Plant mycorrhizal association

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 ... 

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... 

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. 

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. 

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. 

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... 

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... 

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. 

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. 

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,... 

Jakobsen and Rosendahl (1990) observed 20% of plant carbon to be allocated below ground for nonmycorrhizal cucumber plants and 44% for those with mycorrhizal associations. In both cases, about half of this was respired. Working with subtropical Citrus species, Peng et al. (1993) suggested that root respiration rates were about 35% higher for mycorrhizal than for nonmycorrhizal roots. 

It is now well documented that plant roots, bacteria, and fungi (including those involved in mycorrhizal associations) can all excrete organic acids into the soil solution (Marschner, 1995 Jones,... 

Host cells on some plants that are infected with mycorrhizal fungi, especially endotrophic organisms, are able by intracellular digestion of the hyphae to keep the infection within bounds. This lends support to the view expressed by Garrett (1960) that the mycorrhizal association is much more a case of controlled parasitism than of symbiosis. Comprehensive generalizations are, however, not justified. 

Show that radionuclide is present in fungal tissue, which is also a proven mycorrhizal associate of the higher plant being tested and is in physical contact with the labeled litter and living roots. 

Plant-microbial association 5. meliloti S3 +5. plymuthica SI +M. sativa may be applied for remediation of soil polluted with petroleum, industrial oil and diesel fuel. Tbe performed studies have shown that microbial treatment of alfalfa seeds intensified degradation of 1% crude oil in sod-podzol soil (Figure 12.1). Inoculation of alfalfa seeds with nodulating bacteria S. meliloti S3 accelerated oil decomposition (after 3 months) by 13.8% as compared to the control. Exposure of alfalfa seeds to phosphate-mobilizing bacteria S. plymuthica 57 and arbuscular mycorrhizal fungi (AMF) increased the rate of crude oil decay in soil by 28.4% and 22.7%, respectively. Maximal efficiency of petroleum disposal was achieved by plant-microbial association S. meliloti S3 + S. plymuthica 57 +M. sativa realizing the fastest process (58.47% up the control). 

M. J. Harrison, The arbuscular mycorrhizal symbiosis an underground association. Trends Plant Sci. 2 54 (1997). 

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