Fig. 9: Cross-sectional photograph of an Ectomycorrhizal fungus in association with terrestrial a plant, illustrating the complex network of branching mycelium in close association with the plants roots, which helps to aid in water and nutrient uptake. (photo from UCSC instructional computing, http://ic.ucsc.edu/~wxcheng/envs161/Lecture8/Ectomycor_BW.jpg )

Mycorrhizal fungi are classified by structural characteristics at maturity (Harley and Smith 1983) thus not rigid classification system.

  3types:

1)      Ectomycorrhiza.

·        Grow in sub artic and temperate trees and shrubs, form SHEATH 20-40 μm thick around plant roots, thus the fungus maintains connection with soil.

·        Fungus is up to 25% the dry weight of plant and is not distinguishable when in association with host

·        Mainly BASIDOMYCETE partners eg. Tuber

·        Doesn’t penetrate the cortex.

·        Photosynthates passed to fungi (discovered via tracking ¹⁴CO₂ progress from leaves to fungi)

·        Infected plant has additional storage compounds; TREHALOSE and MANNITOL, both of these are unavailable to the plant, thus requires the fungal cells to mobilise it, this maintains a Carbon gradient, which can be useful in resisting water logged areas.

2)      Ectendomycorrhiza.

·        Variant of ectomycorrhiza, sheath has variable development; some may penetrate into the root cortex.

3)      Endomycorrhiza.

3sub-types:

i)                   Vesicular-Arbuscular Mycorrhiza.

·        Very common, associated with HERBACEOUS and Tropical plants (thus includes grasses)

·        Fungal partner is LOWER FUNGI; large spores and dependent on plant for Carbon and energy.

·        Forms APPRESSORIUM at infection point, infection thread then invaginates membrane and forms coils in outer cortex.

·        Forms branched ARBUSCULES in inner cortex BUT NEVER penetrates STELE or CHLOROPHYLLOUS CELLS.

ii)                 Ericalem Mycorrhiza.

3sub-types: a) Ericoid Mycorrhiza.

·        Association with restricted plant groups, includes; Erica (Heather), Calluna (cross heather) and Rhododendriodea.

·        Mainly ASCOMYCETES, form fine root cortical, they are extensively coiled.

·        Large infection number.

·        Fungi intolerant of low pH thus as plants provide a more favourable habitat for fungi to exploit on the basis of a higher pH, that is thought to be the reason for such a symbiosis.

b) Arbutoid Mycorrhiza.

·        Found in woody shrubs, form intracellular hyphae and hartig net aswell as coils.

·        Basidomycete partner.

c) Monotropoid Mycorrhiza.

·        Associated with MONOTROPACEAE (thus they lack chlorophyll), as such the plants depend upon fungal association.

·        Basidomycete partner.

NOTE: in all ENDOMYCORRHIZA the fungal cells are NOT FREE LIVING and therefore depend totally on the plant for obtaining carbon. However as plants generally only respire and undergo photosynthesis at 20% of their maximum rate, plants can easily supply the extra nutrients required by the fungi without any deleterious effect on the plant, by increasing this rate slightly.

iii) Orchidaeceous Mycorrhiza.

• Associated with ORCHIDACEAE (orchids).
• Plant embryo infected during development, plants depend on fungi whilst developing as they lack chlorophyll thus the fungi provide Carbon and vitamins.
• Basidomycete partner, often aggressive; potential classification as pathogenic?
• Form hyphal coils (pelotons). High infection number.

Figure 10; Schematic diagram of illustrating a selection of the types of Mycorrhizal association fungal cells may have with their plant hosts. Including a) Arbuscular Mycorrhiza, b) Orichid endomycorrhizzae, c) Ericoid endomycorrhizae, d) Arbutoid ectendomycorrhizae and e) ectomycorrhizae. (Diagram taken from http://www.ecologischgroen.be/bib/stinzen.html)

General Benefits of Mycorrhiza association:

1.      Increase Drought Resistance; mycorrhizal association increases potential area of water uptake. For instance, Ectomycorrhiza hyphae can be as much as 1-80m per 1cm of plant root.

2.      Increase Resistance to Waterlogging; the external mycelium acts as a protective sheath, and may even aggregate and differentiate to form structures know as strands, to aid in regulation of water transfer.

3.      Increase Transpiration rate of plant host; increase stomatal conductance and change hormone balance, as such cells divide more and don’t elongate, resulting in a larger biomass with more branches.

4.      Increase Hydraulic Conductivity; results due to extra phosphorous supplied by mycorrhiza association which is used to help maintain membranes and efficient function and permeability control. 

5.    Parasitic Nematode control