Variation in pathogenicity of Fusarium verticillioides and resistance of maize genotypes to Fusarium ear rot

Abstract

The ineffective control measures of pathogens is due to variability among their populations. Hence, the study of pathogenic variation of Fusarium verticillioides strains on maize genotypes. Six F. verticillioides infected maize ear were randomly obtained from three agro-ecological zones in Southwest Nigeria. Pathogenicity of F. verticillioides strains at 1.0?×?10⁶ spores/mL were examined in vivo based on the rating scale of 1–7 on maize genotypes T2L COMP.1.STR SYN-W-1, PVA SYN 8F2, and T2L COMP.4. The pathogens were inoculated to the maize genotypes on 8th week for disease severity and were all moderately susceptible, however, genotype T2L COMP.4 was the most susceptible. It was observed that, 9.4% were classified as highly virulent, 12.5% as virulent, 37.5% as moderately virulent, 21.8% as slightly virulent, and 18.8% as non-virulent. In all, Fusarium verticillioides strains displayed different degrees of virulence, however, maize genotype T2L COMP.4 was the most susceptible to ear rot.     

Xylanase from a newly isolated Fusarium verticillioides capable of utilizing corn fiber xylan

A fungus, Fusarium verticillioides (NRRL 26518), was isolated by screening soil samples using corn fiber xylan as carbon source. The extracellular xylanase from this fungal strain was purified to apparent homogeneity from the culture supernatant by ultrafiltration using a 30,000 cut-off membrane, octyl-Sepharose chromatography and Bio gel A-0.5 m gel filtration. The purified xylanase (specific activity 492 U/mg protein; MW 24,000; pI 8.6) displayed an optimum temperature at 50 degrees C and optimum pH at 5.5, a pH stability range from 4.0 to 9.5 and thermal stability up to 50 degrees C. It hydrolyzed a variety of xylan substrates mainly to xylobiose and higher short-chain xylooligosaccharides. No xylose was formed. The enzyme did not require metal ions for activity and stability.     

Genetic variation in Fusarium section Liseola from no-till maize in Argentina

Strains of Fusarium species belonging to section Liseola cause stalk and ear rot of maize and produce important mycotoxins, such as fumonisins. We isolated two species, Fusarium verticillioides (Gibberella fujikuroi mating population A) and Fusarium proliferatum (G. fujikuroi mating population D) from maize cultivated under no-till conditions at five locations in the Córdoba province of Argentina. We determined the effective population number for mating population A (N(e)) and found that the N(e) for mating type was 89% of the count (total population) and that the N(e) for male or hermaphrodite status was 36%. Thus, the number of strains that can function as the female parent limits N(e), and sexual reproduction needs to occur only once every 54 to 220 asexual generations to maintain this level of sexual fertility. Our results indicate that the fungal populations isolated from no-till maize are similar to those recovered from maize managed with conventional tillage. We placed 36 strains from mating population A into 28 vegetative compatibility groups (VCGs). Of the 13 strains belonging to five multimember VCGs, only 2 isolates belonging to one VCG were clones based on amplified fragment length polymorphism (AFLP) fingerprints. Members of the other four multimember VCGs had an average similarity index of 0.89, and members of one VCG were no more closely related to other members of the same VCG than they were to other members of the population as a whole. This finding suggests that the common assumption that strains in the same VCG are either clonal or very closely related needs to be examined in more detail. The variability observed with AFLPs and VCGs suggests that sexual reproduction may occur more frequently than estimated by N(e).     

Molecular characterization of endophytic strains of Fusarium verticillioides (= Fusarium moniliforme) from maize (Zea mays. L)

The species Fusarium verticillioides (= F. moniliforme) is often found in maize seeds, constituting an important source of inoculum in the field. Fusarium spp., associated with symptomatic and asymptomatic plants, may be a primary causal agent of disease, a secondary invader or an endophyte. In the present work, endophytic fungi were isolated from two populations of Zea mays (BR-105 and BR-106) and their respective inbred lines. Within different inbred lines of maize, Fusarium was found at a frequency of 0 to 100% relative to the number of total isolated fungi. The frequency with which the genus occurred was practically the same in the two field sites (around 60%). Twenty-one F. verticillioides strains were analysed using the random amplified polymorphic DNA (RAPD) technique, employing 10 random primers. Variability analysis of endophytic isolates via RAPD showed genome polymorphism taxa of species around 60%. Endophytic isolates were clustered by their sites of origin. RAPD analysis clustered the endophytic isolates by their maize inbred lines hosts (Mil-01 to Mil-06), whereas at site A they clustered into two major groups related to the maize gene pool (BR-105 or BR-106 population). All strains isolated from seeds collected in Site A, except strains L9 and L10, were sub-grouped according to maize inbred lines. The analysis showed a discrete sub-grouping at site B. Results obtained here could be explained by a co-evolution process involving endophytic isolates of F. verticillioides and maize inbred lines.     

Antagonistic potential of Trichoderma longibrachiatum and T. hamatum resident on maize (Zea mays) plant against Fusarium verticillioides (Nirenberg) isolated from rotting maize stem

Antagonistic potential of Trichoderma longibrachiatum and T. hamatum against the pathogen Fusarium verticillioides was examined in the laboratory. This was done by pairing each Trichoderma species with the pathogen on 9 cm Petri plates of acidified potato dextrose agar (APDA). Three pairing methods were employed and gradings were assigned to different radial growth suppression of F. verticillioides by each Trichoderma species. Analysis was done using the GLM procedure of the SAS package. Both Trichoderma species significantly inhibited radial growth of F. verticillioides (P = 0.01, R ² = 0.99) irrespective of pairing method. ‘Inoculating antagonist before pathogen’ supported the best growth inhibition of F. verticillioides by both Trichoderma species. Both Trichoderma species differed significantly (P > 0.0029) in inhibiting radial growth of F. verticillioides. Growth inhibition differed significantly within (P > 0.0059) and among (P > 0.0001) pairing methods. T. longibrachiatum was significantly better than T. hamatum in inhibiting radial growth of F. verticillioides, even at P = 0.01. T. hamatum and T. longibrachiatum could thus be said to show promising antagonistic potential against F. verticillioides with the latter showing better prospects.     

Amylopectin induces fumonisin B1 production by Fusarium verticillioides during colonization of maize kernels

Fusarium verticillioides, a fungal pathogen of maize, produces fumonisin mycotoxins that adversely affect human and animal health. Basic questions remain unanswered regarding the interactions between the host plant and the fungus that lead to the accumulation of fumonisins in maize kernels. In this study, we evaluated the role of kernel endosperm composition in regulating fumonisin B1 (FB1) biosynthesis. We found that kernels lacking starch due to physiological immaturity did not accumulate FB1. Quantitative polymerase chain reaction analysis indicated that kernel development also affected the expression of fungal genes involved in FB1 biosynthesis, starch metabolism, and nitrogen regulation. A mutant strain of F. verticillioides with a disrupted a-amylase gene was impaired in its ability to produce FB1 on starchy kernels, and both the wild-type and mutant strains produced significantly less FB1 on a high-amylose kernel mutant of maize. When grown on a defined medium with amylose as the sole carbon source, the wild-type strain produced only trace amounts of FB1, but it produced large amounts of FB1 when grown on amylopectin or dextrin, a product of amylopectin hydrolysis. We conclude that amylopectin induces FB1 production in F. verticillioides. This study provides new insight regarding the interaction between the fungus and maize kernel during pathogenesis and highlights important areas that need further study.     

Mycotoxin production by Fusarium species isolated from New Zealand maize fields

Forty Fusarium isolates obtained from maize fields were screened for moniliformin production on maize kernels. Twelve isolates, including seven of F. subglutinans, were found to produce moniliformin at levels ranging from 0.4 to 64 ppm. Twenty six isolates were also screened for production of deoxynivalenol, diacetoxyscirpenol, T-2 toxin and zearalenone. Of these, 22, including all 11 isolates of F. graminearum, produced zearalenone at levels ranging from 0.1 to 96.0 ppm, while 13 produced T-2 toxin at low levels, (<1.1 ppm). Deoxynivalenol and diacetoxyscirpenol were each produced by six isolates, also at low levels (<1.0 ppm). Three isolates of F. graminearum and one of F. sambucinum produced four toxins simultaneously.     

Genetic diversity of Fusarium oxysporum populations isolated from different soils in France

The genetic diversity of soil-borne populations of Fusarium oxysporum was assessed using 350 isolates collected from six different French soils. All isolates were characterised by restriction fragment analysis of the PCR-amplified ribosomal intergenic spacer (IGS). Twenty-six IGS types were identified among the 350 isolates analysed. Five to nine different IGS types were detected in each soil. None of the IGS types was common to all of the soils. An analysis of the molecular variance based on IGS type relationships and frequency revealed that the genetic structure of the populations of F. oxysporum varied widely among the soils. Some populations were both highly diverse within the soils and differentiated between the soils. A possible relationship between the intrapopulation or interpopulation level of diversity and some external factors such as the soil type or the crop history was evaluated. A subsample representative of the diversity of the six populations was further characterised by analysing the genomic distribution of two transposable elements, impala and Fot1. One to 10 copies of the impala element were present in most of the isolates, irrespective of their soil of origin. The Fot1 element was only detected in 40% of the isolates originating from the three populations less diverse in terms of IGS types, but in 82.6% of the isolates originating from the three more diverse populations.     

Protein phosphatase 2A regulatory subunits perform distinct functional roles in the maize pathogen Fusarium verticillioides

Fusarium verticillioides is a pathogen of maize causing ear rot and stalk rot. The fungus also produces fumonisins, a group of mycotoxins linked to disorders in animals and humans. A cluster of genes, designated FUM genes, plays a key role in the synthesis of fumonisins. However, our understanding of the regulatory mechanism of fumonisin biosynthesis is still incomplete. We have demonstrated previously that Cpp1, a protein phosphatase type 2A (PP2A) catalytic subunit, negatively regulates fumonisin production and is involved in cell shape maintenance. In general, three PP2A subunits, structural A, regulatory B and catalytic C, make up a heterotrimer complex to perform regulatory functions. Significantly, we identified two PP2A regulatory subunits in the F. verticillioides genome, Ppr1 and Ppr2, which are homologous to Saccharomyces cerevisiae Cdc55 and Rts1, respectively. In this study, we hypothesized that Ppr1 and Ppr2 are involved in the regulation of fumonisin biosynthesis and/or cell development in F. verticillioides, and generated a series of mutants to determine the functional role of Ppr1 and Ppr2. The PPR1 deletion strain (Δppr1) resulted in drastic growth defects, but increased microconidia production. The PPR2 deletion mutant strain (Δppr2) showed elevated fumonisin production, similar to the Δcpp1 strain. Germinating Δppr1 conidia formed abnormally swollen cells with a central septation site, whereas Δppr2 showed early hyphal branching during conidia germination. A kernel rot assay showed that the mutants were slow to colonize kernels, but this is probably a result of growth defects rather than a virulence defect. Results from this study suggest that two PP2A regulatory subunits in F. verticillioides carry out distinct roles in the regulation of fumonisin biosynthesis and fungal development.     

Purification and characterization of mycoferritin from Fusarium verticillioides MRC 826

The fungus Fusarium verticillioides MRC 826 (ascomycetes species), a toxigenic isolate is capable of synthesizing mycoferritin only upon induction with iron in yeast extract sucrose medium. The molecular mass, yield, iron and carbohydrate contents of the purified mycoferritin were 460 kDa, 0.010 mg/g of wet mycelia, 1.0 and 40.2%, respectively. Native gel electrophoresis of the mycoferritin revealed two bands possibly representing isoforms of ferritin. Subunit analysis by SDS–PAGE showed a single protein subunit of ~24 kDa suggesting similar sized subunits in the structure of apoferritin shell. Immunological cross reactivity was observed with the anti-fish liver ferritin. Transmission electron microscopy revealed an apparent particle size of 100 Å. N-terminal amino acid sequencing of mycoferritin showed identities with other eukaryotic ferritin sequences. The spectral characteristics were similar to equine spleen ferritin. However, circular dichroic spectra revealed a higher degree of helicity. Functionally, induction of mycoferritin minimizes the pro-oxidant role of iron.     

Production of trichothecene and non-trichothecene mycotoxins by Fusarium species isolated from maize in Minnesota

Eighty-two cultures of Fusarium species isolated in 1986 from moldy maize in Minnesota were each cultured on rice for 4 weeks and found to produce the following mycotoxins: F. graminearum isolates, deoxynivalenol (DON, 4–225 g/g), 3-acetyldeoxynivalenol (3-ADON, 2–4g/g), 15-acetyldeoxynivalenol (15-ADON, 1–35 g/g) and zearalenone (ZEA, 5–4350 g/g); F. moniliforme, fusarin C (detectable amounts to 1000 g/g); F. mòniliforme, F. oxysporum, F. proliferatum and F. subglutinans isolates, moniliformin (15–6775 g/g); F. moniliforme, F. proliferatum, and F. subglutinans isolates, fusaric acid (detectable amounts). Other mycotoxins screened for in each rice sample and not detected were T-2 toxin, HT-2 toxin, neosolaniol, T-2 tetraol, nivalenol, fusarenon-X, scirpenols, alpha and beta trans-zearalenols, wortmannin, and fusarochromanone. The rat feeding bioassay indicated that other, unidentified toxins may be present.     

Resistance to Fusarium verticillioides in 20 Zambian Maize Hybrids

Visual assessment of maize ears and Fusarium spp. isolation from kernels were compared to determine resistance in 20 Zambian maize hybrids. The mean percentage Fusarium spp. isolations in non-inoculated field experiments varied between years (12–62%). Symptomless infection by Fusarium spp. had domination over symptomatic. More than 95% of the Fusarium species isolated were F. vertcillioides . A disease severity index and the percentage of visibly diseased, discoloured and damaged kernels did not differentiate hybrids with respect to Fusarium spp. ear rot under natural conditions. Artificial inoculation provided a good estimate of Fusarium spp. resistance based on visual symptoms in a year of moderate disease pressure, but not in a year of high disease pressure. The percentage Fusarium spp. isolations showed significant differences between hybrids after inoculation, and it was significantly negatively correlated with the number of days from planting to midsilk. Parental line L5522 contributed to hybrid resistance to Fusarium . The hybrids MM 701-1 and MM 752 were the most resistant among the 20 hybrids.