Found in translation: high-throughput chemical screening in Arabidopsis thaliana identifies small molecules that reduce Fusarium head blight disease in wheat

Overexpression of Liriodendron Hybrid LhGLK1 in Arabidopsis Leads to Excessive Chlorophyll Synthesis and Improved Growth

Chloroplasts is the site for photosynthesis, which is the main primary source of energy for plants. Golden2-like (GLK) is a key transcription factor that regulates chloroplast development and chlorophyll synthesis. However, most studies on GLK genes are performed in crops and model plants with less attention to woody plants. In this study, we identified the LhGLK1 and LhGLK2 genes in the woody plant Liriodendron hybrid, and they are specifically expressed in green tissues. We showed that overexpression of the LhGLK1 gene improves rosette leaf chlorophyll content and induces ectopic chlorophyll biogenesis in primary root and petal vascular tissue in Arabidopsis. Although these exhibit a late-flowering phenotype, transgenic lines accumulate more biomass in vegetative growth with improved photochemical quenching (qP) and efficiency of photosystem II. Taken together, we verified a conserved and ancient mechanism for regulating chloroplast biogenesis in Liriodendron hybrid and evaluated its effect on photosynthesis and rosette biomass accumulation in the model plant Arabidopsis.

PSEUDO-ETIOLATION IN LIGHT proteins reduce greening by binding GLK transcription factors

Knocking out genes encoding proteins that downregulate the accumulation of pigments may lead to increases in crop quality and yield. PSEUDO-ETIOLATION IN LIGHT 1 (PEL1) downregulates the accumulation of carotenoids in carrot and chlorophyll in Arabidopsis and rice and may inhibit GOLDEN 2-LIKE (GLK) transcription factors. PEL1 belongs to a previously unstudied gene family found only in plants. We used CRISPR/Cas9 technology to knock out each member of the 4-member PEL gene family and both GLK genes in Arabidopsis. In pel mutants, chlorophyll levels were elevated in seedlings; after flowering, chloroplasts increased in size, and anthocyanin levels increased. Although the chlorophyll-deficient phenotype of glk1 glk2 was epistatic to pel1 pel2 pel3 pel4 in most of our experiments, glk1 glk2 was not epistatic to pel1 pel2 pel3 pel4 for the accumulation of anthocyanins in most of our experiments. The pel alleles attenuated growth, altered the accumulation of nutrients in seeds, disrupted an abscisic acid-inducible inhibition of seedling growth response that promotes drought tolerance, and affected the expression of genes associated with diverse biological functions, such as stress responses, cell wall metabolism hormone responses, signaling, growth, and the accumulation of phenylpropanoids and pigments. We found that PEL proteins specifically bind 6 transcription factors that influence the accumulation of anthocyanins, GLK2, and the carboxy termini of GLK1 and Arabidopsis thaliana myeloblastosis oncogene homolog 4 (AtMYB4). Our data indicate that the PEL proteins influence the accumulation of chlorophyll and many other processes, possibly by inhibiting GLK transcription factors and via other mechanisms, and that multiple mechanisms downregulate chlorophyll content.

A genome‑wide approach to the systematic and comprehensive analysis of LIM gene family in sorghum (Sorghum bicolor L.)

The LIM domain-containing proteins are dominantly found in plants and play a significant role in various biological processes such as gene transcription as well as actin cytoskeletal organization. Nevertheless, genome-wide identification as well as functional analysis of the LIM gene family have not yet been reported in the economically important plant sorghum (Sorghum bicolor L.). Therefore, we conducted an in silico identification and characterization of LIM genes in S. bicolor genome using integrated bioinformatics approaches. Based on phylogenetic tree analysis and conserved domain, we identified five LIM genes in S. bicolor (SbLIM) genome corresponding to Arabidopsis LIM (AtLIM) genes. The conserved domain, motif as well as gene structure analyses of the SbLIM gene family showed the similarity within the SbLIM and AtLIM members. The gene ontology (GO) enrichment study revealed that the candidate LIM genes are directly involved in cytoskeletal organization and various other important biological as well as molecular pathways. Some important families of regulating transcription factors such as ERF, MYB, WRKY, NAC, bZIP, C2H2, Dof, and G2-like were detected by analyzing their interaction network with identified SbLIM genes. The cis-acting regulatory elements related to predicted SbLIM genes were identified as responsive to light, hormones, stress, and other functions. The present study will provide valuable useful information about LIM genes in sorghum which would pave the way for the future study of functional pathways of candidate SbLIM genes as well as their regulatory factors in wet-lab experiments.

Recent Developments of Gramine: Chemistry and Biological Activity

The natural alkaloid gramine has attracted significant attention in both academic and industrial circles because of its potential and diverse biological activities, including antiviral, antibacterial, antifungal, anti-inflammatory and antitumor activities; application in therapy for Alzheimer's disease; serotonin-receptor-related activity; insecticidal activity; and application as an algicide. In this review, we focus on the research advances that have been made for gramine-based molecules since their discovery, providing key information on their extraction and separation, chemical synthesis and diverse biological activities. Data regarding their mechanisms of action are also presented. This comprehensive and critical review will serve as a guide for developing more drug candidates based on gramine skeletons.

The G2-Like gene family in Populus trichocarpa: identification, evolution and expression profiles

The Golden2-like (GLK) transcription factors are plant-specific transcription factors (TFs) that perform extensive and significant roles in regulating chloroplast development. Here, genome-wide identification, classification, conserved motifs, cis-elements, chromosomal locations, evolution and expression patterns of the PtGLK genes in the woody model plant Populus trichocarpa were analyzed in detail. In total, 55 putative PtGLKs (PtGLK1-PtGLK55) were identified and divided into 11 distinct subfamilies according to the gene structure, motif composition and phylogenetic analysis. Synteny analysis showed that 22 orthologous pairs and highly conservation between regions of GLK genes across P. trichocarpa and Arabidopsis were identified. Furthermore, analysis of the duplication events and divergence times provided insight into the evolutionary patterns of GLK genes. The previously published transcriptome data indicated that PtGLK genes exhibited distinct expression patterns in various tissues and different stages. Additionally, several PtGLKs were significantly upregulated under the responses of cold stress, osmotic stress, and methyl jasmonate (MeJA) and gibberellic acid (GA) treatments, implying that they might take part in abiotic stress and phytohormone responses. Overall, our results provide comprehensive information on the PtGLK gene family and elucidate the potential functional characterization of PtGLK genes in P. trichocarpa.

Strategies for Controlling the Sporulation in Fusarium spp

Fusarium species are the most destructive phytopathogenic and toxin-producing fungi, causing serious diseases in almost all economically important plants. Sporulation is an essential part of the life cycle of Fusarium. Fusarium most frequently produces three different types of asexual spores, i.e., macroconidia, chlamydospores, and microconidia. It also produces meiotic spores, but fewer than 20% of Fusaria have a known sexual cycle. Therefore, the asexual spores of the Fusarium species play an important role in their propagation and infection. This review places special emphasis on current developments in artificial anti-sporulation techniques as well as features of Fusarium's asexual sporulation regulation, such as temperature, light, pH, host tissue, and nutrients. This description of sporulation regulation aspects and artificial anti-sporulation strategies will help to shed light on the ways to effectively control Fusarium diseases by inhibiting the production of spores, which eventually improves the production of food plants.

Identification of GOLDEN2-like transcription factor genes in soybeans and their role in regulating plant development and metal ion stresses

The Golden 2-Like (G2-like or GLK) transcription factors are essential for plant growth, development, and many stress responses as well as heavy metal stress. However, G2-like regulatory genes have not been studied in soybean. This study identified the genes for 130 G2-Like candidates' in the genome of Glycine max (soybean). These GLK genes were located on all 20 chromosomes, and several of them were segmentally duplicated. Most GLK family proteins are highly conserved in Arabidopsis and soybean and were classified into five major groups based on phylogenetic analysis. These GmGLK gene promoters share cis-acting elements involved in plant responses to abscisic acid, methyl jasmonate, auxin signaling, low temperature, and biotic and abiotic stresses. RNA-seq expression data revealed that the GLK genes were classified into 12 major groups and differentially expressed in different tissues or organs. The co-expression network complex revealed that the GmGLK genes encode proteins involved in the interaction of genes related to chlorophyll biosynthesis, circadian rhythms, and flowering regulation. Real-time quantitative PCR analysis confirmed the expression profiles of eight GLK genes in response to cadmium (Cd) and copper (Cu) stress, with some GLK genes significantly induced by both Cd and Cu stress treatments, implying a functional role in defense responsiveness. Thus, we present a comprehensive perspective of the GLK genes in soybean and emphasize their important role in crop development and metal ion stresses.

Proximity-dependent biotinylation identifies a suite of candidate effector proteins from Fusarium graminearum

Fusarium graminearum is a fungal pathogen that causes Fusarium head blight in cereal crops. The identification of proteins secreted from pathogens to overcome plant defenses and cause disease, collectively known as effectors, can reveal the etiology of a disease process. Proximity-dependent biotin identification (BioID) was used to identify potential effector proteins secreted in planta by F. graminearum during the infection of Arabidopsis. Mass spectrometry analysis of streptavidin affinity-purified proteins revealed over 300 proteins from F. graminearum, of which 62 were candidate effector proteins (CEPs). An independent analysis of secreted proteins from axenic cultures of F. graminearum showed a 42% overlap with CEPs, thereby assuring confidence in the BioID methodology. The analysis also revealed that 19 out of 62 CEPs (approx. 30%) had been previously characterized with virulence function in fungi. The functional characterization of additional CEPs was undertaken through deletion analysis by the CRISPR/Cas9 method, and by overexpression into Triticum aestivum (wheat) leaves by the Ustilago hordei delivery system. Deletion studies of 12 CEPs confirmed the effector function of three previously characterized CEPs and validated the function of another four CEPs on wheat inflorescence or vegetative tissues. Lastly, overexpression in wheat showed that all seven CEPs enhanced resistance against the bacterial pathogen Pseudomonas syringae DC3000.

Comprehensive Genomic Analysis of G2-like Transcription Factor Genes and Their Role in Development and Abiotic Stresses in Arabidopsis

GOLDEN2-LIKE (GLK) transcription factors are a subfamily of GARP family transcription factors, which play an essential function in plant growth and development as well as stress response during abiotic and biotic stress conditions. This study reports GLK genes in the Arabidopsis thaliana genome in-depth and identified 55 AtGLK genes in the Arabidopsis genome. Phylogenetic analyses resolved these GLK gene clusters into seven groups. A Ka/Ks ratios analysis indicated that they had experienced purifying selection. Many essential cis elements are present in the promoter regions of AtGLK genes associated with plant hormones, light, and stress. The expression profile from RNA-Seq data revealed that 29.1% of them had relatively high expression in all tested tissues or organs, indicating their crucial housekeeping function in plant growth and development. However, many other GLK members were selectively expressed in particular tissues or organs. In silico study of the transcriptional regulation of AtGLKs indicated that it is strongly regulated by cold, drought, osmotic, salt, and metal ion stressors. Our research provides essential information for the functional studies of each GLK gene in different species in the future.

Genome-wide identification of Tomato Golden 2-Like transcription factors and abiotic stress related members screening

BACKGROUND

Golden 2-Like (G2-like) transcription factors play an important role in plant development. However, the roles of these G2-like regulatory genes in response to abiotic stresses in tomato are not well understood.

RESULTS

In this study, we identified 66 putative G2-like genes in tomato (Solanum lycopersicum) and classified them into 5 groups (I to V) according to gene structure, motif composition and phylogenetic analysis. The G2-like genes were unevenly distributed across all 12 chromosomes. There were nine pairs of duplicated gene segments and four tandem duplicated SlGlk genes. Analysis of the cis-regulatory elements (CREs) showed that the promoter regions of SlGlks contain many kinds of stress- and hormone-related CREs. Based on RNA-seq, SlGlks were expressed in response to three abiotic stresses. Thirty-six differentially expressed SlGlks were identified; these genes have multiple functions according to Gene Ontology (GO) analysis and are enriched mainly in the zeatin biosynthesis pathway. Further studies exhibited that silencing SlGlk16 in tomato would reduce drought stress tolerance by earlier wilted, lower superoxide dismutase (SOD), peroxidase (POD) activities, less Pro contents and more MDA contents.

CONCLUSIONS

Overall, the results of this study provide comprehensive information on G2-like transcription factors and G2-like genes that may be expressed in response to abiotic stresses.

Identification of the Golden-2-like transcription factors gene family in Gossypium hirsutum

Background

Golden2-Like (GLK) transcription factors are a type of transcriptional regulator in plants. They play a pivotal role in the plant physiological activity process and abiotic stress response.

Methods

In this study, the potential function of GLK family genes in Gossypium hirsutum was studied based on genomic identification, phylogenetic analysis, chromosome mapping and cis-regulatory elements prediction. Gene expression of nine key genes were analyzed by qRT-PCR experiments.

Results

Herein, we identified a total of 146 GhGLK genes in Gossypium hirsutum, which were unevenly distributed on each of the chromosomes. There were significant differences in the number and location of genes between the At sub-genome and the Dt sub-genome. According to the phylogenetic analysis, they were divided into ten subgroups, each of which had very similar number and structure of exons and introns. Some cis-regulatory elements were identified through promoter analysis, including five types of elements related to abiotic stress response, five types of elements related to phytohormone and five types of elements involved in growth and development. Based on public transcriptome data analysis, we identified nine key GhGLKs involved in salt, cold, and drought stress. The qRT-PCR results showed that these genes had different expression patterns under these stress conditions, suggesting that GhGLK genes played an important role in abiotic stress response. This study laid a theoretical foundation for the screening and functional verification of genes related to stress resistance of GLK gene family in cotton.

MAMP and DAMP signaling contributes resistance to Fusarium graminearum in Arabidopsis

Plants perceive externally produced microbe-associated molecular patterns (MAMPs) and endogenously produced danger-associated molecular patterns (DAMPs) to activate inducible immunity. While several inducible immune responses have been observed during Fusarium graminearum infection, the identity of the signaling pathways involved is only partly known. We screened 227 receptor kinase and innate immune response genes in Arabidopsis to identify nine genes with a role in F. graminearum resistance. Resistance-promoting genes included the chitin receptors LYK5 and CERK1, and the reactive oxygen species (ROS)-producing NADPH oxidase RbohF, which were required for full inducible immune responses during infection. Two of the genes identified in our screen, APEX and the PAMP-induced peptide 1 (PIP1) DAMP receptor RLK7, repressed F. graminearum resistance. Both RbohF and RLK7 were required for full chitin-induced immune responses and PIP1 precursor expression was induced by chitin and F. graminearum infection. Together, this indicates that F. graminearum resistance is mediated by MAMP and DAMP signaling pathways and that chitin-induced signaling is enhanced by PIP1 perception and ROS production.

Genome-Wide Analysis of the G2-Like Transcription Factor Genes and Their Expression in Different Senescence Stages of Tobacco (Nicotiana tabacum L.)

The Golden2-like (GLK) transcription factors play important roles in regulating chloroplast growth, development, and senescence in plants. In this study, a total of 89 NtGLK genes (NtGLK1-NtGLK89) were identified in the tobacco genome and were classified into 10 subfamilies with variable numbers of exons and similar structural organizations based on the gene structure and protein motif analyses. Twelve segmental duplication pairs of NtGLK genes were identified in the genome. These NtGLK genes contain two conserved helix regions related to the HLH structure, and the sequences of the first helix region are less conserved than that of the second helix motif. Cis-regulatory elements of the NtGLK promoters were widely involved in light responsiveness, hormone treatment, and physiological stress. Moreover, a total of 206 GLK genes from tomato, tobacco, maize, rice, and Arabidopsis were retrieved and clustered into eight subgroups. Our gene expression analysis indicated that NtGLK genes showed differential expression patterns in tobacco leaves at five senescence stages. The expression levels of six NtGLK genes in group C were reduced, coinciding precisely with the increment of the degree of senescence, which might be associated with the function of leaf senescence of tobacco. Our results have revealed valuable information for further functional characterization of the GLK gene family in tobacco.

The transcription factor AtGLK1 acts upstream of MYBL2 to genetically regulate sucrose-induced anthocyanin biosynthesis in Arabidopsis

BACKGROUND

The regulation of anthocyanin biosynthesis by various factors including sugars, light and abiotic stresses is mediated by numerous regulatory factors acting at the transcriptional level. Here experimental evidence was provided in order to demonstrate that the nuclear GARP transcription factor AtGLK1 plays an important role in regulating sucrose-induced anthocyanin biosynthesis in Arabidopsis.

RESULTS

The results obtained using real-time quantitative PCR and GUS staining assays revealed that AtGLK1 was mainly expressed in the green tissues of Arabidopsis seedlings and could be induced by sucrose. The loss-of-function glk1 glk2 double mutant has lower anthocyanin levels than the glk2 single mutant, although it has been determined that loss of AtGLK1 alone does not affect anthocyanin accumulation. Overexpression of AtGLK1 enhances the accumulation of anthocyanin in transgenic Arabidopsis seedlings accompanied by increased expression of anthocyanin biosynthetic and regulatory genes. Moreover, we found that AtGLK1 also participates in plastid-signaling mediated anthocyanin accumulations. Genetic, physiological, and molecular biological approaches demonstrated that AtGLK1 acts upstream of MYBL2, which is a key negative regulator of anthocyanin biosynthesis, to genetically regulate sucrose-induced anthocyanin biosynthesis.

CONCLUSION

Our results indicated that AtGLK1 positively regulates sucrose-induced anthocyanin biosynthesis in Arabidopsis via MYBL2.

DNA Methylation Is Responsive to the Environment and Regulates the Expression of Biosynthetic Gene Clusters, Metabolite Production, and Virulence in Fusarium graminearum

Histone modifications play a significant role in the regulation of biosynthetic gene clusters (BGCs) in the phytopathogen Fusarium graminearum, by contrast, epigenetic regulation by DNA methyltransferases (DNMTs) is less documented. In this study, we characterized two DNMTs (FgDIM-2 and FgRID) in F. graminearum, with homologies to "Deficient in methylation" (DIM-2) and "Repeat-induced point (RIP) deficient" (RID) from Neurospora. The loss of DNMTs resulted in not only a decrease in average methylation density in the nutrient-poor, compared to nutrient-rich conditions, but also differences in the genes expressed between the WT and the DNMT mutant strains, implicating the external environment as an important trigger in altering DNA methylation patterns. Consequently, we observed significant changes in the regulation of multiple BGCs and alterations in the pathogenicity of the fungus.

Identification of two compounds able to improve flax resistance towards Fusarium oxysporum infection

Plants are surrounded by a diverse range of microorganisms that causes serious crop losses and requires the use of pesticides. Flax is a major crop in Normandy used for its fibres and is regularly challenged by the pathogenic fungus Fusarium oxysporum (Fo) f. sp. lini. To protect themselves, plants use "innate immunity" as a first line of defense level against pathogens. Activation of plant defense with elicitors could be an alternative for crop plant protection. A previous work was conducted by screening a chemical library and led to the identification of compounds able to activate defense responses in Arabidopsis thaliana. Four compounds were tested for their abilities to improve resistance of two flax varieties against Fo. Two of them, one natural (holaphyllamine or HPA) and one synthetic (M4), neither affected flax nor Fo growth. HPA and M4 induced oxidative burst and callose deposition. Furthermore, HPA and M4 caused changes in the expression patterns of defense-related genes coding a glucanase and a chitinase-like. Finally, plants pre-treated with HPA or M4 exhibited a significant decrease in the disease symptoms. Together, these findings demonstrate that HPA and M4 are able to activate defense responses in flax and improve its resistance against Fo infection.

Ectopic Expression of AhGLK1b (GOLDEN2-like Transcription Factor) in Arabidopsis Confers Dual Resistance to Fungal and Bacterial Pathogens

GOLDEN2-LIKE (GLK) is a member of the myeloblastosis (MYB) family transcription factor and it plays an important role in the regulation of plastid development and stress tolerance. In this study, a gene named AhGLK1b was identified from a cultivated peanut showing down-regulation in response to low calcium with a complete open reading frame (ORF) of 1212 bp. The AhGLK1b has 99.26% and 96.28% sequence similarities with its orthologs in Arachis ipaensis and A. duranensis, respectively. In the peanut, the AhGLK1b was localized in the nucleus and demonstrated the highest expression in the leaf, followed by the embryo. Furthermore, the expression of AhGLK1b was induced significantly in response to a bacterial pathogen, Ralstonia solanacearum infection. Ectopic expression of AhGLK1b in Arabidopsis showed stronger resistance against important phytopathogenic fungi S. sclerotiorum. It also exhibited high resistance to infection of the bacterial pathogen Pst DC3000. AhGLK1b-expressing Arabidopsis induced defense-related genes including PR10 and Phox/Bem 1 (PBI), which are involved in multiple disease resistance. Taken together, the results suggest that AhGLK1b might be useful in providing dual resistance to fungal and bacterial pathogens as well as tolerance to abiotic stresses.

The Use of PTI-Marker Genes to Identify Novel Compounds that Establish Induced Resistance in Rice

Compounds that establish induced resistance (IR) in plants are promising alternatives for the pesticides that are progressively being banned worldwide. Screening platforms to identify IR-establishing compounds have been developed, but none were specifically designed for monocot plants. Here, we propose the use of an RT-qPCR screening platform, based on conserved immunity marker genes of rice as proxy for IR induction. Central regulators of biotic stress responses of rice were identified with a weighted gene co-expression network analysis (WGCNA), using more than 350 microarray datasets of rice under various sorts of biotic stress. Candidate genes were narrowed down to six immunity marker genes, based on consistent association with pattern-triggered immunity (PTI), both in rice plants as in rice cell suspension cultures (RCSCs). By monitoring the expression of these genes in RCSCs upon treatment with candidate IR-inducing compounds, we showed that our marker genes can predict IR induction in rice. Diproline, a novel IR-establishing compound for monocots that was detected with these marker genes, was shown to induce rice resistance against root-knot nematodes, without fitness costs. Gene expression profiling of the here-described PTI-marker genes can be executed on fully-grown plants or in RCSCs, providing a novel and versatile tool to predict IR induction.

Redox signalling from NADPH oxidase targets metabolic enzymes and developmental proteins in Fusarium graminearum

NADPH oxidase (NOX) is one of the sources of reactive oxygen species (ROS) that modulates the activity of proteins through modifications of their cysteine residues. In a previous study, we demonstrated the importance of NOX in both the development and pathogenicity of the phytopathogen Fusarium graminearum. In this article, comparative proteomics between the wild-type and a Nox mutant of F. graminearum was used to identify active cysteine residues on candidate redox-sensing proteins. A two-dimensional gel approach based on labelling with monobromobimane (mBBR) identified 19 candidate proteins, and was complemented with a gel-free shotgun approach based on a biotin switch method, which yielded 99 candidates. The results indicated that, in addition to temporal regulation, a large number of primary metabolic enzymes are potentially targeted by NoxAB-generated ROS. Targeted disruption of these metabolic genes showed that, although some are dispensable, others are essential. In addition to metabolic enzymes, developmental proteins, such as the Woronin body major protein (FGSG_08737) and a glycosylphosphatidylinositol (GPI)-anchored protein (FGSG_10089), were also identified. Deletion of either of these genes reduced the virulence of F. graminearum. Furthermore, changing the redox-modified cysteine (Cys325 ) residue in FGSG_10089 to either serine or phenylalanine resulted in a similar phenotype to the FGSG_10089 knockout strain, which displayed reduced virulence and altered cell wall morphology; this underscores the importance of Cys325 to the function of the protein. Our results indicate that NOX-generated ROS act as intracellular signals in F. graminearum and modulate the activity of proteins affecting development and virulence in planta.

TOR signaling downregulation increases resistance to the cereal killer Fusarium graminearum

TOR is the master regulator of growth and development that senses energy availability. Biotic stress perturbs metabolic and energy homeostasis, making TOR a good candidate to participate in the plant response. Fusarium graminearum (Fusarium) produces important losses in many crops all over the world. To date, the role of TOR in Fusarium infection has remained unexplored. Here, we show that the resistance to the pathogen increases in different Arabidopsis mutants impaired in TOR complex or in wild-type plants treated with a TOR inhibitor. We conclude that TOR signaling is involved in plant defense against Fusarium.

Holaphyllamine, a steroid, is able to induce defense responses in Arabidopsis thaliana and increases resistance against bacterial infection

MAIN CONCLUSION

A chemical screen of plant-derived compounds identified holaphyllamine, a steroid, able to trigger defense responses in Arabidopsis thaliana and improve resistance against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. A chemical screen of 1600 plant-derived compounds was conducted and allowed the identification of a steroid able to activate defense responses in A. thaliana at a concentration of 1 µM without altering growth. The identified compound is holaphyllamine (HPA) whose chemical structure is similar to steroid pregnanes of mammals. Our data show that HPA, which is not constitutively present in A. thaliana, is able to trigger the formation of reactive oxygen species, deposition of callose and expression of several pathogenesis-related genes of the salicylic and jasmonic acid pathways. In addition, the results show that pre-treatment of A. thaliana seedlings with HPA before infection with the pathogenic bacterium Pseudomonas syringae pv tomato DC3000 results in a significant reduction of symptoms (i.e., reduction of bacterial colonies). Using A. thaliana mutants, we have found that the activation of defense responses by HPA does not depend on BRI1/BAK1 receptor kinases. Finally, a structure/function study reveals that the minimal structure required for activity is a 5-pregnen-20-one steroid with an equatorial nucleophilic group in C-3. Together, these findings demonstrate that HPA can activate defense responses that lead to improved resistance against bacterial infection in A. thaliana.

The world according to GARP transcription factors

Plant specific GARP transcription factor family (made of ARR-B and G2-like) contains genes with very diverse in planta functions: nutrient sensing, root and shoot development, floral transition, chloroplast development, circadian clock oscillation maintenance, hormonal transport and signaling. In this work we review: first, their structural but distant relationships with MYB transcription factors, second, their role in planta, third, the diversity of their Cis-regulatory elements, fourth, their potential protein partners. We conclude that the GARP family may hold keys to understand the interactions between nutritional signaling pathways (nitrogen and phosphate at least) and development. Understanding how plant nutrition and development are coordinated is central to understand how to adapt plants to an ever-changing environment. Consequently GARPs are likely to attract increasing research attentions, as they are likely at the crossroads of these fundamental processes.

Silencing of AtRAP, a target gene of a bacteria-induced small RNA, triggers antibacterial defense responses through activation of LSU2 and down-regulation of GLK1

Plants fine-tune their sophisticated immunity systems in response to pathogen infections. We previously showed that AtlsiRNA-1, a bacteria-induced plant endogenous small interfering RNA, silences the AtRAP gene, which encodes a putative RNA binding protein. In this study, we demonstrate that AtRAP functions as a negative regulator in plant immunity by characterizing molecular and biological responses of the knockout mutant and overexpression lines of AtRAP upon bacterial infection. AtRAP is localized in chloroplasts and physically interacts with Low Sulfur Upregulated 2 (LSU2), which positively regulates plant defense. Our results suggest that AtRAP negatively regulates defense responses by suppressing LSU2 through physical interaction. We also detected downregulation of the transcription factor GOLDEN2-LIKE 1 (GLK1) in atrap-1 using microarray analysis. The glk1 glk2 double mutant showed enhanced resistance to Pseudomonas syringae pv. tomato, which is consistent with a previous study showing enhanced resistance of a glk1 glk2 double mutant to Hyaloperonospora arabidopsidis. Taken together, our data suggest that silencing of AtRAP by AtlsiRNA-1 upon bacterial infection triggers defense responses through regulation of LSU2 and GLK1.

ACCERBATIN, a small molecule at the intersection of auxin and reactive oxygen species homeostasis with herbicidal properties

The volatile two-carbon hormone ethylene acts in concert with an array of signals to affect etiolated seedling development. From a chemical screen, we isolated a quinoline carboxamide designated ACCERBATIN (AEX) that exacerbates the 1-aminocyclopropane-1-carboxylic acid-induced triple response, typical for ethylene-treated seedlings in darkness. Phenotypic analyses revealed distinct AEX effects including inhibition of root hair development and shortening of the root meristem. Mutant analysis and reporter studies further suggested that AEX most probably acts in parallel to ethylene signaling. We demonstrated that AEX functions at the intersection of auxin metabolism and reactive oxygen species (ROS) homeostasis. AEX inhibited auxin efflux in BY-2 cells and promoted indole-3-acetic acid (IAA) oxidation in the shoot apical meristem and cotyledons of etiolated seedlings. Gene expression studies and superoxide/hydrogen peroxide staining further revealed that the disrupted auxin homeostasis was accompanied by oxidative stress. Interestingly, in light conditions, AEX exhibited properties reminiscent of the quinoline carboxylate-type auxin-like herbicides. We propose that AEX interferes with auxin transport from its major biosynthesis sites, either as a direct consequence of poor basipetal transport from the shoot meristematic region, or indirectly, through excessive IAA oxidation and ROS accumulation. Further investigation of AEX can provide new insights into the mechanisms connecting auxin and ROS homeostasis in plant development and provide useful tools to study auxin-type herbicides.

The Arabidopsis miR396 mediates pathogen-associated molecular pattern-triggered immune responses against fungal pathogens

MicroRNAs (miRNAs) play a pivotal role in regulating gene expression during plant development. Although a substantial fraction of plant miRNAs has proven responsive to pathogen infection, their role in disease resistance remains largely unknown, especially during fungal infections. In this study, we screened Arabidopsis thaliana lines in which miRNA activity has been reduced using artificial miRNA target mimics (MIM lines) for their response to fungal pathogens. Reduced activity of miR396 (MIM396 plants) was found to confer broad resistance to necrotrophic and hemibiotrophic fungal pathogens. MiR396 levels gradually decreased during fungal infection, thus, enabling its GRF (GROWTH-REGULATING FACTOR) transcription factor target genes to trigger host reprogramming. Pathogen resistance in MIM396 plants is based on a superactivation of defense responses consistent with a priming event during pathogen infection. Notably, low levels of miR396 are not translated in developmental defects in absence of pathogen challenge. Our findings support a role of miR396 in regulating plant immunity, and broaden our knowledge about the molecular players and processes that sustain defense priming. That miR396 modulates innate immunity without growth costs also suggests fine-tuning of miR396 levels as an effective biotechnological means for protection against pathogen infection.

Thienopyrimidine-type compounds protect Arabidopsis plants against the hemibiotrophic fungal pathogen Colletotrichum higginsianum and bacterial pathogen Pseudomonas syringae pv. maculicola

Plant activators activate systemic acquired resistance-like defense responses or induced systemic resistance, and thus protect plants from pathogens. We screened a chemical library composed of structurally diverse small molecules. We isolated six plant immune-inducing thienopyrimidine-type compounds and their analogous compounds. It was observed that the core structure of thienopyrimidine plays a role in induced resistance in plants. Furthermore, we highlight the protective effect of thienopyrimidine-type compounds against both hemibiotrophic fungal pathogen, Colletotrichum higginsianum, and bacterial pathogen, Pseudomonas syringae pv. maculicola, in Arabidopsis thaliana. We suggest that thienopyrimidine-type compounds could be potential lead compounds as novel plant activators, and can be useful and effective agrochemicals against various plant diseases.

TRI6 and TRI10 play different roles in the regulation of deoxynivalenol (DON) production by cAMP signalling in Fusarium graminearum

The biosynthesis of mycotoxin deoxynivalenol (DON) in Fusarium graminearum is regulated by two pathway-specific transcription factors Tri6 and Tri10 and affected by various host and environmental factors. In this study, we showed that cyclic adenosine monophosphate (cAMP) treatment induced DON production by stimulating TRI gene expression and DON-associated cellular differentiation in F. graminearum. Interestingly, exogenous cAMP had no effects on the tri6 mutant but partially recovered the defect of tri10 mutant in DON biosynthesis. Although the two cAMP phosphodiesterase genes PDE1 and PDE2 had overlapping functions in vegetative growth, conidiation, sexual reproduction and plant infection, deletion of PDE2 but not PDE1 activated intracellular PKA activities and increased DON production. Whereas the tri6 pde2 mutant failed to produce DON, the tri10 pde2 double mutant produced a significantly higher level of DON than the tri10 mutant. Cellular differentiation associated with DON production was stimulated by exogenous cAMP or deletion of PDE2 in both tri10 and tri6 mutants. These data indicate that TRI6 is essential for the regulation of DON biosynthesis by cAMP signalling but elevated PKA activities could partially bypass the requirement of TRI10 for TRI gene-expression and DON production, and Pde2 is the major cAMP phosphodiesterase to negatively regulate DON biosynthesis in F. graminearum.

Intraspecies Interaction of Fusarium graminearum Contributes to Reduced Toxin Production and Virulence

Fusarium graminearum is a pathogenic fungus that causes Fusarium head blight in wheat and lowers the yield and quality of grains by contamination with the trichothecene mycotoxin deoxynivalenol. The fungi coexist and interact with several different fusaria as well as other plant pathogenic fungi and bacteria in the field. In Canada, F. graminearum exists as two main trichothecene chemotypes: 3-acetyldeoxynivalenol and 15-acetyldeoxynivalenol. To understand the potential interactions between two isolates of these chemotypes, we conducted coinoculation studies both in culture and in planta. The studies showed that intraspecies interaction reduces trichothecene yield in culture and disease symptoms in wheat. To elucidate the genes involved in the intraspecies interaction, expression profiling was performed on RNA samples isolated from coinoculated cultures, and potential genes were identified by using the genome sequences of the respective isolates.

Discovery of novel small molecule modulators of Clavibacter michiganensis subsp. michiganensis

Clavibacter michiganensis subsp. michiganensis (Cmm) is a Gram-positive seed-transmitted bacterial phytopathogen responsible for substantial economic losses by adversely affecting tomato production worldwide. A high-throughput, cell-based screen was adapted to identify novel small molecule growth inhibitors to serve as leads for future bactericide development. A library of 4,182 compounds known to be bioactive against Saccharomyces cerevisiae was selected for primary screening against Cmm wild-type strain C290 for whole-cell growth inhibition. Four hundred sixty-eight molecules (11.2% hit rate) were identified as bacteriocidal or bacteriostatic against Cmm at 200 μM. Seventy-seven candidates were selected based on Golden Triangle analyses for secondary screening. Secondary screens showed that several of these candidates were strain-selective. Several compounds were inhibitory to multiple Cmm strains as well as Bacillus subtilis, but not to Pseudomonas fluorescens, Mitsuaria sp., Lysobacter enzymogenes, Lactobacillus rhamnosus, Bifidobacterium animalis, or Escherichia coli. Most of the compounds were not phytotoxic and did not show overt host toxicity. Using a novel 96-well bioluminescent Cmm seedling infection assay, we assessed effects of selected compounds on pathogen infection. The 12 most potent novel molecules were identified by compiling the scores from all secondary screens combined with the reduction of pathogen infection in planta. When tested for ability to develop resistance to the top-12 compounds, no resistant Cmm were recovered, suggesting that the discovered compounds are unlikely to induce resistance. In conclusion, here we report top-12 compounds that provide chemical scaffolds for future Cmm-specific bactericide development.

Host to a Stranger: Arabidopsis and Fusarium Ear Blight

Fusarium ear blight (FEB) is a devastating fungal disease of cereal crops. Outbreaks are sporadic and current control strategies are severely limited. This review highlights the use of Arabidopsis to study plant-FEB interactions. Use of this pathosystem has identified natural variation in Fusarium susceptibility in Arabidopsis, and native plant genes and signalling processes modulating the interaction. Recent breakthroughs include the identification of plant- and insect-derived small molecules which increase disease resistance, and the use of a host-induced gene silencing (HIGS) construct to silence an important Fusarium gene to prevent infection. Arabidopsis has also been used to study other fungi that cause cereal diseases. These findings offer the potential for translational research in cereals which could yield much-needed novel control strategies.

Leucine metabolism regulates TRI6 expression and affects deoxynivalenol production and virulence in Fusarium graminearum

TRI6 is a positive regulator of the trichothecene gene cluster and the production of trichothecene mycotoxins [deoxynivalenol (DON)] and acetylated forms such as 15-Acetyl-DON) in the cereal pathogen Fusarium graminearum. As a global transcriptional regulator, TRI6 expression is modulated by nitrogen-limiting conditions, sources of nitrogen and carbon, pH and light. However, the mechanism by which these diverse environmental factors affect TRI6 expression remains underexplored. In our effort to understand how nutrients affect TRI6 regulation, comparative digital expression profiling was performed with a wild-type F. graminearum and a Δtri6 mutant strain, grown in nutrient-rich conditions. Analysis showed that TRI6 negatively regulates genes of the branched-chain amino acid (BCAA) metabolic pathway. Feeding studies with deletion mutants of MCC, encoding methylcrotonyl-CoA-carboxylase, one of the key enzymes of leucine metabolism, showed that addition of leucine specifically down-regulated TRI6 expression and reduced 15-ADON accumulation. Constitutive expression of TRI6 in the Δmcc mutant strain restored 15-ADON production. A combination of cellophane breach assays and pathogenicity experiments on wheat demonstrated that disrupting the leucine metabolic pathway significantly reduced disease. These findings suggest a complex interaction between one of the primary metabolic pathways with a global regulator of mycotoxin biosynthesis and virulence in F. graminearum.

Synthetic plant defense elicitors

To defend themselves against invading pathogens plants utilize a complex regulatory network that coordinates extensive transcriptional and metabolic reprogramming. Although many of the key players of this immunity-associated network are known, the details of its topology and dynamics are still poorly understood. As an alternative to forward and reverse genetic studies, chemical genetics-related approaches based on bioactive small molecules have gained substantial popularity in the analysis of biological pathways and networks. Use of such molecular probes can allow researchers to access biological space that was previously inaccessible to genetic analyses due to gene redundancy or lethality of mutations. Synthetic elicitors are small drug-like molecules that induce plant defense responses, but are distinct from known natural elicitors of plant immunity. While the discovery of some synthetic elicitors had already been reported in the 1970s, recent breakthroughs in combinatorial chemical synthesis now allow for inexpensive high-throughput screens for bioactive plant defense-inducing compounds. Along with powerful reverse genetics tools and resources available for model plants and crop systems, comprehensive collections of new synthetic elicitors will likely allow plant scientists to study the intricacies of plant defense signaling pathways and networks in an unparalleled fashion. As synthetic elicitors can protect crops from diseases, without the need to be directly toxic for pathogenic organisms, they may also serve as promising alternatives to conventional biocidal pesticides, which often are harmful for the environment, farmers and consumers. Here we are discussing various types of synthetic elicitors that have been used for studies on the plant immune system, their modes-of-action as well as their application in crop protection.

Mutations in the Arabidopsis homoserine kinase gene DMR1 confer enhanced resistance to Fusarium culmorum and F. graminearum

BACKGROUND

Mutation of Arabidopsis DMR1, encoding homoserine kinase, leads to elevation in homoserine and foliar resistance to the biotrophic pathogens Hyaloperonospora arabidopsidis and Oidium neolycopersici through activation of an unidentified defence mechanism. This study investigates the effect of mutation of dmr1 on resistance to the ascomycete pathogens Fusarium graminearum and F. culmorum, which cause Fusarium Ear Blight (FEB) disease on small grain cereals.

RESULTS

We initially found that the dmr1-2 mutant allele confers increased resistance to F. culmorum and F. graminearum silique infection, and decreased colonisation of rosette leaves. Meanwhile the dmr1-1 allele supports less rosette leaf colonisation but has wild type silique resistance. Three additional dmr1 alleles were subsequently examined for altered F. culmorum susceptibility and all showed increased silique resistance, while leaf colonisation was reduced in two (dmr1-3 and dmr1-4). Amino acid analysis of dmr1 siliques revealed homoserine accumulation, which is undetectable in wild type plants. Exogenous application of L-homoserine reduced bud infection in both dmr1 and wild type plants, whilst D-homoserine application did not. Delayed leaf senescence was also observed in dmr1 plants compared to wild type and correlated with reduced Fusarium leaf colonisation.

CONCLUSIONS

These findings suggest that common Arabidopsis DMR1 mediated susceptibility mechanisms occur during infection by both obligate biotrophic oomycete and hemi-biotrophic fungal pathogens, not only in vegetative but also in reproductive plant tissues. This has the potential to aid the development of cereal crops with enhanced resistance to FEB.

Using molecular tools to decipher the complex world of plant resistance inducers: an apple case study

Exogenous application of plant resistance inducers (PRIs) able to activate plant defenses is an interesting approach for new integrated pest management practices. The full integration of PRIs into agricultural practices requires methods for the fast and objective upstream screening of efficient PRIs and optimization of their application. To select active PRIs, we used a molecular tool as an alternative to methods involving plant protection assays. The expressions of 28 genes involved in complementary plant defense mechanisms were simultaneously determined by quantitative real-time PCR in PRI-treated tissues. Using a set of 10 commercial preparations and considering the pathosystem apple/Erwinia amylovora, this study shows a strong correlation between defense activation and protection efficiency in controlled conditions, thus enabling the easy identification of promising PRIs in fire blight protection. Hence this work clearly highlights the benefits of using a molecular tool to discriminate nonactive PRI preparations and provides useful molecular markers for the optimization of their use in orchard.

High-throughput screening of small-molecule libraries for inducers of plant defense responses

Transgenic Arabidopsis seedlings containing a pathogen-responsive reporter gene allow for convenient high-throughput screening of chemical libraries for compounds that induce plant defense responses. Candidates identified by such screens can be further tested for their ability to protect plants from pathogen-caused diseases. Using Arabidopsis defense signaling mutants, defined regulatory processes that are targeted by a given candidate molecule can be easily narrowed down. Here, we provide a detailed high-throughput screening protocol for library compounds that activate a pathogen-responsive reporter gene in liquid-grown Arabidopsis seedlings.

Arabidopsis GOLDEN2-LIKE (GLK) transcription factors activate jasmonic acid (JA)-dependent disease susceptibility to the biotrophic pathogen Hyaloperonospora arabidopsidis, as well as JA-independent plant immunity against the necrotrophic pathogen Botrytis cinerea

Arabidopsis thaliana GOLDEN2-LIKE (GLK1 and 2) transcription factors regulate chloroplast development in a redundant manner. Overexpression of AtGLK1 (35S:AtGLK1) in Arabidopsis also confers resistance to the cereal pathogen Fusarium graminearum. To further elucidate the role of GLK transcription factors in plant defence, the Arabidopsis glk1 glk2 double-mutant and 35S:AtGLK1 plants were challenged with the virulent oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) Noco2. Compared with Col-0, glk1 glk2 plants were highly resistant to Hpa Noco2, whereas 35S:AtGLK1 plants showed enhanced susceptibility to this pathogen. Genetic studies suggested that AtGLK-mediated plant defence to Hpa Noco2 was partially dependent on salicylic acid (SA) accumulation, but independent of the SA signalling protein NONEXPRESSOR OF PATHOGENESIS-RELATED 1 (NPR1). Pretreatment with jasmonic acid (JA) dramatically reversed Hpa Noco2 resistance in the glk1 glk2 double mutant, but only marginally affected the 35S:AtGLK1 plants. In addition, overexpression of AtGLK1 in the JA signalling mutant coi1-16 did not increase susceptibility to Hpa Noco2. Together, our GLK gain-of-function and loss-of-function experiments suggest that GLK acts upstream of JA signalling in disease susceptibility to Hpa Noco2. In contrast, glk1 glk2 plants were more susceptible to the necrotrophic fungal pathogen Botrytis cinerea, whereas 35S:AtGLK1 plants exhibited heightened resistance which could be maintained in the absence of JA signalling. Together, the data reveal that AtGLK1 is involved in JA-dependent susceptibility to the biotrophic pathogen Hpa Noco2 and in JA-independent resistance to the necrotrophic pathogen B. cinerea.

Cytological and molecular characterization of quantitative trait locus qRfg1, which confers resistance to gibberella stalk rot in maize

Tremendous progress has been made recently in understanding plant response to Fusarium graminearum infection. Here, the cytological aspect and molecular mechanism of maize defense to F. graminearum infection were characterized using a pair of near-isogenic lines (NIL), the resistant and the susceptible NIL. F. graminearum primarily penetrated the maize root tip and no penetration structure was found. The fungal biomass within the root correlated well with root-disease severity. Following inoculation, R-NIL and S-NIL plants significantly differed in percentage of diseased primary roots. In R-NIL roots, a fraction of exodermal cells collapsed to form cavities, and hyphae were confined to the outer exodermal cells. However, most exodermal cells shrank and turned brown, and fungi colonized the entire S-NIL root. In the R-NIL roots, the exodermal cells exhibited plasmolysis and atropous hyphal growth whereas, in the exodermal cells of the S-NIL roots, severe cellular degradation and membrane-coated, lushly grown hyphae were found. Transcriptome sequencing revealed comprehensive transcription reprogramming, reinforcement of a complex defense network, to enhance the systemic and basal resistance. This study reports a detailed microscopic analysis of F. graminearum infection on maize root, and provides insights into the molecular mechanisms underlying maize resistance to the pathogen.

Fusarium culmorum: causal agent of foot and root rot and head blight on wheat

Fusarium culmorum is a ubiquitous soil-borne fungus able to cause foot and root rot and Fusarium head blight on different small-grain cereals, in particular wheat and barley. It causes significant yield and quality losses and results in contamination of the grain with mycotoxins. This review summarizes recent research activities related to F. culmorum, including studies into its population diversity, mycotoxin biosynthesis, mechanisms of pathogenesis and resistance, the development of diagnostic tools and preliminary genome sequence surveys. We also propose potential research areas that may expand our basic understanding of the wheat-F. culmorum interaction and assist in the management of the disease caused by this pathogen.

TAXONOMY

Fusarium culmorum (W.G. Smith) Sacc. Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Sordariomycetes; Subclass Hypocreomycetidae; Order Hypocreales; Family Nectriaceae; Genus Fusarium.

DISEASE SYMPTOMS

Foot and root rot (also known as Fusarium crown rot): seedling blight with death of the plant before or after emergence; brown discoloration on roots and coleoptiles of the infected seedlings; brown discoloration on subcrown internodes and on the first two/three internodes of the main stem; tiller abortion; formation of whiteheads with shrivelled white grains; Fusarium head blight: prematurely bleached spikelets or blighting of the entire head, which remains empty or contains shrunken dark kernels. IDENTIFICATION AND DETECTION: Morphological identification is based on the shape of the macroconidia formed on sporodochia on carnation leaf agar. The conidiophores are branched monophialides, short and wide. The macroconidia are relatively short and stout with an apical cell blunt or slightly papillate; the basal cell is foot-shaped or just notched. Macroconidia are thick-walled and curved, usually 3-5 septate, and mostly measuring 30-50 × 5.0-7.5 μm. Microconidia are absent. Oval to globose chlamydospores are formed, intercalary in the hyphae, solitary, in chains or in clumps; they are also formed from macroconidia. The colony grows very rapidly (1.6-2.2 cm/day) on potato dextrose agar (PDA) at the optimum temperature of 25 °C. The mycelium on PDA is floccose, whitish, light yellow or red. The pigment on the reverse plate on PDA varies from greyish-rose, carmine red or burgundy. A wide array of polymerase chain reaction (PCR) and real-time PCR tools, as well as complementary methods, which are summarised in the first two tables, have been developed for the detection and/or quantification of F. culmorum in culture and in naturally infected plant tissue.

HOST RANGE

Fusarium culmorum has a wide range of host plants, mainly cereals, such as wheat, barley, oats, rye, corn, sorghum and various grasses. In addition, it has been isolated from sugar beet, flax, carnation, bean, pea, asparagus, red clover, hop, leeks, Norway spruce, strawberry and potato tuber. Fusarium culmorum has also been associated with dermatitis on marram grass planters in the Netherlands, although its role as a causal agent of skin lesions appears questionable. It is also isolated as a symbiont able to confer resistance to abiotic stress, and has been proposed as a potential biocontrol agent to control the aquatic weed Hydrilla spp.

USEFUL WEBSITES

http://isolate.fusariumdb.org/; http://sppadbase.ipp.cnr.it/; http://www.broad.mit.edu/annotation/genome/fusarium_group/MultiHome.html; http://www.fgsc.net/Fusarium/fushome.htm; http://plantpath.psu.edu/facilities/fusarium-research-center; http://www.phi-base.org/; http://www.uniprot.org/; http://www.cabi.org/; http://www.indexfungorum.org/

Towards systems biology of mycotoxin regulation

Systems biology is a scientific approach that integrates many scientific disciplines to develop a comprehensive understanding of biological phenomena, thus allowing the prediction and accurate simulation of complex biological behaviors. It may be presumptuous to write about toxin regulation at the level of systems biology, but the last decade of research is leading us closer than ever to this approach. Past research has delineated multiple levels of regulation in the pathways leading to the biosynthesis of secondary metabolites, including mycotoxins. At the top of this hierarchy, the global or master transcriptional regulators perceive various environmental cues such as climatic conditions, the availability of nutrients, and the developmental stages of the organism. Information accumulated from various inputs is integrated through a complex web of signalling networks to generate the eventual outcome. This review will focus on adapting techniques such as chemical and other genetic tools available in the model system Saccharomyces cerevisiae, to disentangle the various biological networks involved in the biosynthesis of mycotoxins in the Fusarium spp.

Comparative secretome analysis of Fusarium graminearum and two of its non-pathogenic mutants upon deoxynivalenol induction in vitro

To understand early events in plant-pathogen interactions, it is necessary to explore the pathogen secretome to identify secreted proteins that help orchestrate pathology. The secretome can be obtained from pathogens grown in vitro, and then characterized using standard proteomic approaches based on protein extraction and subsequent identification of tryptic peptides by LC-MS. A subset of the secretome is composed of proteins whose presence is required to initiate infection and their removal from the secretome would result in pathogens with reduced or no virulence. We present here comparative secretome from Fusarium graminearum. This filamentous fungus causes Fusarium head blight on wheat, a serious cereal disease found in many cereal-growing regions. Affected grain is contaminated with mycotoxins and cannot be used for food or feed. We used label-free quantitative MS to compare the secretomes of wild-type with two nonpathogenic deletion mutants of F. graminearum, Δtri6, and Δtri10. These mutations in mycotoxin-regulating transcription factors revealed a subset of 29 proteins whose relative abundance was affected in their secretomes, as measured by spectral counting. Proteins that decreased in abundance are potential candidate virulence factors and these included cell wall-degrading enzymes, metabolic enzymes, pathogenesis-related proteins, and proteins of unknown function.

On the trail of a cereal killer: recent advances in Fusarium graminearum pathogenomics and host resistance

The ascomycete fungal pathogen Fusarium graminearum (sexual stage: Gibberella zeae) causes the devastating head blight or scab disease on wheat and barley, and cob or ear rot disease on maize. Fusarium graminearum infection causes significant crop and quality losses. In addition to roles as virulence factors during pathogenesis, trichothecene mycotoxins (e.g. deoxynivalenol) produced by this pathogen constitute a significant threat to human and animal health if consumed in respective food or feed products. In the last few years, significant progress has been made towards a better understanding of the processes involved in F. graminearum pathogenesis, toxin biosynthesis and host resistance mechanisms through the use of high-throughput genomic and phenomic technologies. In this article, we briefly review these new advances and also discuss how future research can contribute to the development of sustainable plant protection strategies against this important plant pathogen.

Small molecules present large opportunities in plant biology

Since the introduction of chemical genomics to plant biology as a tool for basic research, the field has advanced significantly. There are now examples of important basic discoveries that demonstrate the power and untapped potential of this approach. Given the combination of protein and small-molecule complexity, new phenotypes can be described through the perturbation of cellular functions that can be linked to growth and developmental phenotypes. There are now clear examples of overcoming functional redundancy in plants to dissect molecular mechanisms or critical pathways such as hormone signaling and dynamic intracellular processes. Owing to ongoing advances, including more sophisticated high-content screening and rapid approaches for target identification, the field is beginning to move forward. However, there are also challenges to improve automation, imaging, and analysis and provide chemical biology resources to the broader plant biology community.

Tri6 is a global transcription regulator in the phytopathogen Fusarium graminearum

In F. graminearum, the transcriptional regulator Tri6 is encoded within the trichothecene gene cluster and regulates genes involved in the biosynthesis of the secondary metabolite deoxynivalenol (DON). The Tri6 protein with its Cys₂His₂ zinc-finger may also conform to the class of global transcription regulators. This class of global transcriptional regulators mediate various environmental cues and generally responds to the demands of cellular metabolism. To address this issue directly, we sought to find gene targets of Tri6 in F. graminearum grown in optimal nutrient conditions. Chromatin immunoprecipitation followed by Illumina sequencing (ChIP-Seq) revealed that in addition to identifying six genes within the trichothecene gene cluster, Tri1, Tri3, Tri6, Tri7, Tri12 and Tri14, the ChIP-Seq also identified 192 additional targets potentially regulated by Tri6. Functional classification revealed that, among the annotated genes, ∼40% are associated with cellular metabolism and transport and the rest of the target genes fall into the category of signal transduction and gene expression regulation. ChIP-Seq data also revealed Tri6 has the highest affinity toward its own promoter, suggesting that this gene could be subject to self-regulation. Electro mobility shift assays (EMSA) performed on the promoter of Tri6 with purified Tri6 protein identified a minimum binding motif of GTGA repeats as a consensus sequence. Finally, expression profiling of F. graminearum grown under nitrogen-limiting conditions revealed that 49 out of 198 target genes are differentially regulated by Tri6. The identification of potential new targets together with deciphering novel binding sites for Tri6, casts new light into the role of this transcriptional regulator in the overall growth and development of F. graminearum.

Our knowledge of mechanisms involved in the activation and biosynthesis of DON comes largely from in vitro culture studies. Cumulated knowledge suggests that the physiological status of the fungus and the availability of nutrients are the main determining factors for DON production. Integration of various environmental cues to coordinate expression of secondary metabolic genes is thought to be mediated by a combination of global and pathway-specific transcription factors. While the global transcriptional factors respond to broad range of environmental cues such as the availability of carbon and nitrogen, the pathway-specific transcriptional factors regulate genes within a gene cluster. In F. graminearum, the transcriptional regulator Tri6 is encoded within the trichothecene gene cluster and regulates genes involved in the synthesis and transport of DON. In this report, we utilized ChIP-Seq to demonstrate that Tri6 can potentially bind to promoters and regulate genes not involved in the synthesis of DON and furthermore, many of these non-trichothecene genes are involved in various aspects of cellular metabolism, including transport and energy. Expression profiling revealed that many of the target genes are differentially regulated by Tri6, thus validating our hypothesis that Tri6 is a global regulator involved in cellular metabolism.