Ecological Associations of Hymenomycetes: Wood Decay and Mycorrhizae

1. Ecological Associations of Hymenomycetes: Wood Decay and Mycorrhizae Pl P 421/521 General Mycology

2. Fungi as agents of decomposition Host death Epiphytic fungi Endophytes, weak parasites Pioneer saprotrophic fungi Polymer-degrading fungi Secondary opportunistic fungi Degraders of recalcitrant compounds

3. Epiphytes • Phyllosphere: Bacteria and yeasts growing on soluble nutrients “leaking” from host tissue • Many have carotenoid pigments, capsules or slime for protection and adhesion

4. Endophytes and weak parasites • Endophytes grow in living plant tissue • Many taxa • Weak parasites colonize living tissue, or tissue just starting to senesce • Common parasites include Cladosporium, Alternaria, Colletotrichum

5. Pioneer saprotrophic fungi • Utilize sugars and other simple soluble nutrients • Germinate and grow rapidly • Cannot compete well with other fungi • Includes Mucor, Rhizopus, Pythium

6. Polymer-degrading fungi • Extended growth on structural polymers (cellulose, hemicellulose, chitin) • Defend resource against invaders either through sequestering nutrients or producing antibiotics Includes Fusarium, Chaetomium, Stachybotrys, Trichoderma

7. Fungi that degrade resistant polymers • Utilize cellulose, hemicellulose and lignin in dead plant material • Includes wide range of saprotrophic and wood decay ascomycetes and basidiomycetes

8. Wood Decay—White Rot • Cellulose, lignin and hemicellulose removed at approx. equal rates • Wood becomes pale (bleached) and stringy • Predominant form of wood rot • Fungi are the only organisms able to completely degrade lignin through the production of relatively few enzymes • e.g., laccase, lignin peroxidase, manganese peroxidase http://www.anbg.gov.au/fungi/images/0122.jpg

9. Wood Decay--Brown Rot • Cellulose and hemicellulose are selectively removed by fungus, lignin is slightly modified • Cellulose degraded by oxidative process involving production of hydrogen peroxide • Wood becomes dark and cubical • Brown rot fungi comprise 10% of wood decay taxa, but 80% of brown rot fungi occur on conifers http://forestpathology.coafes.umn.edu

10. Brown Rot • Residues are highly resistant to decomposition • Can remain in the soil as humus for 300 years • Make up 30% of the soil volume in upper layers of conifer forest soils • Soil layers with brown rot residues are the major sites of ectomycorrhizal development

11. Mycorrhiza • Symbiotic association involving mycobiont and photobiont • Mycobiont obtains carbohydrates, vitamins, and spore germination stimulated by root exudates • Up to 20% of the host’s fixed carbon can be “captured” by the fungus • Plant obtains benefits from increased mineral and nutrient uptake, increased water uptake, and protection from root pathogen • up to 80% of plant’s phosphorus needs and 25% of its nitrogen obtained via the fungus

12. Types of mycorrhiza • Endomycorrhiza • Ectomycorrhiza • Ericaceous mycorrhiza • Orchid mycorrhiza

13. Endomycorrhizae • Glomeromycota • 154 species in 7 genera • ubiquitous and physiologically unspecialized • Formed by 80-90% of all vascular plant species

14. Arbuscular Mycorrhizae • Intracellular arbuscules • highly conserved structures • point of exchange between host plant and fungus

15. Features of AM Fungi • Hyphae • Within root (intraradical) and outside root (extraradical) • Arbuscules • Highly branched, thin-walled structures within host cell, short-lived, become digested by plant • Spores • Asexual, in soil or roots, may contain hundreds of nuclei • Storage organs • vesicles or auxilliary cells

16. Arbuscule in host plant cell

17. Spores of Glomus spp.

18. Auxilliary cells Vesicles in host root

19. Ancient Association • AM symbiosis estimated to have originated > 400 million years ago based on fossil record • The fossil record also indicates that the structure of the mycorrhiza has not changed over the course of time

21. Glomites rhyniensis • Fungal hyphae (f) and arbuscules penetrating the outer cortex of an Aglaophyton major stem (scale bar = 100µm) http://www.abdn.ac.uk/rhynie/fungi.htm

22. http://www.xs4all.nl/~steurh/engrhyn/eglomit2.html

23. Aglaophyton from Rhynie Chert • These plants were small (< 0.5 m), and simply structured, lacking leaves and roots • Early Devonian, ~ 408-360 million years ago 16 cm tall max., stem diam 1.5-6 mm http://www.xs4all.nl/~steurh/engrhyn/erhynie.html

24. Ectomycorrhizas • Infection of actively growing secondary roots by spores, hyphae, root-to-root contact • Results in formation of sheathing mantle up to 40 microns thick, extraradical hyphae, and intercellular Hartig net

26. EM roots • Infected roots are short, broad, branched • Branching may increase root surface area 4-fold • Maintained in juvenile condition, no suberization and no root hairs • Rhizomorphs and hyphae extending from the mantle (extraradical mycelium) increase absorption area >40-fold

28. Hebeloma (above) and Tricholoma magnivelare (below) Photos by Randy Molina

29. Mycobionts • 5000-6000 species • Most species belong in Agaricales, Boletales and Gasteromycetes, some belong in Tuberales (Ascomycota), at least one loculoascomycete (Cenococcum), and Endogone (Zygomycota) • ~4500 are epigeous • Most species can be grown in vitro (mycelium)

30. Cenococcum • Dark sterile mycelial fungus that produces sclerotia • Widespread EM fungus • Shown by sequence analysis to belong to loculoascomycetes

31. Rhizopogon on Tsuga mertensiana (B. Zak) Lactarius deliciosus on madrone (Randy Molina) Lactarius deliciousus (Fred Stevens)

32. Cantharellus ectomycorrhizal root photo by Eric Danell

33. Host specificity • Wide EM host potential, low specificity • Examples: Amanita muscaria, Boletus edulis, Laccaria laccata, Lactarius deliciosus, Paxillus involutus, Pisolithus arhizus • Intermediate host potential, specific or limited in basidiocarp/host association • Example: Suillus spp.with conifers • Narrow host potential • Examples: Cortinarius pistorius and Rhizopogoncokeri with Pinus spp.

34. Ericaceous mycorrhizas • Ericaceae often grow in nutrient-poor, acidic soils; these plants have unusually fine roots • Ericoid type with endophytic mycobiont forming intracellular hyphal coils in epidermal cells with members of tribes Ericeae, Vaccineae, Rhododendreae, Calluneae • Ascomycetous fungi, including Hymenoscyphus (discomycete) and Oidiodendron (deuteromycete)

35. Arbutoid type • Ectendotrophic mycelium growing within epidermal cells, forming hyphal coils, and ensheathing root tissue • Mycobionts similar to those forming EM

36. Monotropoid mycorrhizas • Monotropa (Indian pipe) lacks chlorophyll • Seeds germinate poorly, require external source of carbohydrates provided by fungal symbiont (Suillus, Rhizopogon, Russula, Boletus) • Fungus provides link to photosynthetic host (tree)

37. Orchids and their mycobionts The orchid family is one of the largest families of flowering plants, with over 23,000 species, distributed mainly in the tropics. The family includes both terrestrial and epiphytic species.

38. Epiphytic Orchids

39. Terrestrial Orchids Calypso bulbosa Blettia striata

40. Orchid Seed • Orchids produce millions of tiny, dust-like seed that lack endosperm

41. Orchid mycorrhizas • All orchid seed require infection by a species of Rhizoctonia in order to germinate under natural conditions

42. Orchids have a prolonged seedling stage during which they are unable to photosynthesize, and obtain their nutrients via the fungus

43. Commercial Orchid Cultivation Orchids are cultivated under sterile conditions; seed are germinated on nutrient agar, and protocorms are grown in flasks with nutrients supplied via agar.

44. Orchid mycorrhizas • Two types of mycobionts associated with orchids • Intercellular Rhizoctonia-like mycobiont forms hyphal coils (pelotons) and supplies seedling with carbon during heterotrophic state • Mature plants of nonchlorophyllous plants form second type of mycorrhiza with clamp-forming fungus that forms mantle and Hartig net

45. Pelotons inside root cells of mature orchids; hyphal coils break down to release nutrients to the plant cell

46. Coralroot (Corallorhiza) A temperate orchid that lacks chlorophyll and obtains its nutrients from another plant via mycorrhizal fungus