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Fungi and Mycorrhizae
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1. What are fungi and how do they differ from bacteria ?
2. What is the ecological role of fungi in soil and on plant surfaces ?
3. How do agricultural practices affect the fungi in soil ?
4. What are mycorrhizae and what is their role in agriculture ?
5. What is biological control in the context of plant disease management ?
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1. eukaryote, filamentous, yeast, chitinous walls, glucans, conidia, spores, sclerotia
2. saprophyte, parasite, pathogen, biotroph, necrotroph, mildew, root rot, damping off ,antibiosis, mutualism, symbiosis, Rhizoctonia solani, Cylindrocarpon destructans, Pythium spp., Thielaviopsis elegans, nutrient cycling
3. crop rotation, selection pressure, amendment, enrichment, anaerobic, tillage, pesticide residue, drainage,host specificity, propagule survival, dispersal
4. Glomus spp.. Gigaspora spp., vesicular-arbuscular mycorrhizae, endomycorrhizae, ectomycorrhizae, growth promotion, mutualism, nutrient absorption, fertility, phosphorus azygospore, chlamydospore, disease suppression, drought tolerance,carrier, agarose beads, pelletization, peat-based potting medium, obligate symbiont, colonized root fragments
5. rhizosphere, mycorrhizosphere, rhizosphere competence, competitive saprophytic ability, actinomycete, antibiotic, toxic fungal metabolites, Sporothrix spp., Tilletiopsis spp., hyperparasites, antagonists, competition, allelopathy, chelation
1. The Fungi
Several diverse microorganisms are classified within the broad group of protists called the fungi. However, they do share some typical features that distinguish them as fungi different from other microbes. These distinguishing features are based on the physiology and morphology of cells. The fungal cell is distinguished from the bacterial cell by its size (generally greater than 1-2 um in diameter) and its eukaryotic structure similar to that of plant and animal cells (cells having distinct membrane-bound organelles such as nuclei, mitochondria). Bacterial cells which lack these organelles are termed prokaryotic. Fungal cells divide by mitosis (asexual reproduction) and by meiosis (sexual reproduction); bacterial cells divide by binary fission.
Like animals, the fungi are heterotrophic organisms that cannot manufacture their own food by photosynthesis as plants and algae can. They require oxygen for growth (aerobic) and generally prefer an acidic environment (below pH of 7) unlike the bacteria which are anaerobic and aerobic and g enerally grow in basic environments ( at or above pH of 7). Fungi utilize preformed organic material from other organisms as sources of energy and building blocks for their cellular sysnthesis. Soluble nutrients are absorbed from the growth substrate following the breakdown of complex polymers by e xtracellular enzymes (proteinases, cellulases, pectinases etc.) secreted by fungal cells.
The fungi have diverse morphologies especially in spore production which is the basis for identification. But, they are commonly recognized as the yeasts (single-celled thallus), the molds (filamentous thallus called a mycelium consisting of tubular cells in long, branched, thread-like str uctures called hyphae) and the mushrooms (macroscopic fungi with considerable differentiation of tissues and hyphae in the mushroom, the sporulating portion of the thallus, which is fed by a massive underground mycelium). The fungi like bacteria, are very susceptible to drying and, therefore, are generally found in very most, if not aquatic habitats. They can resist desiccation by producing thick, melanized walls which are often seen in aerial spores, in hyphal strands, and in sclerotia that enable the fungus to survive suboptimal growing conditions in the soil for several years. The funga l wall usually consists of layers of chitin, a linear polysaccharide polymer of N-acetylglucosamine, embedded in and often cove ed by glucans, branched polymers of glucose and other sugars. Some species of fungi in the Class Oomycetes such as Phytophthora and Pythium have cellulose walls rather than chitin. The melanin components consist of branched polymers of phenolic material similar to the lignins of plant cell walls.
2. Ecological role of the fungi
The fungi are ubiquitous in nature andconstitute about 85-90 % of a typical organic soil biomass. Saprophytic fungi are the decomposers and live on dead organic material. They play a major role in the re-cycling of nutrients. Decay by fungi is usually considered in a negative context as the r otting of wood products and textiles or the mold or mildew on paint or bathroom tiles. But, fungal decomposition is benefial as can be seen in the thermophilic composting process in the backyard or the barnyard. Think of the incredible build-up of organic leaf litter in the forest without the sap rophytic activity of the fungi.
Fungi as parasites cause serious diseases (pathogenic) to animals, plants and other fungi. As obligate or biotrophic parasites such as wheat rust, they must obtain nutrients from living cells. They are very host specific and do not kill their hosts right away. As faculative or necrotrophic p arasites they kill the host by toxins prior to or at the time of invasion and then continue to live as saprophytes on the decaying tissues. Some fungi such as the rusts attack weed species and are fine candidates for biological control of unwanted plants. Still other fungi attack insect pests an d other pathogenic fungi in the soil or on plant surfaces. They are important sources of biological control for insect problems and plant disease in forestry, agricultural and horticultural crops.
We are well aware of the mutualistc association between fungi and algae or fungi and cyanobacteria in the lichen thallus on rocks, trees and the forest floor. Fungi can be beneficial also to the growth of plants by forming mutualistic symbiotic associations with roots called mycorrhizae. There are numerous kinds of mycorrhizal associations. Mycorrhizae of different types are ubiquitous on most herbaceous plants and tree species in a wide variety of habitats including agricultural systems.
3. Agricultural impact on fungi
Because fungi are everywhere, agricultural practices have profound effects on their growth, distribution and survival. The impact may be positive or negative. The amendment of soil with organic material will enhance the activity of decomposer fungi in the soil. Some of these fungi may also be antagonistic to fungal pathogens of plants and lead to suppression of disease. In any event, the community structure of fungi will be changed through this enrichment process. However, excessive deep plowing may separate the organic material from the fungal decomposers which occur in the top few c entimeters of the soil. This tillage will lead to a general decline in fungal biomass. The addition of pesticides to the soil will also affect the survival of selected fungi.
Specific fungicides used to destroy specific pathogens may also destroy close relatives of these fungi that may be beneficial as saprophytes. This non-target effect is a particular concern in the use of broad-spectrum biocides such as the fumigant, methyl bromide, which destroys all biological activity. So, using this fumigant to kill plant pathogenic nematodes in greenhouse soil will also kill pathogenic fungi and kill the mycorrhizal fungi that are beneficial to plants. Use of a specific nematicide on the other hand, will kill nematodes but not the mycorrhizal fungi. Non-target eff ects on fungi in the soil are now being identified for herbicides used to kill weeds in conservation tillage practices.
Crop rotation can also influence the distribution of fungi in soil because of the known host specificity of certain species of fungi. Rotation of susceptible and non-susceptible crops is a well-established means of managing disease in crop plants. This technique fails, however, for the control of fungi that produce sclerotial structures or spores that survive for long periods in the soil as portentially infective propagules. Moisture manipulation through irrigation and drainage, or compaction of the soil can affect the activity of fungi. Poor drainage can encourage the growth of soil-b orne water molds that cause root rot of some crops. Similarly, dense plantings create humid environments that encourage the growth of foliar fungi that cause leaf disease and mildews.
4. Mycorrhizae and their significance
Mycorrhizae are mutualistc associations between plant roots and fungi. These beneficial symbioses are ubiquitous in nature and almost all plant species have some form of mycorrhizal association with fungi. Herbaceous and tree species, both deciduous and coniferous, are receptive to infection by mycorrhizal fungi. A few crop plants in the cabbage family (Brassicae) and goose-foot families (Chenopodiaceae) are less likely to be mycorrhizal except in very stressful environments low in soil fertility.
The types of mycorrhizal fungi and the associations they form with roots are varied. These associations are classified as endotrophic (fungus inside roots) or ectotrophic (fungus outside the root forming a sheath of mycelium over the root surface). Still other associations in some forest trees are ectendotrophic or combinations of these types. Other forms exist such as the unique mycorrhizal association with Rhizoctonia-like fungi inside the roots of orchids.
The endomycorrhizal fungi generally associated with the roots of agricultural crops are in the Class Zygomycetes to which the common black bread mold belongs. However, these fungi are obligate symbionts and cannot be cultivated outside the living roots of plants. Their colonization is internal t o the root and cannot be seen without staining and microscopy. The common genera are Glomus and Gigaspora producing large, distinctive azygospores that can be wet sieved from the soil. These spore germinate in the presence on plant roots and infect the outer cortical cells. However, the cell is not killed and although the plant cell wall is penetrated the cell membrane is not disrupted. The endomycorrhizal fungus produces a highly branched hyphal structure called an arbuscule within the plant cell by invaginating its cell membrane. This infection creates an absorptive structure with a v ery high surface area of transfer for nutrients between the plant and the fungus.
The plant usually has few root hairs in this area and the fungus provides the intimate contact with the soil through fine extraradical hyphae which extend several millimeters beyond the root. This extensive hyphal network enhances absorption of water and nutrients, particularly phosphorus, and promotes growth of the plant. In exchange for the phosphorus which is transferred to the plant, the fungus obtains sugars and other organics vital to its growth and reproduction. These exchanges have been verified by various histochemical studies and the use of nutrients labelled with radioisoto pes. As the association begins to senesce, the fungus produces vesiculate storage bodies on and within the root cells and produces the distinctive asexual spores. Sexual reproduction has rarely been observed for these fungi.
The ectomycorrhizal fungi are in the Class Basidiomycetes and Class Ascomycetes, the fungi that we usually identify as wild mushrooms in various forest environments. Genera such as Russula, Lactarius, Laccaria, Amanita, Boletus, and Tuber (truffle) or Cenococcum (false truffle) to name a few, are all ectomycorrhizal. They are facultative symbionts of the roots of forest trees and their colonization can seen with the naked eye. These fungi can be cultivated in the laboratory on special nutrient media. They have limited saprophytic abilities and prefer to grow in association with plant r oots. The mutual benefits are similar to those described for the endomycorrhizae. The ectomycorrhizal fungus froms a sheath or mantle of densely packed hyphae on the surface of tree roots. This mantle is often black or brightly colored. The mantle is connected to highly branched hyphae that pene trate the root and grow between but not into the bost cells. This network of hyphae (hartig net) forms the absorptive structur that is the site of nutrient exchange. The sheath is connected to extraradical hyphae that permeate the soil and absorb water and nutrients for the ectomycorrhizal root.
Other benefits of mycorrhizal associations that are of interest to plant pathologists include biological control and the various growth promotion effects that enhance establishment of plants in the field. Mycorrhizal roots are generally more drought tolerant. The general vigour of mycorrhizal p lants makes them more tolerant of limited root loss due to diseases. Another mechanism of biocontrol by mycorrhizal fungi is the competiton for nutrients and space on the root against the pathogenic soilborne fungi. The ectomycorrhizal fungi have the added advantage of being able to produce antib iotic substances that inhibit the fungal pathogens. Prior colonization by mycorrhizal fungi may also stimulate the root to produce natural defensive wall structures and chemicals (chitinases and phytoalexins) that protect the root from attack by pathogens. The mycorrhizosphere (or area on and aroun d the root of mycorrhizal roots) also contains communities of helpful microorganisms including fungi and bacteria that are anta onistic to pathogens and that solubilize nutrients such as rock phosphate.
There is a great potential for the use of mycorrhizal fungi and associated microorganisms as inoculants especially in the production and protection of high-value greenhouse-grown crops or transplanted vegetable crops such as tomatoes and other bedding plants. The challenge is to produce the inoc ulum of endomycorrhizal fungi that cannot as yet be grown in artificial culture. Various stabilization and embedding or pelleting methods are being developed for coating seeds or inoculating potting media with root fragments and spores. These have been adapted from the technology used to develop R hizobium inoculum for leguminous crops. Peat-based potting media with mycorrhizal inoculum are being developed in Canada by companies such as Premier Peat Moss (Riviere-du-Loupe, Quebec). The production of ectomycorrhizal inoculum for forest nurseries based on solid-substrate fermentation and amen dment of potting media is well-underway in The United States of America and Australia.
5. Biological control of plant disease
Biological control of plant disease is the suppression of disease symtoms and disease incidence by the application of a biological agent, usually a microorganism. It is welcomed as a safe and environmentally acceptable alternative to the use of chemical measures for disease control. Various me chanisms exist including the use of microbial antagonists that produce antibiotics or lytic enzymes, that compete for nutrients with the pathogen, that directly invade and kill the pathogen as hyperparasites, that invade and transmit viral avirulence (hypovirulence) factors, or that are non-pathoge nic but trigger or stimulate natural defence mechanisms in the host (induced ressistance and cross protection). Another biological mechanism is the use of microbial agents to modify the chemical environment (allelopathic biocontrol) through the breakdown of organics in the soil to release antimicr obial compounds such as phenolics or to enhance the chelation of essential nutrients for the pathogen such as iron.
Selection of previous or companion crops may affect the availability of these allelopathic compounds that bring about biological control. More than one mechanism may function at the same time in any biocontrol system. The desireable approach is to integrate biological controls with other contro l measure including limited chemical applications, culture practices (crop rotation, tillage, etc) and host resistance. This approach reduces the rate at which disease resistance can develop. In general, resistance to biological agents develops very slowly because of the complex control mechanism involving numerous biochemical systems and associated genes.
Allen, M.F. (Editor). 1992. Mycorrhizal Functioning: An Integrative Plant-Fungal Process. Chapman and Hall, London
Altman, J.(Editor) 1993. Pesticide Interactions in Crop Production: Beneficial and Deleterious Effects. CRC Press, Boca Raton FL
Carlile, M.J., and Watkinson, S.C. 1994. The Fungi. Academic Press, Inc. New York.
Cook, R.J., and Baker, K.F. (2nd edition). 1983. The Nature and Practice of Biological Control of Plant Pathogens. The American Phytopathological Society Press, St Paul MN.
Harley, J.L. and Smith, S.E. 1983. Mycorrhizal Symbiosis. Academic Press, London.
Jones, D.G. (Editor) 1993. Exploitation of Microorganisms. Chapman and Hall, London.
Metting, F.B. Jr.(Editor). 1993. Soil Microbial Ecology: Applications in Agricultural and Environmental Management. Marcel Dekker, Inc. New York.
Pfleger, F.L., and Linderman, R.G.(Editors) 1994. Mycorrhizae and Plant Health. American Phytopathological Society Press, St Paul, MN .
WFCC World Data Center for Microorganisms (WDCM) provides a comprehensive directory of culture collecions, databases on microbes and cell lines, and the gateway to biodiversity, molecular biology and genome projects. Try the STRAINS - fungi Search Interface
California State University Biological Sciences WWW Server. The purpose of this server is to consolidate existing WWW Biological Science teaching and research resources and to create and distribute original multimedia resources for the teaching of bio logy. Try the FUNGI-related links
Symptoms of Disease - Alberta Agriculture, Food and Rural Development
Root Biology and Mycorrhiza Research Group, Department of Botany, University of Guelph, Ontario, Canada
Mycological Resources on the Internet - Cornell University
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