1. Associations (symbiotic ?) between plant roots and fungi.
Mycorrhizae
Associations (symbiotic ?) between plant roots and fungi.
“Most woody plants require mycorrhizae to survive;
most herbaceous plants need them to thrive”

2. Mycorrhizae
plants leak complex nutrients (sugars, amino-acids, vitamins)
fungi store above compounds and provide efficient water and mineral gathering - esp. in poor soils. eg PO4, Fe
Mycorrhizae very important - > 90% of plants depend on them - some absolutely - some partially (helps counter stress due to pollution or environment, need less fertilizer, grow in more extreme conditions etc).

3. Mycorrhizae 1. Ectotrophic mycorrhizae (EM)
grow on exterior and between cortex cells but never penetrate root cells
2000 sp of plants incl. many important trees eg pines, firs, oaks, beeches and eucalypts depend on EM.
about 5000 fungal sp mostly Holobasidiomycetes + a few Ascomycetes

4. Mycorrhizae
2. Endotrophic or vesicular-arbuscular mycorrhizae (VAM or just AM)
penetrate inside cells of cortex of root
over 300,000 sp of plants - only a few plant families lack VAM (e.g Brassicaceae)
only 130 sp of fungi so far - all obligately mycorrhizal - mostly asexual Zygomycetes in the order GLOMALES
BUT taxonomy confused - may be many more species.

6. Ectotrophic Mycorrhizae
spores or hyphae establish dense infection on root surface (mantle)
some hyphae penetrate between cortical cells forming the Hartig net - the main interface for exchange
fungal hormones make root cells grow to become thicker and branched, root hairs suppressed - result = short stubby roots + extensive fungal hyphae
fungal mantle can act as a sink - store plant sugars etc. and also soil minerals
a single tree may have several sp. of EM fungi associated with it.

7. Ectotrophic Mycorrhizae
Mycobiont usually a Holobasidiomycete (eg Amanita), sometimes an Ascomycete (eg Tuber - truffles) - fruit bodies develop above or below ground
Some fungi have wide host range, others species specific.
Hosts in Gymnosperms (pines and other conifers), Angiosperms (oaks and other hardwoods) - mostly woody plants.
Selection and encouragement of EM very important in forestry.
How to select best EM ? - see text pages

8. Larix laricina(larch) with Suillus cavipes mycobiont
Pinus strobus with. Amanita muscaria
short stubby roots
fungus hyphae

9. Douglas fir tree 100 feet (30 metres) tall will probably have about 108 ectomycorrhizas. These may collectively involve ten or a dozen different fungi
Individual ectomycorrhizas remain active for up to 3 years.
Forests seem to consist of a mass of interconnected organisms, cooperating and competing together. A single 'clone' of Armillaria ostoyae has colonized an area of about acres in Washington State, and probably has a mass of hundreds of tons.

10. Short stubby roots (often branched) after infection
dichotomously branched ectomycorrhizas of a
basidiomycete with a conifer.
Short stubby roots (often branched) after infection
ectomycorrhizas of Laccaria bicolor with Populus tremuloides.

11. Transverse sections showing Hartig Net
Side view of cortical cell surface showing dense coverage of the hyphae (Hartig Net)

12. Sugars translocated from root to fungal mantle - converted into trehalose, mannitol and glycogen
The glycogen is insoluble - unavailable for possible reabsorption by the plant.
Mannitol and trehalose remain in solution in the fungus, but the tree cannot reabsorb them. Fungus acts as a sink storing carbohydrates .
So (1) as autumn approaches, fungi mobilize stored carbohydrates and produce flushes of large mushrooms near the tree. (2) Carbohydrates can be translocated through mycelia from established trees to seedlings of the same or different species. (3) The tree can reclaim some of the stored energy and minerals if conditions become appropriate for a new surge of growth..
Flush of fruit bodies from the extensive underground hyphae of the mycobiont

13. Seedlings of Douglas fir with and without ectomycorrhizal partners .
Photosynthetically fixed carbon has been shown to move from birch to adjacent Douglas fir trees (they have at least 10 mycobionts in common).

14. Ectomycorrhizae may also utilize N from animals
Recent research (Guelph):
fungus Laccaria bicolor on Pinus trees can immobilize and infect Springtails. (Other fungi cannot).
Use of N-labelled springtails (dead or alive) established that the N was ending up in the pine tree ( up to 25% of all plant N acquired).
Expts. with other ectomycorrhizal fungi indicated that they were only able to get N from dead fungi.
Klironomos and Hart, Nature, April 5, 2001

15. Vesicular-Arbuscular Mycorrhizae (VAM) (Endotrophic)
not detectable to naked eye (no short, stubby and branched roots like EM), no flush of mushrooms
Fungus infects root or root hair, grows into cortical region and penetrates cells, developing ‘tree-like’ highly branched hyphae inside - arbuscules - main interface for nutrient exchange.
most sp. produce large vesicles - full of lipids

16. Vesicular-Arbuscular Mycorrhizae (VAM) (Endotrophic)
Like the EM types, VAM fungi develop an extensive network of hyphae in a wide zone round root
has been estimated that 1gm of soil may contain 14km. of hyphae !!!.
20-25% of the soil volume near roots may be hyphae
fungus may have a very wide host range - but situation confused by poor taxonomy (what is a species ?)
Usually not found on those hosts that have EM
Types of VAM present in soil + the pathogens - may be the key factor determining plant diversity at that site.

18. Extramatrical hyphae of Glomus mosseae, an AM fungus - these allow a mycorrhizal plant to exploit several times the volume of soil available to a non-mycorrhizal plant. These hyphae obtain phosphorus and other minerals, plus water for the plant. In return they receive photosynthetic products.

19. Vesicular-Arbuscular Mycorrhizae (VAM) (Endotrophic)
Mycobiont - in order Glomales (Zygomycetes) - now raised to phylum Glomeromycota
most have no sexual stage
produce large lipid filled asexual spores in soil (are vesicles spores in root ?).
obligately biotrophic - can’t be grown in pure culture
VAM not usually essential but plants do much better with them (particularly in less fertile situations)
Plant nurseries need to find ways to ensure good infection with VAM - see pages

20. arbuscles inside host cortical cells
hyphae growing between host cells

21. Close-up of arbuscules

22. Vesicles - large inflated - used for lipid storage

23. Above left are asexual spores of Glomus versiforme, sieved from the soil. On the right, a single spore of Glomus mosseae.
Spores usually form in the soil singly, but some taxa develop aggregations up to 2 cm in diameter called sporocarps
Spores germinate - hyphae - enter new host (not host specific sp. of fungi, 300,000 potential host species))
These VAM fungi in the Glomales are unusual Zygomycetes in that they lack sexual spores (zygospores).

24. Other types of mycorrhizae
Orchidaceae - very tiny seeds - for first 2-11 yrs couldn’t survive without fungus (basidiomycete) partner - till first green leaf. The seed may either:-
be colonized and thrive;
be parasitized and die;
or fungal invasion fails and seed dies
Coils not arbuscules)
Basidiomycetes fungus - but forms endomycorrhizae (penetrates cells of cortex)

25. Other types of mycorrhizae
Ericaceae - mycorrhizae intermediate between EM and VAM - have mantle plus stubby roots as in EM but also penetrate cortical cells as in VAM
Coils not arbuscules)

26. Other types of mycorrhizae
Monotropa - has no chlorophyll. Monotropa seems to parasitize pines using the fungus as intermediate !
mycorrhizae have mantles, Hartig Net, but penetrate cortex cells via a peg.
A COMPLEX RELATIONSHIP. Pine tree makes sugars and passes them to fungus (e.g. Boletus). Fungus translocates them through its mycelium and hands over part to the Monotropa. Thus Monotropa seems to exploit the fungus directly, and the pine at second hand.
peg

27. Endophytes
Estimated that most plants carry non-symptomatic fungi in their above ground portions - endophytes. Beneficial, even necessary for growth ?
recent work in turfgrasses suggest that such endophytes confer some benefits:-
increased growth, increased drought tolerance (eg arid lands and deserts), and resistance to certain insects. Fungus seems to help in water and mineral uptake/usage
but also some drawbacks to agriculture:-
endophyte toxins cause sickness in grazing animals -ryegrass staggers, fescue foot

28. Lecture review. Read Chap. 17 in text View Chap. 17 on CDROM or Web
Review lecture slides on 318b Web page
Terms :-
ectotrophic/endotrophic; EM/VAM
Hartig net / mantle, arbuscules, vesicles,