Mycorrhizal symbioses

a symbiotic association in between plant and fungi

1. Mycorrhizal Symbioses by Fariha Ilyas

2. What is Mycorrhiza? In 1885, a German forest pathologist A. B. Frank discovered it An association, or symbiosis, between plants and fungi that colonize the cortical tissue of roots during plant active growth periods. Characterized by the movement of: Plant produced carbon to the fungus and Fungal-acquired nutrients to the plants. 95% plant species are able to form it. Improve plant growth and yield. 2 Fariha Ilyas

3. Mechanism of Mycorrhiza 3 Fariha Ilyas

4. Global Perspective There is broad generalization about latitude, soil properties, and structure and function different mycorrhizal types that colonize the dominant vegetation in a specific climate zone. Ericoid mycorrhiza: Dominate in acidic soils, having organic matter, can breakdown the organic matter. (Ericaceous plants) Ectomycorrhiza: Colonize coniferous trees. Can also mineralize O.M and Litter layer. Arbuscular mycorrhiza: Dominate in warmer and hot climate. Diversity provides strategies for efficient functioning of mycorrhiza. 4 Fariha Ilyas

5. Vegetation Form of N Mycorrhizal type Organic Nitrate Ammonium Ericoid Ecto Arbuscular Grassland Hardwood forest Conifer forest Heathland Decreasing Latitude or Altitude Depth of Organic Matter key Figure : Changes in dominant vegetation, N form, and Myc. Type with decreasing latitude or altitude 5 Fariha Ilyas

6. Types of Mycorrhizae Ectomycorrhizae Arbuscular mycorrhizae Ericaceous mycorrhizae Orchidaceous mycorrhizae Ectendomycorrhizae 6 Fariha Ilyas

7. 1. Ectomycorrhizae Hyphae present between root cortical cells, producing a netlike structure, hartig net. Have sheath or mantle of fungal tissue that may completely cover the roots of plants. Mantle can vary widely in thickness, color & texture depending upon fungal-plant combination It enhance the surface area of roots Contiguous with mantle are hyphal strand, that extend into soil. Hyphal strand will aggregate to form rhizomorphs. Visible to unaided eye Internal portion of rhizomorphs can differentiate into tubelike structures specialized for long distance transport of nutrients and water. Discovered by “ Robert Hartig” , Father of forest Biology 7 Fariha Ilyas

8. Occurrence: Found on woody plants ranging from shrubs to forest trees. Mostly gymnosperms and some angiosperms. Plants belonging to family Pinaceae, Fagaceae, Betulaceae and Myrtaceae. Nearly 4000 species of fungi, belonging to Basidiomycota and Ascomycota. Many of these fungi form mushrooms and puffballs on forest floor. Some fungi have narrow host range. Boletus betulicola form association on Betula Spp. Some have broad host range. Pisolitbus arbizus , form association with more than 46 trees. 8 Fariha Ilyas

9. 9 Fariha Ilyas

10. 2. Arbuscular Mycorrhizae Mycorrhiza develop highly branched structure within root cortical cells. Initially grows between cortical cells, but soon penetrates in host cells & grows there. Fungus invaginates in host cells, a new compartment is formed where material is deposited  Arbuscules. Avoid direct contact between fungus and plant cytoplasm. Allow efficient transfer of nutrients. Arbuscules are relatively short-lived, less than 15 days. General term for fungus grows within the host cell is Endomycorrhiza 10 Fariha Ilyas

11. Structures Produced by Mycorrhizae 11 Fariha Ilyas

12. Structures Produced by Mycorrhizae Formed in soil. Coiled or knobby Formed from hyphae after spores germination & before mycorrhizae development. Function of these structures is still unknown Auxiliary Cells 12 Fariha Ilyas

13. Structures Produced by Mycorrhizae Either be formed in root or in soil Distinctive, 10µm- >1000µm Color: Hyaline – Black. Texture: Smooth - Rough Asexual: Formed by differentiation of vegetative hyphae, Vesicles (Root)  Secondary thickening, septum is formed across hyphal attachment Reproductive Spores Mycorrhizal Spores 13 Fariha Ilyas

14. 2. Arbuscular Mycorrhizae (Cont.) Classified in the order Glomales (Morton, 1988). Glomales Gigasporineae Glomineae Vesicles absent Acaulospora Entrophospora Glomus & Sclerocystis Gigaspora Scutellospora AM is now the preferred Acronym From end of hyphae From neck of swollen hyphae Forms within the neck of hyphae Spores formation Inner membranous walls & Germination Shield Above structure absent 14 Fariha Ilyas

15. 3. Ericaceous Mycorrhizae Association found in Ericles order of plants. Hyphae in root can penetrate cortical cells, no arbuscules formed. Three major forms of ericaceous mycorrhiza: Ericoid Arbutoid Monotropoid Erica vagans Primula patens 15 Fariha Ilyas

16. Ericoid Mycorrhizae Inner cortex packed with fungal hyphae. Hyphae grows over the root surface but a true mantle is not formed. Found in acidic soils. Fungi belonged to ascomycetes Plants involved in symbiosis: Heather, blueberries, azaleas. Heather Blueberries  Coiled hyphae inside epidermal cells. From Kendrick (1999)  The Fifth Kingdom . Hyphae 16 Fariha Ilyas

17. Arbutoid Mycorrhizae Characteristics of both, i.e. EM and AM. Intercellular penetration occur, mantle forms, hartig net is present. Association is found on Arbutus, Arctostaphylos and several species of Pyrolaceae. Fungi belongs to basidiomycetes and identical to EM fungi. Arbutus menziesii   Native to the western coastal areas of North America, from British Columbia to California Mycorrhizal association showing paraepidermal hartig net (HN) and intracellular hyphae (IH). Reprinted from Smith & Read (1997) Intr. Hyphae 17 Fariha Ilyas

18. Monotropoid Mycorrhizae Fungi colonize “achlorophyllous” plants in Monotropaceae. Produce hartig net and mantle Fungi are basidiomycetes, linked with other plant (EM). Achlorophyllous plant depends upon fungus for its food source. Indian pipe Monotropoid fungi 18 Fariha Ilyas

19. 4. Orchidaceous Mycorrhizae Role in Orchidaceae. In achlorophyllous plant spp. Plant depend upon fungal partner for food. Fungi colonize even after germination, fungus supply C to plant. Fungi grows into the cell, formed coils Short lifespan, after they lose turgor, nutrient contents release in plant cell Plant cell functions normally. Fungi belongs to basidiomycetes, after infection, 3 outcomes are possible: Symbiotic interaction, as described above. Parasitic interaction, embryo dies. Orchid cell rejects infection. Hyphal coils stained red. In mature orchids, mycorrhizae have role in nutrient uptake & transformation 19 Fariha Ilyas

20. 5. Mixed Infection A host can support more than one type of mycorrhizal association. Alnus, willows, populus and eucalyptus can have both AM and EM association on same plant. Intermediate mycorrhizal type can be formed on coniferous and deciduous host. EM structure forms Mantle is thin or absent, hartig net is present in cortical cells. This ectendo -association will replaced with EM as seedling matures. Fungi belongs to ascomycetes 20 Fariha Ilyas

21. Uptake and transfer of Soil Nutrients Root uptake nutrient form soil Create (immobile) Nutrient Depletion Zone Around Roots Mycorrhizae Enhance root surface area Small Diameter of Hyphae Small hyphae can grow in small pores Effective absorption 21 Fariha Ilyas

22. Accessing P Pools Production of Low-mol. Wt. anions, i.e. Oxalate Replace sorbed P, at metal OH surface, through ligand exchange Dissolve Metal oxide surface that sorbed P Complex metals in solution & Prevent P precipitation Mineralization Organic P (C-O-P) Ester bonds P-mediated Hydrolysis Positive correlation b/w phosphatase & hyphae length 22 Fariha Ilyas

23. N Mineralization Ericoid & EM Fungi can get N from secondary sources , i.e. dead microbial biomass Depolymerize Organic N Hydrolytic enzyme action, i.e. Cellulose, pectinase Uptake by plants 23 Fariha Ilyas

24. Carbon Fluxes in Mycorrhizal Plants Mycorrhizal fungi ranges from obligate symbionts (AM) to facultative symbionts (EM). Only obtain C from host plant. Mineralize organic C form nonliving source. EM & Ericoid mycorrhizae transform host carbohydrate ( e.g. mannitol, trehalose ), produce sink. In AM, lipids accumulates in Vesicles, provide analogous sink for host photosynthates. Mycorrhizae take 20% of total plant photosynthates  host plant enhance photosynthetic rate. Extramatrical hyphae secrete some enzymes (Proteases, phosphatase): Mineralize nutrients Binds soil particles. 24 Fariha Ilyas

25. Mycorrhizal vs Non-Mycorrhizal Roots Fariha Ilyas 25 A= Uninoculated, B= Inoculated

26. Interaction With Other Soil Organisms Mycorrhizae interaction may be positive, neutral and negative with other rhizosphere microbes. Mycorrhization helper bacteria stimulate EM formation in conifers nursery. Interaction between AM and Rhizobia in legumes: High Phosphorous demand in N 2 fixation. legume have coarse root texture  inefficient in P uptake Myc. fulfil P nutrition, enhance nitrogenase activity that promotes root and myc. Growth. Negative interaction with root pathogens: Myc. colonize feeder roots, thereby interact with root pathogens (e.g. Fusarium spp.) Myc. protect root from pathogens. 26 Fariha Ilyas

27. Interaction With Other Soil Organisms Mycorrhizae reduce the incidence and severity of root diseases. Following protective mechanisms: Development of mechanical barrier, mantle of EM, to infection by pathogens. Production of antibiotic compounds suppress pathogens. Competition for nutrients with pathogens. Induction of generalization of host defense mechanisms. Fariha Ilyas 27

28. Management of Mycorrhizae Factors should be considered when assessing the potential role of myc. In agroecosystem include: Mycorrhizal Dependency (MD) of Host Crop : this is growth response of myc. (M) versus nonmyc . (NM) plants at a given P level: MD = {( M - NM) / NM )} × 100 Generally coarse rooted plants benefit more than fine rooted plants. Nutrient Status of the Soil : Major benefit of myc. is improved P uptake. High P status of soil is mainly myc. limiting factor. Many tropical soils have low P, & proper myc. colonization for plant to obtain proper P. However increased concern about environmental quality, P based fertilizers use in developed country is reduced. Another factor to consider is interaction b/w water stress & nutrient availability. As soils dry, P become limit even in high available P status soils. Fariha Ilyas 28

29. Management of Mycorrhizae Inoculum Potential : It is a product of abundance & vigor of propagules in soil, can be quantified by determining the rate of colonization of a susceptible host under a standard set of conditions. It can be adversely affected by management practices, like: Fertilizer & lime application Pesticide use (fungicides) Crop rotation Fallowing, tillage Topsoil removal Fariha Ilyas 29

30. Problems & Potential for Inoculum Potential & Use Inoculation is best technique, providing fungi for plant, where native mycorrhizal inoculum potential is low or ineffective. First step in any inoculation is the isolation and selection of mycorrhizal fungus, that is effective and infective. Individual isolates of myc. Vary widely in nature, so screening is important for selection. Screening under natural process is best. Isolation & Production of EM & AM: EM fungi can be culture on artificial media, isolates can be obtained by placing myc. Short root on agar medium. Resulting fungal biomass can be used as inoculum AM fungi will not grow apart from their hosts. Spores can be sieved from soil, surface disinfected & used to initiate pot cultures on host under sterile conditions. Fariha Ilyas 30

31. Use of inoculum The goal of inoculation is to induce propagules of selected mycorrhizal fungi into rhizosphere of target plant. Most common method is to place inoculum below seed or seedling prior to planting. Other strategy include coating seed Fariha Ilyas 31

32. Fariha Ilyas 32