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| Frequently Asked Questions About Fungi and Mycorrhizae= |
Contact: Dr. James Tra=
quair
FAQ Menu URL: http://= res.agr.ca/lond/pmrc/faq/menu.html
1. What are fungi and how do they differ fro= m bacteria ?
2. What is the ecological= role of fungi in soil and on plant surfaces ?
3. How do agricultural pr= actices affect the fungi in soil ?
4. What are mycorrh= izae and what is their role in agriculture ?
5. What is biologi= cal control in the context of plant disease management ?
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Special Topics:
- Microbiology of Composting
- Rhizosphere Microbiology
- Nutrient Cycling
- Non-target Effects of Pesticides
- Allelochemistry
- Crop Residue Management
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Key Words:
1. eukaryote, filamentous, yeast, chitinous walls, glucans, conidia, s= pores, sclerotia
2. saprophyte, parasite, pathogen, biotroph, necrotroph, mildew, root = rot, damping off ,antibiosis, mutualism, symbiosis, Rhizoctonia solani, C= ylindrocarpon destructans, Pythium spp., Thielaviopsis elegans, nutrient = cycling
=3. crop rotation, selection pressure, amendment, enrichment, anaerobic= , tillage, pesticide residue, drainage,host specificity, propagule surviv= al, dispersal
4. Glomus spp.. Gigaspora spp., vesicular-arbuscular mycorrhizae, endo= mycorrhizae, ectomycorrhizae, growth promotion, mutualism, nutrient absor= ption, fertility, phosphorus azygospore, chlamydospore, disease suppressi= on, drought tolerance,carrier, agarose beads, pelletization, peat-based p= otting 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, competi= tion, allelopathy, chelation
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Brief Responses to FAQs:
Several diverse microorganisms are classified within the broad gr= oup of protists called the fungi. However, they do share some typical fe= atures that distinguish them as fungi different from other microbes. Thes= e distinguishing features are based on the physiology and morphology of c= ells. The fungal cell is distinguished from the bacterial cell by its si= ze (generally greater than 1-2 um in diameter) and its eukaryotic struct= ure similar to that of plant and animal cells (cells having distinct memb= rane-bound organelles such as nuclei, mitochondria). Bacterial cells whi= ch lack these organelles are termed prokaryotic. Fungal cells divide by = mitosis (asexual reproduction) and by meiosis (sexual reproduction); bact= erial cells divide by binary fission.
Like animals, the fungi are heterotrophic organisms that cannot m= anufacture their own food by photosynthesis as plants and algae can. They= require oxygen for growth (aerobic) and generally prefer an acidic envir= onment (below pH of 7) unlike the bacteria which are anaerobic and aerobi= c and generally grow in basic environments ( at or above pH of 7). Fungi = utilize preformed organic material from other organisms as sources of ene= rgy and building blocks for their cellular sysnthesis. Soluble nutrients = are absorbed from the growth substrate following the breakdown of complex= polymers by extracellular enzymes (proteinases, cellulases, pectinases e= tc.) secreted by fungal cells.
The fungi have diverse morphologies especially in spore productio= n which is the basis for identification. But, they are commonly recogniz= ed as the yeasts (single-celled thallus), the molds (filamentous thallus = called a mycelium consisting of tubular cells in long, branched, thread-l= ike structures called hyphae) and the mushrooms (macroscopic fungi with c= onsiderable differentiation of tissues and hyphae in the mushroom, the sp= orulating portion of the thallus, which is fed by a massive underground m= ycelium). The fungi like bacteria, are very susceptible to drying and, t= herefore, are generally found in very most, if not aquatic habitats. The= y can resist desiccation by producing thick, melanized walls which are of= ten seen in aerial spores, in hyphal strands, and in sclerotia that enabl= e the fungus to survive suboptimal growing conditions in the soil for sev= eral years. The fungal wall usually consists of layers of chitin, a linea= r polysaccharide polymer of N-acetylglucosamine, embedded in and often co= ve ed by glucans, branched polymers of glucose and other sugars. Some speci= es of fungi in the Class Oomycetes such as Phytophthora and Pythium have = cellulose walls rather than chitin. The melanin components consist of br= anched polymers of phenolic material similar to the lignins of plant cell= walls.
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2. Ecological role of the fungi
The fungi are ubiquitous in nature andconstitute about 85-90 % of a ty= pical organic soil biomass. Saprophytic fungi are the decomposers and liv= e 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 a= s the rotting of wood products and textiles or the mold or mildew on pain= t or bathroom tiles. But, fungal decomposition is benefial as can be see= n in the thermophilic composting process in the backyard or the barnyard.= Think of the incredible build-up of organic leaf litter in the forest w= ithout the saprophytic activity of the fungi.
Fungi as parasites cause serious diseases (pathogenic) to animals, pla= nts and other fungi. As obligate or biotrophic parasites such as wheat r= ust, they must obtain nutrients from living cells. They are very host s= pecific and do not kill their hosts right away. As faculative or necrotr= ophic parasites they kill the host by toxins prior to or at the time of i= nvasion and then continue to live as saprophytes on the decaying tissues.= Some fungi such as the rusts attack weed species and are fine candidat= es for biological control of unwanted plants. Still other fungi attack i= nsect pests and 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 alga= e or fungi and cyanobacteria in the lichen thallus on rocks, trees and th= e 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 d= ifferent types are ubiquitous on most herbaceous plants and tree species = in a wide variety of habitats including agricultural systems.
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3. Agricultural impact on fungi
Because fungi are everywhere, agricultural practices have profound eff= ects on their growth, distribution and survival. The impact may be posit= ive or negative. The amendment of soil with organic material will enhanc= e the activity of decomposer fungi in the soil. Some of these fungi may = also be antagonistic to fungal pathogens of plants and lead to suppressio= n of disease. In any event, the community structure of fungi will be cha= nged through this enrichment process. However, excessive deep plowing may= separate the organic material from the fungal decomposers which occur in= the top few centimeters of the soil. This tillage will lead to a general= decline in fungal biomass. The addition of pesticides to the soil will a= lso affect the survival of selected fungi.
Specific fungicides used to destroy specific pathogens may also destro= y close relatives of these fungi that may be beneficial as saprophytes. = This non-target effect is a particular concern in the use of broad-spectr= um biocides such as the fumigant, methyl bromide, which destroys all biol= ogical activity. So, using this fumigant to kill plant pathogenic nemato= des in greenhouse soil will also kill pathogenic fungi and kill the mycor= rhizal 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 effects on fungi in the soil are now being identified for herb= icides used to kill weeds in conservation tillage practices.
Crop rotation can also influence the distribution of fungi in soil bec= ause 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 surviv= e for long periods in the soil as portentially infective propagules. Mois= ture manipulation through irrigation and drainage, or compaction of the s= oil can affect the activity of fungi. Poor drainage can encourage the gr= owth of soil-borne water molds that cause root rot of some crops. Similar= ly, dense plantings create humid environments that encourage the growth o= f foliar fungi that cause leaf disease and mildews.
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4. Mycorrhizae and their significance
Mycorrhizae are mutualistc associations between plant roots and fungi.= These beneficial symbioses are ubiquitous in nature and almost all plan= t species have some form of mycorrhizal association with fungi. Herbaceou= s and tree species, both deciduous and coniferous, are receptive to infec= tion by mycorrhizal fungi. A few crop plants in the cabbage family (Bras= sicae) and goose-foot families (Chenopodiaceae) are less likely to be myc= orrhizal except in very stressful environments low in soil fertility.
=The types of mycorrhizal fungi and the associations they form with roo= ts 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 fores= t trees are ectendotrophic or combinations of these types. Other forms e= xist such as the unique mycorrhizal association with Rhizoctonia-like fun= gi inside the roots of orchids.
The endomycorrhizal fungi generally associated with the roots of agric= ultural crops are in the Class Zygomycetes to which the common black brea= d mold belongs. However, these fungi are obligate symbionts and cannot be= cultivated outside the living roots of plants. Their colonization is int= ernal to the root and cannot be seen without staining and microscopy. Th= e common genera are Glomus and Gigaspora producing large, distinctive azy= gospores 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 t= he cell membrane is not disrupted. The endomycorrhizal fungus produces a= highly branched hyphal structure called an arbuscule within the plant ce= ll by invaginating its cell membrane. This infection creates an absorpti= ve structure with a very high surface area of transfer for nutrients betw= een the plant and the fungus.
The plant usually has few root hairs in this area and the fungus provi= des 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 phosphor= us, and promotes growth of the plant. In exchange for the phosphorus whi= ch is transferred to the plant, the fungus obtains sugars and other organ= ics vital to its growth and reproduction. These exchanges have been veri= fied by various histochemical studies and the use of nutrients labelled w= ith radioisotopes. As the association begins to senesce, the fungus prod= uces vesiculate storage bodies on and within the root cells and produces = the distinctive asexual spores. Sexual reproduction has rarely been obse= rved for these fungi.
The ectomycorrhizal fungi are in the Class Basidiomycetes and Class As= comycetes, the fungi that we usually identify as wild mushrooms in vario= us forest environments. Genera such as Russula, Lactarius, Laccaria, Ama= nita, Boletus, and Tuber (truffle) or Cenococcum (false truffle) to name = a few, are all ectomycorrhizal. They are facultative symbionts of the roo= ts of forest trees and their colonization can seen with the naked eye. T= hese fungi can be cultivated in the laboratory on special nutrient media.= They have limited saprophytic abilities and prefer to grow in associatio= n with plant roots. The mutual benefits are similar to those described f= or the endomycorrhizae. The ectomycorrhizal fungus froms a sheath or man= tle of densely packed hyphae on the surface of tree roots. This mantle is= often black or brightly colored. The mantle is connected to highly bran= ched hyphae that penetrate the root and grow between but not into the bos= t cells. This network of hyphae (hartig net) forms the absorptive struct= ur that is the site of nutrient exchange. The sheath is connected to extrar= adical hyphae that permeate the soil and absorb water and nutrients for t= he ectomycorrhizal root.
Other benefits of mycorrhizal associations that are of interest to pla= nt pathologists include biological control and the various growth promoti= on effects that enhance establishment of plants in the field. Mycorrhizal= roots are generally more drought tolerant. The general vigour of mycorr= hizal plants makes them more tolerant of limited root loss due to disease= s. Another mechanism of biocontrol by mycorrhizal fungi is the competito= n for nutrients and space on the root against the pathogenic soilborne fu= ngi. The ectomycorrhizal fungi have the added advantage of being able to= produce antibiotic substances that inhibit the fungal pathogens. Prior c= olonization by mycorrhizal fungi may also stimulate the root to produce n= atural defensive wall structures and chemicals (chitinases and phytoalexi= ns) that protect the root from attack by pathogens. The mycorrhizosphere = (or area on and around the root of mycorrhizal roots) also contains commu= nities of helpful microorganisms including fungi and bacteria that are an= ta onistic to pathogens and that solubilize nutrients such as rock phosphate= =2E
There is a great potential for the use of mycorrhizal fungi and associ= ated microorganisms as inoculants especially in the production and protec= tion of high-value greenhouse-grown crops or transplanted vegetable crops= such as tomatoes and other bedding plants. The challenge is to produce t= he inoculum of endomycorrhizal fungi that cannot as yet be grown in artif= icial culture. Various stabilization and embedding or pelleting methods = are being developed for coating seeds or inoculating potting media with r= oot fragments and spores. These have been adapted from the technology use= d to develop Rhizobium inoculum for leguminous crops. Peat-based potting = media with mycorrhizal inoculum are being developed in Canada by companie= s such as Premier Peat Moss (Riviere-du-Loupe, Quebec). The production o= f ectomycorrhizal inoculum for forest nurseries based on solid-substrate = fermentation and amendment of potting media is well-underway in The Unite= d States of America and Australia.
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5. Biological control of plant disease
Biological control of plant disease is the suppression of disease symt= oms and disease incidence by the application of a biological agent, usual= ly a microorganism. It is welcomed as a safe and environmentally accepta= ble alternative to the use of chemical measures for disease control. Var= ious mechanisms exist including the use of microbial antagonists that pro= duce antibiotics or lytic enzymes, that compete for nutrients with the pa= thogen, that directly invade and kill the pathogen as hyperparasites, tha= t invade and transmit viral avirulence (hypovirulence) factors, or that a= re non-pathogenic but trigger or stimulate natural defence mechanisms in = the host (induced ressistance and cross protection). Another biological m= echanism is the use of microbial agents to modify the chemical environmen= t (allelopathic biocontrol) through the breakdown of organics in the soi= l to release antimicrobial 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 o= f these allelopathic compounds that bring about biological control. More= than one mechanism may function at the same time in any biocontrol syste= m. The desireable approach is to integrate biological controls with other= control measure including limited chemical applications, culture practic= es (crop rotation, tillage, etc) and host resistance. This approach redu= ces the rate at which disease resistance can develop. In general, resist= ance to biological agents develops very slowly because of the complex con= trol mechanism involving numerous biochemical systems and associated gene= s.
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Selected References:
=Allen, M.F. (Editor). 1992. Mycorrhizal Functioning: An Integrative Pl= ant-Fungal Process. Chapman and Hall, London
Altman, J.(Editor) 1993. Pesticide Interactions in Crop Production: Be= neficial and Deleterious Effects. CRC Press, Boca Raton FL
Carlile, M.J., and Watkinson, S.C. 1994. The Fungi. Academic Pres= s, Inc. New York.
Cook, R.J., and Baker, K.F. (2nd edition). 1983. The Nature and Practi= ce of Biological Control of Plant Pathogens. The American Phytopatho= logical Society Press, St Paul MN.
Harley, J.L. and Smith, S.E. 1983. Mycorrhizal Symbiosis. Academi= c 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 Plan= t Health. American Phytopathological Society Press, St Paul, MN .
Other Related WEB Links
WFCC World Data Center for Mi= croorganisms (WDCM) provides a comprehensive directory of culture col= lecions, databases on microbes and cell lines, and the gateway to biodive= rsity, molecular biology and genome projects. Try the STRAINS - fungi Search Inter= face
California State University= Biological Sciences WWW Server. The purpose of this server is to con= solidate existing WWW Biological Science teaching and research resources = and to create and distribute original multimedia resources for the teachi= ng of biology. Try the FUNG= I-related links
S= ymptoms of Disease - Alberta Agriculture, Food and Rural Development =
Root Biology = and Mycorrhiza Research Group, Department of Botany, University of G= uelph, Ontario, Canada
Mycologica= l Resources on the Internet - Cornell University
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Last Revised: Thursday, August 10, 1995
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