Fungal Symbiosis. Mycorrhizas. Types of Mycorrhiza. Arbuscular Mycorrhiza (AM) Arbuscular Mycorrhiza (AM) Arbuscular Mycorrhiza (AM) 11/23/09

Transcription

1 Mycorrhizas Fungal Symbiosis Cassie Doinoff, Andreal Reed, Ka8e Allen, and Adie Kurtanich Literally means fungus root Fungi form many types of symbio8c associa8ons Roots Underground organs of plants Mutualis8c associa8ons Fungus sends nutrients to plant Plant provides fungus with sugar Types of Mycorrhiza Arbuscular mycorrhizas Ectomycorrhizas Ericoid mycorrhizas Orchid mycorrhizas Monotropoid mycorrhizas Most common type of mycorrhiza Found worldwide Crop plants Trees Herbaceous plants Originally classified as Zygomycota Now classified in new monophyle8c group Glomeromycota Roots show no outward signs of infec8on Hyphae can only be seen by special techniques Differen8al interference contrast microscopy Staining Treat root with strong alkali Stain with fungal dyes such as trypan blue Fungal hyphae extensively colonize roots 1. Grow between root cor8cal cells Produce large swollen vesicles Storage func8on 1

2 Fig 13.1 Fungal hyphae extensively colonize roots 2. Penetrate individual root cor8cal cells Form tree- like branches known as arbuscules Main site of nutrient exchange between fungus and cell root Remain alive for short period of 8me Degenerated and replaced Fig The principal features of arbuscular mycorrhizal (AM) fungi, observed by clearing the root 8ssues with strong alkali and then staining roots with the fungal dye, trypan blue. Top le>: A root heavily colonised by AM fungi, with hyphae that radiate into the soil. Top right: When observed through the depth of the root cortex, AM fungal hyphae are o^en seen to run parallel to the root axis, growing between the root cor8cal cells. These hyphae are irregular, with constric8ons and bulges, quite unlike the hyphae of most other fungi. They frequently produce large, swollen vesicles within the root 8ssues. BoBom le>: some of the external hyphae and hyphal aggregates produce clusters of spores in the soil. BoBom right: Some of the root cor8cal cells are penetrated by hyphae that branch repeatedly to produce intricately branched arbuscules, o^en completely filling the root cells. [! Jim Deacon] Spores Large, up to 400 um Germinate and infect the roots From an appressorium- like infec8on structure on the root surface Ecological Significance of AM AM fungi provides plants with mineral nutrients from soil Especially phosphorous Produce extensive hyphal networks in soil Release acid phosphatases Absorb phosphate in excess and store it in form of polyphosphates Fig 13.4 Host ranges and communi8es of AM fungi AM have a wide range of host Experimental study by Van den Heijden et al. Plants in natural communi8es are colonized preferen8ally by different strains of AM fungi The diversity of AM fungi in a site can influence the plant biodiversity in natural ecosytems Fig An increase in the number of different arbuscular mycorrhizal fungi in a soil leads to an increase in plant produc8vity and plant biodiversity. 2

3 Fig 13.5 Ectomycorrhizas Found mainly on woody plants Coniferous and broad- leaved trees outside of tropics Such as pine, spruce, oak, beech, and birch Members of Basidiomycota or Ascomycota Characteris8c feature Presence of substan8al sheath of fungal 8ssue Incases the terminal nutrient absorbing rootlets Fig Total biomass and the biomass of three representa8ve plant species grown in soil with no mycorrhizal fungus (0) or with four separate AM fungal species (A,B,C,D) or a combina8on of all four AM species. Note that the ver8cal scale of each histogram is different but the largest biomass is shown in each case. Ectomycorrhizas Ecologically adapted to grow as symbionts Can grow in laboratory on simple media Have lifle to no ability to degrade cellulose and lignin Rarely have high degree of host- specificity Ectomycorrhizas Grouped into two types Generalists Wide host range Young trees Such as Laccaria, Hebeloma, Thelephora terrestris Ectomycorrhizas Grouped into two types Mature Host restricted Mature trees Release proteases Providing ectomycorrhiza plants with nitrogen in the form of amino acids Ectomycorrhizas Fungal Networking Extensive network of hyphae and mycelial cords Link different plants within a habitat Mycelial connec8ons retain mineral nutrients Withdraws nutrients from degenera8ng mycorrhizas Transports nutrients to other func8oning mycorrhizas Fungal network extends beyond root zone Mycelial cords transport water from deeper in the soil, beyond the reach of the roots 3

4 Fig 13.7 Fig 13.8 Fig Le>: Scanning electron micrograph of a cross sec8on of part of a mycorrhizal root, showing the fungal sheath that surrounds the root. Right: Thin sec8on of part of an ectomycorrhizal root. The arrowheads show hyphae invading between the root cor8cal cells, forming the Har8g net. Nutrient- exchange between the fungus and the root is thought to occur in this interfacial region. Figure 13.8 A young larch seedling, about 3 cm high, growing in a peatbased substrate against a sloping face of an observa8on chamber. Mycoohizas can be seen at the base of the stem but almost all the visible growth is mycelial cords that explore the soil for nutrients. Ericoid Mycorrhizas Typical host families Ericaceae Epacridaceae Both families produce coils of hyphae termed hair roots Fungi that produce this mycorrhiza are free- living saprotrophs in soil Ascomycota Fig Fig Sec8on through part of the protocorm (basal stem region) of an orchid, Neo4a, showing coils of hyphae (termed 'peletons') within the orchid cells.the plant cells were alive at the 8me of sec8oning, evidenced by presence of nuclei (the dark granular structures) in two of the orchid cells. [! Jim Deacon] Ericoid Mycorrhizas Grown in culture Produce septate hyphae with fragmented zigzag growth Primary role To provide host plants with nitrogen Secrete proteinase Release amino acids from soil organic mafer Orchid Mycorrhizas Parasi8c on fungus Orchid seeds Small, consists of embryo and few nutrient reserves Triggered to germinate Produce few root hairs Must be colonized by a fungus at an early stage or seedling will die 4

5 Orchid Mycorrhizas Fungus penetrates orchid embryo and produces hyphal coils Peletons Only last a few days Degenerate and replaced by further coils in other cells Repeated process provides main source of sugar to developing orchid Orchid Mycorrhizas Provide orchids with sole source of carbohydrates during early years of life Most do not emerge above ground or produce chlorophyll un8l 3-5 years old As many as 200 species never produce chlorophyll Remain dependent on mycorrhizal fungus Monotropoid Mycorrhizas Plants of family Monotropaceae lack chlorophyll Dependent on mycorrhiza fungi for all their nutrients Found in shade beneath forest trees Fungi involved are Basidiomycota Radiate into the soil as hyphal networks or mycelial cords Form hyphal sheath around roots Monotropoid Mycorrhizas Three membered symbiosis Direct nutri8onal connec8on Tree host Mycorrhizal fungus Parasi8c higher plant Summary Lichens Mycorrhizal type Typical host plants Fungi involved Major Significance Arbucular mycorrhizas Many Glomeromycota Phosphorous uptake from soil Ectomycorrhizas Forest trees Basidiomycota, Ascomycota Nitrogen uptake from soil Ericoid mycorrhizas Heathland plants Ascomycota Nitrogen uptake from soil Orchid mycorrhizas Orchids Basidiomycota Fungi supply plant with sugars Monotropoid mycorrhizas Nonphotosynthe8c plants Basidiomycota Plants obtain sugars from ectomycorrhizas Lichens symbio8c associa8ons of fungus & photosynthe8c partner (green alga or cyanobacterium) Involve 2 or more partners Come together to form dis8nct new organism Form long- term symbio8c rela8onships 5

6 Lichen Partners 13,500 to 17,000 species of lichens Difficult to name & classify since composed of 2 or more organisms Mycobiont fungal partner Usually type of Ascomycota or rarely Basidiomycota Photobiont photosynthe8c partner Green alga or cyanobacterium Examples of Lichens Fungal Lichen Partners Lichen fungi are very specialized & mainly found only in lichen partnerships Few species are found in non- symbio8c states Some species have been grown in lab Slow growth Lack enzymes needed to live freely Algal Lichen Partners About 100 types of green algae are lichen partners Trebouxia Most common genus associated with lichen partnerships Single- celled rarely found as free- living Found in temperate regions Trentepohlia Found in tropical regions Cyanobacteria Lichen Partners Found in about 10% of lichens Pel9gera Most common genus Found in dog lichen Some lichens have green algae & cyanobacteria Cephalodia wart- like structures on lichen surface Thought to use nitrogen fixing abili8es of cyanobacteria Forms of Lichens Lichen thallus body of lichen Categories Foliose Flat & leaf- like Fru8cose Erect & branch- like Squamulose Small & scale- like Crustose Flat & crust- like 6

7 Structural OrganizaUon Many have well- defined structures Typical structure Upper cortex Medulla Lower cortex Rhizinae Algal associa8ons also require gas exchange Hydrophobin coated fungal hyphae create spaces for gas exchange Physiology: Water Requirements Lack of water Lichens containing green algae absorb water from humid air & resume photosynthesis Lichens containing cyanobacteria must absorb liquid water to resume photosynthesis Drought tolerance is likely conferred by hydrophobins produced by lichen fungi Physiology: Nutrient Exchange Lichen fungi protect photosynthe8c partner & absorb nutrients Very efficient at absorbing nutrients from limited environmental resources Unfortunately for lichens, also absorb toxins & pollutants Thus, lichens are rarely found in polluted environments Physiology: Nutrient Exchange Photosynthe8c partners provide sugars for the growth of lichen Give up to 90% of photosynthate to fungal partner Carbohydrates found in lichens Mannitol Arabitol Ribitol Glucose ReproducUon & Dispersal Propaga8on In some lichens, fragments break off & are transported by wind or animals Isidia stalk- like structures produced by some lichens that break off Soredia powdery propagules Dispersal of fungal ascospores & reassembly with photosynthe8c partner Ecology & Significance Lichens can live in a variety of environments From tundra to cooled lava flows to tropical forests Most significant role of lichens Contribu8on to soil forma8on 7

8 Summary Geosiphon pysiform Symbio8c associa8ons between fungal, cyanobacterial &/or algal partners Can survive in many environments Have defined structures Have several ways to propagate Significant contribu8on to soil forma8on Unique rela8onship between mycorrhizal and cyanobacteria Duel organism Discovered in Germany 1996 Belongs to the arbuscular mycorrhizal (AM) and related fungi Bladders of Geosiphon pyriforme growing on the surface of soil. Courtesy of A. Schuessler Geosiphon pysiform Geosiphon pysiform Endocytose cyanobacteria Leads to a membrane bound symbiosome containing cyanobacteria Provides fungus with source of sugar Symbio8c Cyanobacteria depends on phosphate from fungus Mutualis8c Image courtesy of A. Scheussler & M. Kluge, 2001 Fungus/Insect Mutualism Several insects have mutalis8c associa8ons with fungi Provide the insect with a food source Insect ensures mutualism Carrying and transmiqng the fungus Mycangia Inoculate suitable substrate with spores on the fungus Tending the substrate to promote growth of fungus Food source Sirex Wood Wasp Female injects toxic mucus and Amylostereum areolatum Laying eggs in bark of damaged trees Mucus kills tree cells Fungus feeds on the killed wood Insect larva feed on fungus A^er pupa8on, the female adults collect the spores from the roqng wood Store in mycangia to repeat the cycle 8

9 Leaf- Cu^ng Ants The wood wasp, Sirex noc9lio, boring a hole in a weakened tree to deposit eggs and fungal spores. Courtesy of M. P. CouDs, J. E. Dolezal and the University of Tasmania Aqne Ants Produce large nests Carries fungus, Leucoagricus, in pouch of mouth Deposits fungus on suitable plant Eggs laid as fungus grows Worker ants bring leaf pieces back to nest, inoculate with fungus Fungus produces hyphae Gongylidia or broma8a Fed to larvae Provide enzymes needed for degrading plants Gardening Termites Do not digest wood Evolved symbiosis with the Termitomyces Found in fungus gardens Termites weed out any contaminant fungi Heavy rainfall Fungus produces large, mushroom fruitbodies Wood Boring Beetle Produce tunnels in damaged trees Deposit eggs Smear with ambrosia fungi Spores stored in special sacs of beetle Mycangia Developing beetle larvae Create galleries beneath bark Feed on fungus Fungus/Insect Mutualism About million years ago, these three types independently evolved the ability to culture fungi as a source of food. Now all of these fungus- cul8va8ng insects and their fungal partners are mutually dependent on another. There is no evidence that they have ever reverted to an independent existence. QuesUons 1. Which fungal phylum is associated with arbuscular mycorrhizas? a. Zygomycota b. Ascomycota c. Basidiomycota d. Glomeromycota 9

10 QuesUons 2. T/F Ectomycorrhizal fungi are classified as either mature or immature. 3. T/F The primary role of the ericoid mycorrhizas are to provide the host plants with sugar. QuesUons 4. Stalk- like structures that aid in propaga8on a. Soredia b. Cephalodia c. Isidia d. Rhizinae QuesUons 5. Lichens that produce flat crusts on rock, soil, or tree surfaces a. Squamulose b. Fru8cose c. Foliose d. Crustose Sources Deacon, J. Fungal Biology. Cambridge, MA. Blackwell hfp:// lichens.html www- biol.paisley.ac.uk/biorefchlorophyta www3.isrl.uiuc.edu/~openkeyalgae/image/ Papilla- pore?n=d hfp:// 10