Fusarium Sporotrichioides

''Fusarium sporotrichioides'' is a fungal plant pathogen, one of various ''Fusarium'' species responsible for damaging crops, in particular causing a condition known as Fusarium head blight in wheat, consequently being of notable agricultural and economic importance. The species is ecologically widespread, being found across tropical and temperate regions, and is a significant producer of mycotoxins, particularly trichothecenes. Although mainly infecting crops, ''F. sporotrichioides''-derived mycotoxins can have repercussions for human health in the case of the ingestion of infected cereals. One such example includes the outbreak of alimentary toxic aleukia (ATA) in Russia, of which ''F. sporotrichioides''-infected crop was suspected to be the cause. Although current studies on ''F. sporotrichioides'' are somewhat limited in comparison to other species in the genus, ''Fusarium sporotrichioides'' has found several applications as a model system for experimentation in molecular biology.

History and taxonomy

The genus ''Fusarium'' was first characterised by the German botanist Johann Link in 1809, prior to the recognition of fungal involvement in plant disease. Over a thousand different species of ''Fusarium'' were identified by the 1930s, however, upon further analysis, these were narrowed down to 65 different species. Despite this reduction in the recognised number of species, differentiating one from the other remained difficult and unclear.

Lacking a satisfactory system of classification and identification with which to organise these many, seemingly similar ''Fusarium'' species, the mycologists Snyder and Hansen collected samples of various fusaria from laboratories worldwide, isolated single spores and cultured them for subsequent analysis of their genetic variation. Their work indicated the existence of 9 distinct species of ''Fusarium'', but there have since been several different classification systems that reach different conclusions, and consensus around this has been difficult to establish, perhaps in part because research focuses predominantly on the agriculturally or botanically more significant fusaria. ''F. sporotrichioides'' exemplifies these classification difficulties, as it is usually designated as belonging to the section Sporotrichiella, along with other similar species such as the more well-studied ''F. poae'' and ''F. tricinctum'', although other classification systems have placed ''F. sporotrichioides'' in the ''Arthrosporiella'' section, based on the similarity of its conidial morphology to other species in the section.

More currently, the taxonomy of the genus ''Fusarium'' is studied using high-performance liquid chromatography, with each of the peaks on the resulting chromatograph being detected by a photodiode array and grouped into chromophore families. As each species produces a different pattern of peaks, this technique allows accurate species identification, and has successfully been applied to several fusaria, including ''F. sporotrichioides''. For larger samples, thin-layer chromatography is usually used as a less expensive alternative.

Ecology

''Fusarium sporotrichioides'' are found in many tropical and temperate regions, usually in the soil or on wheat. Several members of the genus ''Fusarium'' are often found colonising the same area, with the proportion of the total ''Fusarium'' population each species constitutes fluctuating depending on extant weather conditions, as for example, colder weather hinders the growth of dominant species such as ''F. avenaceum'', allowing others such as ''F. culmorum'' to dominate. In contrast to other taxonomically-related fusaria that are mainly found on cereal crops, ''F. sporotrichioides'' is often soil-dwelling. Furthermore, other non-pathogenic or opportunistic ''Fusarium'' species are frequently found alongside ''F. sporotrichioides'' as part of a phylogenetically similar species complex.

Morphology

''Fusarium sporotrichioides'' is usually white in early growth, but syellow, brownish, red, pink, or purple later on. The hyphae are usually trinucleated, but can have up to eight nuclei. ''F. sportotrichioides'' usually has many aerial mycelia, and may form reddish- or yellow-brown clusters of hyphae, called sporodochia. Yellow sporodochia turn purple upon addition of alkaline substances such as ammonia, whilst reddish-brown clusters turn yellow under acidic conditions.

Members of this species have irregularly shaped, almost globular microconidia (referred to as subglobose), that are usually 5-7 μm in diameter, whilst their macroconidia are slightly curved and usually have three to five septa. Many have numerous brown, globose chlamydospores that are 7 to 15 μm in diameter, and serve as an important feature for their distinction from other fusaria.

Growth and reproduction

''Fusarium sporotrichioides'' carries out both sexual and asexual mechanisms of reproduction, with its teleomorphs including ''Nectria'' and ''Gibberella''. It is a fast-growing fungus, usually able to grow up to 8-8.8 cm in diameter within four days. Its optimal growth temperature ranges from , with the minimum and maximum temperatures required for growth being and , respectively. The minimum humidity level required for vegetative growth is 88%.

This species uses mainly maltose, starch and raffinose as carbon sources for growth. Fusarial growth is also iron-dependent, and is therefore inhibited by siderophores. These are small molecules with a high affinity for iron, that are produced by other soil-dwelling microorganisms and act as their iron-delivery system, thus interfering with the uptake of iron by ''Fusarium'' species and consequently preventing their germination.

Identification and detection methods

In earlier work, the differentiation of ''F. sporotrichioides'' from other fusaria is mainly based on differences in conidial morphology. For example, the basal cells of macroconidia in some ''Fusarium'' species have hooks or notches whilst others do not, but these differences are not always sufficient to distinguish closely related fusaria from one another. A feature unique to ''F. sporotrichioides'' compared to taxonomically related species is the presence of multiporous cells, known as polyphialides, which are now carefully considered in its identification. The fact that these polyphialides produce pyriform as well as fusiform microconidia is a further distinctive feature of ''F. sporotrichioides'', and the blastospores of ''F. sporotrichioides'' are a crucial characteristic in its distinction from the similar ''F. tricinctum''.

Over the past years, advances in molecular biology and the introduction of the polymerase chain reaction (PCR) have made the identification of ''Fusarium'' species a far more precise process. Today, species of ''Fusarium'' can be identified through the cloning and sequencing of RAPD fragments to produce primers for use in PCR that will consequently only amplify the DNA sequence of a specific species. Restriction Fragment Length Polymorphisms (RFLP) are also useful in the differentiation of fusaria, as differences in base-pair sequence cause the sample DNA sequences to be fragmented at different sites by restriction enzymes, resulting in DNA fragments of different lengths. This identification method is particularly useful for screening large numbers of samples.

Media for identification

As conidial morphology can vary depending not only on factors such as temperature, but also on the components of the growth medium, precision, and consistency in the production and use of growth media is important. potato dextrose agar

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