Categories: Animal-plant interactions; fungi; microorganisms

Fungi that spend at least a part of their lives within the aboveground parts of living plants—in leaves, stems, and in some cases reproductive organs—but cause no outward signs of infection are called endophytes. Some endophytes protect the host plant by deterring grazing animals or pathogenic fungi.

In the 1980's scientists began to realize that a great variety of microscopic fungal species live benignly within plants, as endophytes (from the Greek words endos, meaning "inside," and phyton, for "plant"), in contrast to fungi living on the surfaces of plants, as epiphytes (from the Greek epi, meaning "upon," plus phyton). Most endophytic fungi are ascomycetes. Many appear to be close relatives of plant pathogens.

Most endophytic fungi live and feed between the host plant's cells. Those endophytes that provide a benefit to the plant in return for their keep are considered to be partners with their host, in a symbiotic relationship called mutualism. Endophytic mutualism is well developed in some grasses, in which the fungal partner produces alkaloid substances that deter herbivores and pathogens.

Some fungi live within a plant benignly or mutu-alistically for a time, and then, if environmental stress or senescence afflicts the host or conditions otherwise change, the fungi turn pathogenic. For example, in a drought-weakened tree, previously benign fungi may initiate disease symptoms. Such fungi are said to have both an endophytic and a pathogenic phase. Other fungi may have a dormant, endophytic phase, then eventually become dependent on dead organic matter for sustenance.

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