Lecture #9

Ectomycorrhiza

Ectomycorrhiza:

• hyphae do not penetrate cells of cortex in root tips
• hartig net: growth of hyphae in between and around cortex cells
• fungal mantle
• only form in actively growing roots
• 2000 spp. of plants
• 5000 spp. fungi
• most ectomyco. fungi unable to degrade cellulose
• obligate vs. facultative; enhanced by high light, decreased by high N & P levels
• e.g. Amanita, Russula, Lactarius, Laccaria, Cortinarius, Inocybe, Boletus, Suillus
• know the difference between endo- and ectomycorrhiza and be able to give examples

How do mycorrhizal relationships form?

• feeder roots colonized by hyphae or germinating spore
• root exudate promotes spore germination and growth
• penetrate between cortex cells; form hartig net
• form fungal mantel
• extension of hyphae from mantel into soil

Unique root growth and morphology

• fungus secretes auxins, giberellins, & cytokinens; all plant growth hormones
• enhances root growth
• maintains juvenile roots; no root hairs
• mycorrhizal root tip is more often branched than nonmycorrhizal root tip, e.g., dicotomous branching in Pinaceae

What does fungus get from plant?

• sugars (monosaccharides) that are transported from the plant to the fungus and converted into trehalose, mannitol & glycogen, which are generally not permeable back across fungal cell membrane
• B vitamins; stimulate spore germination
• estimated that trees invest 10% of photosynthates to their mycobionts

What does plant get from fungus?

• phosphorus, calcium, potassium, copper, molybdenum, magnesium, zinc
• nitrogen
• water: source-sink and water potential
• protection against pathogens

Ectomycorrhiza dynamic process

• succession:
  • individual mycorrhiza active for 1-3 yrs
  • root may out grow fungus and be colonized by a alternative ectomycorrhizal fungus or a more aggressive ectomycorrhizal fungus may replace the previous
• a single tree will often have several mycorrhizal partners throughout its root system; its not one tree one fungus
• the most conspicuous and prolific mycorrhizal fruiting population above ground may not be the dominant mycorrhizal partner
• not all mycorrhizae are beneficial to all stages of host life period

Ectendomycorrhiza

• characteristics of ectos but exhibit intracellular penetration
• MRAs Mycelium radicis atrovirens; dark septate mycorrhiza; ascomycetes

Orchid mycorrhiza

• orchid seeds and young plants associated with Rhizoctonia
• seeds and seedlings apparently lack some enzymatic machinery; can't metabolize sulfer
• many orchids are achlorophyllous; get carbon from fungus
• Rhizoctoniais normally a potent pathogen of plant; often considered to be captured by the orchid; orchids produce many secondary products that may inhibit fungus
• don't have any major effect on root morphology
• form loops in root cells
• other orchid mycorrhizae (JS Edits LATER)

Ericoid mycorrhiza

• mycorrhiza of the Ericales (Ericaceae, Monotropaceae); some do have true ectomycorrhiza
• 2 forms; one with mantel, Hartig net; one with no mantel, Hartig net but invades cortex cells
• on fine root system
• extensive hyphal development around root; slight development on root surface; within cells extensive hyphal coils
• fungus eventually degenerates but so does cortex cell
• Mycelium radicis myrtillis

Arbutoid mycorrhiza

• described in Arbutus
• similar to ectomycorrhiza in root morphology
• mantle, Hartig net
• epidermal cells are colonized by hyphae that form coils; coils disintegrate but cells do not; may be colonized again
• Cortinarius, Cenococcum
• Review general ectomycorrhiza notes from lecture #5