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An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life without causing apparent disease. Endophytes are ubiquitous and have been found in all the species of plants studied to date; however, most of these endophyte/plant relationships are not well understood. Many economically important grasses (e.g., Festuca spp., Lolium spp., Zea) carry fungal endophytes which enhance their growth, may improve the plant's to tolerate abiotic stresses such as drought, as well as improve their resistance to insect and mammalian herbivores.
Endophytes may be transmitted either vertically (directly from parent to offspring) or horizontally (among individuals). Vertically transmitted fungal endophytes are typically considered sterile and transmit via fungal hyphae penetrating the host’s seeds (e.g., Neotyphodium). Since their reproductive fitness is intimately tied to that of their host plant, these fungi are often mutualistic. Conversely, horizontally transmitted fungal endophytes are fertile, and reproduce through asexual or sexual spores that can be spread by wind and/or insect vectors. Because of the complex mating systems of fungi, some endophytes normally considered to be vertically transmitted may occasionally become sexually reproductive, producing spores that can be transmitted horizontally (e.g., Epichloë). Some endophytic fungi are actually latent pathogens or saprotophs that only become active and reproduce when their host plants are stressed or begin to senesce, respectively
Endophytes may benefit host plants by preventing pathogenic organisms from colonizing them. Extensive colonization of the plant tissue by endophytes creates a "barrier effect", where the local endophytes outcompete and prevent pathogenic organisms from taking hold. Endophytes may also produce chemicals which inhibit the growth of competitors, including pathogenic organisms. Some bacterial endophytes have proven to increase plant growth. The presence of fungal endophytes can cause higher rates of water loss in leaves. However, certain fungal endophytes help plants survive drought and heat. Fungal endophyte-related host benefits are common phenomena, and have been the focus of much research, particularly among the grass endophytes (see below).
The wide range of compounds produced by endophytes have been shown to combat pathogens and even cancers in animals including humans. One notable endophyte with medicinal benefits to humans was discovered by Gary Strobel: Pestalotiopsis microspora, an endophytic fungus of Taxus wallachiana (Himalayan Yew) was found to produce taxol. Endophytes are also being investigated for roles in agriculture and biofuels production. Inoculating crop plants with certain endophytes may provide increased disease or parasite resistance  while others may possess metabolic processes that convert cellulose and other carbon sources into "myco-diesel" hydrocarbons and hydrocarbon derivatives. Piriformospora indica is an interesting endophytic fungus of the order Sebacinales, the fungus is capable of colonising roots and forming symbiotic relationship with every possible plant on earth. P. indica has also been shown to increase both crop yield and plant defence of a variety of crops(barley, tomato, maize etc.) against root-pathogens.
It is speculated that there may be many thousands of endophytes useful to mankind but since there are few scientists working in this field, and since forests and areas of biodiversity are rapidly being destroyed, many useful endophytes for curing disease might be permanently lost for medicinal use before they are discovered. The effects of climate change on endophytes is being investigated. Studies of plants grown at different climates or at increased carbon dioxide levels have different distributions of endophytic species.
While many endophytes are known to colonize multiple species of plants, some are host specific. Endophytic species are very diverse; it is thought that only a small minority of all existing endophytes have been characterized. A single leaf of a plant can harbor many different species of endophytes, both bacterial and fungal.
Endophytes can be identified in several ways, usually through amplifying and sequencing a small piece of DNA. Some endophytes can be cultured from a small piece of their host plant in an appropriate growth medium. Not all endophytes present can be cultured in this way because amplification of ground up plant tissue using fungal or bacterial specific primers has revealed the existence cryptic species. Some grass endophytes can be seen as coiled tubes of hyphae under the microscope at 400X following clearing of the leaf sheaths in ethanol and staining with aniline blue.
Diversity of Fungal Endophytes
Fungal endophytes are generally from the phylum Ascomycota. Some specific examples of which are found in orders Hypocreales and Hylareales of the Sordariomycete (Pyrenomycete) class. Additionally the class of Loculoascomycetes includes endophytes.
Diversity of Algal Endophytes
A number of Endophytic algae species are now known that grows within other seaweeds. One such example is Ulvella leptochaete, which have recently been discovered from host algae including Cladophora and Laurentia from India.
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