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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. Endophytes are also known to occur within lichens and algae. Many economically important grasses (e.g., Festuca spp., Lolium spp., Zea) carry fungal endophytes, some of which may enhance host growth and may improve the plant's ability to tolerate abiotic stresses, such as drought, and resistance to insects 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 many plants. P. indica symbiosis has been shown to increase crop yield for a variety of crops(barley, tomato, maize etc.) and provide a measure of protection 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 deforestation and biodiversity loss is widespread, many endophytes might be permanently lost before their utility is explored. 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.
ᾼᾶhost specific. Endophytic species are very diverse; only a small minority of existing endophytes have been characterized. A single leaf of a plant can harbor many different species of endophytes, both bacterial and fungal. Additionally, some endophytic bacteria may live within endophytic fungi.
Endophytes can be identified in several ways, usually through amplifying and sequencing a small piece of DNA. Some endophytes can be cultured from a piece of their host plant in an appropriate growth medium. Not all endophytes can be cultured in this way, as shown by discovery of cryptic, unculturable endophyte species through DNA based analysis of leaf tissue. Some grass endophytes can be seen as coiled tubes of hyphae under the microscope following clearing of the leaf tissue and staining with aniline blue. Many endophytes do not sporulate when cultured. Since fungal identification by morphology is based primarily on spore-bearing structures, this fact makes visual identification of endophytic cultures challenging.
Diversity of Fungal Endophytes
Fungal endophytes are generally from the phylum Ascomycota, though other phyla are represented. 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 endophytes are now known that grow within seaweeds and algae. One such example is Ulvella leptochaete, which has recently been discovered from host algae including Cladophora and Laurentia from India.
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