1.1.1 Importance of fungal organisms on our planet
Fungi are very successful organisms in the adaptation to their environment for many hundred million years. They developed many different largely still unknown mechanisms to maintain the habitat they exist in. Fungi are potential sources for the discovery of secondary metabolites e.g. antibiotics, mycotoxins, phytotoxins etc. and for the understanding of such mechanisms (Bhetariya, et al., 2011, Dickman & Figueiredo, 2011, Khlangwiset, et al., 2011, Bayram & Braus, 2012, Scharf, et al., 2012). Some secondary metabolites are either carcinogenic or anti-therapeutic (Wainright, 1992). On the other hand some fungal derived secondary metabolites have therapeutic relevance. For instance terrequinone A initially found in the filamentous ascomycete Aspergillus terreus (He, et al., 2004) possesses anti-tumor properties. Fungi are crucial for the recycling of organic material within the terrestrial ecosystem.
Fungi also serve as basis for our food. Among them are prominent examples like the baker’s yeast Saccharomyces cerevisiae for bread baking, beer and wine production, for which another (filamentous) ascomycete Aspergillus oryzae is also used. Food industry utilizes Aspergillus niger for citrate synthesis in huge industrial scales (Bomstein & Johnson, 1952, Papagianni, 2007, Dhillon, et al., 2011, Acourene & Ammouche, 2012). For instance, citrate serves as cleaning and preservative agent as well as dietary supplement.
Fungi are part of our daily life and have enormous biotechnological potentials.
Filamentous fungi serve as excellent platforms to analyze and understand the regulation of biology, physiology, genetics and biochemistry of a eukaryotic cell. This allows us to understand the mechanism of eukaryotic cells and also provide information about the human pathogenic fungi.
1.1.2 Pathogenic fungi
Pathogenesis describes the competence of an organism to cause deceases in another organism. Among countless pro- and eukaryotes the fungal kingdom contains numerous pathogenic representatives possessing harming effects on organisms of the two animal and plant kingdoms (Hof, 2003). Damages caused by fungi result in massive financial losses in our agriculture and health system every year (Agrios, 1997, Latge, 1999, Agrios, 2005).
Phytophthora infestans is a plant pathogenic oomycote causing the serious potato
disease known as late blight (Nowicki, et al., 2012). The effects of Phytophthora infestans infection of potatoes in Ireland from 1845 to 1857 caused over one million to starve to death and forced another two million to emigrate from affected countries. Another plant pathogenic fungus is Verticillium dahliae belonging to the class of sordariomycetes. This fungus causes verticillium wilt in many plant species showing symptoms like leaves to curl and discolour (Douglas, 2011).
In contrast, Candida albicans is a human pathogen that belongs to the Saccharomycetales, the real yeasts. This pathogen causes candidasis in immunocompromized patients suffer in AIDS, cancer and diabetis mellitus (Nielsen & Heitman, 2007). Invasive mycoses cause high morbidity and mortality rates in severely ill patients. Candida, Cryptococcus, Pneumocystis and Aspergillus are most prevalent agents for mycoses (Peman
& Salavert, 2012).
Aspergilli are heterogeneous according to their benefits and disadvantages they bring to mankind. Among the Aspergillus genus that comprises about 190 species most Aspergilli are non-pathogenic saprophytic soil organisms. Despite of this, inhaling their spores can result in different types of respiratory hypersensitive disorder. Mainly three Aspergillus species were classified as human pathogens in immunocompromized patients. These are A. flavus, A.
terreus and A. fumigatus. A. flavus favors hot climate, whereas A. fumigatus is mainly present in temperate climates. Both species cause invasive pulmonary aspergillosis leading to death in more than 90% of the patients (Sethi, et al., 2012, Pabst, et al., 2013). A. parasiticus and A.
flavus produce the carcinogenic aflatoxin and are often found in crops representing a permanent problem in food industry (De Lucca, 2007).
1.1.3 Symbiotic fungi
During symbiosis a community between fungus and another organism, mostly plants and trees, are formed. In contrast to parasites, this relationship is in mutual advantage and is called mycorrhiza. Fungi often have important mycorrhizal symbiosis with other organisms.
Particularly 90% of plants have some kind of mycorrhizal relationship with various fungi and are dependent upon this relationship for their survival (Smith & Read, 1997). For instance, mycorrhiza is specifically employed to give roses an excellent start for their growth. In this symbiosis fungi enhance the uptake of water and minerals for the plant and get sugar compounds in return. A successful growth and development of many orchids also requires symbiosis with specific fungi.
The Agaricomycetes Amanita muscaria is capable to undergo symbiosis with different
trees, whereas Leccinum scabrum and Suillus viscidus form their symbiosis with a specific tree (Reis, et al., 2011). Fungi form another symbiosis together with algae result in lichens. As for the symbiosis between fungus and plant, algae deliver the fungus with carbohydrates synthesized during photosynthesis and get water and minerals by the fungus (Perrine-Walker, et al., 2011, Ba, et al., 2012, Zambare & Christopher, 2012).
1.1.4 Saprophytic fungi
Fungal growth and differentiation are very energy consuming processes and require exploitation of external energy sources. Along with fungi numerous saprophytic pro- and eukaryotic organism genomes contain and express several genes encoding enzymes secreted for the hydrolysis of cell wall material derived from dead animals, plants, fungi and bacteria.
Cell wall composition of the different organisms is specific for the kingdoms. Plant cell wall degraded by saprophytic organisms like A. nidulans is primarily composed of a primary, secondary layer and middle lamella (Buchanan, et al., 2000). The primary layer consists of pectins, cellulose, hemicellulose and glycoproteins. Xylan belongs to hemicelluloses and is also part of the primary plant cell wall. The epidermis an outer part of the primary plant cell wall consists of cutin and wax generating the plant cuticle a permeability barrier. Waxes protect the plant from drying-out. Suberin or cutin two epidermal polyester-like polymers protect the cell from herbivores (Moire, et al., 1999). The secondary plant cell wall named cuticula consists of microfibrilcellulose and hemicellulose which strengthen and waterproof the wall additionally. Plant cell wall hydrolysis requires specific enzymes. Among them are xylanases, pectinases, cutinases, polygalacturonase etc. In contrast, bacterial cell walls are mainly composed of peptidoglycan which is also called murein (van Heijenoort, 2001).
Muramidases are able to hydrolyse murein.
Polysaccharides like starch and lichenin which assure energy storage in plants are also at fungal disposal. Thereby lichenin is mainly synthesized in moos and lichen for long-term energy storage. Saprophytic fungi contain and secret enzymes like amylases and licheninase for the utilization of such external polysaccharides. Hydrolysis of starch requires amylases.
The A. nidulans genome comprises seven known amylase genes (amyA – amyF, glaA, glaB) (Nakamura, et al., 2006). Beside starch, lichenin presents a further polysaccharide with an immense meaning for the survival of countless organisms since licheninases are conserved from prokaryotes to eukaryotes. For instance, the eng2 orthologue xgeA (AN2385) from A.
nidulans is also thought to be a putative GPI anchored endo-1,3(4)-beta-glucanase (Bauer, et al., 2006, de Groot, et al., 2009). XgeA possesses also licheninase activity. Licheninases are
also present in other Aspergilli like A. japonicus and were tested extensively (Grishutin, et al., 2006).