Endophytic fungi: definitions, diversity, distribution and their significance in plant life.

Endophytes are set opposite to pathogens and therefore should colonize plants asymptomatically. However, as will be illustrated, endophytic fungi may behave differently under various biotic and abiotic circumstances, in which the host plant can play a defining role as well. The genetic differences between an endophytic fungus and a phylogenetically related pathogenic fungus may vary significantly. Nevertheless, over the years endophytic fungi have frequently been isolated and never elicit disease symptoms in various host plants. Such true endophytes are considered mutually beneficial; the endophyte, embedded in the stable, protective and resource-rich environment of the host plant, supports the host plant to sustain biotic and abiotic stress conditions. The mechanisms by which endophytic fungi protect the host plant against biotic stress factors are generally diverse because they can directly antagonize pests or pathogens, trigger plant defence mechanisms or do both simultaneously.