Mycorrhiza

Land plants never had an independence (from fungi); for if they had, they could never have colonized land.

KA. Pirozynski and D.W. Malloch (1975)

About 90% of all terrestrial plants have underground fungal partners known as mycorrhiza, which means "fungus-root" or a root colonized by a symbiotic fungus. The fungus invades tree roots and obtains nourishment by tapping into the plant's vascular system. The hyphae enmesh the root and extend into the soil particles or leaf-litter, tapping a larger volume of soil and increasing the plant's access to water and relatively immobile nutrients. The underground mycelium can even interconnect plants. Many trees are so dependent on their mycorrhizal partners that they languish or die without them. The recognition of mycorrhizal association between roots and fungi provides an explanation for the paradox of the luxuriance of rain forests growing in soil from which soluble minerals have been leached by torrential rains over millennia and are of extremely low fertility. Mycorrhizal fungi provide the primary mechanism for the uptake of nutrients by the forest trees and thus contribute to the green cover on Earth. In most natural habitats, mycorrhizal plants compared with non-mycorrhizal plants show a greater uptake of mineral ions such as nitrogen, potassium and particularly phosphorus. Mycorrhizal symbiosis is estimated to be several hundred million years old and the primitive root systems of the earliest known plants are associated with fungi. Spores with characteristics similar to that of mycorrhiza are recorded in fossils.

3.1.1 Types of Mycorrhiza

The truffles (Ascomycotina), the agarics and the boletes (Basidiomycotina) are common examples of mycorrhizal fungi. They are now placed in a separate phylum called Glom-eromycota or Glomeromycotina. Depending upon whether the bulk of the fungus is outside or inside the root, mycorrhizas are divided into ectomycorrhiza and endomycorrhiza, respectively. The ectomycorrhiza has a conspicuous pseudoparenchymatous tissue ensheathing the root which sends hyphae between the cortical cells of the root and outwards spreading into the surrounding soil and litter. In a cross section, hyphae are seen penetrating the cortex and forming a dense intercellular network of hyphae, called the Hartig net (Figure 3.1). Ectotrophic mycorrhiza are common in the temperate zone. The root tips proliferate and root cells become coral-like in appearance. In endomycorrhiza, the sheath is reduced or absent. Most trees have vesicular-arbuscular mycorrhizal fungi. Arbuscular mycorrhiza are so-called because the fungal hyphae ramify extensively into tree-like structures called arbuscules within the root cells, invaginated by the host plasma membrane. The arbuscules provide a large surface symbiotic interface for the exchange of mineral nutrients from the fungus to the plant.

For a hypha to enter into a root, it adheres by the formation of a swollen structure called appressorium (Figure 3.2). This appears to be formed as a result of topographical or biochemical signals. Following adhesion, the colonization of the root cortex occurs by an intradical mycelium. The intercellular fungal hypha develops highly branched tree-like fungal structures inside the arbuscules. Arbuscules occupy a major portion of plant cell volume but are separated from the host protoplast by a periarbusular membrane which greatly increases surface area and is the site of nutrient and signal exchange. Arbuscule development in a root cortex cell is accompanied by plastid proliferation, pointing to a highly regulated exchange of compounds and/or signals between the two partners (Hans et al., 2004). Arbuscules degenerate in a few days and the fungus develops ovoid or spherical vesicles which become thick walled and contain fat globules. These asexually formed spores (chlamydospores) are 20 to 1000 ^m or more in diameter and persist in the soil for long periods of time. The identification of the vesicular arbuscular mycorrhizal (commonly abbreviated as the VAM or the AM) fungi is based on the size, color, number of wall layers and surface features of the spores. The non-septate, coenocytic mycelium suggests that the VAM fungi belong to the phylum Zygomycotina (see Appendix). The absence of host specificity in VAM fungi suggests that mycorrhizal symbiosis is conserved between evolutionary distant plant species.

3.1.2 Techniques of Studying Mycorrhizal Symbiosis

The study of mycorrhiza is complicated by the biotrophic nature of the fungus. A com-partmented pot system has been used to quantitatively estimate the contribution of the mycorrhizal uptake pathway to total plant phosphate (P) supply. Experimental plants (two

Phosphorus Ectomycorrhizal

Figure 3.1 Ectomycorrhiza. (a) Boletus edulis, one of the Basidiomycotina that forms mycorrhiza. (b) A seedling of Douglas fir (Pseudotsuga menziesii) colonized by Lec-cinium sp. The fungal mycelium has developed ectomycorrhizas on the root and has produced a basidiocarp above ground. (c) Short roots ensheathed by an ectomycorrhizal fungus. (d) Transverse section of a Eucalyptus/Pisolithus ectomycorrhiza showing the external (EM) and internal (IM) mantles of hyphae; the fungal hyphae penetrating between the epidermal cells of the root cortex (RC) to form the Hartig net (HN). Extramatrical hyphae (EH) are exploring the medium. From Martin et al. (2001). With permission of Blackwell Publishing.

Figure 3.1 Ectomycorrhiza. (a) Boletus edulis, one of the Basidiomycotina that forms mycorrhiza. (b) A seedling of Douglas fir (Pseudotsuga menziesii) colonized by Lec-cinium sp. The fungal mycelium has developed ectomycorrhizas on the root and has produced a basidiocarp above ground. (c) Short roots ensheathed by an ectomycorrhizal fungus. (d) Transverse section of a Eucalyptus/Pisolithus ectomycorrhiza showing the external (EM) and internal (IM) mantles of hyphae; the fungal hyphae penetrating between the epidermal cells of the root cortex (RC) to form the Hartig net (HN). Extramatrical hyphae (EH) are exploring the medium. From Martin et al. (2001). With permission of Blackwell Publishing.

per pot) were grown (Smith et al., 2003) and a mycorrhizal spore inoculum covered with nylon mesh was placed in the soil, allowing the hyphae but not roots to penetrate into the soil in which 33P-labeled orthophosphate of high specific activity was mixed. The 33P from the soil solution could only reach the plants via the hyphae; unlabeled phosphate could be absorbed directly. After a period of growth, comparison of specific activity of 33P in Glomus-inoculated and uninoculated plants showed that mycorrhizal plants grew better in terms of dry weight production. The pathway of phosphorus transport is thought to

Root Anatomy Mycorrhizal Aquatic Plants
Figure 3.2 Morphology of an arbuscular mycorrhizal fungus. The fungal structures are visualized after staining with trypan blue. Redrawn from Ingold and Hudson, The Biology of Fungi (1993), London: Chapman and Hall.

involve the uptake of phosphates by fungal transporters located in external hyphae and then its delivery into cortical cells of the root. Mycorrhizal uptake replaced direct uptake pathways in roots colonized by fungi, presumably due to down-regulation of plant genes encoding phosphate transporters, indicating a molecular cross-talk between plant and fungus.

To illustrate the movement of carbohydrates (Figure 3.3) by experiment, 14C-labelled sucrose was supplied to mycorrhiza via beech root tissue (Harley, 1969). The fungus converted sugars taken up in the root compartment into trehalose—a typical fungal sugar, not found in the higher plants, mannitol and lipids (Pfeffer et al., 1999). From results of such experiments arose the concept of bidirectional transfer of nutrients in mycorrhizal symbiosis: whereas the photosynthetic plant benefits from the transports of phosphates to the root tissue, it supplies the fungus with carbohydrates. The metabolism of the fungal partner was recently examined by combining an in vitro compartmentalized system with 13C-labeling and spectroscopic nuclear magnetic resonance (NMR) analyses. Using plugs of carrot roots colonized by Glomus intraradicus, isotopically labeled substrates were added either to the roots or the extraradical hyphae which grew out of root and analyzed by NMR spectroscopy. Labeling patterns indicated that 13C-labelled glucose and fructose were taken up by the fungus within the root and converted into trehalose, mannitol and lipids (Pfeffer et al., 1999).

Since the growth of mycorrhizal fungi on artificial media has not been successful, it is assumed they are wholly dependent upon a photosynthetic plant. The in vitro root-organ cultures are increasingly used in investigations of VAM symbioses. Several VAM species have been cultivated on root organ cultures (Fortin et al., 2002), providing extraradical

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