Mycorrhizal associations

M. J. Harrison and R. A. Dixon, Isoflavonoids accumulation and expression of defense gene transcripts during the establi.shment of vesicular-arbuscular mycorrhizal associations in roots of Medicago truncatula. Mol. Plant-Microbe Interact. 6 ... 

J. S. Pate, The mycorrhizal association just one of many nutrient acquiring specializations in natural ecosystems. Management of Mycorrhizas in Agriculture, Horticulture and Forestry (A. D. Robson, L. K. Abbott, and N. Malajezuk. eds.). Kluwer Academic Publishers, 1994, pp. 1-10. 

Although some authors exclude the mycorrhizal associations from general discussions of the rhizosphere, we have used the term in its broadest sense. Since we will be unable to cover all the literature, we refer the reader to several general references on the rhizosphere (8-12) and on mycorrhizae (13-18). 

As stated earlier, mycorrhizae enhance nutrient absorption. Greater soil exploitation by mycorrhizal roots as a means of increasing phosphate uptake is well established. The normal phosphate depletion zone around non-mycorrhizal roots is 1-2 mm, but an endomycorrhizal root symbiont increased this zone to 7 cm (140). This ability to increase the nutritional level (particularly with regard to phosphorus), and subsequently the overall better growth dynamics of the mycorrhizal plant has been suggested as the reason for the salt (43) and drought (44-46) tolerance and increased nodulation (47) observed in mycorrhizal associations. Another interesting aspect of this enhanced nutrient uptake is the possible effect of mycorrhizae on competitive ability between two plant species. Under some conditions, mycorrhizal... 

Although there is no doubt as to the importance of mycorrhizae in nutrient absorption, reviews on ion uptake have generally not considered it. Hatling et al. (143) made this same point more than 10 years ago. In addition, although phenolic acids inhibit phosphate (144, 145) and potassium (146) uptake, no work has examined the effects of these compounds on nutrient absorption of mycorrhizal associations. Since soil microorganisms produce the bulk of the volatile compounds emitted from soil, which are known to inhibit or stimulate fungal development (147-148), this group of compounds from microbial sources should receive more attention. 

If mycorrhizae are sites of action for allelochemicals, this is an important indirect aspect of allelopathic interaction among plants. Inhibition of mycorrhizal formation or a reduction in the efficiency of mycorrhizal association would reduce the nutrient level of the mycorrhizal plant and subsequently its competitiveness, stress tolerance or nodulation. Although allelochemicals have been implicated in the reduction of nodulation and nodule size, possible mycorrhizal involvement has not been examined. This is a difficult area of research but one that will provide better understanding of this complex situation. 

Brundrett M, Diversity and classihcation of mycorrhizal associations, Biol Rev... 

Handley, W. R. C. "Mycorrhizal associations and Calluna heathland afforestation" Bull. For. Coromun. London, No. 36. Persidsky, D. J. Lowenstein, H. Wilde, S. A. Agron. J. 

Little is known about the potential to exploit mutualism in agricultural systems. Almost all the work to date has concentrated on symbiotic nitrogen-fixers and mycorrhizal associations. Availability of soil nitrogen and phosphorus is a severe problem in many areas of the world. Allelochemicals may impact the availability of these nutrients through effects on the symbiotic microbes. 

Abstract. Allelochemals induced in mycorrhizal plants play an important role in disease resistance. Mycorrhizal associations are the most important symbiosis systems in terrestrial ecosystems and offer many benefits to die host plant. Arbuscular mycorrhizal associations can reduce damage caused by soil and root - borne pathogens. 

This review describes the role of mycorrhizal associations in disease control. 

Abuzinadah, R. A. Read, D. J. (1986a). The role of proteins in the nitrogen nutrition of ectomycorrhizal plants. III. Protein utilization by Betula, Picea, and Pinus in mycorrhizal association with Hebeloma crustuliniforme. New Phytologist, 103, 507-14. 

Alexander, I. J. (1981). Picea sitchensis and Lactarius rufus mycorrhizal association and its effects on seedling growth and development. Transactions of the British Mycological Society, 76, 417-23. 

Warnock, A. J., Fitter, A. H. Usher, M. B. (1982). The influence of a springtail Folsomia Candida (Insecta, Collembola) on the mycorrhizal association of leek Allium porrum and the vesicular-mycorrhizal endophyte Glomus fasciculatus. New Phytologist, 90, 285-92. 

Increasing attention has been given to the potential of mycorrhizal associations for reforestation and clean-up of metal-contaminated areas (Van der Lelie et al. 2001). Mycorrhizas may enhance metal phytoextraction by host plants by increasing plant biomass and increasing metal... 

Meharg, A. A. (2003). The mechanistic basis of interactions between mycorrhizal associations and toxic metal cations. Mycological Research, 107, 1253-65. 

Mycorrhizal associations with vascular plants apparently evolved about 400 million years ago and are present in about 80% of our plants (Simon et al., 1993 Remy et al., 1994). Ecto- and endomycorrhiza are common associations with roots they enlarge the root surface area and thus help to catch more... 

Mycorrhizal associations can increase the absorptive capacity of sparse root systems. Fungal hyphae can penetrate the soil surrounding the infected root, absorbing P that is otherwise too far from the root surface for absorption to take place. Our... 

Fig. 93 Arbuscular mycorrhizal associations in fine roots of adjacent mature forest, secondary forest on abandoned cattle pasture (l6 years old), and active cattle pasture planted with Brachiaria brizantha forage grass. Each data point is the average value for six root samples, with 75 1-cm root segments stained and analyzed per sample following Phillips and Hayman (1970). Percent infection refers to the percent of each root segment s length in which fungal hyphae or arbuscles were visible.

Mutualistic species interactions contribute directly to many essential ecosystem processes, such as nutrient inputs through nitrogen fixation and mycorrhizal associations that govern phosphorus and organic nitrogen uptake by plants (Read, 1991). Other mutualisms, such as pollination and seed dispersal, have indirect effects, influencing the presence or absence of species that may have strong ecosystem effects. 

The key step in this process seems to be the below-ground, symbiotic fungus-root structures (mycorrhizal association) studied closely by Sir Albert Howard. See chapter 7. 

Mycorrhizal association" refers to the symbiotic relation between the mycelium of certain fungi and the roots of a seed plant. 

If you examine the roots of almost any plant, you ll find that they are infected with a beneficial type of fungus called mycorrhizal fungus. Infection actually helps the plants by improving the uptake of nutrients, especially phosphorus. The fungi also influence a plant s ability to tolerate drought and to ward off microorganisms that attack roots. Soil sterilization and certain pesticides will discourage mycorrhizal associations. 

Fay, D. A., and Mitchell, D. T. (1999). A preliminary study of the mycorrhizal associations of the tree seedlings growing on mine spoil at Avoca, Co. Wicklow. Biol, and Environ. Proc. R. Ir. Acad. 99B, 19-26. 

Generally speaking, most tine roots in tropical forest soils are found in the upper 0.5 m (Kerfoot, 1963), with a marked concentration of roots into a root mat close to the soil surfiice and within the litter layer being especially common on low-fertility soils (Stark and Jordan, 1978 Medina and Cuevas, 1989). It is generally considered that these root mats serve to ensure the maximum retention of nutrients by the vegetation and to minimize any leaching losses. Surveys of tropical forests have indicated almost ubiquitous mycorrhizal associations for such roots (Alexander, 1989 Janos, 1989). 

As for temperate plants, it is widely assumed that mycorrhizal associations in tropical forests serve to improve the uptake of mineral nutrients, particularly phosphorus (Bolan, 1991 Koide, 1991 Smith and Read, 1997). Growth stimulations and enhanced P uptake in response to mycorrhizal infection have been reported for tropical tree seedlings (Janos, 1989 Lovelock etai, 1996, 1997). 

This improved P uptake occurs in exchange for the provision of G from the host plant, and the carbon requirements of the mycorrhizal association can be substantial. For example. Baas et al. (1989) showeci root respiration rates of mycorrhizal plant to be 20-30% higher than those of nonmycorrhizal plants. Similarly,... 

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