mycorrhizal symbiosis Flashcards
tell me some facts about mycorrhizal symbiosis
- in 80% terrestrial plants in almost all ecosystems
- provide up to 80% of plant N+P
- 50,000 fungi and 250,000 plants
- benefits plant by increasing resistance to pathogens and abiotic stress
what are the 4 main types of mycorrhizal fungi?
arbuscular (AM)
ectomycorrhiza (ECM)
orchid (OMF)
ericoid (ErM)
Tell me about ericoid symbiosis
symbiosis is formed with Ericaceae family of plants.
initial growth is on the root hair surface and a loose hyphal sheath is established.
Hyphae penetrate epidermis and cortical cells to form densely packed, intracellular fungal cells called coils (nutrient transfer site)
some ErM can colonise root stems, leaves and flowers
Tell me about Orchids development
Orchids are mycoheterotrophic (non-photosynthetic, all nutrients from fungi) during seed germination and early seedling development, however some rely on OMF during adulthood too (this is called mixotrophy)
Tell me about OMF symbiosis
OMF is an endomycorrhizal fungi that forms pelotons in cortical cells.
penetration is via the apoplast, and is surrounded by plant derived membrane that acts as a rich in transporters interfacial zone.
What is different about OMF symbiosis?
nutrients movement in OMF is not bi-directional:
During early seedling stages, the plant recieved N,P,C from the mycobiont.
When the Orchid grows green leaves it can provide the fungi with C, however cannot receive N anylonger.
tell me about arbuscular mycorrhizal fungi lifecycle
arbuscular mycorrhizal fungi is a obligate biotroph - meaning it relies on its autotroph host to complete its lifecycle and produce next gen of spores.
spores can germinate without presence of host, however respond better to exudates
Tell me about arbuscular hyphae
AM contain coenocytic hyphae - containing hundreds of unseparated nuclei
how does AM breach the plant?
A hyphopodium is established on the root surface. From this podium, hyphae enter apoplast in cortex and grow laterally along root axis, penetrating root cortical cells
there are two types of mycelium formed:
- intraradical (in root)
- extraradical (in soil)
How do the IRM and ERM mobilise nutrients?
NH4+/NO3- are taken up into the extraradical mycelium and assimilated into arginine (arg+). P is absorbed by phosphate importers and transported as polyphosphate granules. their negative charge makes them possible carriers of arg+ and metals. They are transported from the ERM to IRM, which exchanges nutrients at the interfacial apoplast
what is nutrients used for in the AM
nutrients are used to maintain and enlarge ERM (i.e. energy used in cell respiration) and to produce spores
arum-type vs paris-type colonisation
most plants have arum-type - with intercellular hyphae and intracellular arbuscules
Paris-type - has intracellular hyphae and coils
Tell me about signalling in AM
very similar to rhizobium etc.
Myc factors are COs and LCOs perceived by LysM.
Subset of genes required to decode Ca2+ spike: i.e. symbiosis receptor like kinase, two ion channels and nucleoporin proteins
In the nucleus, Ca2+ spikes CCaMK and CYCLOPS that activate transcription factors
Tell me about ECM hosts and conditions
Ectomycorrhizal symbiosis is in 6% of land plant species, mostly timber producing plants (making it economically important). It can grow without a host and is part of the Basidiomycota phylum
Tell me about ECM colonisation
form mantle of hyphae enveloping roots and extending into soil. This extends into the root cortex forming an an intercellular hyphal network called a hartig net. This is because ECM has lost the ability to penetrate cell walls, and allows for fungal and plant cell walls to always be in contact. The hartig net can penetrate from 1 to 4 cortical cells deep.
