Cytology of Infection of Maize Seedlings by Fusarium moniliforme and Immunolocalization of the Pathogenesis-Related PRms Protein

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.

Pathogen and drought stress affect cell wall and phytohormone signaling to shape host responses in a sorghum COMT bmr12 mutant

BACKGROUND

As effects of global climate change intensify, the interaction of biotic and abiotic stresses increasingly threatens current agricultural practices. The secondary cell wall is a vanguard of resistance to these stresses. Fusarium thapsinum (Fusarium stalk rot) and Macrophomina phaseolina (charcoal rot) cause internal damage to the stalks of the drought tolerant C4 grass, sorghum (Sorghum bicolor (L.) Moench), resulting in reduced transpiration, reduced photosynthesis, and increased lodging, severely reducing yields. Drought can magnify these losses. Two null alleles in monolignol biosynthesis of sorghum (brown midrib 6-ref, bmr6-ref; cinnamyl alcohol dehydrogenase, CAD; and bmr12-ref; caffeic acid O-methyltransferase, COMT) were used to investigate the interaction of water limitation with F. thapsinum or M. phaseolina infection.

RESULTS

The bmr12 plants inoculated with either of these pathogens had increased levels of salicylic acid (SA) and jasmonic acid (JA) across both watering conditions and significantly reduced lesion sizes under water limitation compared to adequate watering, which suggested that drought may prime induction of pathogen resistance. RNA-Seq analysis revealed coexpressed genes associated with pathogen infection. The defense response included phytohormone signal transduction pathways, primary and secondary cell wall biosynthetic genes, and genes encoding components of the spliceosome and proteasome.

CONCLUSION

Alterations in the composition of the secondary cell wall affect immunity by influencing phenolic composition and phytohormone signaling, leading to the action of defense pathways. Some of these pathways appear to be activated or enhanced by drought. Secondary metabolite biosynthesis and modification in SA and JA signal transduction may be involved in priming a stronger defense response in water-limited bmr12 plants.

Induction of defense-related enzymes and enhanced disease resistance in maize against Fusarium verticillioides by seed treatment with Jacaranda mimosifolia formulations

Fusarium verticillioides is an important fungal pathogen of maize, causing stalk rot and severely affecting crop production. The aim of this study was to characterize the protective effects of formulations based on Jacaranda mimosifolia leaf extracts against F. verticillioides in maize. We compared different seed treatments comprising J. mimosifolia extracts, chemical fungicide (mefenoxam) and salicylic acid to modulate the defense system of maize host plants. Both aqueous and methanolic leaf extracts of J. mimosifolia (1.2% w/v) resulted in 96-97% inhibition of mycelial growth of F. verticillioides. While a full-dose (1.2%) extract of J. mimosifolia provided significant protective effects on maize plants compared to the inoculated control, a half-dose (0.6% w/v) application of J. mimosifolia in combination with half-strength mefenoxam was the most effective treatment in reducing stalk rot disease in pot and field experiments. The same seed treatment significantly upregulated the expression of genes in the leaves encoding chitinase, glucanase, lipid transfer protein, and pathogenesis-related proteins PR-1, PR-5 and PR-10, 72 h after inoculation. This treatment also induced the activities of peroxidase, polyphenol oxidase, protease, acid invertase, chitinase and phenylalanine ammonia lyase. We conclude that seed pre-treatment with J. mimosifolia extract with half-strength chemical mefenoxam is a promising approach for the management of stalk rot in maize.

Rice Stomatal Mega-Papillae Restrict Water Loss and Pathogen Entry

Rice (Oryza sativa) is a water-intensive crop, and like other plants uses stomata to balance CO₂ uptake with water-loss. To identify agronomic traits related to rice stomatal complexes, an anatomical screen of 64 Thai and 100 global rice cultivars was undertaken. Epidermal outgrowths called papillae were identified on the stomatal subsidiary cells of all cultivars. These were also detected on eight other species of the Oryza genus but not on the stomata of any other plant species we surveyed. Our rice screen identified two cultivars that had "mega-papillae" that were so large or abundant that their stomatal pores were partially occluded; Kalubala Vee had extra-large papillae, and Dharia had approximately twice the normal number of papillae. These were most accentuated on the flag leaves, but mega-papillae were also detectable on earlier forming leaves. Energy dispersive X-Ray spectrometry revealed that silicon is the major component of stomatal papillae. We studied the potential function(s) of mega-papillae by assessing gas exchange and pathogen infection rates. Under saturating light conditions, mega-papillae bearing cultivars had reduced stomatal conductance and their stomata were slower to close and re-open, but photosynthetic assimilation was not significantly affected. Assessment of an F₃ hybrid population treated with Xanthomonas oryzae pv. oryzicola indicated that subsidiary cell mega-papillae may aid in preventing bacterial leaf streak infection. Our results highlight stomatal mega-papillae as a novel rice trait that influences gas exchange, stomatal dynamics, and defense against stomatal pathogens which we propose could benefit the performance of future rice crops.

Genomics of Maize Resistance to Fusarium Ear Rot and Fumonisin Contamination

Food contamination with mycotoxins is a worldwide concern, because these toxins produced by several fungal species have detrimental effects on animal and/or human health. In maize, fumonisins are among the toxins with the highest threatening potential because they are mainly produced by Fusarium verticillioides, which is distributed worldwide. Plant breeding has emerged as an effective and environmentally safe method to reduce fumonisin levels in maize kernels, but although phenotypic selection has proved effective for improving resistance to fumonisin contamination, further resources should be mobilized to meet farmers' needs. Selection based on molecular markers linked to quantitative trait loci (QTL) for resistance to fumonisin contamination or/and genotype values obtained using prediction models with markers distributed across the whole genome could speed up breeding progress. Therefore, in the current paper, previously identified genomic regions, genes, and/or pathways implicated in resistance to fumonisin accumulation will be reviewed. Studies done until now have provide many markers to be used by breeders, but to get further insight on plant mechanisms to defend against fungal infection and to limit fumonisin contamination, the genes behind those QTLs should be identified.

Infection cycle of maize stalk rot and ear rot caused by Fusarium verticillioides

Fusarium verticillioides, an important maize pathogen produces fumonisins and causes stalk and ear rot; thus, we are aimed to clarify its infection cycle by assessing enhanced green fluorescent protein (EGFP) expression in stalk and ear rot strains. Maize seeds were inoculated with stable and strongly pathogenic transformants. To investigate the degree of infection, inoculated plants were observed under a stereo fluorescence microscope, and affected tissue strains were detected using PCR. We found that both transformants infected maize. Hyphae infected the plants from radical to the stem and extended to the ear and infected ear kernels caused a second infection. This process formed the infection cycle.

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.

The Pathogenesis-Related Maize Seed (PRms) Gene Plays a Role in Resistance to Aspergillus flavus Infection and Aflatoxin Contamination

Aspergillus flavus is an opportunistic plant pathogen that colonizes and produces the toxic and carcinogenic secondary metabolites, aflatoxins, in oil-rich crops such as maize (Zea mays ssp. mays L.). Pathogenesis-related (PR) proteins serve as an important defense mechanism against invading pathogens by conferring systemic acquired resistance in plants. Among these, production of the PR maize seed protein, ZmPRms (AC205274.3_FG001), has been speculated to be involved in resistance to infection by A. flavus and other pathogens. To better understand the relative contribution of ZmPRms to A. flavus resistance and aflatoxin production, a seed-specific RNA interference (RNAi)-based gene silencing approach was used to develop transgenic maize lines expressing hairpin RNAs to target ZmPRms. Downregulation of ZmPRms in transgenic kernels resulted in a ∼250-350% increase in A. flavus infection accompanied by a ∼4.5-7.5-fold higher accumulation of aflatoxins than control plants. Gene co-expression network analysis of RNA-seq data during the A. flavus-maize interaction identified ZmPRms as a network hub possibly responsible for regulating several downstream candidate genes associated with disease resistance and other biochemical functions. Expression analysis of these candidate genes in the ZmPRms-RNAi lines demonstrated downregulation (vs. control) of a majority of these ZmPRms-regulated genes during A. flavus infection. These results are consistent with a key role of ZmPRms in resistance to A. flavus infection and aflatoxin accumulation in maize kernels.

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.

Comparative Histological and Transcriptional Analysis of Maize Kernels Infected with Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides infect maize kernels and contaminate them with the mycotoxins aflatoxin, and fumonisin, respectively. Genetic resistance in maize to these fungi and to mycotoxin contamination has been difficult to achieve due to lack of identified resistance genes. The objective of this study was to identify new candidate resistance genes by characterizing their temporal expression in response to infection and comparing expression of these genes with genes known to be associated with plant defense. Fungal colonization and transcriptional changes in kernels inoculated with each fungus were monitored at 4, 12, 24, 48, and 72 h post inoculation (hpi). Maize kernels responded by differential gene expression to each fungus within 4 hpi, before the fungi could be observed visually, but more genes were differentially expressed between 48 and 72 hpi, when fungal colonization was more extensive. Two-way hierarchal clustering analysis grouped the temporal expression profiles of the 5,863 differentially expressed maize genes over all time points into 12 clusters. Many clusters were enriched for genes previously associated with defense responses to either A. flavus or F. verticillioides. Also within these expression clusters were genes that lacked either annotation or assignment to functional categories. This study provided a comprehensive analysis of gene expression of each A. flavus and F. verticillioides during infection of maize kernels, it identified genes expressed early and late in the infection process, and it provided a grouping of genes of unknown function with similarly expressed defense related genes that could inform selection of new genes as targets in breeding strategies.

Constitutive expression of pathogenesis-related proteins and antioxydant enzyme activities triggers maize resistance towards Fusarium verticillioides

Fusarium verticillioides is a fungal pathogen of maize that causes ear rot and contaminates the grains with fumonisin mycotoxins. Breeding for resistance to Fusarium emerged as the most economic and environmentally safe strategy; therefore the discovery of resistant sources and effective molecular markers are a priority. Ears of resistant (CO441 and CO433) and susceptible (CO354 and CO389) maize lines were inoculated with F. verticillioides and the expression of pathogenesis-related (PR) genes (PR1, PR5, PRm3, PRm6) and genes that protect from oxidative stress (peroxidase, catalase, superoxide dismutase and ascorbate peroxidase) were evaluated in the kernels at 72h post inoculation. In addition, the oxidation level and the enzymatic activity of ascorbate-glutathione cycle, catalase, superoxide dismutase and cytosolic and wall peroxidases were investigated. The uninoculated kernels of the resistant lines showed higher gene expression and enzymatic activities, highlighting the key role of constitutive resistance in limiting pathogen attack. In contrast, the susceptible lines activated defensive genes only after pathogen inoculation, resulting in increased levels of H2O2 and lipid peroxidation, as well as lower enzymatic activities. The constitutive defenses observed in this study from seed could be profitably exploited to develop markers to speed up conventional breeding programs in the selection of resistant genotypes.

Histopathological assessment of the infection of maize leaves by Fusarium graminearum, F. proliferatum, and F. verticillioides

Young maize plants were inoculated on unfolded mature leaves and on folded immature leaves with Fusarium graminearum, Fusarium proliferatum, and Fusarium verticillioides suspensions. Infection and symptom development of disease on these asymptomatic mature leaves and immature leaves were then documented. Subcuticular infection was found by the three Fusarium species on both symptomatic and symptomless leaves. The three Fusarium species penetrated the stomata of immature leaves by the formation of appressoria-like structures, infection cushions or by direct penetration. Infection by the three species of Fusarium via stomata is reported here for the first time. The superficial hyphae and re-emerging hyphae of the three species produced conidia. The macroconidia of F. graminearum produced secondary macroconidia and F. proliferatum formed microconidia inside the leaf tissues that sporulated through stomata and trichomes. The infection of maize leaves by the three species of Fusarium and their sporulation may contribute inoculum to cob and kernel infection.

Maize leaf trichomes represent an entry point of infection for Fusarium species

Fifteen day old maize seedlings were inoculated with Fusarium graminearum, Fusarium proliferatum, and Fusarium verticillioides. More than 90 % F. proliferatum and F. verticillioides conidia and 50 % of F. graminearum formed one germ tube whereas the other 50 % of F. graminearum conidia formed two to three germ tubes. The germ tubes of F. graminearum conidia were longer than those of F. proliferatum and F. verticillioides. The three species of Fusarium infected bi-cellular trichomes by adhering and growing along the trichomes or by attaching to the cap cell of the trichomes 48 h after inoculation. Hyphae penetrated into the trichomes at the base, the side or at the top of the cap cells. The hyphae colonized the cap cells and then spread to base cells. Prickle trichomes were infected 72 h after inoculation. The hyphae either wrapped around prickle trichomes or formed a mass of hyphae around the top of prickle trichomes or formed appressorium. Macro trichomes were infected by F. graminearum 7 d after inoculation. Following penetration, the fungus spread to adjacent epidermal cells and to the subcuticle. This investigation provides the first assessment of F. graminearum, F. proliferatum, and F. verticillioides infection via trichomes of maize leaves.

Tissue-specific gene expression in maize seeds during colonization by Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides are fungal pathogens that colonize maize kernels and produce the harmful mycotoxins aflatoxin and fumonisin, respectively. Management practice based on potential host resistance to reduce contamination by these mycotoxins has proven difficult, resulting in the need for a better understanding of the infection process by these fungi and the response of maize seeds to infection. In this study, we followed the colonization of seeds by histological methods and the transcriptional changes of two maize defence-related genes in specific seed tissues by RNA in situ hybridization. Maize kernels were inoculated with either A. flavus or F. verticillioides 21-22 days after pollination, and harvested at 4, 12, 24, 48, 72, 96 and 120 h post-inoculation. The fungi colonized all tissues of maize seed, but differed in their interactions with aleurone and germ tissues. RNA in situ hybridization showed the induction of the maize pathogenesis-related protein, maize seed (PRms) gene in the aleurone and scutellum on infection by either fungus. Transcripts of the maize sucrose synthase-encoding gene, shrunken-1 (Sh1), were observed in the embryo of non-infected kernels, but were induced on infection by each fungus in the aleurone and scutellum. By comparing histological and RNA in situ hybridization results from adjacent serial sections, we found that the transcripts of these two genes accumulated in tissue prior to the arrival of the advancing pathogens in the seeds. A knowledge of the patterns of colonization and tissue-specific gene expression in response to these fungi will be helpful in the development of resistance.

Defense Responses to Mycotoxin-Producing Fungi Fusarium proliferatum, F. subglutinans, and Aspergillus flavus in Kernels of Susceptible and Resistant Maize Genotypes

Developing kernels of resistant and susceptible maize genotypes were inoculated with Fusarium proliferatum, F. subglutinans, and Aspergillus flavus. Selected defense systems were investigated using real-time reverse transcription-polymerase chain reaction to monitor the expression of pathogenesis-related (PR) genes (PR1, PR5, PRm3, PRm6) and genes protective from oxidative stress (peroxidase, catalase, superoxide dismutase and ascorbate peroxidase) at 72 h postinoculation. The study was also extended to the analysis of the ascorbate-glutathione cycle and catalase, superoxide dismutase, and cytosolic and wall peroxidases enzymes. Furthermore, the hydrogen peroxide and malondialdehyde contents were studied to evaluate the oxidation level. Higher gene expression and enzymatic activities were observed in uninoculated kernels of resistant line, conferring a major readiness to the pathogen attack. Moreover expression values of PR genes remained higher in the resistant line after inoculation, demonstrating a potentiated response to the pathogen invasions. In contrast, reactive oxygen species-scavenging genes were strongly induced in the susceptible line only after pathogen inoculation, although their enzymatic activity was higher in the resistant line. Our data provide an important basis for further investigation of defense gene functions in developing kernels in order to improve resistance to fungal pathogens. Maize genotypes with overexpressed resistance traits could be profitably utilized in breeding programs focused on resistance to pathogens and grain safety.

Fusarium verticillioides SGE1 is required for full virulence and regulates expression of protein effector and secondary metabolite biosynthetic genes

The transition from one lifestyle to another in some fungi is initiated by a single orthologous gene, SGE1, that regulates markedly different genes in different fungi. Despite these differences, many of the regulated genes encode effector proteins or proteins involved in the synthesis of secondary metabolites (SM), both of which can contribute to pathogenicity. Fusarium verticillioides is both an endophyte and a pathogen of maize and can grow as a saprophyte on dead plant material. During growth on live maize plants, the fungus can synthesize a number of toxic SM, including fumonisins, fusarins, and fusaric acid, that can contaminate kernels and kernel-based food and feed. In this study, the role of F. verticillioides SGE1 in pathogenicity and secondary metabolism was examined by gene deletion analysis and transcriptomics. SGE1 is not required for vegetative growth or conidiation but is required for wild-type pathogenicity and affects synthesis of multiple SM, including fumonisins and fusarins. Induced expression of SGE1 enhanced or reduced expression of hundreds of genes, including numerous putative effector genes that could contribute to growth in planta; genes encoding cell surface proteins; gene clusters required for synthesis of fusarins, bikaverin, and an unknown metabolite; as well as the gene encoding the fumonisin cluster transcriptional activator. Together, our results indicate that SGE1 has a role in global regulation of transcription in F. verticillioides that impacts but is not absolutely required for secondary metabolism and pathogenicity on maize.

Induction of salicylic acid-mediated defense response in perennial ryegrass against infection by Magnaporthe oryzae

Incorporation of plant defense activators is an innovative approach to development of an integrated strategy for the management of turfgrass diseases. The effects of salicylic acid (SA), benzothiadiazole (BTH, chemical analog of SA), jasmonic acid (JA), and ethephon (ET, an ethylene-releasing compound) on development of gray leaf spot in perennial ryegrass (Lolium perenne L.) caused by Magnaporthe oryzae were evaluated. Gray leaf spot disease incidence and severity were significantly decreased when plants were treated prior to inoculation with SA, BTH, and partially by ET but not by JA. Accumulation of endogenous SA and elevated expression of pathogenesis-related (PR)-1, PR-3.1, and PR-5 genes were associated with inoculation of plants by M. oryzae. Treatment of plants with SA enhanced expression levels of PR-3.1 and PR-5 but did not affect the PR-1 level, whereas BTH treatment enhanced relative expression levels of all three PR genes. Microscopic observations of leaves inoculated with M. oryzae revealed higher frequencies of callose deposition at the penetration sites in SA- and BTH-treated plants compared with the control plants (treated with water). These results suggest that early and higher induction of these genes by systemic resistance inducers may provide perennial ryegrass with a substantial advantage to defend against infection by M. oryzae.

Expression of the maize ZmGF14-6 gene in rice confers tolerance to drought stress while enhancing susceptibility to pathogen infection

14-3-3 proteins are found in all eukaryotes where they act as regulators of diverse signalling pathways associated with a wide range of biological processes. In this study the functional characterization of the ZmGF14-6 gene encoding a maize 14-3-3 protein is reported. Gene expression analyses indicated that ZmGF14-6 is up-regulated by fungal infection and salt treatment in maize plants, whereas its expression is down-regulated by drought stress. It is reported that rice plants constitutively expressing ZmGF14-6 displayed enhanced tolerance to drought stress which was accompanied by a stronger induction of drought-associated rice genes. However, rice plants expressing ZmGF14-6 either in a constitutive or under a pathogen-inducible regime showed a higher susceptibility to infection by the fungal pathogens Fusarium verticillioides and Magnaporthe oryzae. Under infection conditions, a lower intensity in the expression of defence-related genes occurred in ZmGF14-6 rice plants. These findings support that ZmGF14-6 positively regulates drought tolerance in transgenic rice while negatively modulating the plant defence response to pathogen infection. Transient expression assays of fluorescently labelled ZmGF14-6 protein in onion epidermal cells revealed a widespread distribution of ZmGF14-6 in the cytoplasm and nucleus. Additionally, colocalization experiments of fluorescently labelled ZmGF14-6 with organelle markers, in combination with cell labelling with the endocytic tracer FM4-64, revealed a subcellular localization of ZmGF14-6 in the early endosomes. Taken together, these results improve our understanding of the role of ZmGF14-6 in stress signalling pathways, while indicating that ZmGF14-6 inversely regulates the plant response to biotic and abiotic stresses.

Ultrastructural studies of Phellinus sulphurascens infection of Douglas-fir roots and immunolocalization of host pathogenesis-related proteins

Interactions between roots of Douglas-fir (DF; Pseudotsuga menziesii) seedlings and the laminated root rot fungus Phellinus sulphurascens were investigated using scanning and transmission electron microscopy and immunogold labelling techniques. Scanning electron micrographs revealed that P. sulphurascens hyphae colonize root surfaces and initiate the penetration of root epidermal tissues by developing appressoria within 2 d postinoculation (dpi). During early colonization, intra- and intercellular fungal hyphae were detected. They efficiently disintegrate cellular components of the host including cell walls and membranes. P. sulphurascens hyphae penetrate host cell walls by forming narrow hyphal tips and a variety of haustoria-like structures which may play important roles in pathogenic interactions. Ovomucoid-WGA (wheat germ agglutinin) conjugated gold particles (10 nm) confirmed the occurrence and location of P. sulphurascens hyphae, while four specific host pathogenesis-related (PR) protein antibodies conjugated with protein A-gold complex (20 nm) showed the localization and abundance of these PR proteins in infected root tissues. A thaumatin-like protein and an endochitinase-like protein were both strongly evident and localized in host cell membranes. A DF-PR10 protein was localized in the cell walls and cytoplasm of host cells while an antimicrobial peptide occurred in host cell walls. A close association of some PR proteins with P. sulphurascens hyphae suggests their potential antifungal activities in DF roots.

Host-Pathogen Interactions in Douglas-Fir Seedlings Infected by Phellinus sulphurascens

ABSTRACT Several aspects of the host-pathogen interaction between Douglas-fir (Pseudotsuga menziesii) and the fungal pathogen Phellinus sulphurascens were investigated in an in vitro inoculation system using young seedlings and fungal mycelia. Light microscopy confirmed that P. sulphurascens mycelia can successfully penetrate host epidermal cells within 3 days postinoculation (dpi). Extensive fungal colonization and cortical cell decay occurred within 14 dpi. Western immunoblot studies showed significant upregulation (five to sixfold) of four specific pathogenesis-related (PR) proteins in infected roots. These proteins were a Douglas-fir thaumatin-like protein (PmTLP), an endochitinase protein (ECP), a Douglas-fir PR10 (DF-PR10) protein (PsemI), and a 10.6-kDa antimicrobial peptide (PmAMP1). The highest accumulation of PmTLP and PmAMP1 occurred at 12 dpi, whereas accumulations of the ECP and DF-PR10 proteins peaked at 7 dpi. For both inoculated and control Douglas-fir seedlings, only one of the four PR proteins, PmAMP1, was clearly detectable in needles. Immunolocalization experiments using fluorescein isothiocyanate-conjugated secondary antibodies confirmed accumulation of all four PR proteins mainly in and around cell walls of root cortical tissues. Overall, the highest immunofluorescence was observed in infected roots at 12 dpi, whereas labeling in control roots was negligible at all sample times. The ECP produced the highest fluorescence; the DF-PR10 the lowest. Upregulation and localization of these PR proteins in cortical tissues of inoculated roots suggest that they play a defensive role in response to infection by P. sulphurascens. This in vitro inoculation system will facilitate further proteomic and genomic studies of this important pathosystem.

Maize (Zea mays L.) genetic factors for preventing fumonisin contamination

Fusarium moniliforme and Fusarium proliferatum are the most frequently isolated fungi from maize (Zea mays L.) in Spain. Both Fusarium species produce toxins potentially dangerous for animals and humans, the fumonisins being the most significant of those toxins. White maize is preferred for human consumption, and extra care should be taken to avoid kernel mycotoxin contamination. The objectives of this study were to identify and quantify kernel infection by Fusarium spp. and contamination by fumonisin on white maize hybrids, to search for white maize sources of resistance to infection by Fusarium spp. and mycotoxin contamination, and to preliminarily study the genetics involved in such resistances. Ten F(1) single crosses derived from a diallel mating design among five white maize inbreds were evaluated in a randomized complete block design with three replications in 2002 at two locations. Fusarium verticilloides and F. proliferatum were detected on kernels of white maize hybrids cultivated in northwestern Spain. No differences in fungal infection were found among maize genotypes, but differences in fumonisin contamination were significant and could be related, in part, to differences in husk tightness. Among the genotypes studied, general combining ability (GCA) effects were the most important for resistance to fumonisin contamination. Inbreds EP10 and EC22 showed the most favorable GCA effects for husk tightness and fumonisin content, and the cross between them, EP10 x EC22, had the most favorable specific combining ability (SCA) effect for husk tightness. Inbreds EP10 and EC22 showed favorable GCA effects for fumonisin contamination and husk tightness, and the cross EP10 x EC22 was the only one with an average fumonisin level below 1 mug/g. Although this should be confirmed with more extensive studies, white maize inbreds developed from white maize landraces could be sources of resistance to fumonisin contamination.

Early events in the Fusarium verticillioides-maize interaction characterized by using a green fluorescent protein-expressing transgenic isolate

The infection of maize by Fusarium verticillioides can result in highly variable disease symptoms ranging from asymptomatic plants to severe rotting and wilting. We produced F. verticillioides green fluorescent protein-expressing transgenic isolates and used them to characterize early events in the F. verticillioides-maize interaction that may affect later symptom appearance. Plants grown in F. verticillioides-infested soil were smaller and chlorotic. The fungus colonized all of the underground parts of a plant but was found primarily in lateral roots and mesocotyl tissue. In some mesocotyl cells, conidia were produced within 14 to 21 days after infection. Intercellular mycelium was detected, but additional cells were not infected until 21 days after planting. At 25 to 30 days after planting, the mesocotyl and main roots were heavily infected, and rotting developed in these tissues. Other tissues, including the adventitious roots and the stem, appeared to be healthy and contained only a small number of hyphae. These results imply that asymptomatic systemic infection is characterized by a mode of fungal development that includes infection of certain tissues, intercellular growth of a limited number of fungal hyphae, and reproduction of the fungus in a few cells without invasion of other cells. Development of visibly rotted tissue is associated with massive production of fungal mycelium and much less organized growth.

Latency- and Defense-Related Ultrastructural Characteristics of Apple Fruit Tissues Infected with Botryosphaeria dothidea

ABSTRACT Apple fruit tissues infected with Botryosphaeria dothidea were examined by transmission electron microscopy using susceptible cv. Fuji and resistant cv. Jonathan. Immature (green) and mature (red) fruits of cv. Fuji with restricted or expanding lesions were also examined to reveal subcellular characteristics related with latent and restricted disease development. In infected susceptible mature fruits, cytoplasmic degeneration and organelle disruption commonly occurred, accompanying cell wall dissolution around invading hyphae. Cell wall dissolution around invading hyphae in subepidermis was rare in immature, red halo-symptomed cv. Fuji and resistant cv. Jonathan fruits. In infected immature fruits of cv. Fuji, presumably at the latent state of disease development, cellular degeneration was less severe, and invading hyphae contained prominent microbody-lipid globule complexes or the deposition of thin electron-dense outer layer around cell wall of intercellular hyphae. Both mature fruits with red halos and resistant apple fruits formed cell wall protuberances at the outside of cell walls. In addition, electron-dense extramural layers were formed in the resistant apple fruits. Aberrant hyphal structures such as intrahyphal hyphae were found only in resistant fruit tissues, indicating the physiologically altered fungal growth. These ultrastructural changes of host tissues and fungal hyphae may reflect the pathogenesis of apple white rot under varying conditions of apple fruits.