Biomedical Engineering Reference
In-Depth Information
INTRODUCTION
Fungi are eukaryotic, carbon-heterotrophic microorganisms. To satisfy
their need for organic nutrients, most fungal species live a saprophytic
lifestyle. It has been estimated that the fungal kingdom contains more than
1.5 million species, but only around 100,000 have so far been described,
with yeast, mold and mushroom being the most familiar (Hawksworth
1991). Fungi are important pathogens of plants that cause more signifi cant
yield losses than bacteria or viruses. Numerous fungi are devastating to
human and plants and are a serious threat to agricultural industry and
human health (Tournas 2005, San-Blas et al. 2000).
Toxins produced by plant pathogenic fungi differ in structure as well
as in their role in disease and mode of action (Knoche et Duvick 1987).
Toxins play diverse roles in disease, from impacting symptom expression
and disease progress to being absolutely required for pathogenesis. Some
toxins are directly toxic, killing cells and allowing for infection of other
cells. Others interfere with induction of defense responses or induce
programmed cell death-mediated defense responses in order to generate
necrosis required for pathogenesis (Wolpert et al. 2002). All of the identifi ed
perylenequinone toxins are produced by members of the Ascomycota, the
largest phylum within the fungal kingdom. The similarity of the fungal
erylenequinone structure to hypericin led to investigations of the fungal
compounds as hotosensitizers (Daub 1982, Yamazaki et al. 1975), which play
diverse roles as defense compounds in plants, pathogenesis determinants
in fungi and as molecules responsible for photomovement of protozoans
(Heitz et Downum 1995).
Visual assessment of disease caused by fungal species is often
insuffi cient to diagnose causal agent of the disease, particularly where
several organisms induce similar symptoms within a disease niche. Accurate
identifi cation and early detection of pathogens is the cornerstone of disease
management in many crops. Conventional methods for the identifi cation
of fungal species in animal and plant tissues generally involve isolation
of the fungus into axenic culture. Once isolated, the organism is generally
identifi ed on the basis of morphological characteristics of the colony, conidia
and conidiogenous cells. Many plant pathogens are diffi cult to identify using
morphological criteria, which can be time consuming and challenging and
requires extensive knowledge in taxonomy. Reliable identifi cation requires
considerable expertise and is greatly complicated by the plasticity and
instability of fungal species in culture. It is thus, perhaps, unsurprising that
alternative methods have been sought to enable detection and diagnosis of
plant and animal pathogens. Although the detection and identifi cation of a
particular fungal species is often desirable it may be secondary to the other
considerations. Advances in genetics, molecular biology and analyses of
