Introduction

A wide diversity of fungi are isolated from the tissues of most terrestrial and aquatic plants, and red and brown algae. Fungi are present in most plant parts, especially the leaves. Where the tissue is apparently healthy, the fungi may be either endophytes, epiphytes or latent pathogens. Endophytes are contained within the plant without disease. Plant tissues remain entire and functional.

Endophytic Fungi

Endophytes colonise plant tissue and remain within the tissue, except that fruiting structures may emerge through the surface of the plant tissue. Indeed, leaves may be fully colonised by a variety of fungi within a few weeks of leaf emergence. The colonies remain asymptomatic and some in perennial plant parts may have a very long life.

Endophytic fungi are found in all divisions of fungi so have presumably evolved the association independently on many occasions. The most common endophytes are anamorphic members of the Ascomycota, and they are often closely related to fungi known to cause disease. Phylogenetic evidence is used to suggest that some endophytes have evolved from pathogens and for others, vice versa. The mechanisms of host recognition and development of colonisation may also be common among closely related endophytic and pathogenic fungi.

A wide range of plants have now been examined for endophytes, and endophytes have been found in nearly all of them. An enormous number of different fungi can be isolated from plants growing in their native habitat. Most of the fungi are uncommon and narrowly distributed, taxonomically and geographically. However a few fungi are widely distributed with the host, suggesting a long standing, close and mutually beneficial interaction. Some fungi are found in many different terrestrial hosts, especially endophytes of crop plants. While most information has been gathered from terrestrial ecosystems, fungi are found in algae and seagrasses. Just as we know less about marine ecosystems, our understanding of the biology of marine endophytes is extremely limited and will not be discussed further.

Apart from Neothyphodium and related species, endophytes are transmitted horizontally. That is, each plant is colonised by fungal propagules that arrive from the environment. The source of transmission has been determined in only a few cases. Propagules of some endophytes have been found in the body of insect pests of the host. Intriguingly, at least two entomopathogens have been documented as endophytic fungi. Thus insects may disperse some fungi from host to host.

Aerial dispersal either in the wind or on vectors is probably the most common mechanism for fungal dispersal. Endophytic fungi colonise various parts of the plant. Many of the fungi sporulate in culture indicating the potential to release spores in the air. Indeed, sporulation is seen after senescence of plant tissues. However, few cases of dispersal have been documented in the wild and the various mechanisms remain unexplored.

Nutrients are cycled between the host and fungus. The endophytic fungus gains a predictable environment in which nutrients are readily available. Thus the benefits are clear for fungi establishing endophytic associations. The loss of plant resources to the fungus, and the potential of some fungi to grow rapidly, indicates that the host regulates development of colonies.

Each plant host has a range of physical, chemical, constitutive and induced controls over the spread of fungi within tissues. An enormous diversity of phenolic and other deterrent plant compounds are associated with the presence of endophytic fungi, in fact more than are associated with potent pathogens in the same host. In addition, presence of endophytes upregulates plant responses to pathogens. In the absence of plant controls, proliferation of endophytes through tissues would be expected.

The reaction of the plant to endophytes suggest that the interaction is one of confinement by the plant. Colonisation by endophytes ranges from single cells (Rhabdocline parkeri) to patchy distribution through leaves and stems (Chaetomium globosum).

Plants may benefit from the presence of endophytes in many ways. Potential plant benefits have been examined in only a few cases. Rhabdocline parkeri produces a compound that reduces needle attack by borers. Metabolites produced by Phomopsis sp in cotton appear to deter larvae of Helicoverpa from feeding on leaves. The parallels with Neotyphodium are clear. In addition, aphids feeding on leaves of cotton may become colonised by Lecanicillium lecanii, when conditions permit. Thus the aphid may be killed or it may transfer the fungus to another leaf.

In addition, endophytes may upregulate host responses to pathogens and pests. Chaetomium globosum has been shown to increase host resistance to rust and tan spot pathogens in wheat. Direct interactions appear to be too small to measure in this case. Presence of Lecanicillium lecanii appears to reduce the feeding by aphids from leaves of cotton. The interaction is probaly due to induction of host responses, which is perplexing because plant regulatory pathways for responses to insects and pathogens are not thought to be complementary.

Endophytes appear to have direct and induced effects on plant responses to biotic agents. The interaction with abiotic agents remains largely unexplored.

The broader, ecological function of endophytic associations is still being debated. Many fungi that are associated with the initial stages of litter decomposition are found in healthy tissue of the same plants. LINK Thus they are involved in the initial stages of resource recycling. Endophytes are also associated with aquatic activities. Many aquatic fungi have an endophytic stage in their life cycle. LINK An enormous diversity of endophytes are found and endophytes are probably associated with a wide variety of host functions.

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Latent Pathogens

Latent infection of plants by pathogens is the state where a host is infected by a potential pathogen and the signs of infection are absent. Latent infection is eventually followed the expression of macroscopic disease. Disease may be induced by changes to the host physiology due to the activity of the fungus, or changes in the environment that increase the stress on the plant. For instance, a decline in available light, or water, or increased herbivory may trigger development of disease signs. In some cases, the signs of disease do not appear until the host tissue has reached maturity or senescence. The causes of disease are possibly due to significant removal of host resources, response to toxins released as secondary metabolites by the fungus, or alteration of host metabolic pathways which reduce host growth rates. At the very least, pathogens would dwarf the infected host. LINK

The period of latency has been defined as the time between initiation of infection to the expression of macroscopic signs. Thus the length of the latency period may be days or years.

Latent infections are important for the damage they cause directly. They may also weaken the host plant, predisposing it to infection by other pathogens and pests. Further, infection in one plant is likely to spread to adjacent plants as happens with many pathogens of crops. Because the host lacks obvious signs, the presence of latent pathogens is likely to go undetected and their effect ascribed to other causes.

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Epiphytes

Fungi that exist on the surfaces of plants are called epiphytes. While some fungi are adapted to the plant surface, the community also includes propagules of air-borne species, fungi that would not otherwise be considered epiphytes. The interaction of epiphytes with their host is largely unknown. The surface of the plant, especially the leaf is a challenging environment. Though plants express metabolites through the cuticle, the surface is dry, waxy and affectedby UV radiation.

Epiphytic fungi differ markedly from endophytes. Epiphytes have many obvious characteristics that enable continuation through the inclement conditions of the leaf surface. In brief, epiphytes are likely to be coloured (melanised), and thus able to resist UV radiation. Some epiphytes can digest lipidic substrates, and thus may utilise the waxy layer covering the leaf. Finally the yeast form has a relatively short life cycle enabling epiphytes to multiply during the short periods of appropriate environment.

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Conclusion

Endophytes include fungi that have one or more of a variety of interactions with their host plant: some fungi are widespread and found on many different plant species; others are highly specific to single hosts in single environments. Further, a diverse array of interactions between plant and fungus have been found. Given that a huge array of fungi may be isolated from any one host, it seems possible that endophytes will have one or more of a wide array of functions, most of which are unknown at present.

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References

Redlin RC and Carris LM (1985) Endophytic Fungi of Grasses and Woody Plants. APS Press, St Paul, Minnesota, USA.

van Bael et al. (2005) eds Dighton J, White JF & Oudemans P. The Fungal Community. Taylor & Francis.

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