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Symbiosis Sym biosis together life Type of Interaction Direct Effect on Species 1 Direct Effect on Species 2 Neutral 0 0 Commensalism Symbionts: organisms involved Host: larger organism, if there is one 0 Mutualism Interspecific Competition Predation Parasitism Mutualism: both symbionts benefit Commensalism: one symbiont receives benefit while neither harming nor helping the other in any significant way Parasitism: one symbiont, called a parasite, benefits at the expense of the other, usually a host Anthropomorphism in biological definitions? Examples Ants and plants (greehouse examples): mutualism with a large host Epiphytic plants (many orchids): commensalism with a large host Parasitism: Giardia, Plasmodium Parasites need to be distinguished between cases where a pathogen may lead to death and where there may be a balance in the host-parasite relationship. For example, there can be genetic balances in virulence and resistance that operate at the populations level. Myxomatosis and rabbits Mutualism: Nitrogen fixing bacteria and Plants In biological nitrogen fixation two moles of ammonia are produced from one mole of nitrogen gas, using 16 moles of ATP and a supply of electrons and protons (hydrogen ions): N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an enzyme complex termed nitrogenase. This enzyme consists of two proteins an iron protein and a molybdenum-iron protein. A point of special interest is that the nitrogenase enzyme complex is highly sensitive to oxygen The mutualism: plant gains nitrogen compounds, the bacterium gains carbohydrate and an environment with reduced oxygen infection thread root hair Soya bean Glycine max and Rhizobium root nodule Nodule formation 1 Roots emit chemical signals that attract Rhizobium bacteria. The bacteria emit signals that stimulate root hairs to elongate, and to form an infection thread by an invagination of the plasma membrane Soya bean infection with Rhizobium The bacteria penetrate the root cortex within the infection thread. Plant cells start dividing and vesicles containing the bacteria, bacteriods, bud into the cells from the branching infection thread Nodule formation 2 Nodule formation 3 Growth continues in the affected regions of the cortex and pericycle and these fuse to form the nodule Nodule formation 4 The nodule grows and vascular tissue connecting it to the plant’s xylem and phloem develops Lens culinaris (lentil) root nodulation. 7 days 12 days Small numbers of bacteria Large numbers of bacteria TEM photomicrographs http://www.sunderland.ac.uk/~es0man/tem2.htm In symbiotic nitrogen-fixing organisms such as Rhizobium, root nodules can contain oxygen-scavenging molecules such as leghaemoglobin., This shows as a pink colour when the active nitrogen-fixing nodules are cut open. Leghaemoglobin may regulate the supply of oxygen to the nodule tissues in the same way as haemoglobin regulates the supply of oxygen to mammalian tissues Clover root nodules. Leghaemoglobin Leghaemoglobin is found only in the nodules and is not produced by either the bacterium or the plant when grown alone. Some legumes, e.g., peanuts, cowpeas, soybeans, and faba beans are good nitrogen fixers, and will fix all of their N needs – up to 250 lbs of N per acre and are not usually fertilized. If large amounts of nitrogen are applied, the plant slows or shuts down the nitrogen fixation process. Common beans, Phaseolus vulgaris, are poor fixers (< 50 lbs per acre) and fix less than their N needs. Maximum economic yield in New Mexico requires an additional 30-50 lbs of fertilizer N per acre. However, if beans are not nodulated, yields often remain low, regardless of the amount of nitrogen applied. There are many research programs attempting genetic improvement of nitrogen fixation, e.g., alfalfa. Genetic modification for tropical crops. G Myco rrhizae Fungus Root Mycorrhizas are highly evolved, mutualistic associations between soil fungi and plant roots. The host plant receives mineral nutrients while the fungus obtains photosynthetically derived carbon compounds. Almost 80 percent of all terrestrial plants can form mycorrhizal associations. There are two major types: ectotrophic mycorrhizae and endotrophic mycorrhizae called vesiculararbuscular mycorrhizae because of the structures they produce inside roots Ectomycorrhizae Mycorrhizal fungi produce a hyphal network in soils consisting of individual strands of hyphae or relatively undifferentiated bundles of hyphae called mycelial strands. Some fungi produce rhizomorphs containing specialised conducting hyphae, or sclerotia, which are resistant storage structures that survivein the soil and then infect other plants. Fungal structures in soil Mycellial strand Absorptive hyphae Scleridia Mycorrhizal root Rhizomorphs Soil mycellium Early stage of colonisation of pine short root by Pisolithus tinctorius. Hyphae (arrows) have contacted the root and are starting to proliferate on its surface near the apex (A). SEM image showing the next stage of pine root colonisation by Pisolithus tinctorius. Mantle hyphae (arrows) have formed a dense covering on the root surface (arrows). http://www.ffp.csiro.au/research/mycorrhiza/ecm.html Example of ECM short roots (arrows) of birch (Betula alleghaniensis), an angiosperm tree. The mycorrhizal short roots are thicker than other laterals of the same order due to the mycorrhizal infection. Pinus radiata and Amanita muscaria ECM synthesised under sterile conditions. This association has highly branched short roots with many root tips (arrows). Eucalyptus maculata and Astraeus pteridis association synthesised under sterile conditions with relatively unbranched ECM and attached mycelial strands (star). Hand section cleared and stained with Chlorazol black E and viewed with interference contrast microscopy Populus tremuloides ECM root cross section showing labyrinthine net hyphae (arrows) around elongated epidermal cells. This complex hyphal branching pattern is considered to increase the fungal surface area in contact with the root. http://www.ffp.csiro.au/research/mycorrhiza/ecm.html Vesicular arbuscular mycorrhizae Endotrophic (VAM) The network of hyphae in the soil is only connected to roots by the entry points that initiate mycorrhizal associations Mycorrhizal root system washed carefully from coarse sand to reveal the intact network with external hyphae (arrow) with spores (S) produced by Glomus mosseae. http://www.ffp.csiro.au/research/mycorrhiza/ecm.html Appressorium At entry point Epidermis Hypodermis Intracellular hyphae Intercellular hyphae in air channel Arbuscules Vesicle Cortex A colony of VAM refers to hyphal growth within a root resulting from the same external hyphae (1 or more connected entry points). These are also called infection units. The colony may produce arbuscules, exchange structures Vesicles appear to be storage structures Arbuscules (A) and convoluted hyphae (arrow) in the inner cortex of an Asarum canadense root. Arbuscules only form in the innermost cortex cell layer next to the endodermis in this species. Vesicles (V) produced by a Glomus species in a leek root. This root also contains many intercellular hyphae. (Bar = 100 um) http://www.ffp.csiro.au/research/mycorrhiza/ecm.html