I. Introduction
Symbiosis is defined in the original sense as the living together of different organisms (de Bary 1879). Perhaps two-thirds of the known species of fungi form intimate relationships with other living organisms in parasitic, commensalistic, or mutualistic symbiosis (Pirozynski and Hawksworth 1988). There exist different fungus–plant interactions, which range from mycorrhiza to plant pathogens. Arbuscular mycorrhizal fungi are ancient Zygomycetes and are associated with the roots of about 80% of plant species (Bonfante and Perotto 1995). This fact supports the hypothesis of Pirozynski and Malloch (1975) that the colonization of land and the evolution of plants were possible only through the establishment of symbiotic association of a semiaquatic ancestral alga and an aquatic fungus.
Of the estimated 1.5 *106 fungal species on earth (Hawksworth 2001) about 1.30 *106 species are estimated to live as endophytes in plants (Dreyfuss and Chapela 1994), so most of the fungi are endophytes. They are ubiquitous, and probably all vascular and non-vascular terrestrial plants harbor them (Weber and Anke 2006). The term ‘‘endophyte’’ includes all organisms that, at some time of their life cycle, live symptomlessly within plant tissue (Petrini 1996). This definition also includes pathogens with extended latency periods. Endophyte association may range from intimate contact where the fungus inhabits the intercellular spaces and xylem vessels within the plant, to more or less superficial colonization of peripheral, often dying or dead tissues such as bark layers on plants with secondary growth (Petrini 1996). Ascomycetes, Basidiomycetes, Deuteromycetes, and Oomycetes were isolated as endophytes (Sinclair and Cerkauskas 1996); of these the Ascomycetes and imperfect fungi constitute the largest group.
In some cases this fungus–plant interaction has mutualistic features. Enhanced rate of growth (Bradshaw 1959; Kackley et al. 1990; Hill et al. 1991), resistance and toxicity against herbivores (Wallner et al. 1983; Read and Camp 1986), deterrence of insects (Clay 1988b), resistance against nematodes (Kimmons et al. 1990; West et al. 1990) and pathogens (White and Cole 1985; Yshihara et al. 1985), and enhanced drought tolerance (Read and Camp 1986; Arechavaleta et al. 1989; West et al. 1993) are observed as advantages to the host plants. The benefits for the fungal symbionts can be greater access to nutrients, protection against desiccation, insects, and parasitic fungi, and competition from other microbes (White et al. 2000). During the coevolution, endophytic microbes improved the resistance of the host plants to adverse conditions by secreting bioactive secondary metabolites (Ge et al. 2008).
Although the first discovery of an endophytic fungus was in 1902 by Freeman, little attention was paid to this group of fungi until the recent realization of their ecological relevance and their potential as source for new bioactive metabolites (Gunatilaka 2006).
II. The Ecological Relevance of Endophytic Fungi
Two ecological groups of endophytes can be distinguished: grass endophytes and endophytes of woody plants. Endophytic fungi were found in all woody plants examined for endophytes (Saikkonen et al. 1998). Despite the great diversity and abundancy of endophytes in woody plants, these endophytes and their interaction with their host plants receive less attention than those of grass endophytes (Saikkonen et al. 1998). The production of alkaloids in infected grasses (in planta) can cause intoxication of grazing livestock. Because of this economic importance, endophytes of poaceous grasses are the best-examined (Weber and Anke 2006).
In the family Clavicipitaceae there is a development from parasitic grass endophytes like Claviceps to mutualistic species like Acremonium (anamorphic form of Epichloe¨; Clay 1988a). Species of the genus Claviceps cause local infections of the inflorescences of grasses. Sclerotia develop at the site of infection instead of seeds of the host plant. The local infection evolved to a systemic infection, which can be found in Acremonium species (Clay 1988a). Reduction of sexual reproduction can be observed in this group of obligate endophytic fungi (Saikkonen et al. 1998). The asexual or anamorphic form is transmitted vertically by the growth of fungal hyphae into the host plant’s seeds (Arnold et al. 2003). Vogl already discovered the mycelium of an endophytic fungus in seeds of Lolium temulentum in 1898. Infection of the inflorescenses by parasitic species results in sexual sterilization of the host plant. This is in contrast to the loss of sexual reproduction of endophytes in mutualistic symbiosis. In these cases propagation of the fungus is ensured by viable seeds of the host plant.
The endophytic fungi of woody plants show a high diversity and belong to different taxonomic groups. These fungal endophytes are horizontally transmitted via spores and are not known to grow into seeds and systemically infecting the plant following seed germination (Wilson 2000). The infections are often highly localized within leaves, petioles, bark, or stems (Saikkonen et al. 1998).
Many endophytes of woody plants are closely related to pathogenic species. Freeman and Rodriguez (1993) were able to show that the mutation at a single genetic locus can change an isolate of Colletotrichum magna from a pathogen to a nonpathogenic endophytic mutualist. The wild-type fungus and the mutant of the species (path 1) were capable to infect the host plant and systemic growth was observed. The host plants infected by path 1 produced no visible effect on any plant and were protected from the wild-type fungus. Freeman and Rodriguez (1993) assumed that a portion of the host defense system is activated by path 1 so that, when exposed to the wild-type fungus, there was no delay in the defense response.
In contrast to the fungi of the Clavicipitaceae the endophytes of woody plants are not intensively investigated. One exception is the family of the Xylariaceae, which attracts attention because of its fruiting bodies and its host specificity. Most xylariaceous species are collected from living or dead angiospermous plants (Rogers 1979); most members of the family are wood inhabitants but representatives are also found in litter, soil, dung, and associated with insects (Whalley 1996). The greatest diversity of xylariaceous fungi can be found in the tropics (Petrini et al. 1995). Whalley (1996) distinguishes three groups: saprophytes, phytopathogens, and endophytes. Endophytic species, e.g. Hypoxylon and related genera, can colonize the xylem of host plants (Stone et al. 2000). Other xylem-colonizing endophytes have been identified as species of the Diaporthales (e. g. Phragmoporthe, Amphiporthe, Phomopsis), Hypocreales (Nectria spp.), and a few Basidiomycetes (e.g. Coniophora; Stone et al. 2000). Water deficiency and drought in the host plant can cause a change in the lifecycle of endophytic species. Many plant-pathogenic species of the Xylariaceae have typical characteristics of endophytes; they live as ‘‘latent invaders’’. Water stress triggers the switch to the pathogenic phase of the fungus (Whalley 1996). A switch from the latent state to the saprophytic stage is observed in Hypoxylon fragiforme when the colonized host tissue dies. The death of the tissue is accompanied by loss of water, which triggers the switch to the saprophytic stage of fungus (Weber and Anke 2006).
Some of the xylariaceous fungi have developed host specificity, e.g. Hypoxylon fragiforme is consistently isolated from healthy beech trees in Europe and H. mammatum is isolated from aspen trees in the northeast of the United States (Chapela and Boddy 1988; Chapela 1989). The wide distribution of the Xylariaceae, as endophytes and as saprophytes and their ability to produce bioactive compounds (cytochalasins, melleins, and xylarin) suggest that these fungi have an important ecological role (Petrini et al. 1995). Compounds from the bark of beech trees can trigger the eclosion of Hypoxylon fragiforme ascospores, which is a precondition for germination of the spores (Chapela et al. 1993). This can partly explain the host specificity of H. fragiforme. The eclosion of the spores can be observed 10 min after the addition of the beech extract (Webster and Weber 2004). Two substances have been identified which trigger the release of the spores at micromolar concentrations: the two monolignol glucosides Z-syringin and Z-isoconiferin (Fig. 8.1; Chapela et al. 1991).
Symbiosis is defined in the original sense as the living together of different organisms (de Bary 1879). Perhaps two-thirds of the known species of fungi form intimate relationships with other living organisms in parasitic, commensalistic, or mutualistic symbiosis (Pirozynski and Hawksworth 1988). There exist different fungus–plant interactions, which range from mycorrhiza to plant pathogens. Arbuscular mycorrhizal fungi are ancient Zygomycetes and are associated with the roots of about 80% of plant species (Bonfante and Perotto 1995). This fact supports the hypothesis of Pirozynski and Malloch (1975) that the colonization of land and the evolution of plants were possible only through the establishment of symbiotic association of a semiaquatic ancestral alga and an aquatic fungus.
Of the estimated 1.5 *106 fungal species on earth (Hawksworth 2001) about 1.30 *106 species are estimated to live as endophytes in plants (Dreyfuss and Chapela 1994), so most of the fungi are endophytes. They are ubiquitous, and probably all vascular and non-vascular terrestrial plants harbor them (Weber and Anke 2006). The term ‘‘endophyte’’ includes all organisms that, at some time of their life cycle, live symptomlessly within plant tissue (Petrini 1996). This definition also includes pathogens with extended latency periods. Endophyte association may range from intimate contact where the fungus inhabits the intercellular spaces and xylem vessels within the plant, to more or less superficial colonization of peripheral, often dying or dead tissues such as bark layers on plants with secondary growth (Petrini 1996). Ascomycetes, Basidiomycetes, Deuteromycetes, and Oomycetes were isolated as endophytes (Sinclair and Cerkauskas 1996); of these the Ascomycetes and imperfect fungi constitute the largest group.
In some cases this fungus–plant interaction has mutualistic features. Enhanced rate of growth (Bradshaw 1959; Kackley et al. 1990; Hill et al. 1991), resistance and toxicity against herbivores (Wallner et al. 1983; Read and Camp 1986), deterrence of insects (Clay 1988b), resistance against nematodes (Kimmons et al. 1990; West et al. 1990) and pathogens (White and Cole 1985; Yshihara et al. 1985), and enhanced drought tolerance (Read and Camp 1986; Arechavaleta et al. 1989; West et al. 1993) are observed as advantages to the host plants. The benefits for the fungal symbionts can be greater access to nutrients, protection against desiccation, insects, and parasitic fungi, and competition from other microbes (White et al. 2000). During the coevolution, endophytic microbes improved the resistance of the host plants to adverse conditions by secreting bioactive secondary metabolites (Ge et al. 2008).
Although the first discovery of an endophytic fungus was in 1902 by Freeman, little attention was paid to this group of fungi until the recent realization of their ecological relevance and their potential as source for new bioactive metabolites (Gunatilaka 2006).
II. The Ecological Relevance of Endophytic Fungi
Two ecological groups of endophytes can be distinguished: grass endophytes and endophytes of woody plants. Endophytic fungi were found in all woody plants examined for endophytes (Saikkonen et al. 1998). Despite the great diversity and abundancy of endophytes in woody plants, these endophytes and their interaction with their host plants receive less attention than those of grass endophytes (Saikkonen et al. 1998). The production of alkaloids in infected grasses (in planta) can cause intoxication of grazing livestock. Because of this economic importance, endophytes of poaceous grasses are the best-examined (Weber and Anke 2006).
In the family Clavicipitaceae there is a development from parasitic grass endophytes like Claviceps to mutualistic species like Acremonium (anamorphic form of Epichloe¨; Clay 1988a). Species of the genus Claviceps cause local infections of the inflorescences of grasses. Sclerotia develop at the site of infection instead of seeds of the host plant. The local infection evolved to a systemic infection, which can be found in Acremonium species (Clay 1988a). Reduction of sexual reproduction can be observed in this group of obligate endophytic fungi (Saikkonen et al. 1998). The asexual or anamorphic form is transmitted vertically by the growth of fungal hyphae into the host plant’s seeds (Arnold et al. 2003). Vogl already discovered the mycelium of an endophytic fungus in seeds of Lolium temulentum in 1898. Infection of the inflorescenses by parasitic species results in sexual sterilization of the host plant. This is in contrast to the loss of sexual reproduction of endophytes in mutualistic symbiosis. In these cases propagation of the fungus is ensured by viable seeds of the host plant.
The endophytic fungi of woody plants show a high diversity and belong to different taxonomic groups. These fungal endophytes are horizontally transmitted via spores and are not known to grow into seeds and systemically infecting the plant following seed germination (Wilson 2000). The infections are often highly localized within leaves, petioles, bark, or stems (Saikkonen et al. 1998).
Many endophytes of woody plants are closely related to pathogenic species. Freeman and Rodriguez (1993) were able to show that the mutation at a single genetic locus can change an isolate of Colletotrichum magna from a pathogen to a nonpathogenic endophytic mutualist. The wild-type fungus and the mutant of the species (path 1) were capable to infect the host plant and systemic growth was observed. The host plants infected by path 1 produced no visible effect on any plant and were protected from the wild-type fungus. Freeman and Rodriguez (1993) assumed that a portion of the host defense system is activated by path 1 so that, when exposed to the wild-type fungus, there was no delay in the defense response.
In contrast to the fungi of the Clavicipitaceae the endophytes of woody plants are not intensively investigated. One exception is the family of the Xylariaceae, which attracts attention because of its fruiting bodies and its host specificity. Most xylariaceous species are collected from living or dead angiospermous plants (Rogers 1979); most members of the family are wood inhabitants but representatives are also found in litter, soil, dung, and associated with insects (Whalley 1996). The greatest diversity of xylariaceous fungi can be found in the tropics (Petrini et al. 1995). Whalley (1996) distinguishes three groups: saprophytes, phytopathogens, and endophytes. Endophytic species, e.g. Hypoxylon and related genera, can colonize the xylem of host plants (Stone et al. 2000). Other xylem-colonizing endophytes have been identified as species of the Diaporthales (e. g. Phragmoporthe, Amphiporthe, Phomopsis), Hypocreales (Nectria spp.), and a few Basidiomycetes (e.g. Coniophora; Stone et al. 2000). Water deficiency and drought in the host plant can cause a change in the lifecycle of endophytic species. Many plant-pathogenic species of the Xylariaceae have typical characteristics of endophytes; they live as ‘‘latent invaders’’. Water stress triggers the switch to the pathogenic phase of the fungus (Whalley 1996). A switch from the latent state to the saprophytic stage is observed in Hypoxylon fragiforme when the colonized host tissue dies. The death of the tissue is accompanied by loss of water, which triggers the switch to the saprophytic stage of fungus (Weber and Anke 2006).
Some of the xylariaceous fungi have developed host specificity, e.g. Hypoxylon fragiforme is consistently isolated from healthy beech trees in Europe and H. mammatum is isolated from aspen trees in the northeast of the United States (Chapela and Boddy 1988; Chapela 1989). The wide distribution of the Xylariaceae, as endophytes and as saprophytes and their ability to produce bioactive compounds (cytochalasins, melleins, and xylarin) suggest that these fungi have an important ecological role (Petrini et al. 1995). Compounds from the bark of beech trees can trigger the eclosion of Hypoxylon fragiforme ascospores, which is a precondition for germination of the spores (Chapela et al. 1993). This can partly explain the host specificity of H. fragiforme. The eclosion of the spores can be observed 10 min after the addition of the beech extract (Webster and Weber 2004). Two substances have been identified which trigger the release of the spores at micromolar concentrations: the two monolignol glucosides Z-syringin and Z-isoconiferin (Fig. 8.1; Chapela et al. 1991).
