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 mutualistically 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.
Two Growth Patterns
The endophytes of grasses differ in growth habit from those of woody plants (both coniferous and angiospermous). Grass endophytes have been found to grow systemically, throughout the stems and leaves, of themature plant, producing substantial fungal biomass.
Hyphae, or filaments, of the fungal body even penetrate the grass ovule, which is the reproductive structure that develops into the seed. Via the infected seed, the fungus is transmitted to the next generation and thus is perpetuated down a plant’s lineage.
In contrast, in most of the woody plants that have been investigated, individual endophytes are not systemic but instead are localized within leaves or stems, where they may be confined to specific plant tissues, such as bark or xylem(wood).
Woody plant endophytes typically propagate not by invading the host’s ovule but rather via spores, which are carried to other plants by air, water, or animals.
Presumably, the spores are able to disperse because they are not produced inside the plant host but rather on plant parts that have dropped off or are dying. This subject has been little investigated, however.
Abundance and Diversity
|Abundance and Diversity|
Scientists have suggested that endophytic fungi may be as widespread among plants as are mycorrhizae (associations between certain fungi and plant roots), which characterize the vast majority of vascular plants. In mycorrhizae, part of the fungal body is external to the plant,whereas endophytes are wholly internal.
Endophytes are diverse, especially in trees and shrubs. Individual woody species and even individual plants typically harbor scores of fungal species, as has been shown for alder, oak, beech, maple, birch, ash, pine, spruce, fir, and other plants.
Nevertheless, just one or a few species or genera of fungi usually dominate the fungal community of each woody plant species. These dominants commonly do not occur in plants other than the host species or closely related species.
Tropical trees are only beginning to be investigated for endophytes, but indications are that they are particularly rich in endophytic species. Given the high diversity of tropical trees, the endophytes still awaiting discovery may represent an enormous reservoir of biodiversity. In contrast to the wide variety of endophytes in woody plants, endophytes in grasses seem to be of low diversity.
Grass endophytes are all closely related species, and each grass species seems to host only one or a few of them. Grasses, however, have been little examined for the nonsystemic kinds of endophytes that woody plants harbor in such variety.
Mutualisms and Ecology
Endophytic fungi that are mutualistic, or protective of the plant, are particularly well known in the pasture grasses tall fescue (Festuca arundinacea) and perennial ryegrass (Loliumperenne).
The endophytes produce alkaloid toxins that defend these plants against insect attack and also cause serious illness in grazing livestock. Some endophytes, especially in grasses, increase host tolerance of other environmental stresses, such as drought.
Many of the diverse endophytic species in woody plants have no known protective function, and some seemto be latent pathogens. Among those that are protective is one in the needles of Douglas fir (Pseudotsuga menziesii). It produces substances toxic to midges that form galls on the needles.
The ecological effects of endophytes in natural populations seem to be very complex. Endophytes sometimes are actually antagonistic to the host plant rather than mutualistic with it. The ecological role of endophytes and their host plants in natural environments has been little studied, however.
The chemical substances produced by endophytic fungi are of considerable commercial interest to the pharmaceutical and food-processing industries and as potential biocontrol agents for plant diseases and pests. Endophytic fungi may eventually help farmers grow crops with minimal use of water, fertilizers, and pesticides.