Mycorrhizae structure

Jakobsen, I. (1999) Transport of phosphorus and carbon in arbuscular mycorrhizas. In Varma, A. and Hock, B. (eds) Mycorrhiza Structure, Function, Molecular Biology and Biotechnology, 2nd edn. Springer, Berlin, pp. 305-332. 

Mycorrhizae Host plants Fungal symbionts Fungal structures... 

Early interactions between the cell walls of the plant and the fungus and changes in their composition are essential morphogenetic events in the constitution of a functioning mycorrhiza. Cellular and molecular approaches have provided new insights into the complex and ever-changing scenario of these interactions. Cytochemical and in situ immunological techniques have demonstrated that both structure and function are less complex when assessed at the cell level (10,11,71,72). 

R. L. Peterson and P. Bonfante, Comparative structure of vesicular-arbuscular mycorrhizas and ectomycorrhizas. Plant Soil 159 19 (1994). 

For most of the world s plants mycorrhizae are the primary interface between physiologically active areas of the root and the external environment. Hence it is likely that many, perhaps most, allelochemical interactions Involving higher plant roots are mediated by the fungal symbiont. Herein we briefly review past research dealing with allelopathic effects on mycorrhizae, and discuss how interaction between mycorrhizae and chemicals may influence structural and functional aspects of ecosystems. Most studies to date have dealt with EM in forest ecosystems, and this is where our discussion will focus. 

The rapid allocation of trace quantities of to external AM fungal hyphae provides independent support for the observations by Johnson et al. (2002a, b) that carbon transfer to mycorrhizas occurs very quickly. Staddon et al. (2003b) concluded that the turnover rate of AMF hyphae was likely to be just 5-6 days. This corresponds well with the turnover rates by absorptive hyphal structures of 5-7 days observed in Artemisia triden-tata and Oryzopsis hymenoides communities (Friese Allen, 1991). However, it is not known from the Staddon et al. (2003b) study if the carbon allocated to the AMF was used for production of mycelial biomass or allocated into compounds that are rapidly respired, in accordance with the observations of Johnson et al. (2002a, b). 

Order 4.— Tuberales, the truffles. Fungi whose septate mycelium is often connected with the roots of trees forming the structure known as mycorrhiza. Several species of the genus Tuber growing in woods of France, Germany and Italy produce tuberous subterranean bodies called Truffles, which are highly prized as a table delicacy by the inhabitants of these countries. 

In soil, mycorrhiza produces vegetative structures like chlamydospores and zygospores, which become dominant during the period of environmental stress and germinate with the return of favourable conditions (Bhoon, 2005 Kumari et al, 2003). Thus, they are better equipped for combating the unfavorable conditions and have larger shelf lives as compared to bacterial systems. 

AM fungi contribute to plant growth and plant nutrition as their extraradical hyphae increase the volume of the explored soil. These extraradical hyphae can extend up to more than 10 cm from the roots (Harley and Smith, 1983) and make up a hyphal density of 1-30 m g soil (Smith and Read, 1997). Therefore, AM roots can more easily access soil nutrients localized outside the root depletion zone, especially the nonmobile elements such as P and Zn. Mycorrhizas also improve plant water uptake, stress tolerance, and affect the microbial community structure (loner et al, 2001). In polluted soils, mycorrhizal plants also benefit from the presence of the AM fungi. 

The hyphae of AM grow into the root and also penetrate the root cells. Inside the root cells the hyphae branch to form an arbuscule (a tree like structure) that works as an organ for nutrient exchange (Fig 6.2, left). Sometimes one can see lipid rich storage vesicles in the root as well (Fig 6.2, right). In the slightly older literature the AM mycorrhiza were called vesicular arbuscular mycorrhiza (VAM) but this name was dropped since not all AM form vesicles. 

All plants live in more or less prolonged association with fungi. They may live as symbionts between the plant cells and develop either in a harmonious manner (endoph3Tes), or on the contrary provoke illness (ph5Topathogens). Mycorrhizal fungi, meanwhile, are among mutualistic S5mibionts the most important of the rhizosphere. They form particular structures with the roots of plants, mycorrhizae, which participate in the nutrition of almost all plants. 

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