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FIGURE 10.2 Colonization of pine short roots by ectomycorrhizal fungi. (A) Early stage: hyphae (Cenococcum sp.) are starting to proliferate on the root surface near the apex (arrow). (B) Late stage: mantle hyphae (Rhizopogon sp.) cover the entire root surface. (Images courtesy of Larry Peterson.)

Mycorrhizas are ancient; fossil evidence from the Devonian (400 million years ago (Mya)) indicates early land plants formed endophytic associations with fungi resembling the modern AM fungi that colonize modern bryophytes. Fossils of spores resembling AM fungi have been found in rocks from the Ordovician (460 Mya), the period during which plants first colonized land (Fig. 10.3). Early land plants resembled modern bryophytes, had poorly developed root systems, and would have benefited greatly from mycorrhizal symbioses.

FIGURE 10.3 (A-C, E-G) Fossil spores from the Ordovician and (D, H) spores from extant arbuscular mycorrhizal fungi. Scale bars, 50 p,m. (Reprinted with permission from Redecker, Kodner, and Graham, 2000; copyright 2000, AAAS.)

The primary benefit derived by plants involved in mycorrhizal symbioses is generally perceived to be enhanced nutrient uptake, achieved by the fungus expanding the zone of nutrient uptake farther away from the rhizosphere and/or more efficiently taking up and transporting nutrients. The nutrients that are taken up by the fungus depend on the type of mycorrhiza. Generally, all types of mycorrhizal fungi are able to transport N and P. Ericoid mycorrhizal fungi primarily increase the plant's access to N, but also to P, Ca, and Fe. AM fungi have been studied mostly for their ability to enhance P nutrition, but also play a role in the uptake of N, K, and Zn. EM fungi have saprotrophic abilities and are able to access organic forms of N and P. As described above, monotropoid and orchid mycorrhizal fungi transport C, in addition to N and P, to their mycoheterotrophic hosts.

However, the symbiotic role of mycorrhizal fungi is not limited to enhancing the plant's nutritional status (Fig. 10.4). Mycorrhizal fungi play an important role in the formation of soil macroaggregates, which help maintain soil stability during alternating periods of wetting and drying. Other benefits accrued by the plant may result from altered biotic interactions and take the form of defense, with mycorrhizal roots being colonized by fewer disease-causing organisms or suffering less grazing by root herbivores than nonmycorrhizal roots. The consequences of these interactions are not confined to the soil: plant fitness may be altered by mycorrhizal colonization as a result of altered interactions with insects, including herbivores, natural enemies of herbivores, and pollinators. Performance of plants and herbivores in response to mycorrhizal colonization varies depending on the feeding strategy of the herbivore and differs among fungal species (Gehring and Whitham, 2002). Parasitism of a leaf-mining insect, Chromatomyia syngenesiae, by a generalist parasitoid Diglyphus isaea was reduced on Glomus mosseae-colonized plants but increased on Glomus caledonium- and Glomus fasciculatum-colonized plants, possibly due

FIGURE 10.4 The multifunctional nature of AM fungi. Morphological requirements for different mycorrhizal functions include diffuse extraradical mycelium far from the root (P uptake) and in the rhizosphere (plant_soil water relations) and extensive intraradical colonization by hyphae (pathogen protection) and arbuscules (P transfer). (Reprinted from Fitter, 2005, with permission; copyright 2005, Blackwell Publishing.)

FIGURE 10.4 The multifunctional nature of AM fungi. Morphological requirements for different mycorrhizal functions include diffuse extraradical mycelium far from the root (P uptake) and in the rhizosphere (plant_soil water relations) and extensive intraradical colonization by hyphae (pathogen protection) and arbuscules (P transfer). (Reprinted from Fitter, 2005, with permission; copyright 2005, Blackwell Publishing.)

to altered host searching efficiency by the parasitoid on larger and smaller plants (Gange et al., 2003). Plant_pollinator interactions may be affected by AM fungi via changes in floral morphology (Philip et al., 2001) and enhanced pollen quality (Poulton et al., 2001) such that pollinator visitation rates are enhanced (Wolfe et al., 2005). Mycorrhizal fungi, particularly EM and/or AM fungi, also show promise for the breakdown of organic pollutants, protection of plants from metal phytotoxicity, and sequestration of toxic elements in contaminated soils.

Recently, as mycorrhizal researchers have learned more about the ecological outcomes of mycorrhizal symbioses, the concept of "mycorrhiza = mutualism" has been challenged. In orchid mycorrhizas, for example, the plant partner is achloro-phyllous for an extended period following seedling emergence, thus the fungal partner receives no known benefit from the association. Achlorophyllous plants within the Monotropaceae obtain photosynthate by establishing monotropoid myc-orrhizas with EM fungi already associated with a gymnosperm host; as a result, the achlorophyllous plant effectively parasitizes the gymnosperm via the fungus, although the consequence for fungal fitness is not known. The degree to which the associates derive benefits may differ depending on environmental conditions. AM symbioses are expensive for plants to maintain, with AM fungi requiring up to 20% of host photosynthate during colonization and while functioning (Graham, 2000). Under conditions of high nutrient availability, if C costs exceed the benefits accrued by mycorrhizal uptake of nutrients, the association may actually be detrimental to the plant host. AM fungal colonization of plants is reduced under conditions of high P availability, probably due to reduced exudation of compounds stimulatory to spore germination and hyphal growth, but possibly even due to presence of plant-derived inhibitory compounds in root exudates, suggesting plant control over colonization events (Vierheilig, 2004). In addition, the species composition of an AM fungal community differs depending on soil fertility; fertilization can select for AM fungal species that promote growth to a lesser extent than those found in unfertilized soil (Johnson, 1993). The outcome of the symbiosis is also dependent on the plant and fungal species involved in the association, with effects on plant growth ranging from positive to negative across a variety of plant-fungal species combinations (Klironomos, 2003; Fig. 10.5).

Mycorrhizal symbioses are also important for shaping the environment in which plants and fungi grow. The microbial composition of mycorrhizal root systems is very different from that of nonmycorrhizal roots, a phenomenon known as the "mycorrhizosphere effect" (Linderman, 1988; Vierheilig, 2004), due to chemical and physical effects on the soil environment. Roots colonized by mycorrhizal fungi are hypothesized to have altered root membrane permeability, which may affect the abundance and composition of root exudates; few studies have measured these changes in root exudate composition, and in those studies only certain groups of identifiable compounds (e.g., phenolics) were considered. Reduced mineral and nutrient concentrations in soil, as a result of increased uptake by mycorrhizal fungi, as well as higher C inputs due to high turnover of the fungal mycelium may also contribute to changes in microbial community composition. Some mycorrhizal fungi can secrete compounds into the rhizosphere that are antagonistic to soil microorganisms; for example, Paxillus involutus, an EM fungus, produces ethanol-soluble substances that suppress sporulation in a pathogenic fungus, Fusarium oxysporum (Duchesne et al., 1988).

While mycorrhizal fungi influence the composition and activity of soil microbial communities, soil microbes also reciprocate by influencing fungal activity and functioning. Microscopic examination of the surfaces of AM fungal spores and hyphae reveals colonization of surfaces by bacteria. Xavier and Germida (2003) isolated several bacterial species associated with Glomus luteum (formerly Glomus clarum) spores reared from greenhouse cultures, some of which had stimulatory or inhibitory effects on spore germination and/or hyphal growth. Both diffusible and nonvolatile substances produced by bacteria are thought to be involved in these effects on AM fungal activity. Bacillus chitinosporus, which inhibited spore germination in the in vitro bioassay, reduced G. luteum colonization of pea roots

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