Fdb1 and Fdb2, Fusarium verticillioides loci necessary for detoxification of preformed antimicrobials from corn

A Fusarium verticillioides MAT1-2 Strain near Isogenic to the Sequenced FGSC7600 Strain for Producing Homozygous Multigene Mutants

Fungal genetic systems ideally combine molecular tools for genome manipulation and a sexual reproduction system to create an informative assortment of combinations of genomic modifications. When employing the sexual cycle to generate multi-mutants, the background genotype variations in the parents may result in progeny phenotypic variation obscuring the effects of combined mutations. Here, to mitigate this variation in Fusarium verticillioides, we generated a MAT1-2 strain that was near isogenic to the sequenced wild-type MAT1-1 strain, FGSC7600. This was accomplished by crossing FGSC7600 with the divergent wild-type MAT1-2 strain FGSC7603 followed by six sequential backcrosses (e.g., six generations) of MAT1-2 progeny to FGSC7600. We sequenced each generation and mapped recombination events. The parental cross involved twenty-six crossovers on nine of the eleven chromosomes. The dispensable chromosome 12, found in FGSC7603 but lacking in FGSC7600, was not present in the progeny post generation five. Inheritance of complete chromosomes without crossover was frequently observed. A deletion of approximately 140 kilobases, containing 54 predicted genes on chromosome 4, occurred in generation 4 and was retained in generation 5 indicating that these genes are dispensable for growth and both asexual and sexual reproduction. The final MAT1-2 strain TMRU10/35 is about 93% identical to FGSC7600. TMRU10/35 is available from the Fungal Genetics Stock Center as FGSC27326 and from the ARS Culture Collection as NRRL64809.

Adaptation of Ustilago maydis to phenolic and alkaloid responsive metabolites in maize B73

The adaptation of pathogenic fungi to plant-specialized metabolites is necessary for their survival and reproduction. The biotrophic fungus Ustilago maydis can cause maize smut and produce tumors in maize (Zea mays), resulting in reduced maize yield and significant economic losses. Qualitative analysis using UPLC-MS/MS revealed that the infection of maize variety B73 with U. maydis resulted in increased levels of phytohormones, phenolics, and alkaloids in maize seedling tissues. However, correlation analysis showed that nearly all compounds in the mechanical damage group were significantly negatively correlated with the shoot growth indexes of maize B73. The correlation coefficients of 2-hydroxy-7-methoxy-1,4-benzoxazin-3-one (HMBOA) and maize B73 shoot length and shoot weight were r = -0.56 (p < 0.01) and r = -0.75 (p < 0.001), respectively. In the inoculation group, these correlations weakened, with the correlation coefficients between HMBOA and maize B73 shoot length and shoot weight being r = 0.02 and r = -0.1, respectively. The correlation coefficients between 6-methoxy-2-benzoxazolinone (MBOA) and the shoot weight were r = -0.73 (p < 0.001) and r = -0.15 in the mechanical damage group and inoculation group, respectively. These findings suggest that increased concentrations of these compounds are more positively associated with mechanical damage than with U. maydis infection. At high concentrations, most of these compounds had an inhibitory effect on U. maydis. This study investigated the ability of U. maydis to regulate various compounds, including phytohormones, phenolic acids, and alkaloids in maize B73, providing evidence that U. maydis has adapted to the specialized metabolites produced by maize B73.

Isolation and identification of allelochemicals and their activities and functions

Allelopathy is the interaction between donor plants and receiver plants through allelochemicals. According to a great number of publications, allelopathy may be involved in several ecological aspects such as the formation of monospecific stands and sparse understory vegetation for certain plant species. Allelopathy also contributes to the naturalization of invasive plant species in introduced ranges. Autotoxicity is a particular type of allelopathy involving certain compounds. Many medicinal plants have been reported to show relatively high allelopathic activity. We selected plant species that show high allelopathic activity and isolated allelochemicals through the bioassay-guided purification process. More than 100 allelochemicals, including novel compounds have been identified in some medicinal and invasive plants, plants forming monospecific stands, plants with sparse understory vegetation, and plants showing autotoxicity. The allelopathic activity of benzoxazinones and related compounds was also determined.

The impact of climate change on maize chemical defenses

Climate change is increasingly affecting agriculture, both at the levels of crops themselves, and by altering the distribution and damage caused by insect or microbial pests. As global food security depends on the reliable production of major crops such as maize (Zea mays), it is vital that appropriate steps are taken to mitigate these negative impacts. To do this a clear understanding of what the impacts are and how they occur is needed. This review focuses on the impact of climate change on the production and effectiveness of maize chemical defenses, including volatile organic compounds, terpenoid phytoalexins, benzoxazinoids, phenolics, and flavonoids. Drought, flooding, heat stress, and elevated concentrations of atmospheric carbon dioxide, all impact the production of maize chemical defenses, in a compound and tissue-specific manner. Furthermore, changes in stomatal conductance and altered soil conditions caused by climate change can impact environmental dispersal and effectiveness certain chemicals. This can alter both defensive barrier formation and multitrophic interactions. The production of defense chemicals is controlled by stress signaling networks. The use of similar networks to co-ordinate the response to abiotic and biotic stress can lead to complex integration of these networks in response to the combinatorial stresses that are likely to occur in a changing climate. The impact of multiple stressors on maize chemical defenses can therefore be different from the sum of the responses to individual stressors and challenging to predict. Much work remains to effectively leverage these protective chemicals in climate-resilient maize.

Genome sequence resource of Pectobacterium polaris QK413-1 that causes blackleg on potato in Fujian Province, China

The Pectobacterium pathogens cause soft rot and blackleg diseases on many plants and crops, including potatoes. Here we first report a high-quality genome assembly and announcement of the P. polaris strain QK413-1, which causes blackleg disease in potatoes in China. The QK413-1 genome was sequenced and assembled using the PacBio Sequel II and Illumina sequencing platform. The assembled genome has a total size of 5,005,507bp with a GC content of 51.81%, encoding 4782 open reading frames, including 639 virulence genes, 273 drug resistance genes, and 416 secreted proteins. The QK413-1 genome sequence provides a valuable resource for the control of potato blackleg and research into its mechanism.

Transcriptomic Responses of Fusarium verticillioides to Lactam and Lactone Xenobiotics

The important cereal crops of maize, rye, and wheat constitutively produce precursors to 2-benzoxazolinone, a phytochemical having antifungal effects towards many Fusarium species. However, Fusarium verticillioides can tolerate 2-benzoxazolinone by converting it into non-toxic metabolites through the synergism of two previously identified gene clusters, FDB1 and FDB2. Inspired by the induction of these two clusters upon exposure to 2-benzoxazolinone, RNA sequencing experiments were carried out by challenging F. verticillioides individually with 2-benzoxazolinone and three related chemical compounds, 2-oxindole, 2-coumaranone, and chlorzoxazone. These compounds all contain lactam and/or lactone moieties, and transcriptional analysis provided inferences regarding the degradation of such lactams and lactones. Besides induction of FDB1 and FDB2 gene clusters, four additional clusters were identified as induced by 2-benzoxazolinone exposure, including a cluster thought to be responsible for biosynthesis of pyridoxine (vitamin B6), a known antioxidant providing tolerance to reactive oxygen species. Three putative gene clusters were identified as induced by challenging F. verticillioides with 2-oxindole, two with 2-coumaranone, and two with chlorzoxazone. Interestingly, 2-benzoxazolinone and 2-oxindole each induced two specific gene clusters with similar composition of enzymatic functions. Exposure to 2-coumranone elicited the expression of the fusaric acid biosynthetic gene cluster. Another gene cluster that may encode enzymes responsible for degrading intermediate catabolic metabolites with carboxylic ester bonds was induced by 2-benzoxazolinone, 2-oxindole, and chlorzoxazone. Also, the induction of a dehalogenase encoding gene during chlorzoxazone exposure suggested its role in the removal of the chlorine atom. Together, this work identifies genes and putative gene clusters responsive to the 2-benzoxazolinone-like compounds with metabolic inferences. Potential targets for future functional analyses are discussed.

Molecular Insights into Defense Responses of Vietnamese Maize Varieties to Fusarium verticillioides Isolates

Fusarium ear rot (FER) caused by Fusarium verticillioides is one of the main fungal diseases in maize worldwide. To develop a pathogen-tailored FER resistant maize line for local implementation, insights into the virulence variability of a residing F. verticillioides population are crucial for developing customized maize varieties, but remain unexplored. Moreover, little information is currently available on the involvement of the archetypal defense pathways in the F. verticillioides-maize interaction using local isolates and germplasm, respectively. Therefore, this study aims to fill these knowledge gaps. We used a collection of 12 F. verticillioides isolates randomly gathered from diseased maize fields in the Vietnamese central highlands. To assess the plant's defense responses against the pathogens, two of the most important maize hybrid genotypes grown in this agro-ecological zone, lines CP888 and Bt/GT NK7328, were used. Based on two assays, a germination and an in-planta assay, we found that line CP888 was more susceptible to the F. verticillioides isolates when compared to line Bt/GT NK7328. Using the most aggressive isolate, we monitored disease severity and gene expression profiles related to biosynthesis pathways of salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), benzoxazinoids (BXs), and pathogenesis-related proteins (PRs). As a result, a stronger induction of SA, JA, ABA, BXs, and PRs synthesizing genes might be linked to the higher resistance of line Bt/GT NK7328 compared to the susceptible line CP888. All these findings could supply valuable knowledge in the selection of suitable FER resistant lines against the local F. verticllioides population and in the development of new FER resistant germplasms.

Disarming the Host: Detoxification of Plant Defense Compounds During Fungal Necrotrophy

While fungal biotrophs are dependent on successfully suppressing/subverting host defenses during their interaction with live cells, necrotrophs, due to their lifestyle are often confronted with a suite of toxic metabolites. These include an assortment of plant defense compounds (PDCs) which can demonstrate broad antifungal activity. These PDCs can be either constitutively present in plant tissue or induced in response to infection, but are nevertheless an important obstacle which needs to be overcome for successful pathogenesis. Fungal necrotrophs have developed a number of strategies to achieve this goal, from the direct detoxification of these compounds through enzymatic catalysis and modification, to the active transport of various PDCs to achieve toxin sequestration and efflux. Studies have shown across multiple pathogens that the efficient detoxification of host PDCs is both critical for successful infection and often a determinant factor in pathogen host range. Here, we provide a broad and comparative overview of the various mechanisms for PDC detoxification which have been identified in both fungal necrotrophs and fungal pathogens which depend on detoxification during a necrotrophic phase of infection. Furthermore, the effect that these mechanisms have on fungal host range, metabolism, and disease control will be discussed.

Smut fungal strategies for the successful infection

The smut fungi include a large number of plant pathogens that establish obligate biotrophic relationships with their host. Throughout the whole life inside plant tissue, smut fungi keep plant cells alive and acquire nutrients via biotrophic interfaces. This mini-review mainly summarizes the interactions between smut fungi and their host plants during the infection process. Despite various strategies recruited by plants to defense invading pathogens, smut fungi successfully evolved an arsenal for colonization. Mating of two compatible haploids gives rise to parasitic mycelium, which can sense plant surface cues such as fatty acids and hydrophobic surface, and induce the formation of appressoria for surface penetration. Plants can recognize fungal invading and activate defense response, including callose and lignin deposition, programmed cell death, and SA signaling pathway. To suppress plant immunity and alter the metabolic pathway of host plants, a cocktail of effectors is secreted by smut fungi depending on the plant organ and cell type that is infected.

The Trichoderma viride F-00612 consortium tolerates 2-amino-3H-phenoxazin-3-one and degrades nitrated benzo[d]oxazol-2(3H)-one

Numerous allelopathic plant secondary metabolites impact plant–microorganism interactions by injuring plant-associated beneficial bacteria and fungi. Fungi belonging to the genus Trichoderma positively influence crops, including benzoxazinone-containing maize. However, benzoxazinones and their downstream metabolites such as benzoxazolinone and phenoxazinones are often fungitoxic. Specimen Trichoderma viride F-00612 was found to be insensitive to 100-µM phenoxazinone and 500-µM benzoxazolinone. Screening of 46 additional specimens of ascomycetes revealed insensitivity to phenoxazinones among fungi that cause disease in benzoxazinone-producing cereal crops, whereas many other ascomycetes were highly sensitive. In contrast, most of the screened fungi were insensitive to benzoxazolinone. T. viride F-00612 was associated with bacteria and, thus, existed as a consortium. By contrast, Enterobacter species and Acinetobacter calcoaceticus were prominent in the original specimen, and Bacillus species predominated after antibiotic application. Prolonged cultivation of T. viride F-00612 in liquid medium and on Czapek agar in the presence of < 100 µM phenoxazinone and < 500 µM benzoxazolinone resulted in a massive loss of bacteria accompanied by impacted fungal growth in the presence of phenoxazinone. The original consortium was actively involved in implementing metabolic sequences for the degradation and detoxification of nitrated benzoxazolinone derivatives. The 2-aminophenol was rapidly converted into acetamidophenol, but benzoxazolinone, methoxylated benzoxazolinone, and picolinic acid remained unchanged. Excluding phenoxazinone, none of the tested compounds markedly impaired fungal growth in liquid culture. In conclusion, members of the T. viride F-00612 consortium may contribute to the ability to manage benzoxazinone downstream products and facilitate BOA-6-OH degradation via nitration.

Deletion of the benzoxazinoid detoxification gene NAT1 in Fusarium graminearum reduces deoxynivalenol in spring wheat

Benzoxazinoid (Bx) metabolites produced by wheat and other members of the Poaceae have activity against Fusarium sp. that cause cereal diseases including Fusarium head blight (FHB) on wheat and barley. Certain Bx metabolites can be detoxified by Fusarium sp. with the arylamine N-acetyltransferase NAT1. Investigation of this pathway may reveal strategies for increasing FHB resistance, such as selection for higher levels of Bx metabolites within existing germplasm and/or engineering fungal susceptibility via host induced silencing of NAT1. We assessed the reactions of fifteen wheat cultivars or breeding lines adapted to the Northwestern United States to infection with F. graminearum Δnat1 mutants that should be sensitive to Bx metabolites. Significant differences were noted in disease severity and deoxynivalenol (DON) among the cultivars 21 d after inoculation with either mutant or wildtype (PH1) strains. Mutant vs. wildtype strains did not result in significant variation for infection severity (as measured by % infected florets), but inoculation with Δnat1 mutants vs. wildtype resulted in significantly lower DON concentrations in mature kernels (p < 0.0001). Of the cultivars tested, HRS3419 was the most resistant cultivar to PH1 (severity = 62%, DON = 45 ppm) and Δnat1 mutants (severity = 61%, DON = 30 ppm). The cultivar most susceptible to infection was Kelse with PH1 (severity = 100%, DON = 292 ppm) and Δnat1 mutants (severity = 100%, DON = 158 ppm). We hypothesized that sub-lethal Bx metabolite levels may suppress DON production in F. graminearum Δnat1 mutants. In vitro assays of Bx metabolites BOA, MBOA, and DIMBOA at 30 μM did not affect growth, but did reduce DON production by Δnat1 and PH1. Although the levels of Bx metabolites are likely too low in the wheat cultivars we tested to suppress FHB, higher levels of Bx metabolites may contribute towards reductions in DON and FHB.

Plant Protection by Benzoxazinoids—Recent Insights into Biosynthesis and Function

Benzoxazinoids (BXs) are secondary metabolites present in many Poaceae including the major crops maize, wheat, and rye. In contrast to other potentially toxic secondary metabolites, BXs have not been targets of counter selection during breeding and the effect of BXs on insects, microbes, and neighbouring plants has been recognised. A broad knowledge about the mode of action and metabolisation in target organisms including herbivorous insects, aphids, and plants has been gathered in the last decades. BX biosynthesis has been elucidated on a molecular level in crop cereals. Recent advances, mainly made by investigations in maize, uncovered a significant diversity in the composition of BXs within one species. The pattern can be specific for single plant lines and dynamic changes triggered by biotic and abiotic stresses were observed. Single BXs might be toxic, repelling, attractive, and even growth-promoting for insects, depending on the particular species. BXs delivered into the soil influence plant and microbial communities. Furthermore, BXs can possibly be used as signalling molecules within the plant. In this review we intend to give an overview of the current data on the biosynthesis, structure, and function of BXs, beyond their characterisation as mere phytotoxins.

Fusarium crown rot caused by Fusarium pseudograminearum in cereal crops: recent progress and future prospects

Diseases caused by Fusarium pathogens inflict major yield and quality losses on many economically important plant species worldwide, including cereals. Fusarium crown rot (FCR), caused by Fusarium pseudograminearum, is a cereal disease that occurs in many arid and semi-arid cropping regions of the world. In recent years, this disease has become more prevalent, in part as a result of the adoption of moisture-preserving cultural practices, such as minimum tillage and stubble retention. In this pathogen profile, we present a brief overview of recent research efforts that have not only advanced our understanding of the interactions between F. pseudograminearum and cereal hosts, but have also provided new disease management options. For instance, significant progress has been made in the genetic characterization of pathogen populations, the development of new tools for disease prediction, and the identification and pyramiding of loci that confer quantitative resistance to FCR in wheat and barley. In addition, transcriptome analyses have revealed new insights into the processes involved in host defence. Significant progress has also been made in understanding the mechanistic details of the F. pseudograminearum infection process. The sequencing and comparative analyses of the F. pseudograminearum genome have revealed novel virulence factors, possibly acquired through horizontal gene transfer. In addition, a conserved pathogen gene cluster involved in the degradation of wheat defence compounds has been identified, and a role for the trichothecene toxin deoxynivalenol (DON) in pathogen virulence has been reported. Overall, a better understanding of cereal host-F. pseudograminearum interactions will lead to the development of new control options for this increasingly important disease problem. Taxonomy: Fusarium pseudograminearum O'Donnell & Aoki; Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Sordariomycetes; Subclass Hypocreomycetidae; Order Hypocreales; Family Nectriaceae; Genus Fusarium. Disease symptoms: Fusarium crown rot caused by F. pseudograminearum is also known as crown rot, foot rot and root rot. Infected seedlings can die before or after emergence. If infected seedlings survive, typical disease symptoms are browning of the coleoptile, subcrown internode, lower leaf sheaths and adjacent stems and nodal tissues; this browning can become evident within a few weeks after planting or throughout plant development. Infected plants may develop white heads with no or shrivelled grains. Disease symptoms are exacerbated under water limitation. Identification and detection: Fusarium pseudograminearum macroconidia usually contain three to five septa (22-60.5 × 2.5-5.5 μm). On potato dextrose agar (PDA), aerial mycelia appear floccose and reddish white, with red or reddish-brown reverse pigmentation. Diagnostic polymerase chain reaction (PCR) tests based on the amplification of the gene encoding translation elongation factor-1a (TEF-1a) have been developed for molecular identification. Host range: All major winter cereals can be colonized by F. pseudograminearum. However, the main impact of this pathogen is on bread (Triticum aestivum L.) and durum (Triticum turgidum L. spp. durum (Dest.)) wheat and barley (Hordeum vulgare L.). Oats (Avena sativa L.) can be infected, but show little or no disease symptoms. In addition, the pathogen has been isolated from various other grass genera, such as Phalaris, Agropyron and Bromus, which may occur as common weeds. Useful websites: https://nt.ars-grin.gov/fungaldatabases/; http://plantpath.psu.edu/facilities/fusarium-research-center; https://nt.ars-grin.gov/fungaldatabases/; http://www.speciesfungorum.org/Names/Names.asp.

Highly diverse endophytes in roots of Cycas bifida (Cycadaceae), an ancient but endangered gymnosperm

As an ancient seed plant, cycads are one of the few gymnosperms that develop a root symbiosis with cyanobacteria, which has allowed cycads to cope with harsh geologic and climatic conditions during the evolutionary process. However, the endophytic microbes in cycad roots remain poorly identified. In this study, using next-generation sequencing techniques, we investigated the microbial diversity and composition of both the coralloid and regular roots of Cycas bifida (Dyer) K.D. Hill. Highly diverse endophytic communities were observed in both the coralloid and regular roots. Of the associated bacteria, the top five families were the Nostocaceae, Sinobacteraceae, Bradyrhizobiaceae, Bacillaceae, and Hyphomicrobiaceae. The Nectriaceae, Trichocomaceae, and Incertae sedis were the predominant fungal families in all root samples. A significant difference in the endophytic bacterial community was detected between coralloid roots and regular roots, but no difference was observed between the fungal communities in the two root types. Cyanobacteria were more dominant in coralloid roots than in regular roots. The divergence of cycad root structures and the modified physiological processes may have contributed to the abundance of cyanobionts in coralloid roots. Consequently, the colonization of cyanobacteria inhibits the assemblage of other endophytes. Our results contribute to an understanding of the species diversity and composition of the cycad-endophyte microbiome and provide an abbreviated list of potential ecological roles of the core microbes present.

Fusarium verticillioides: Advancements in Understanding the Toxicity, Virulence, and Niche Adaptations of a Model Mycotoxigenic Pathogen of Maize

The importance of understanding the biology of the mycotoxigenic fungus Fusarium verticillioides and its various microbial and plant host interactions is critical given its threat to maize, one of the world's most valuable food crops. Disease outbreaks and mycotoxin contamination of grain threaten economic returns and have grave implications for human and animal health and food security. Furthermore, F. verticillioides is a member of a genus of significant phytopathogens and, thus, data regarding its host association, biosynthesis of secondary metabolites, and other metabolic (degradative) capabilities are consequential to both basic and applied research efforts across multiple pathosystems. Notorious among its secondary metabolites are the fumonisin mycotoxins, which cause severe animal diseases and are implicated in human disease. Additionally, studies of these mycotoxins have led to new understandings of F. verticillioides plant pathogenicity and provide tools for research into cellular processes and host-pathogen interaction strategies. This review presents current knowledge regarding several significant lines of F. verticillioides research, including facets of toxin production, virulence, and novel fitness strategies exhibited by this fungus across rhizosphere and plant environments.

A high-resolution genetic map of the cereal crown rot pathogen Fusarium pseudograminearum provides a near-complete genome assembly

Fusarium pseudograminearum is an important pathogen of wheat and barley, particularly in semi-arid environments. Previous genome assemblies for this organism were based entirely on short read data and are highly fragmented. In this work, a genetic map of F. pseudograminearum has been constructed for the first time based on a mapping population of 178 individuals. The genetic map, together with long read scaffolding of a short read-based genome assembly, was used to give a near-complete assembly of the four F. pseudograminearum chromosomes. Large regions of synteny between F. pseudograminearum and F. graminearum, the related pathogen that is the primary causal agent of cereal head blight disease, were previously proposed in the core conserved genome, but the construction of a genetic map to order and orient contigs is critical to the validation of synteny and the placing of species-specific regions. Indeed, our comparative analyses of the genomes of these two related pathogens suggest that rearrangements in the F. pseudograminearum genome have occurred in the chromosome ends. One of these rearrangements includes the transposition of an entire gene cluster involved in the detoxification of the benzoxazolinone (BOA) class of plant phytoalexins. This work provides an important genomic and genetic resource for F. pseudograminearum, which is less well characterized than F. graminearum. In addition, this study provides new insights into a better understanding of the sexual reproduction process in F. pseudograminearum, which informs us of the potential of this pathogen to evolve.

Molecular Basis of Resistance to Fusarium Ear Rot in Maize

The impact of climate change has been identified as an emerging issue for food security and safety, and the increased incidence of mycotoxin contamination in maize over the last two decades is considered a potential emerging hazard. Disease control by chemical and agronomic approaches is often ineffective and increases the cost of production; for this reason the exploitation of genetic resistance is the most sustainable method for reducing contamination. The review focuses on the significant advances that have been made in the development of transcriptomic, genetic and genomic information for maize, Fusarium verticillioides molds, and their interactions, over recent years. Findings from transcriptomic studies have been used to outline a specific model for the intracellular signaling cascade occurring in maize cells against F. verticillioides infection. Several recognition receptors, such as receptor-like kinases and R genes, are involved in pathogen perception, and trigger down-stream signaling networks mediated by mitogen-associated protein kinases. These signals could be orchestrated primarily by hormones, including salicylic acid, auxin, abscisic acid, ethylene, and jasmonic acid, in association with calcium signaling, targeting multiple transcription factors that in turn promote the down-stream activation of defensive response genes, such as those related to detoxification processes, phenylpropanoid, and oxylipin metabolic pathways. At the genetic and genomic levels, several quantitative trait loci (QTL) and single-nucleotide polymorphism markers for resistance to Fusarium ear rot deriving from QTL mapping and genome-wide association studies are described, indicating the complexity of this polygenic trait. All these findings will contribute to identifying candidate genes for resistance and to applying genomic technologies for selecting resistant maize genotypes and speeding up a strategy of breeding to contrast disease, through plants resistant to mycotoxin-producing pathogens.

The novel fungal-specific gene FUG1 has a role in pathogenicity and fumonisin biosynthesis in Fusarium verticillioides

Fusarium verticillioides is a globally important pathogen of maize, capable of causing severe yield reductions and economic losses. In addition, F. verticillioides produces toxic secondary metabolites during kernel colonization that pose significant threats to human and animal health. Fusarium verticillioides and other plant-pathogenic fungi possess a large number of genes with no known or predicted function, some of which could encode novel virulence factors or antifungal targets. In this study, we identified and characterized the novel gene FUG1 (Fungal Unknown Gene 1) in F. verticillioides through functional genetics. Deletion of FUG1 impaired maize kernel colonization and fumonisin biosynthesis. In addition, deletion of FUG1 increased sensitivity to the antimicrobial compound 2-benzoxazolinone and to hydrogen peroxide, which indicates that FUG1 may play a role in mitigating stresses associated with host defence. Transcriptional profiling via RNA-sequencing (RNA-seq) identified numerous fungal genes that were differentially expressed in the kernel environment following the deletion of FUG1, including genes involved in secondary metabolism and mycelial development. Sequence analysis of the Fug1 protein provided evidence for nuclear localization, DNA binding and a domain of unknown function associated with previously characterized transcriptional regulators. This information, combined with the observed transcriptional reprogramming in the deletion mutant, suggests that FUG1 represents a novel class of fungal transcription factors or genes otherwise involved in signal transduction.

Simplified and representative bacterial community of maize roots

Plant-associated microbes are important for the growth and health of their hosts. As a result of numerous prior studies, we know that host genotypes and abiotic factors influence the composition of plant microbiomes. However, the high complexity of these communities challenges detailed studies to define experimentally the mechanisms underlying the dynamics of community assembly and the beneficial effects of such microbiomes on plant hosts. In this work, from the distinctive microbiota assembled by maize roots, through host-mediated selection, we obtained a greatly simplified synthetic bacterial community consisting of seven strains (Enterobacter cloacae, Stenotrophomonas maltophilia, Ochrobactrum pituitosum, Herbaspirillum frisingense, Pseudomonas putida, Curtobacterium pusillum, and Chryseobacterium indologenes) representing three of the four most dominant phyla found in maize roots. By using a selective culture-dependent method to track the abundance of each strain, we investigated the role that each plays in community assembly on roots of axenic maize seedlings. Only the removal of E. cloacae led to the complete loss of the community, and C. pusillum took over. This result suggests that E. cloacae plays the role of keystone species in this model ecosystem. In planta and in vitro, this model community inhibited the phytopathogenic fungus Fusarium verticillioides, indicating a clear benefit to the host. Thus, combined with the selective culture-dependent quantification method, our synthetic seven-species community representing the root microbiome has the potential to serve as a useful system to explore how bacterial interspecies interactions affect root microbiome assembly and to dissect the beneficial effects of the root microbiota on hosts under laboratory conditions in the future.

Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

Metabolic Pathway Involved in 6-Chloro-2-Benzoxazolinone Degradation by Pigmentiphaga sp. Strain DL-8 and Identification of the Novel Metal-Dependent Hydrolase CbaA

6-Chloro-2-benzoxazolinone (CDHB) is a precursor of herbicide, insecticide, and fungicide synthesis and has a broad spectrum of biological activity. Pigmentiphaga sp. strain DL-8 can transform CDHB into 2-amino-5-chlorophenol (2A5CP), which it then utilizes as a carbon source for growth. The CDHB hydrolase (CbaA) was purified from strain DL-8, which can also hydrolyze 2-benzoxazolinone (BOA), 5-chloro-2-BOA, and benzamide. The specific activity of purified CbaA was 5,900 U · mg protein(-1) for CDHB, with Km and kcat values of 0.29 mM and 8,500 s(-1), respectively. The optimal pH for purified CbaA was 9.0, the highest activity was observed at 55°C, and the inactive metal-free enzyme could be reactivated by Mg(2+), Ni(2+), Ca(2+), or Zn(2+) Based on the results obtained for the CbaA peptide mass fingerprinting and draft genome sequence of strain DL-8, cbaA (encoding 339 amino acids) was cloned and expressed in Escherichia coli BL21(DE3). CbaA shared 18 to 21% identity with some metal-dependent hydrolases of the PF01499 family and contained the signature metal-binding motif Q127XXXQ131XD133XXXH137 The conserved amino acid residues His288 and Glu301 served as the proton donor and acceptor. E. coli BL21(DE3-pET-cbaA) resting cells could transform 0.2 mM CDHB into 2A5CP. The mutant strain DL-8ΔcbaA lost the ability to degrade CDHB but retained the ability to degrade 2A5CP, consistent with strain DL-8. These results indicated that cbaA was the key gene responsible for CDHB degradation by strain DL-8.

IMPORTANCE

2-Benzoxazolinone (BOA) derivatives are widely used as synthetic intermediates and are also an important group of allelochemicals acting in response to tissue damage or pathogen attack in gramineous plants. However, the degradation mechanism of BOA derivatives by microorganisms is not clear. In the present study, we reported the identification of CbaA and metabolic pathway responsible for the degradation of CDHB in Pigmentiphaga sp. DL-8. This will provide microorganism and gene resources for the bioremediation of the environmental pollution caused by BOA derivatives.

The Fdb3 transcription factor of the Fusarium Detoxification of Benzoxazolinone gene cluster is required for MBOA but not BOA degradation in Fusarium pseudograminearum

A number of cereals produce the benzoxazolinone class of phytoalexins. Fusarium species pathogenic towards these hosts can typically degrade these compounds via an aminophenol intermediate, and the ability to do so is encoded by a group of genes found in the Fusarium Detoxification of Benzoxazolinone (FDB) cluster. A zinc finger transcription factor encoded by one of the FDB cluster genes (FDB3) has been proposed to regulate the expression of other genes in the cluster and hence is potentially involved in benzoxazolinone degradation. Herein we show that Fdb3 is essential for the ability of Fusarium pseudograminearum to efficiently detoxify the predominant wheat benzoxazolinone, 6-methoxy-benzoxazolin-2-one (MBOA), but not benzoxazoline-2-one (BOA). Furthermore, additional genes thought to be part of the FDB gene cluster, based upon transcriptional response to benzoxazolinones, are regulated by Fdb3. However, deletion mutants for these latter genes remain capable of benzoxazolinone degradation, suggesting that they are not essential for this process.

Two Horizontally Transferred Xenobiotic Resistance Gene Clusters Associated with Detoxification of Benzoxazolinones by Fusarium Species

Microbes encounter a broad spectrum of antimicrobial compounds in their environments and often possess metabolic strategies to detoxify such xenobiotics. We have previously shown that Fusarium verticillioides, a fungal pathogen of maize known for its production of fumonisin mycotoxins, possesses two unlinked loci, FDB1 and FDB2, necessary for detoxification of antimicrobial compounds produced by maize, including the γ-lactam 2-benzoxazolinone (BOA). In support of these earlier studies, microarray analysis of F. verticillioides exposed to BOA identified the induction of multiple genes at FDB1 and FDB2, indicating the loci consist of gene clusters. One of the FDB1 cluster genes encoded a protein having domain homology to the metallo-β-lactamase (MBL) superfamily. Deletion of this gene (MBL1) rendered F. verticillioides incapable of metabolizing BOA and thus unable to grow on BOA-amended media. Deletion of other FDB1 cluster genes, in particular AMD1 and DLH1, did not affect BOA degradation. Phylogenetic analyses and topology testing of the FDB1 and FDB2 cluster genes suggested two horizontal transfer events among fungi, one being transfer of FDB1 from Fusarium to Colletotrichum, and the second being transfer of the FDB2 cluster from Fusarium to Aspergillus. Together, the results suggest that plant-derived xenobiotics have exerted evolutionary pressure on these fungi, leading to horizontal transfer of genes that enhance fitness or virulence.

Degradation of the benzoxazolinone class of phytoalexins is important for virulence of Fusarium pseudograminearum towards wheat

Wheat, maize, rye and certain other agriculturally important species in the Poaceae family produce the benzoxazolinone class of phytoalexins on pest and pathogen attack. Benzoxazolinones can inhibit the growth of pathogens. However, certain fungi can actively detoxify these compounds. Despite this, a clear link between the ability to detoxify benzoxazolinones and pathogen virulence has not been shown. Here, through comparative genome analysis of several Fusarium species, we have identified a conserved genomic region around the FDB2 gene encoding an N-malonyltransferase enzyme known to be involved in benzoxazolinone degradation in the maize pathogen Fusarium verticillioides. Expression analyses demonstrated that a cluster of nine genes was responsive to exogenous benzoxazolinone in the important wheat pathogen Fusarium pseudograminearum. The analysis of independent F. pseudograminearum FDB2 knockouts and complementation of the knockout with FDB2 homologues from F. graminearum and F. verticillioides confirmed that the N-malonyltransferase enzyme encoded by this gene is central to the detoxification of benzoxazolinones, and that Fdb2 contributes quantitatively to virulence towards wheat in head blight inoculation assays. This contrasts with previous observations in F. verticillioides, where no effect of FDB2 mutations on pathogen virulence towards maize was observed. Overall, our results demonstrate that the detoxification of benzoxazolinones is a strategy adopted by wheat-infecting F. pseudograminearum to overcome host-derived chemical defences.

Accumulation of terpenoid phytoalexins in maize roots is associated with drought tolerance

Maize (Zea mays) production, which is of global agro-economic importance, is largely limited by herbivore pests, pathogens and environmental conditions, such as drought. Zealexins and kauralexins belong to two recently identified families of acidic terpenoid phytoalexins in maize that mediate defence against both pathogen and insect attacks in aboveground tissues. However, little is known about their function in belowground organs and their potential to counter abiotic stress. In this study, we show that zealexins and kauralexins accumulate in roots in response to both biotic and abiotic stress including, Diabrotica balteata herbivory, Fusarium verticillioides infection, drought and high salinity. We find that the quantity of drought-induced phytoalexins is positively correlated with the root-to-shoot ratio of different maize varieties, and further demonstrate that mutant an2 plants deficient in kauralexin production are more sensitive to drought. The induction of phytoalexins in response to drought is root specific and does not influence phytoalexin levels aboveground; however, the accumulation of phytoalexins in one tissue may influence the induction capacity of other tissues.

A γ-lactamase from cereal infecting Fusarium spp. catalyses the first step in the degradation of the benzoxazolinone class of phytoalexins

The benzoxazolinone class of phytoalexins are released by wheat, maize, rye and other agriculturally important species in the Poaceae family upon pathogen attack. Benzoxazolinones show antimicrobial effects on plant pathogens, but certain fungi have evolved mechanisms to actively detoxify these compounds which may contribute to the virulence of the pathogens. In many Fusarium spp. a cluster of genes is thought to be involved in the detoxification of benzoxazolinones. However, only one enzyme encoded in the cluster has been unequivocally assigned a role in this process. The first step in the detoxification of benzoxazolinones in Fusarium spp. involves the hydrolysis of a cyclic ester bond. This reaction is encoded by the FDB1 locus in F. verticillioides but the underlying gene is yet to be cloned. We previously proposed that FDB1 encodes a γ-lactamase, and here direct evidence for this is presented. Expression analyses in the important wheat pathogen F. pseudograminearum demonstrated that amongst the three predicted γ-lactamase genes only the one designated as FDB1, part of the proposed benzoxazolinone cluster in F. pseudograminearum, was strongly responsive to exogenous benzoxazolinone application. Analysis of independent F. pseudograminearum and F. graminearum FDB1 gene deletion mutants, as well as biochemical assays, demonstrated that the γ-lactamase enzyme, encoded by FDB1, catalyses the first step in detoxification of benzoxazolinones. Overall, our results support the notion that Fusarium pathogens that cause crown rot and head blight on wheat have adopted strategies to overcome host-derived chemical defences.

Homologues of xenobiotic metabolizing N-acetyltransferases in plant-associated fungi: Novel functions for an old enzyme family

Plant-pathogenic fungi and their hosts engage in chemical warfare, attacking each other with toxic products of secondary metabolism and defending themselves via an arsenal of xenobiotic metabolizing enzymes. One such enzyme is homologous to arylamine N-acetyltransferase (NAT) and has been identified in Fusarium infecting cereal plants as responsible for detoxification of host defence compound 2-benzoxazolinone. Here we investigate functional diversification of NAT enzymes in crop-compromising species of Fusarium and Aspergillus, identifying three groups of homologues: Isoenzymes of the first group are found in all species and catalyse reactions with acetyl-CoA or propionyl-CoA. The second group is restricted to the plant pathogens and is active with malonyl-CoA in Fusarium species infecting cereals. The third group generates minimal activity with acyl-CoA compounds that bind non-selectively to the proteins. We propose that fungal NAT isoenzymes may have evolved to perform diverse functions, potentially relevant to pathogen fitness, acetyl-CoA/propionyl-CoA intracellular balance and secondary metabolism.

Bioguided isolation, characterization, and biotransformation by Fusarium verticillioides of maize kernel compounds that inhibit fumonisin production

Fusarium verticillioides infects maize ears, causing ear rot disease and contamination of grain with fumonisin mycotoxins. This contamination can be reduced by the presence of bioactive compounds in kernels that are able to inhibit fumonisin biosynthesis. To identify such compounds, we used kernels from a maize genotype with moderate susceptibility to F. verticillioides, harvested at the milk-dough stage (i.e., when fumonisin production initiates in planta), and applied a bioguided fractionation approach. Chlorogenic acid was the most abundant compound in the purified active fraction and its contribution to fumonisin inhibitory activity was up to 70%. Moreover, using a set of maize genotypes with different levels of susceptibility, chlorogenic acid was shown to be significantly higher in immature kernels of the moderately susceptible group. Altogether, our data indicate that chlorogenic acid may considerably contribute to either maize resistance to Fusarium ear rot, fumonisin accumulation, or both. We further investigated the mechanisms involved in the inhibition of fumonisin production by chlorogenic acid and one of its hydrolyzed products, caffeic acid, by following their metabolic fate in supplemented F. verticillioides broths. Our data indicate that F. verticillioides was able to biotransform these phenolic compounds and that the resulting products can contribute to their inhibitory activity.

A secreted Ustilago maydis effector promotes virulence by targeting anthocyanin biosynthesis in maize

The biotrophic fungus Ustilago maydis causes smut disease in maize with characteristic tumor formation and anthocyanin induction. Here, we show that anthocyanin biosynthesis is induced by the virulence promoting secreted effector protein Tin2. Tin2 protein functions inside plant cells where it interacts with maize protein kinase ZmTTK1. Tin2 masks a ubiquitin-proteasome degradation motif in ZmTTK1, thus stabilizing the active kinase. Active ZmTTK1 controls activation of genes in the anthocyanin biosynthesis pathway. Without Tin2, enhanced lignin biosynthesis is observed in infected tissue and vascular bundles show strong lignification. This is presumably limiting access of fungal hyphae to nutrients needed for massive proliferation. Consistent with this assertion, we observe that maize brown midrib mutants affected in lignin biosynthesis are hypersensitive to U. maydis infection. We speculate that Tin2 rewires metabolites into the anthocyanin pathway to lower their availability for other defense responses. DOI: http://dx.doi.org/10.7554/eLife.01355.001.

A secreted Ustilago maydis effector promotes virulence by targeting anthocyanin biosynthesis in maize

The biotrophic fungus Ustilago maydis causes smut disease in maize with characteristic tumor formation and anthocyanin induction. Here, we show that anthocyanin biosynthesis is induced by the virulence promoting secreted effector protein Tin2. Tin2 protein functions inside plant cells where it interacts with maize protein kinase ZmTTK1. Tin2 masks a ubiquitin-proteasome degradation motif in ZmTTK1, thus stabilizing the active kinase. Active ZmTTK1 controls activation of genes in the anthocyanin biosynthesis pathway. Without Tin2, enhanced lignin biosynthesis is observed in infected tissue and vascular bundles show strong lignification. This is presumably limiting access of fungal hyphae to nutrients needed for massive proliferation. Consistent with this assertion, we observe that maize brown midrib mutants affected in lignin biosynthesis are hypersensitive to U. maydis infection. We speculate that Tin2 rewires metabolites into the anthocyanin pathway to lower their availability for other defense responses. DOI: http://dx.doi.org/10.7554/eLife.01355.001.

Interactions between Fusarium verticillioides, Ustilago maydis, and Zea mays: an endophyte, a pathogen, and their shared plant host

Highly diverse communities of microbial symbionts occupy eukaryotic organisms, including plants. While many well-studied symbionts may be characterized as either parasites or as mutualists, the prevalent but cryptic endophytic fungi are less easily qualified because they do not cause observable symptoms of their presence within their host. Here, we investigate the interactions of an endophytic fungus, Fusarium verticillioides with a pathogen, Ustilago maydis, as they occur within maize (Zea mays). We used experimental inoculations to evaluate metabolic mechanisms by which these three organisms might interact. We assessed the impacts of fungal-fungal interactions on endophyte and pathogen growth within the plant, and on plant growth. We find that F. verticillioides modulates the growth of U. maydis and thus decreases the pathogen's aggressiveness toward the plant. With co-inoculation of the endophyte with the pathogen, plant growth is similar to that which would be gained without the pathogen present. However, the endophyte may also break down plant compounds that limit U. maydis growth, and obtains a growth benefit from the presence of the pathogen. Thus, an endophyte such as F. verticillioides may function as both a defensive mutualist and a parasite, and express nutritional modes that depend on ecological context.

Structure-activity relationship of benzoxazinones and related compounds with respect to the growth inhibition and alpha-amylase activity in cress seedlings

Benzoxazinones and their degradation compounds inhibited root growth and alpha-amylase activity in cress seedlings. The inhibitory activity of these compounds was divided into three groups: the high active group; 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one, 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one, 4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one, 4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one, the moderate active group; 7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one, (2H)-1,4-benzoxazin-3(4H)-one, 6-methoxy-benzoxazolin-2(3H)-one, benzoxazolin-2(3H)-one and 2-amino-phenoxazine-3-one, and the low active group; 2-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one, 2-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one, 2-amino-7-hydroxyphenoxazine-3-one and 2-amino-7-methoxyphenoxazine-3-one. The structure-activity of these compounds suggests that compounds that have benzoxazinone skeletons are the most active structure, and a hydroxyl group at position C-2 on the benzoxazinone skeleton may not affect inhibitory activity, whereas a hydroxyl group at position N-4 on the skeleton is essential for inhibitory activity. However, the concentration-response curves of these compounds and the I(50) values (the concentrations required for 50% inhibition) for root growth and alpha-amylase indicated that root growth was positively correlated with the alpha-amylase activity in the seedlings. alpha-Amylase is required not only for seed germination, but also subsequent seedling growth until photosynthesis is sufficient to support seedling growth. Therefore, these results suggest that the compounds studied here may inhibit the root growth of cress seedlings by inhibiting alpha-amylase activity.

FDB2 encodes a member of the arylamine N-acetyltransferase family and is necessary for biotransformation of benzoxazolinones by Fusarium verticillioides

AIMS

To clone and characterize genes from the mycotoxigenic fungus, Fusarium verticillioides, which are associated with its ability to biotransform allelopathic benzoxazolinones produced by maize, wheat, and rye.

METHODS AND RESULTS

Suppression subtractive hybridization identified F. verticillioides genes up-regulated in response to 2-benzoxazolinone (BOA), including a cluster of genes along chromosome 3. One of these genes, putatively encoding an arylamine N-acetyltransferase (NAT), was highly represented in the subtracted library and was of particular interest since previous analyses identified the FDB2 locus as possibly encoding transferase activity. The gene was subcloned and complemented a natural fdb2 mutant. Conversely, disruption of the gene eliminated the ability of F. verticillioides to metabolize BOA. Other genes in the cluster also were assessed using a complementation assay. Metabolic profiles of fdb2 mutants suggest that minor acylation activity occurred independently of the NAT activity encoded by FDB2.

CONCLUSIONS

The previously defined FDB2 locus was functionally associated with the gene encoding putative NAT activity, and the FDB2 gene was essential for biotransformation of BOA. The flanking gene FDB3 encodes a putative Zn(II)2Cys6 transcription factor and contributes to efficient BOA biotransformation but was not essential.

SIGNIFICANCE AND IMPACT OF THE STUDY

Biotransformation of benzoxazolinones by F. verticillioides may enhance its ecological fitness in maize field environments and our results provide greater understanding of the genes that modulate the biotransformation process. Additionally, this is the first homologue of the NAT gene family to be characterized in a filamentous fungus.

Hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one: key defense chemicals of cereals

Many cereals accumulate hydroxamic acids derived from 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one. These benzoxazinoid hydroxamic acids are involved in defense of maize against various lepidopteran pests, most notably the European corn borer, in defense of cereals against various aphid species, and in allelopathy affecting the growth of weeds associated with rye and wheat crops. The role of benzoxazinoid hydroxamic acids in defense against fungal infection is less clear and seems to depend on the nature of the interactions at the plant-fungus interface. Efficient use of benzoxazinoid hydroxamic acids as resistance factors has been limited by the inability to selectively increase their levels at the plant growth stage and the plant tissues where they are mostly needed for a given pest. Although the biosynthesis of benzoxazinoid hydroxamic acids has been elucidated, the genes and mechanisms controlling their differential expression in different plant tissues and along plant ontogeny remain to be unraveled.

Effects of four benzoxazinoids on gibberellin-induced alpha-amylase activity in barley seeds

Germination of barley seeds was inhibited by 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA) and 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one (DIBOA) at concentrations greater than 0.03mmol/L, and 6-methoxy-benzoxazolin-2(3H)-one (MBOA) and benzoxazolin-2(3H)-one (BOA) at concentrations greater than 0.1mmol/L. These benzoxazinoids also inhibited the induction of alpha-amylase activity in the barley seeds, and inhibited gibberellin-induced alpha-amylase activity in de-embryonated barley seeds. Significant inhibition in the germination and alpha-amylase induction were observed as concentrations of DIMBOA, DIBOA, MBOA and BOA increased. These results suggest that DIMBOA, DIBOA, MBOA and BOA may inhibit the germination of barley seeds by inhibiting the gibberellin-induced process, leading to alpha-amylase production. The inhibitory activities of germination and alpha-amylase induction of DIMBOA and DIBOA were greater than those of their degraded substances MBOA and BOA, respectively, and the inhibitory activities of DIMBOA and MBOA were greater than those of their demethoxylated analogues DIBOA and BOA, respectively.

Transformation-mediated complementation of a FUM gene cluster deletion in Fusarium verticillioides restores both fumonisin production and pathogenicity on maize seedlings

The filamentous ascomycete Fusarium verticillioides is a pathogen of maize and produces the fumonisin mycotoxins. However, a distinct population of F. verticillioides is pathogenic on banana and does not produce fumonisins. Fumonisin-producing strains from maize cause leaf lesions, developmental abnormalities, stunting, and sometimes death of maize seedlings, whereas fumonisin-nonproducing banana strains do not. A Southern analysis of banana strains did not detect genes in the fumonisin biosynthetic gene (FUM) cluster but did detect genes flanking the cluster. Nucleotide sequence analysis of the genomic region carrying the flanking genes revealed that the FUM cluster was absent in banana strains except for portions of FUM21 and FUM19, which are the terminal genes at each end of the cluster. Polymerase chain reaction analysis confirmed the absence of the cluster in all banana strains examined. Cotransformation of a banana strain with two overlapping cosmids, which together contain the entire FUM cluster, yielded fumonisin-producing transformants that were pathogenic on maize seedlings. Conversely, maize strains that possess the FUM cluster but do not produce fumonisins because of mutations in FUM1, a polyketide synthase gene, were not pathogenic on maize seedlings. Together, the data indicate that fumonisin production may have been lost by deletion of the FUM cluster in the banana population of F. verticillioides but that fumonisin production could be restored by molecular genetic complementation. The results also indicate that fumonisin production by F. verticillioides is required for development of foliar disease symptoms on maize seedlings.

Interactions of Bacillus mojavensis and Fusarium verticillioides with a benzoxazolinone (BOA) and its transformation product, APO

The benzoxazolinones, specifically benzoxazolin-2(3H)-one (BOA), are important transformation products of the benzoxazinones that can serve as allelochemicals providing resistance to maize from pathogenic bacteria, fungi, and insects. However, maize pathogens such as Fusarium verticillioides are capable of detoxifying the benzoxazolinones to 2-aminophenol (AP), which is converted to the less toxic N-(2-hydroxyphenyl) malonamic acid (HPMA) and 2-acetamidophenol (HPAA). As biocontrol strategies that utilize a species of endophytic bacterium, Bacillus mojavensis, are considered efficacious as a control of this Fusarium species, the in vitro transformation and effects of BOA on growth of this bacterium was examined relative to its interaction with strains of F. verticillioides. The results showed that a red pigment was produced and accumulated only on BOA-amended media when wild type and the progeny of genetic crosses of F. verticillioides are cultured in the presence of the bacterium. The pigment was identified as 2-amino-3H-phenoxazin-3-one (APO), which is a stable product. The results indicate that the bacterium interacts with the fungus preventing the usual transformation of AP to the nontoxic HPMA, resulting in the accumulation of higher amounts of APO than when the fungus is cultured alone. APO is highly toxic to F. verticillioides and other organisms. Thus, an enhanced biocontrol is suggested by this in vitro study.

Fumonisin disruption of ceramide biosynthesis in maize roots and the effects on plant development and Fusarium verticillioides-induced seedling disease

The fungus Fusarium verticillioides infects maize and produces fumonisins, inhibitors of ceramide synthase. Seeds of the cultivar Silver Queen were inoculated with fumonisin-producing or non-fumonisin-producing strains of F. verticillioides. Leaf lesion incidence and severity of effects on root and stalk growth were significantly correlated with fumonisin in roots and disruption of sphingolipid metabolism in roots. Uninoculated seeds grown in soil watered with solutions of fumonisin B1 exhibited above-ground symptoms indicative of F. verticillioides-induced seedling disease and dose-dependent reduction in root mass that was inversely correlated with fumonisin B1, sphingoid bases, and sphingoid base 1-phosphates in roots. There was also evidence of an adaptive response to disrupted sphingolipid metabolism in both the virulence and watering assays, suggesting induction of pathways responsible for metabolism of sphingoid base 1-phosphates after prolonged exposure. The results suggest that fumonisin, and its effects on sphingolipids, could contribute to all aspects of F. verticillioides maize seedling disease.

Possible Mechanism of Inhibition of 6-Methoxy-Benzoxazolin-2(3H)-One on Germination of Cress (Lepidium sativum L.)

6-Methoxy-benzoxazolin-2(3H)-one (MBOA) inhibited the germination of cress (Lepidium sativum L.) seeds at concentrations greater than 0.03 mM. Inhibition was overcome by sucrose, suggesting that MBOA may inhibit sugar metabolism in cress seeds. Induction of alpha-amylase activity in seeds was also inhibited by MBOA at concentrations greater than 0.03 mM. Inhibition of both germination and induction of alpha-amylase activity increased with increasing concentrations of MBOA, and the extent of germination correlated positively with the activity of alpha-amylase in the seeds. MBOA added to a reaction mixture for alpha-amylase assay did not affect enzyme activity, indicating that MBOA does not inhibit in vitro alpha-amylase activity. Cress seeds germinated approximately 16 hr after incubation, and inhibition of alpha-amylase by MBOA occurred within 6 hr after incubation. These results suggest that MBOA may inhibit the germination of cress seeds by inhibiting the induction of alpha-amylase activity, because alpha-amylase plays a key role in the conversion of reserve carbohydrate into soluble sugars, a prerequisite for seed germination.

Natural variation of ascospore and conidial germination by Fusarium verticillioides and other Fusarium species

Fusarium verticillioides and other Fusarium species were examined for their spore germination phenotypes. In general, germinating spores of F. verticillioides formed germ tubes that immediately penetrated into agar. Such invasive germination was the predominant growth phenotype among 22 examined field isolates of F. verticillioides from a broad range hosts and locations. However, two of the field isolates were unique in that they formed conidial germ tubes and hyphae that grew along the surface of agar before penetration eventually occurred. Conidia of 22 other Fusarium species were assessed for their germination phenotypes, and only some strains of F. annulatum, F. fujikuroi, F. globosum, F. nygamai, and F. pseudoanthophilum had the surface germination phenotype (21% of the strains assessed). Sexual crosses and segregation analyses involving one of the F. verticillioides surface germination strains, NRRL 25059, indicated a single locus, designated SIG1 (surface vs. invasive germination), controlled the germ tube growth phenotypes exhibited by both conidia and ascospores. Perfect correlation was observed between an ascospore germination phenotype and the germination phenotype of the conidia produced from the resulting ascospore-derived colony. Recombination data suggested SIG1 was linked ( approximately 7% recombination frequency) to FPH1, a recently described locus necessary for enteroblastic conidiogenesis. Corn seedling blight assays indicated surface germinating strains of F. verticillioides were less virulent than invasively germinating strains. Assays also indicated pathogenicity segregated independently of the FPH1 locus. Invasive germination is proposed as the dominant form of spore germination among Fusarium species. Furthermore, conidia were not necessary for corn seedling disease development, but invasive germination may have enhanced the virulence of conidiating strains.

Effects of 6-methoxy-2-benzoxazolinone on the germination and alpha-amylase activity in lettuce seeds

Germination of lettuce seeds was inhibited by 6-methoxy-2-benzoxazolinone (MBOA) at concentrations greater than 0.03 mmol/L. MBOA also inhibited the induction of alpha-amylase activity in the lettuce seeds at concentrations greater than 0.03 mmol/L. These two concentration-response curves for the germination and alpha-amylase indicate that the percentage of the germination was positively correlated with the activity of alpha-amylase in the seeds. Lettuce seeds germinated around 18h after incubation and inhibition of alpha-amylase by MBOA occurred within 6h after seed incubation. These results show that MBOA may inhibit the germination of lettuce seeds by inhibiting the induction of alpha-amylase activity.

Comparative analysis of 87,000 expressed sequence tags from the fumonisin-producing fungus Fusarium verticillioides

Fusarium verticillioides (teleomorph Gibberella moniliformis) is a pathogen of maize worldwide and produces fumonisins, a family of mycotoxins that have been associated with several animal diseases as well as cancer in humans. In this study, we sought to identify fungal genes that affect fumonisin production and/or the plant-fungal interaction. We generated over 87,000 expressed sequence tags from nine different cDNA libraries that correspond to 11,119 unique sequences and are estimated to represent 80% of the genomic complement of genes. A comparative analysis of the libraries showed that all 15 genes in the fumonisin gene cluster were differentially expressed. In addition, nine candidate fumonisin regulatory genes and a number of genes that may play a role in plant-fungal interaction were identified. Analysis of over 700 FUM gene transcripts from five different libraries provided evidence for transcripts with unspliced introns and spliced introns with alternative 3' splice sites. The abundance of the alternative splice forms and the frequency with which they were found for genes involved in the biosynthesis of a single family of metabolites as well as their differential expression suggest they may have a biological function. Finally, analysis of an EST that aligns to genomic sequence between FUM12 and FUM13 provided evidence for a previously unidentified gene (FUM20) in the FUM gene cluster.

Transformation products of 2-benzoxazolinone (BOA) in soil

Three degradation experiments were performed to examine the formation of transformation products from 2-benzoxazolinone (BOA) in different soil types and concentrations. In two experiments using BOA in low concentration (400 microgkg(-1)) only one unidentified transformation product was found, whereas in the degradation experiment in high concentration (400 mgkg(-1)) several metabolites occurred. Two of these metabolites, 2-amino-(3H)-phenoxazin-3-one (APO); and 2-acetylamino-(3H)-phenoxazin-3-one (AAPO) were synthesized to prove their identity. This is the first time that the successive formation of these types of compounds from BOA is shown in soil. A number of other APO related transformation products were detected and provisionally characterized. The formation of APO, which is a much more biologically active compound than BOA, and the concurrent formation of a number of other APO-related metabolites in soil underline the importance of performing transformation studies as part of the evaluation of the effect of allelochemicals on weeds and soil-borne diseases.