its ability to systemically colonize wheat via root infection. However, the molecular response of wheat to Fusarium root rot (FRR) infection and systemic spread is still unknown. As a molecular camera, mass spectrometry (MS) imaging combines label-free and multiplex metabolite profiling with histopathology.

RESULTS

Atmospheric-pressure (AP)-SMALDI-MS imaging was combined with optical microscopy to study wheat-F. graminearum interaction at the root-shoot junction, which is a crucial line of defense against a pathogen that can invade all distal plant parts. To scope the functional, temporal and local aspects of FRR disease spread, metabolic changes were simultaneous visualized in diseased and healthy stem bases of the resistant cultivar Florence-Aurore at 10, 14 and 21 days after root inoculation. Histological information was used to identify disease relevant tissues and to assist the interpretation of molecular images. Detected mycotoxin compounds secreted by F. graminearum showed a route of stem infection that was consistent with observations made by microscopy. The outer epidermis and vasculature of leaf sheath were, at different disease stages, identified as prominent sites of pathogen migration and wheat protection. Wheat metabolites mapped to these relatively small tissues indicated cell wall strengthening and antifungal activity as direct defenses as well as conservation in the wheat reactions to F. graminearum diseases that affect different plant organs.

CONCLUSIONS

AP-SMALDI-MS imaging at high spatial resolution is a versatile technique that can be applied to basic and applied aspects of agricultural research. Combining the technology with optical microscopy was found to be a powerful tool to gain in-depth information on almost unknown crop disease. Moreover, the approach allowed studying metabolism at the host-pathogen interface. The results provide important hints to an understanding of the complex spatio-temporal organization of plant resistance. Defense-on-demand responses to pathogen ingress were found, which provide opportunities for future research towards an improved resistance that does not negatively impact yield development in the field by saving plant resources and, moreover, may control different Fusarium diseases.