Endophyte

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.[1][2] 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.[3][4] Endophytes are also known to occur within lichens[5] and algae. Many economically important grasses (e.g., Festuca spp., Lolium spp., Zea) carry fungal endophytes, some of which may enhance host growth,[6] may improve the plant's ability to tolerate abiotic stresses such as drought, as well as improve their resistance to insect and mammalian herbivores.[7][8][9]

Transmission

Endophytes may be transmitted either vertically (directly from parent to offspring) or horizontally (among individuals).[10] 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[11]

Endophyte-host interactions

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.[12] The presence of fungal endophytes can cause higher rates of water loss in leaves. However, certain fungal endophytes help plants survive drought and heat.[13] Fungal endophyte-related host benefits are common phenomena, and have been the focus of much research, particularly among the grass endophytes (see below).

Applications

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.[14] 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 [9] while others may possess metabolic processes that convert cellulose and other carbon sources into "myco-diesel" hydrocarbons and hydrocarbon derivatives.[15] 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.[16][17]

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.[18] 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.[19]

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.[20]

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.[21]

See also

References

  1. ↑ Clay K, Schardl C (October 2002). "Evolutionary origins and ecological consequences of endophyte symbiosis with grasses". The American Naturalist 160 (Suppl 4): S99–S127. doi:10.1086/342161. PMID 18707456.
  2. ↑ Carroll, G. C. (1986). "The biology of endophytism in plants with particular reference to woody perennials". In Fokkema, N. J.; Van den Heuvel, J. Microbiology of the phyllosphere. Cambridge: Cambridge University Press. pp. 205–22. ISBN 978-0-521-32344-4.
  3. ↑ Faeth, Stanley H. (2002). "Are endophytic fungi defensive plant mutualists?". Oikos 98 (1): 25–36. doi:10.1034/j.1600-0706.2002.980103.x. JSTOR 3547609.
  4. ↑ Faeth SH (May 2009). "Asexual fungal symbionts alter reproductive allocation and herbivory over time in their native perennial grass hosts". The American Naturalist 173 (5): 554–65. doi:10.1086/597376. PMID 19296736.
  5. ↑ Peters, A. F., et al. "Molecular identification, distribution and taxonomy of brown algal endophytes, with emphasis on species from Antarctica." Proceedings of the 17th International Seaweed Symposium, Cape Town, South Africa, 28 January-2 February 2001.. Oxford University Press, 2003.
  6. ↑ Nassar, Amr H.; El-Tarabily, Khaled A.; Sivasithamparam, Krishnapillai (November 2005). "Promotion of plant growth by an auxin-producing isolate of the yeast Williopsis saturnus endophytic in maize (Zea mays L.) roots". Biology and Fertility of Soils 42 (2): 97–108. doi:10.1007/s00374-005-0008-y.
  7. ↑ Clay, Keith (February 1988). "Fungal Endophytes of Grasses: A Defensive Mutualism between Plants and Fungi". Ecology 69 (1): 10–6. doi:10.2307/1943155. JSTOR 1943155.
  8. ↑ Cheplick, G. P. and S. H. Faeth. 2009. Ecology and Evolution of the Grass-Endophyte Symbiosis. Oxford University Press, Oxford.
  9. ↑ 9.0 9.1 "University of Rhode Island GreenShare Factsheets: Endopyhte-Enhanced Grasses". Archived from the original on 2006-03-12. Retrieved June 14, 2009.
  10. ↑ Carroll, George (February 1988). "Fungal Endophytes in Stems and Leaves: From Latent Pathogen to Mutualistic Symbiont". Ecology 69 (1): 2–9. doi:10.2307/1943154. JSTOR 1943154.
  11. ↑ Petrini, Orlando (1991). "Fungal Endophytes of Tree Leaves". In Andrews, John H.; Hirano, Susan S. Microbial Ecology of Leaves. Brock/Springer Series in Contemporary Bioscience. pp. 179–97. doi:10.1007/978-1-4612-3168-4_9. ISBN 978-1-4612-7822-1.
  12. ↑ Taghavi S, Garafola C, Monchy S et al. (February 2009). "Genome survey and characterization of endophytic bacteria exhibiting a beneficial effect on growth and development of poplar trees". Applied and Environmental Microbiology 75 (3): 748–57. doi:10.1128/AEM.02239-08. PMC 2632133. PMID 19060168. Lay summary – Brookhaven National Laboratory (January 26, 2009).
  13. ↑ Brink, G. E.; Casler, M. D.; Martin, N. P. (2010). "Meadow Fescue, Tall Fescue, and Orchardgrass Response to Defoliation Management". Agronomy Journal 102 (2): 667. doi:10.2134/agronj2009.0376. Lay summary – Agricultural Research Service (March 15, 2011).
  14. ↑ Strobel G, Yang X, Sears J, Kramer R, Sidhu RS, Hess WM (February 1996). "Taxol from Pestalotiopsis microspora, an endophytic fungus of Taxus wallachiana". Microbiology 142 (2): 435–40. doi:10.1099/13500872-142-2-435. PMID 8932715.
  15. ↑ Strobel GA, Knighton B, Kluck K et al. (November 2008). "The production of myco-diesel hydrocarbons and their derivatives by the endophytic fungus Gliocladium roseum (NRRL 50072)". Microbiology 154 (11): 3319–28. doi:10.1099/mic.0.2008/022186-0. PMID 18957585.
  16. ↑ Varma A, Savita Verma, Sudha, Sahay N, Butehorn B, Franken P (June 1999). "Piriformospora indica, a cultivable plant-growth-promoting root endophyte". Applied and Environmental Microbiology 65 (6): 2741–4. PMC 91405. PMID 10347070.
  17. ↑ Waller F, Achatz B, Baltruschat H et al. (September 2005). "The endophytic fungus Piriformospora indica reprograms barley to salt-stress tolerance, disease resistance, and higher yield". Proceedings of the National Academy of Sciences of the United States of America 102 (38): 13386–91. doi:10.1073/pnas.0504423102. PMC 1224632. PMID 16174735.
  18. ↑ L HAWKSWORTH, David. "The magnitude of fungal diversity: the 1· 5 million species estimate revisited." Mycological research 105, no. 12 (2001): 1422-1432.
  19. ↑ Hoffman, M. T., Gunatilaka, M. K., Wijeratne, K., Gunatilaka, L., & Arnold, A. E. (2013). Endohyphal bacterium enhances production of indole-3-acetic acid by a foliar fungal endophyte. PloS one, 8(9), e73132.
  20. ↑ Singh, D.; Mathur, S.B. (2004). Histopathology of Seed-Borne Infections. Taylor & Francis. p. 149. ISBN 9781420038170.
  21. ↑ BAST, F., BHUSHAN, S AND JOHN, A.A. 2014. DNA Barcoding of a new record of epi-endophytic green algae Ulvella leptochaete (Ulvellaceae, Chlorophyta) in India. Journal of Biosciences 39:711-716

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