Mycorrhizae are mutually beneficial associations formed between the majority of plant and tree species and certain biotrophic soil fungi. The evolution of such a symbiosis provides a means through which the mycobiont can acquire most of its carbon from the photobiont mainly in the form of hexose, whilst donating mineral nutrients, particularly phosphorus and nitrogen, and supplementing plant water requirements. Plant resource acquisition and uptake is enhanced by the presence of a massive mycobiont hyphal surface-area, the activity of which improves mobile resource uptake and is usually solely responsible for uptake of most immobile resource. Affinity for and uptake of nitrogen as ammonia and phosphorus as phosphate is greater for the mycobiont than for the uninfected root. Fungi as mycorrhizal associates may protect against soil pathogens and environmental stress and contribute to the survival of most plants within the natural woodland environment (Smith and Reid 1997).
Some mycorrhizal fungi show little host specificity; e.g., Amanita muscaria may associate with conifers such as
Pinus and Picea, and with deciduous species such as Betula. Such nebulous specificity may allow formation of connecting networks, linking individual plants of the same and different species, in source-sink relationships via communal mycorrhizal fungi. Such conducting and communicating networks may be most significant during for e.g., the translocation of nutrients by a more mature plant community, to support the adjacent growth of stressed or shaded seedling plants (Trappe and Luoma 1992). The ecological significance of such resource sharing in sustaining primary production in natural forests, heathlands, and grasslands is currently the focus of research attention (e.g., Simard et al. 1997). Other mycorrhizal fungi exhibit more strict host specificity; for e.g., Alpova diplophloeus will only associate with Alnus spp. Nevertheless, the development of general successions of mycorrhizal fungi are often recognized during forest maturation, the individual occupants of which have been associated with r- and s-selected strategies and the nature of the litter resource. The ecological strategies of individual mycorrhizae have important implications for forestry practices with respect to choice of mycorrhiza inoculum for plantation within woodlands at different stages of development.
Four broad types exist; the vesicular-arbuscular- (VAM) or arbuscular- (AMF), orchid-, ericoid-, and ecto- mycorrhizae. The VAM or AMF generally occupy environments where phosphorus is the main growth limiting nutrient. They dominate forests in tropical climates, but some temperate trees such as sycamore, ash, and poplars have AM, and some such as willow can form both AM and ectomycorrhizal associations. Ectomycorrhizae, however, are supreme in forests on moder, mull, or brown earth soils in temperate regions of moderate latitude and altitude (e.g., Read 1991), and in relatively infertile soils particularly where nitrogen and phosphorus uptake is curtailed. The ectomycorrhizal fungal mycelium sheaths root cortical cells, forming a Hartig net, and extends through the litter and surface soil layers forming a foraging, ramifying network. The substantial mycelial investment involved implies that prolonged associations tend to occur. The biology, ecology, and biotechnological application of mycorrhizae have been reviewed and described in many excellent texts, for e.g., Allen and Allen (1992); Carroll (1992); Smith and Reid (1997); Podila and Douds (2000). This section will focus only on the saprophytic activity of mycorrhizae in temperate forest soils.
Conventional belief has been that the main function of mycorrhizal fungi is uptake and translocation of mainly immobile mineral nutrients released by free-living sapro-trophs and that saprotrophy is generally lacking in mycorrhizal fungi. Indeed, nutrient flow between plant roots, ectomycorrhizae, and interacting saprotrophic non-mycorrhizal species has been demonstrated. For example, the wood-decay species Hypholoma fasciculare evidently lost phosphorus to mycorrhizal Suillus variegatus (Lindahl et al. 1999), whereas, carbon donation from Pinus sylvestris seedlings to mycorrhizal partner Suillus bovinus was visualized and declined in the presence of the decomposer species Phanerochaete velutina. This latter species subsequently accumulated the labeled carbon source (Figure 1; Leake et al. 2001). Other examples of competition between mycorrhizae and saprotrophs have been reported (Shaw et al. 1995). However, the extreme mycorrhizal dependency exhibited by achlorophyllous plants is testimony to the role of fungal associates in translocating carbon to host plants (Leake 1994). The significance of similar carbon donation by
Figure 1 (a) Localized proliferation of mycelium of Paxillus involutus in mycorrhizal association with Pinus sylvestris on entering discrete patches of nonsterile forest litter in trays placed in an otherwise homogeneous peat substrate. (b) A digital autoradiograph of the microcosm shown in Figure a, 5 days later. The autoradiograph was aquired 48 h after 14C pulse labeling of the shoot of the P. sylvestris seedling. Over 50% of the 14C detected in the mycorrhizal mycelium (excluding roots) was allocated in the recently colonized litter patch labeled T2. (c) A control 20 X 20 cm microcosm with mycelium Suillus bovinus mycorrhizal with P. sylvestris, forming an almost complete covering of the surface of the peat. (d) Similar microcosm in which the mycelium of S. bovinus met and interacted with the mycelial cords of the wood-decomposer fungus Phanerochaete velutina growing from a wood block. Note the difference in density of cover and extent of mycorrhizal mycelium compared with (c). From Leake et al. (2001); Reproduced in black and white from color images.
mycobionts to autotrophic plants is now being investigated. Thus, it is becoming increasingly evident that, particularly in more acidic woodland environments, mycorrhizal fungi influence the degradation of leaf litter and woody debris. For example, mycorrhizal activity is stimulated by addition of litter to microcosms, and is maximal in temperate and boreal forests when nutrient litter supply peaks (Leake and Reid 1997; Unestam 1991). Furthermore, visual evidence of penetration of litter material by ectomycorrhizal hyphae (Leake et al. 2001; Figure 1; Ponge 1990), and nutrient release measurements following ectomycorrhizal (Suillus bovinus or Thelephora terrestris with Pinus sylvestris) colonization of litter patches (Bending and Reid 1995), all implicate a saprotrophic role for the mycobiont. Ericoid- and some ecto-mycorrhizal fungi, commonly found dominating ecosystems where nitrogen and phosphorus reserves often reside within accumulated organic matter, are known to produce extracellular enzymes capable of decaying complex carbon polymers present in litter and soil, thereby exposing and mobilizing otherwise trapped mineral nutrient reserves (Leake and Reid 1997). Proteins, amino acids, chitin, nucleic acids, phospholipids, and sugar phosphates all serve as nitrogen- and phosphorus-rich organic materials. Enzyme capacities so far identified include proteinase, peptidase, chitinase, acid and alkaline phosphatase, phytase, DNAase and RNAase, polygalacturonase, cellulase, xylanase, tyrosi-nase, peroxidase, polyphenoloxidase (including laccase), and ligninolytic activity (see Leake and Reid 1997). The hydrolytic capability of different individual ectomycorrhizal fungi is diverse (Hutchinson 1990), some being capable of degrading holocellulose, lignin, and lignocellulose (Trojanowski et al. 1984), while others utilize protein for carbon and nitrogen and translocate derivatives to host plants (Abuzinadah et al. 1986). The latter capabilities are of primary importance where successful mycorrhizal associations are established within ecosystems where the carbon may be supplied to the mycobiont often entirely by the plant symbiont, but available nitrogen and phosphorus supplies are growth limiting to the fungus and then ultimately to the plant host.
It has been estimated that maintenance of the mycorrhizal association costs the host between 9 and 28% of the total photosynthetic production annually (Finlay and Soderstrom 1992; Leake et al. 2001; Vogt et al. 1992). The benefits of such a costly relationship to the photobiont are not only limited to nutrient scavenging and mobilization, and to the saprotrophic release of otherwise unavailable resource, but extend to the ability of the mycobiont to construct extensive and persistent mycelial networks. This allows retention of a mineral budget within a fluctuating and discontinuous nutritional environment, as the mycorrhizal rootlets undergo dynamic degradation and formation, thereby preventing leaching of important nutrient reserves to the surrounding rhizosphere. Leake and Reid (1997) discuss the ecological significance of degradative activities of mycorrhizal and nonmycorrhizal fungi. They argue that the massive mycelial biomass and localized degradative activities produced within woodlands by mycorrhizal species, together with donation of carbon supply by host plant species, may render the degradative role of mycorrhizal fungi as significant compared to nonmycorrhizal saprotrophs, at an ecosystem scale.
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