Martha M. Vaughan

Climate change impacts on the ecology of Fusarium graminearum species complex and susceptibility of wheat to Fusarium head blight: a review

Fusarium head blight (FHB) of wheat, caused mainly by a few members of the Fusarium graminearum species complex (FGSC), is a major threat to agricultural grain production, food safety, and animal health. The severity of disease epidemics and accumulation of associated trichothecene mycotoxins in wheat kernels is strongly driven by meteorological factors. The potential impacts of change in climate are reviewed from the perspective of the FGSC life cycle and host resistance mechanisms influenced by abiotic pressures at the ecological, physiological and molecular level. Alterations in climate patterns and cropping systems may affect the distribution, composition and load of FGSC inoculum, but quantitative information is lacking regarding the differential responses among FGSC members. In general, the coincidence of wet and warm environment during flowering enhances the risk of FHB epidemics, but the magnitude and direction of the change in FHB and mycotoxin risk will be a consequence of a multitude of effects...

Fusarium dactylidis sp. nov., a novel nivalenol toxin-producing species sister to F. pseudograminearum isolated from orchard grass (Dactylis glomerata) in Oregon and New Zealand

The B trichothecene toxin-producing clade (B clade) of Fusarium includes the etiological agents of Fusarium head blight, crown rot of wheat and barley and stem and ear rot of maize. B clade isolates also have been recovered from several wild and cultivated grasses, including Dactylis glomerata (orchard grass or cock’s foot), one of the world’s most important forage grasses. Two isolates from the latter host are formally described here as F. dactylidis. Phenotypically F. dactylidis most closely resembles F. ussurianum from the Russian Far East. Both species produce symmetrical sporodochial conidia that are similar in size and curved toward both ends. However, conidia of F. ussurianum typically end in a narrow apical beak while the apical cell of F. dactylidis is acute. Fusarium dactylidis produced nivalenol mycotoxin in planta as well as low but detectable amounts of the estrogenic mycotoxin zearalenone in vitro. Results of a pathogenicity test revealed that F. dactylidis induced mild head blight on wheat.

Soybean SDS in South Africa is Caused by Fusarium brasiliense and a Novel Undescribed Fusarium sp.

Soybean sudden death syndrome (SDS) was detected in South Africa for the first time during pathogen surveys conducted in 2013 to 2014. The primary objective of this study was to characterize the 16 slow-growing Fusarium strains that were isolated from the roots of symptomatic plants. Molecular phylogenetic analyses of a portion of translation elongation factor 1-α (TEF1) and the nuclear ribosomal intergenic spacer region (IGS rDNA) indicated that the etiological agents were Fusarium brasiliense and a novel, undescribed Fusarium sp. This is the first report of F. brasiliense outside of Brazil and Argentina and the novel Fusarium sp. causing soybean SDS. Kochs postulates were completed for both fusaria on seven soybean cultivars that are commercially available in South Africa. Results of the pathogenicity experiment revealed that the strains of F. brasiliense and Fusarium sp. differed in aggressiveness to soybean, as reflected in differences in foliar symptoms, root rot, and reduction in shoot length. Cell-free culture filtrates of the two soybean SDS pathogens from South Africa and two positive control strains of F. virguliforme from the United States induced typical SDS symptoms on susceptible soybean cultivars in a whole-seedling assay, indicating that they contained phytotoxins.

Fusarium algeriense, sp. nov., a novel toxigenic crown rot pathogen of durum wheat from Algeria is nested in the Fusarium burgessii species complex

ABSTRACT A novel crown rot pathogen of wheat discovered during pathogen surveys in Algeria in 2014 and 2015 is formally described as Fusarium algeriense. Multilocus molecular phylogenetic data resolved the eight isolates of this pathogen as a genealogically exclusive species lineage in the F. burgessii species complex. The previously described species of this complex, F. burgessii and F. beomiforme, produce abundant chlamydospores in culture, and their optimal temperature for growth is 30 C. In comparison, F. algeriense did not produce chlamydospores under the conditions tested and its optimal temperature for growth is 25 C. Furthermore, F. algeriense differs from F. burgessii because it does not produce polyphialides and F. beomiforme, because it does not produce globose-to-napiform conidia in the aerial mycelium. Isolates of F. algeriense induced moderate crown rot on the susceptible spring wheat cultivar Norm in a temperature-controlled incubator. Fusarium burgessii and F. beomiforme, in contrast, only induced mild symptoms of this disease. BLASTn searches of the whole-genome sequence of F. algeriense strains NRRL 66647 and 66648, using homologs of genes that are responsible for synthesis of toxic secondary metabolites, indicated that they have the potential to produce several polyketide and non-ribosomal peptide-derived mycotoxins. However, moniliformin and 2-AOD-ol (2-amino-14,16-dimethyloctadecan-3-ol) were the only mycotoxins detected by liquid chromatography–mass spectrometry (LC-MS) analyses of strains cultivated in vitro on a solid medium. A polymerase chain reaction (PCR) assay for MAT idiomorph revealed that MAT1-1 and MAT1-2 strains of F. algeriense were present in Algeria, which suggests this pathogen might possess a heterothallic sexual reproductive mode.

Biosynthesis and function of terpenoid defense compounds in maize (Zea mays)

Main conclusionMaize produces an array of herbivore-induced terpene volatiles that attract parasitoids to infested plants and a suite of pathogen-induced non-volatile terpenoids with antimicrobial activity to defend against pests.Plants rely on complex blends of constitutive and dynamically produced specialized metabolites to mediate beneficial ecological interactions and protect against biotic attack. One such class of metabolites are terpenoids, a large and structurally diverse class of molecules shown to play significant defensive and developmental roles in numerous plant species. Despite this, terpenoids have only recently been recognized as significant contributors to pest resistance in maize (Zea mays), a globally important agricultural crop. The current review details recent advances in our understanding of biochemical structures, pathways and functional roles of maize terpenoids. Dependent upon the lines examined, maize can harbor more than 30 terpene synthases, underlying the inherent diversity of maize terpene defense systems. Part of this defensive arsenal is the inducible production of volatile bouquets that include monoterpenes, homoterpenes and sesquiterpenes, which often function in indirect defense by enabling the attraction of parasitoids and predators. More recently discovered are a subset of sesquiterpene and diterpene hydrocarbon olefins modified by cytochrome P450s to produce non-volatile end-products such kauralexins, zealexins, dolabralexins and β-costic acid. These non-volatile terpenoid phytoalexins often provide effective defense against both microbial and insect pests via direct antimicrobial and anti-feedant activity. The diversity and promiscuity of maize terpene synthases, coupled with a variety of secondary modifications, results in elaborate defensive layers whose identities, regulation and precise functions are continuing to be elucidated.

Fusarium subtropicale, sp. nov., a novel nivalenol mycotoxin–producing species isolated from barley (Hordeum vulgare) in Brazil and sister to F. praegraminearum

ABSTRACT Surveys were conducted in commercial wheat and barley fields in the south central production regions of state of Paraná, Brazil, from 2011 to 2015. Spikes displaying visible Fusarium head blight symptoms were collected and the pathogen isolated from the tissues. The 754 Fusarium isolates recovered were identified by a high-throughput multilocus genotyping assay (MLGT) designed to identify trichothecene toxin–producing fusaria (i.e., formerly B-clade, but referred to here as F. sambucinum species complex lineage 1 [FSAMSC-1]) together with sequencing a portion of the translation elongation factor 1-α (TEF1) gene. One strain was discovered that appeared to be closely related to but phylogenetically distinct from F. praegraminearum based on the relatively low 97.7% TEF1 identity and positive genotype obtained with one of the two F. praegraminearum species–specific MLGT probes. Molecular phylogenetic analyses of a 10-gene data set resolved this novel FSAMSC-1 species and F. praegraminearum as sisters. Formally described herein as F. subtropicale, it is phenotypically distinct from the 22 other FSAMSC-1 species in that it produces mostly 1–3-septate macroconidia. Whole-genome sequence data were used to predict its potential to produce mycotoxins. Chemical analyses confirmed that F. subtropicale could produce the mycotoxins 4,15-diacetylnivalenol, butenolide, culmorin, and fusarin C in vitro, and the pathogenicity experiment revealed that F. subtropicale could infect but not spread in susceptible hard red spring wheat cultivar “Norm.”

Fusarium mycotoxins: a trans-disciplinary overview

Abstract Due to health risks and economic losses associated with mycotoxins produced by Fusarium species, there is a compelling need for an improved understanding of these fungi from across diverse perspectives and disciplinary approaches. In this article, we provide a transdisciplinary overview of: (i) Fusarium phylogenetics; (ii) linkages between mycotoxin biosynthetic gene clusters and chemical structures; (iii) biotransformation of mycotoxins to reduce toxicity; (iv) Fusarium population biology; (v) genomics of secondary metabolite production; and (vi) mycotoxigenic fusaria in a phytobiomes context. Phylogenetic studies have made tremendous progress in delineating the species that comprise the genus Fusarium, many of which are morphologically cryptic. Accurate species identification and a thorough understanding of the distribution of mycotoxin biosynthetic genes among those species will facilitate control of mycotoxin contamination. The biochemical pathways leading to the formation of several Fusarium mycotoxins have been elegantly linked with the genes responsible for each chemical transformation during synthesis, and for most structural differences among chemotypes. Screens for the biotransformation of mycotoxins have led to the description of chemical modifications that impact bioactivity and have implications for monitoring and testing of the food supply. Population biology studies have revealed the potential for introductions of foreign genotypes to alter regional populations of mycotoxigenic fusaria. Genomic analyses have begun to reveal the complex evolutionary history of the genes responsible for mycotoxin production, both across and within lineages. Improved understanding of how climate variability impacts plant–Fusarium interactions and mycotoxin accumulation is necessary for effective plant resistance. Additionally, improved understanding of interactions between Fusarium and other members of crop microbiomes is expected to produce novel strategies for limiting disease and mycotoxin accumulation.

Targeting Fumonisin Biosynthetic Genes

The fungus Fusarium is an agricultural problem because it can cause disease on most crop plants and can contaminate crops with mycotoxins. There is considerable variation in the presence/absence and genomic location of gene clusters responsible for synthesis of mycotoxins and other secondary metabolites among species of Fusarium. Here, we describe a quantitative real-time PCR (qPCR) method for distinguishing between and estimating the biomass of two closely related species, F. proliferatum and F. verticillioides, that are pathogens of maize. The qPCR assay is based on differences in the two species with respect to the genomic location of the gene cluster responsible for synthesis of fumonisins, a family of carcinogenic mycotoxins. Species-specific qPCR primers were designed from unique sequences that flank one end of the cluster in each species. The primers were used in qPCR to estimate the biomass of each Fusarium species using DNA isolated from pure cultures and from maize seedlings resulting from seeds inoculated with F. proliferatum alone, F. verticillioides alone, or a 1:1 mixture of the two species. Biomass estimations from seedlings were expressed as the amount of DNA of each Fusarium species per amount of maize DNA, as determined using maize-specific qPCR primers designed from the ribosomal gene L17. Analyses of qPCR experiments using the primers indicated that the assay could distinguish between and quantify the biomass of the two Fusarium species. This finding indicates that genetic diversity resulting from variation in the presence/absence and genomic location of SM biosynthetic gene clusters can be a valuable resource for development of qPCR assays for distinguishing between and quantifying fungi in plants.