Mycorrhizal Symbiosis Flashcards
What is mycorrhizal symbiosis?
a mutualistic association between soil fungi and plant roots (both the mycobiont and phytobiont benefit)
What are the 4 types of mycorrhizal associations?
Ectomycorrhizae (ECM)
endomycorrhizae (arbuscular - AM)
Ericaceous (Ericoid, Arbutoid, Monotropoid)
Orchid
How does the mycobiont strategy for ECM differ from AM?
ECM mycobionts are facultative so they can survive without hosts whereas AM mycobionts are obligate and cannot exist without host (or may exist as a resting spore)
How does the number of plant and fungal species vary for ECM vs AM? What implications does this have?
ECM has fewer plant species than fungal species
AM has a VERY high number of plant species and much fewer fungal species
this means that ECM fungi have a much more specialized host range than AM fungi
What plant taxa are involved in ECM?
mostly trees (and mostly conifers)
All Pinaceae, Cupressaceae, Salicaeae and others
many Myrtaceae and leguminous trees (ex. Acacia)
What fungal taxa are involved in ECM?
mostly Agaricomycetes (from Agaricales and Boletales orders)
some Ascomycetes (Pezizomycotina - including truffles)
What plant taxa are involved in AM?
huge range of herbaceous and woody plants
What fungal taxa are involved in AM?
ONLY species from Glomeromycota
What are the 2 distinguishing features of ectomycorrhizae?
a hyphal mantle: hyphae that surround epidermal cells
and
a Hartig net: a network of intercellular hyphae within the plant root and surrounds cortical cells
NO intracellular hyphae
What are the distinguishing features of endomycorrhizae?
arbuscules: grow intracellularly in cortical cells of plants
- identifying feature - probably serves as nutrient transfer
vesicles: inter and intracellular hyphae that swell up - functions as nutrient storage
appressorium: a specialized cell in a mycorrhizal fungi which penetrates the host tissue
What are chlamydospores?
asexual spores produced within hyphae
likely function to allow fungus to persist through adverse conditions
Where are AM distributed?
most habitats around the world
Where are ECM distributed?
they are dominate in boreal and alpine forests
common in temperate broad-leaved forests
some tropical, subtropical savanna, rain forests
What are non-mycorrhizal or facultatively non-mycorrhizal plants?
bryophytes, hydrophytes, some ferns
and plants growing under extreme conditions (water, salinity, disturbances)
What are the 4 benefits of mycorrhizae to plant nutrition and physiology ?
increased water uptake efficiency
increased nutrient uptake efficiency and access to otherwise unavailable forms of nutrients
plant protection against pathogens, parasites, or conditions
resource sharing between other plants or plant species
How do mycorrhizae increase water and nutrient uptake in associated plants?
mycorrhizae extend the volume of soil that a plant’s root reaches = more exposure
mycorrhizal fungi may release enzymes which can decompose insoluble organic substrates and inorganic nutrients
What are 3 methods of access of mycorrhizal fungi to insoluble nutrient substrates?
phosphatases for increased P uptake from leaf litter
oxidative and hydrolytic enzymes (ex. proteinases) for increased N uptake from organic nitrogen substrates such as lignin, protein, and amino acids
organic acids (ex. oxalic acid) to dissolve inorganic phosphate (mineral) from rocks
How do mycorrhizae provide protection to plant hosts?
they are a physical barrier because they coat the plant roots = reduced pathogens, parasites and nematodes
they may release antibiotic and antifungal compounds into the soil-root area
can increase plant tolerance to adverse soil conditions
How might plants be resource sharing if they have a shared mycorrhizal network? what are the benefits of this?
ECM fungal mycelia may be transferring photosynthates (C14) amongst different plant species
benefits:
- increased stability of ecosystems
- increased diversity of ecosystems
- mycorrhiosphere is improved and microbial biota can be supported
- growth of seedlings and understorey plants may be supported by dominant trees
What are the costs of mycorrhizae?
resource sharing: up to 20% of the photosynthate produced by the plant may be shared with the ECM fungus
fungi usually store sugars as sugar alcohols
Which form of mycorrhizae likely evolved first? what evidence supports this?
Endomycorrhizae - phylogenetics suggest it arose 353-462 mya
consistent with appearance of first land plants and fossil evidence in Rhynie Chert (arbuscule and vesicle like structures)
symbiotic fungi were important in plant colonization of land
What applications do mycorrhizae have?
reforestation - ECM
agriculture - AM
How can ECM be involved in reforestation efforts? what are the challenges?
ECM inoculation programs for reforestation (cautious success) - lots of variables like species combination, forest management strategies, site conditions (soil toxicity, pH, water), exotics/introduced species
How can AM be involved in agriculture? what are the challenges?
AM fungi have associations with many crop species and are very beneficial
challenges include monitoring phosphorous levels in fertilizers, pesticides, soil tillage and compaction, crop rotation/fallowing and the combination of species
** AM are obligate mycorrhizal species so they require a host to exist
