Edition 11 - May, 2002
Edition 11 - May, 2002 |
Fumonisins : mycotoxins produced by Fusarium species of the Gibberella fujikuroi complex (section Liseola) |
Five toxic fungal secondary metabolites (mycotoxins) are considered to be economically and toxicologically important worldwide: aflatoxin, ochratoxin, deoxynivalenol (DON = vomitoxin) and derivatives, zearalenone (ZON) and derivatives, and fumonisins (FB1, FB2). The last three of these, DON, ZON, and fumonisins, are produced by various Fusarium species. FAO experts estimate that 25% of the world's food crops are affected by mycotoxins each year, which means that the Fusarium species contributes substantially to food contamination (Charmley et al., 1994).
The increasing number of reports on fumonisin contamination and intoxication along with recent advances in phylogeny and taxonomy led us to undertake a research program on the Fusarium strains of the MUCL collection. The aims of the program are (1) to provide the scientific community with biochemically, genetically, and molecularly well-described Fusarium strains and (2) to contribute to the elucidation of the fumonisin biosynthetic pathway. To reach this goal, we developed an up-to-date identification scheme of the Fusarium species using molecular tools and sensitive ELISA and HPLC fumonisin determination methods.
Fumonisins are produced by Fusarium species of the Gibberella fujikuroi complex
(section Liseola). Taxonomically, the section Liseola is recognized on the basis
of morphological characteristics, but the G. fujikuroi complex is highly controversial,
comprising from 2 to more than 40 species according to the various authors.
In recent studies using nucleotide characters from four loci [nuclear large
subunit 28S rDNA, internal transcribed spacer regions (ITS1 and ITS2), mitochondrial
small subunit (mtSSU) ribosomal DNA, and b-tubulin] and two protein-encoding
nuclear genes [translation elongation factor 1a (EF-1a) and calmodulin], O'Donnell
et al. recognized 46 species within this complex (O'Donnell et al., 2000). Eight
of these species had already been distinguished as mating populations (A to
H), 2 being known to produce high amounts of fumonisin (A and D).
Six others are weak- or non-producers.
In F. verticillioides (mating A), a cluster of 9 genes (FUM1 to FUM9) is involved
in the biosynthesis of fumonisins. The potential biological functions of 5 genes
(FUM5 to FUM9) have already been described (Seo et al., 2001).
 Fusarium subglutinans:
formation of microconidia in false-head. |
The reference strains for mating and fumonisin production (Table 1) were used as internal references for the development of the different experiments. We designed specific primers to four genes involved in the fumonisin production (FUM6 to 9) and demonstrated the presence of these genes in five tested strains (nos. 1 to 5; Table 1) including non-producing strains. These preliminary results confirm that fumonisin production results from a complex biosynthetic pathway. Our fumonisin production results obtained with reference strains agreed well with published data since only strains from mating populations A and D turned out to be good fumonisin producers. Further analysis of the FUM 1 to 9 genes in the producing and non-producing strains of the species F. verticillioides, as well as in the other species of the complex, will help us to evaluate the presence or the absence of genes in the various species and to determine which genes are required for fumonisin production.
In-depth analysis of the MUCL strains belonging to the G. fujikuroi complex (Table 2) required a more accurate identification up to the species level. For this identification, we used morphological and molecular tools. The mating population of each strain was deduced from the identification and from crossings with reference strains. The mating types (1 or 2) were described using mating type-specific primers (Steenkamp et al., 2000).
The interest of this re-identification is confirmed by the fact that some isolates
stored in the MUCL collection for several decades (e.g. no. 30, Table 2) were
re-identified with new species described in 1998 by Nirenberg and O'Donnell.
Moreover, no information was available on the mating population and the fumonisin
production level of F. phyllophilum, and we demonstrated here that the strain
no. 30 belongs to the mating type 1 and is a relatively weak fumonisin producer.
 Fusarium subglutinans:
formation of microconidia on SNA. Note presence of polyphialides. |
Although morphology remains the first key for taxonomy, molecular biology has
become an essential tool for the accurate and rapid identification of the Fusarium
species of the G. fujikuroi complex and , also the other Fusarium species. A
better knowledge of relationships between the mycotoxigenic potential and taxonomy
is fundamental, and can help the food industry evaluate the risk of mycotoxin
contaminations. Furthermore, the characterization of the MUCL strains using
rapid and appropriate methods provides a higher value-added patrimony.
Authors: Françoise Munaut, Cynthia Langue, François Van Hove
Contact
Dr. François Van Hove
Tel.: +32 (0)10 47 30 84
Fax: +32 (0)10 45 15 01
E-mail: vanhove@mbla.ucl.ac.be
Charmley L.L., Rosenberg A. & Trenholm H.L. (1994) Factors responsible for economic losses due to Fusarium mycotoxin contamination of grain, foods and feedstuffs. In: Mycotoxins in Grains. Miller J.D. and Trenholm H.L. (eds). St. Paul, MN: Eagan Press. p471.
Nirenberg H.I. & O'Donnell K. (1998)
New Fusarium species and combinations within the Gibberella fujikuroi species
complex. Mycologia 90: 434-458.
O'Donnell K., Nirenberg H.I., Aoki T. & Cigelnik E. (2000) A multigene phylogeny of the Gibberella fujikuroi species complex: detection of additional phylogenetically distinct species. Mycoscience 41: 61-78.
Seo J.A., Proctor R.H. & Plattner R.D. (2001) Characterization of four
clustered and
coregulated genes associated with fumonisin biosynthesis in Fusarium verticillioides.
Fungal Genet. Biol. 34:155-165.
Steenkamp E.T., Wingfield B.D., Coutinho T.A., Zeller K.A., Wingfield M.J.,
Marasas W.F.O. & Leslie J.F. (2000)
PCR-based identification of MAT-1 and MAT-2 in the Gibberella fujikuroi species
complex. Appl. Environ. Microbiol. 66: 4378-4382.
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