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Symbioses between various fungi and the roots of terrestrial plants , called Mycorrhizae, confer a number of benefits on the plants. These interactions depend on a fungal species successfully infecting and living in or on the roots of a plant. Mycorrhizae are often mutualistic symbiosesČin which the fungus aids the plant in obtaining certain materials from the soil, and is in turn provided with carbon and energy in the form of carbohydrates by the plant (Fox, 2002).
Mycorrhizae are not only common, but they are found on almost all terrestrial plants and in almost all ecosystems. The ability of plant species to survive in many environments depends on mycorrhizae. They are critical components of ecosystem function in many environments as well, and play a major role in nutrient uptake by plants and in nutrient recycling in soils. About 80% of angiosperms and all gymnosperms are involved in some form of mycorrhyizal symbiosis (Fox, 2002).
There are two major, common groups of mycorrhizae, the ectomycorrhizae (ECM) and endomycorrhizae. The endomycorrhizae are subdivided into three groups. The most important of the endomycorrhizae are the vesicular-arbuscular mycorrhizae (VAM). Ericoid and orchidaceous (which will be discussed further later) are specialized for certain plant genera are sometimes classified on their own and sometimes as members of the endomycorrhizae. The ectendomycorrhizae have characteristics of both the ECM and ericoid groups (Fox, 2002).
The most common type of mycorrhizae is the VAM, which occur in both herbaceous and woody species. VAM are most abundant in systems where phosphorus is limited and in warmer or drier climates. They predominate in tropical ecosystems and are especially important to many crops. The body of the VAM fungus grows branched inside the cortex cells of the roots, forming a structure called and arbuscule, and in the intercellular spaces between root cells, with hyphae extending out several millimeters or more into the soil (Fox, 2002).
Ectomycorrhizae are most commonly found on woody plant species. ECM are particularly common in northern coniferous and temperate deciduous forests and in soils where nitrogen is particularly limiting. In fact, coniferous trees cannot grow in nature without these fungal symbionts. ECM are distinguished by two structures, one inside the roots and one outside it: called the Hartig net, a complex of mycelia that grows in between the root cortical cells, enmeshing them, and the mantle, a dense network of hyphae that partially or fully ensheaths the outside of the root. Plant species with ECM grow unusual-looking, short, stubby roots; this growth form may be controlled hormonally by the fungus (Fox, 2002).
The different branched and finely divided structures of VAM and ECM, including hyphal strands growing in the soil, function to increase nutrient uptake from the soil. Nutrients are taken up from the soil by the fungal mycelium and transferred from the fungal cells to the root cells of the host plant.
Only in the last few decades have botanists and mycologists realized that most terrestrial plants live in symbiosis with soil fungi (Mosse, 1956). The term mycorrhiza, created to reflect this reality, comes to us, moreover, from the combination of two words, one Greek miks (fungus) and the other Latin rhiza (roots). It therefore basically designates the symbiotic association between fungi and plant roots. Among the types of mycorrhizae observed in nature, one is found on the vast majority of cultivated plants. It is the arbuscular mycorrhiza, which lives in association with approximately 85% of herbaceous plants (Reinikka, 1972). This means therefore that in the plant world, mycorrhizal symbiosis is the rule rather than the exception.
Four hundred million years ago, when the continents were virtually deserted, plants and fungi formed symbiotic systems. Plants used solar energy to grow, while fungi specialized in absorbing nutrients from the soil. Thanks to the complementary role and function of these organisms, a wide variety of terrestrial plants emerged over the course of subsequent geological eras. Today, arbuscular mycorrhizal fungi (AMF) are found under all climates and in all ecosystems, regardless of the type of soil, vegetation or growing conditions (Reinikka, 1972).
A symbiosis refers to an association of living organisms that benefits both partners, enabling them to survive, grow and reproduce more effectively. AMF, which are microscopic soil fungi, simultaneously colonize the roots and their rhizosphere and spread out over several centimeters in the form of ramified filaments (Smith, 1997). This filamentous network dispersed inside as well as outside the roots allows the plant to have access to a greater quantity of water and soil minerals required for its nutrition. In return, the plant provides the fungus with sugars, amino acids and vitamins essential to its growth (Harley and Smith, 1983). As a result of its improved nourishment, a mycorrhiza-colonized plant has better growth. It fructifies abundantly and, above all, acquires increased resistance to environmental stresses such as drought, cold and root pathogens (Sylvie and Williams, 1992).
FUNCTION OF ORCHID MYCORRHIZAS
Orchids attract considerable attention largely because of their extraordinary diversity. They are found throughout all moist habitats, and a few are found in deserts. Species are found in terrestrial and epiphytic locations. Some regenerate from tubers each year, others have perennial leaves. None are known to be annuals.
Some are completely subterranean while most have much of their body above ground for much of the growing season. Most are commonly chlorophyllous, and a few are parasitic on their fungus. All form very small seeds, and require a fungus to germinate and nourish their young plants in the wild. It is this latter interaction that is the subject of this page.
Seeds of orchids are undifferentiated, and lack significant reserves of nutrients. Germination depends on colonisation by a specific mycorrhizal fungus. Germination follows a similar pattern in most cases. The seed imbibes water. The fungus penetrates the testa of the seed and enters either through epidermal hairs or the suspensor of the undifferentiated embryo. The fungus forms a tight coil or peleton following invagination of the plasmamembrane and extension into the cell. The peleton remains active for some time, but then collapses. The fungus colonises further cells, and the mycorrhiza spreads (Dodson, 1966).
Initial contact between fungus and imbibed seed can have one of three results: the fungus and plant form a functional mycorrhiza, the fungus can parasitise the seed, or the fungus remains outside the seed. In any one batch of seed, all three processes seem to take place. That is, some seed are potentially germinable, some are parasitised and die, and others remain ungerminated, because of a lack of recognition between symbionts. The issue of specificity will be covered later.
The process of differentiation follows initiation of colonisation. The imbibed seed starts to expand and cells multiply. The fungus colonises further cells. Cells of the end away from the peletons start to differentiate to eventually become the shoot tissue. The tip becomes slightly pointed. From this time, the plant is called a protocorm. The uncolonized tip then continues to grow and becomes the shoot. The basal region remains the absorbing region, eventually differentiating into structures that resemble roots (Reinikka, 1972).
Orchid mycorrhizas are different from other types of mycorrhizas in the nature of the nutrient exchange. After establishment of a mycorrhiza, organic carbon and other nutrients are passed from the fungus to the seed. Because asymbiotic germination requires sugars, amino acids and vitamins, it is assumed that these are also obtained from the fungus. The fungus continues to supply the protocorm with all its organic energy until such times as the plant starts to photosynthesise. Even then, it appears that the fungus does not gain significant supplies of carbon from the photosynthetic partner.
The source of carbon is assumed to be organic materials surrounding the plant. Mycorrhizal fungi have the potential to solubilise carbohydrates, including cellulose. The fungi translocate trehalose in the hyphae and the sugar is made available at the interface. Thus the fungus acquires and translocates organic energy to the plant. This is significant even in adult plants, especially epiphytes that exist in shadows of the canopy. The exception is for achlorophyllous orchids. In the few cases where they have been examined, it appears that the fungus is also attached to adjacent plants in an ectomycorrhiza. Thus transfer of photosynthates from photosynthetic host to orchid via the interconnected fungus is most likely (Smith, 1997). Transfer from photosynthetic hosts to photosynthetic orchids also needs closer examination.
Control of fungal Colonisation
The interaction between plant and fungus is highly regulated by the plant. The plant releases orchinol, a phytoalexin that causes the peletons to collapse. The degree of colonisation changes over the season, indicating that the orchid is controlling uptake of nutrients while preventing parasitism by the fungus.
Some terrestrial orchids have an annual cycle, whereby a period of growth is followed by loss of leaves and/or roots. The orchid then is maintained below ground until conditions become suitable for further growth. The fungus is commonly excluded from the orchid during these periods of dormancy. It can be isolated from the surface of the tuber or root, but peletons are absent inside the tissues (Allen, 1992).
Similarly, epiphytic orchids may have periods where they are poorly colonised. The fungus is retained only in the velamen of the root, and the cortex remains free of the fungus. As the season improves, fresh root growth is accompanied by fresh fungal colonisation.
The orchid can become colonised in “roots” or stems. Where the plants have a "root", the fungus usually penetrates the cortex root close to the root tip. Entry is via a passage cell of the exodermis.
The fungus then spreads rapidly through the cortex, forming peletons in each of the cells. The fungus does not penetrate between the cells, and in this is similar to Paris – type arbuscular mycorrhizas, and the coils of ectendomycorrhizas (Masuhara, 1994).
Orchids have a unique association with a group of fungi. They use their symbiont to gain access to organic and mineral nutrients. Further, the plant controls the degree of development of the mycorrhiza, both in space and time. The fungus appears to gain nothing from the association. Yet because the fungus recolonises the plant each year, and continues to benefit the host, it might be assumed that the plant exudes a powerful attraction. These and many other issues remain to be resolved.
Dodson, Calaway H. Orchid Flowers, their Pollination and Evolution. University of Miami press. 1966
Fox, Gordon A. The Ecology of Plants. Sinauer Associates, Inc., Publishers. Sunderland, Mass. 2002.
Harley, J.L. & Smith, S.E., 1983. Mycorrhizal Symbiosis. Academic Press, New York. 483 pages.
Krichner, John. A Neotropical Companion. Princeton University Press, New Jersey. 1997.
Masuhara, G and Katsuya, K.In situ and in vivo specificity between Rhizoctonia spp and Spiranthes sinensis (Persoon) Ames var amoena (Orchidaceae). New Phytologist 127, 711 – 718. 1994
Mosse, B., 1956. Fructifications of an Endogone species causing endotrophic mycorrhizae in fruit plants. Ann. Bot. (London) 20: 349-362.
MF, Allen (ed). Mycorrhizal Functioning. Chapman & Hall, New York. 1992
Reinikka, Merle A. A History of the Orchid. University of Miami press. 1972
Smith, SE and Read, DJ. Mycorrhizal Symbiosis. Academic Press, London. 1997
Sylvia, D.M. & Williams, S.E., 1992. Vesicular-arbuscular mycorrhizae and environmental stress. In, Mycorrhizae in Sustainable Agriculture. G.J. Bethlenfalvay & R.G. Linderman Eds. ASA Special Publication Number 54, Madison Wisconsin 101-124.
The Microbial World, Mycorrhizas. Http://helios.bto.ed.ac.uk/microbes/mycorrh.htm, 2000
Biodiversity of Mycorrhizal Fungi. http://res2.agr.gc/ecorc/mycor/bio_sols_e.htm, 1997
Function of Orchid Mycorrhizas. http://bugs.bio.usyd.edu.au/mycology/plantinteractions/mycorrhiza/orchidmycorrhiza/function/text.htm 1999.
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