| Arthrobotrys oligospora | |
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| Species: | A. oligospora |
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| Arthrobotrys oligospora Fresen. (1850) | |
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Arthrobotrys oligospora was discovered in Europe in 1850 by Georg Fresenius.[1][2] A. oligospora is the model organism for interactions between fungi and nematodes.[2] It is the most common nematode-capturing fungus,[3][4][5] and most widespread nematode-trapping fungus in nature.[2][6] It was the first species of fungi documented to actively capture nematodes.[2][6]
Growth and morphology
This fungus reproduces by means of 2-celled, pear-shaped conidia, in which the cells are of unequal size with the smaller cell nearer to the attachment point on the conidiophore.[4][7] During germination, the germ tube typically erupts from the smaller cell.[7] In environments rich with nematodes, the spores range from 22-32 by 12-20 µm,[4][7] though the spores are smaller in environments devoid of nematodes.[4][7] Conidium germination has a success rate of 100% but the formation of trapping organs are not always observed.[6] Conidia have been found to disintegrate both in the air and on impact with an agar plate.[8] Conidiophores and conidia grow from hyphae sprouted outside of a trapped dead nematode,[2] and conidiophores have been found to change and grow into part of the adhesive net.[2] Under ideal conditions, a colony can reach 65 mm in diameter after 7 days incubation,[6] with colourless, pale pink or yellow mycelium.[6] The optimal growth temperature for the fungus in nematode-free and nematode-infested environments is 20 °C (68 °F) and 25 °C (77 °F), respectively.[6] The growth rate of colonies is greater in the presence of light than in darkness.[6]
Physiology
A. oligospora is considered a saprobe and is more saprotrophic than other nematode capturing fungi.[2][6] At first the fungus was considered largely saprophytic in nature but this interpretation was later questioned.[4] Saprophytic growth uses D-xylose, D-mannose, and cellobiose.[6] The fungus uses nitrite, nitrate, and ammonium for its nitrogen sources and uses pectin, cellulose, and chitin for its carbon sources.[6] When predating on nematodes, the fungus uses cellobiose, L-asparagine, L-arginine, DL glutamic acid for its carbon and nitrogen sources.[6]
Nematode capturing
Predation of nematodes occurs in low nitrogen environments,[9] as the nematode becomes the main nitrogen source for the fungi.[2] It has been found that the presence of ammonium causes a higher decrease of predation when compared to presence of nitrate or nitrite.[3] Adding green manure or carbohydrates has been found to increase nematode trapping behaviors.[6] A complex 3-dimensional net of hyphae is formed to trap the nematodes under conditions of pH 4.9-8.1 and a temperature less than 37 °C (99 °F).[5][6][8][9] Nematodes, and specifically "nemin" (an extract derived from nematodes) were found to stimulate net formation.[2][6] Nematodes are not as attracted to A. oligospora colonies that have not manifested traps, suggesting that these structures serve an additional attractant role possibly through the expression of pheromones.[9][10]
A full net is not needed to catch nematodes as smaller nematodes can be caught with a single loop.[2] Lectins are used in attaching nematode to fungi[9] The entire surface of net is covered in adhesive material.[2][8] Strong adhesion keeps the nematode trapped and when the nematode struggles, it often results in multiple points of adhesion of the nematode to the net.[8][10] It was even found that the adhesion of the nematode to the fungus remained under washing of agar plate with water.[8] The net is flexible which results in 'hyphal drag' tiring the nematode.[8] Multiple points of adhesion and 'hyphal drag' allow the net to be capable of catching both large and small nematodes easily.[8] In vitro, bait nematodes are consumed often leaving Bunonema nematodes.[8]
A substance found in paralyzed nematodes was found to be capable of paralyzing healthy nematodes,[6][8] and it was later determined that a paralyzing substance, Subtilisin (A serine protease),[11] is excreted into nematodes.[2][8] An unstable toxin was thought to be made by the fungus,[6][8] and it was later found that toxic levels of linoleic acid for nematodes (lethal dose of linoleic acid for C. elegans is 5–10 μg/ml)[12] were found in the fungus.[5][12] Enzymatic hyphal invasion, likely using collagenases which are found in 'Arthrobotrys',[2] of a trapped nematode is followed by the digestion of contents of the nematode.[8][9] Shortly after hyphal invasion, a hyphal bulb appears where hyphae grow outwards from the bulb along the entire body of the nematode.[8]
Not all nematodes are caught by the net as the nematode needs to be in contact with the net for a short period of time in order for adhesion to occur.[8] Nematodes were found to quickly move away from any net followed by curling if instantaneous contact occurs.[8][13] The nematode then proceeds to move forward until out of the area of the net and unless prolonged contact is made the nematode is safe.[8] This means one or several instantaneous contacts are not enough for adhesion between the nematode and net to occur.[8]
No competing fungi or bacteria are found in nematodes which are being consumed by the fungus which means it is possible an antibiotic is released inside the nematode.[8] In 1993, secondary metabolites (oligosporon, oligosporol A, and oligosporal B) which can act as antibiotics were found in the fungus.[2][12] Oligosporon, oligosporol A, oligosporal B have hemolytic effects and are cytotoxic to nematodes, however they are not toxic to the C. elegans.[12] Other oligosporon-type secondary metabolites also found in A. oligospora include (4S,5R,6R)-4′,5′- dihydrooligosporon, (4S,5R,6R)-hydroxyoligosporon, and (4S,5R,6R)-10′11 ′-epoxyoligosporon.[12]
Net formation
A branch of hyphae grows out of a vegetative hyphae eventually arching back to the parent hyphae and fuses with it to make a loop.[3][7][8] This process repeats from any hyphae along any existing branches or a new parent hyphae.[3][8] The nets are immediately adhesive,[8] and hyphae in the loop have different organelles to trap nematodes which are not found in vegetative cells.[2]
Habitat and ecology
A. oligospora has been found in many different geographical regions which include Asia, Africa, North America and South America and Australasia.[2] Some countries it has been found in include Turkmenistan, Azerbaijan, Poland, Canada, New Zealand, and India.[6] The presence of insects infected by nematodes increased presence of A. oligospora but not other nematode capturing fungi.[2]
The fungus can be found in soil in grassland, shrubland, plantations, sheep and cattle yards,[6] and domesticated and non-domesticated animal feces.[2] It colonizes forest steppe soil, mixed forest soil, and Mediterranean brown soil (pH 6.9-8.0) where the pH can be as low as 4.5, but is typically above 5.5.[6] The fungus has also been found in aquatic environments,[2] and heavily polluted areas, specifically heavy metal poisoned mines, fungicide, or nematicide infested soil,[2][5] decayed plant material, leaves, roots, moss,[6] and in the rhizosphere of various bean plants, barley,[2][6] and the tomato plant.[2] Larger populations of the fungus can be found in late spring and summer.[5]
Industrial uses
The fungus is a biological indicator of nematodes.[2] The annual global cost of plant-parasitic nematodes is approximately 100 billion USD.[12] Nematode capturing fungi such as the A. oligospora can be used to control growth of nematodes.[5][6] This means that they can be potentially used as a bio-control agent to protect crops against nematode infestations.[2] This may not be feasible since the nematodes occasionally eat the fungi.[6]
References
- ↑ Fresenius, Georg (1850). Beiträge zur mykologie. p. 18.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Niu, Xue-Mei; Zhang, Ke-Qin (2011). "Arthrobotrys oligospora a model organism for understanding the interaction between fungi and nematodes". Mycology. 2 (2): 59–78. doi:10.1080/21501203.2011.562559.
- 1 2 3 4 Duddington, C; Wyborn, C (1972). "Recent Research on the Nematophagous Hyphomycetes". Botanical Review. 38 (4): 545–562. doi:10.1007/bf02859251. S2CID 29933497.
- 1 2 3 4 5 Dreschler, Charles (1937). "Some Hyphomycetes That Prey on Free-Living Terricolous Nematodes". Mycologia. 29 (4): 447–552. doi:10.2307/3754331. JSTOR 3754331.
- 1 2 3 4 5 6 Zhang, Ke-Qin; Hyde, Kevin; Zhang, Ying; Yang, Jinkui; Li, Guo-Hang (2014). Nematode-trapping Fungi. New York: Dordrecht: Springer. pp. 213, 215, 222, 316.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Domsch, Klaus; Gams, Walter; Traute-Heidi, Anderson (1980). Compendium of soil fungi. New York: Academic Press (London) LTD. pp. 60–63.
- 1 2 3 4 5 Duddington, C (1955). "Fungi That Attack Microscopic Animals". Botanical Review. 21 (7): 377–439. doi:10.1007/bf02872434. S2CID 37392081.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Barron, George (1977). The Nematode-Destroying Fungi. Guelph: Canadian Biological Publications Ltd. pp. 27–37, 93–95, 106, 111.
- 1 2 3 4 5 Alexopoulos, Constantine; Mims, Charles; Blackwell, Meredith (1996). Introductory Mycology (4th ed.). Toronto: John Wiley & Sons, Inc. p. 235.
- 1 2 Nordbring-Hertz, Birgit; Jansson, Hans‐Börje; Stålhammar-Carlemalm, Margaretha (1977). "Interactions Between Nematophagous Fungi and Nematodes". Ecological Bulletins. 25: 483–484.
- ↑ Nordbring-Hertz, Birgit (2004). "Morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora – an extensive plasticity of infection structures". Mycologist. 18 (3): 125–133. doi:10.1017/s0269915x04003052.
- 1 2 3 4 5 6 Degenkolb, Thomas; Vilcinskas, Andreas (2016). "Metabolites from nematophagus fungi and nematicidal natural products from fungi as an alternative for biological control. Part 1: metabolites from nematophagous ascomycetes". Applied Microbiology and Biotechnology. 100 (9): 3799–3812. doi:10.1007/s00253-015-7233-6. PMC 4824826. PMID 26715220.
- ↑ Dreschler, Charles (1934). "Organs of Capture in Some Fungi Preying on Nematodes". Mycologia. 26 (2): 135–144. doi:10.2307/3754035. JSTOR 3754035.