Mycorrhiza Flashcards
(34 cards)
What percentage of land plant species form mycorrhizal associations?
At least 80%
Are mycorrhizal relationships always mutualistic?
No, they lie on a continuum from mutualism to parasitism
e.g. Ghost orchid is parasitic on mycorrhiza, it does not photosynthesis
What are the two main types of mycorrhiza?
- Arbuscular mycorrhiza (AM):
Colonises >80% of plants.
Aseptate (lack cross cell walls)
Intracellular colonisation - Ectomycorrhiza:
More common in trees, limited to certain species of gymnosperm and angiosperm
Septate
No intracellular colonisation
What are the three key benefits of mycorrhiza to wild plants?
- Enhanced nutrient access
- Disease / pest resistance
- Drought tolerance
What are the 6 steps of mycorrhizal colonisation?
- Pre-symbiotic signalling
- Hyphal growth and root recognition
- Root penetration and intracellular colonisation
- Arbuscule maturation and functionality
- Fungal proliferation and extraradical mycelium growth
- Arbuscule senescence and symbiosis termination
What happens in the first stage of mycorrhizal colonisation?
Pre-symbiotic signalling
- Plants release strigolactones into the rhizosphere, stimulating AM fungal spore germination and hyphal growth
AM fungi secretes Myc factors, triggering plant root response
What happens in the second stage of mycorrhizal symbiosis?
Hyphal growth and root recognition
- The fungal hyphae grow towards the root, guided by plant secreted signals
- AM hyphae attach to root surface and form hyphopodium over epidermal cells
- Pre-penetration apparatus (PPA) forms, facilitating penetration
- Involves recognition via plant receptor-like kinases and activation of the common symbiosis signalling pathway (CSSP)
What is the common symbiosis signalling (CSS) pathway?
AMF and rhizobium share a common symbiosis (SYM) signalling pathway, but AM specific receptors and signals remain unidentified.
Key components:
- DMI1: ion channel for calcium influx and oscillations
- DMI2: receptor kinase for signal transduction
- DMI3 / CCaMK: activates CYCLOPS transcription factor to control AM fungal colonisation
- RAM1: triggers arbuscule formation
What happens in the 3rd stage of mycorrhizal symbiosis?
Root penetration and intracellular colonisation
- Fungus enters the root through epidermal cells, typically avoiding cell wall damage by forming membranous packets around the fungal hyphae (perifungal membrane)
- Gros intercellularly through the cortex and forms highly branches arbuscules inside cortical cells
What happens in the 4th stage of the mycorrhizal symbiosis pathway?
Arbuscule maturation and functionality
- Fully developed arbuscules facilitate bidirectional nutrient exchange
- Fungi deliver phosphorus, nitrogen and other minerals to the plant
- Plants supply sugars and lipids to the fungi
- Regulated by symbiosis-specific transporters (e.g. PT4 - phosphate transporter)
What happens in the 5th stage of mycorrhizal symbiosis?
Fungal proliferation and extraradical mycelium growth
- Some AMF structures develop vesiclces (lipid storage organs)
- Extraradical mycelium extends into the soil, allowing for enhanced nutrient acquisition beyond the root depletion zone
- Connects multiple plant roots, forming common mycorrhizal networks (CMNs)
What happens in the 6th stage of mycorrhizal symbiosis?
Arbuscule senescence and symbiosis termination
- Arbuscules have a short lifespan (~5-10 days) and eventually degrade
- The fungal hyphae can remain in the root or detach and contribute to soil mycelium networks
What receptor complex recognises myc factors (lipochito-oligosaccharides)?
SYMRK
What ion channels are involved in perinuclear calcium spiking?
CASTOR
POLLUX
How is RAM1 transcription activated?
DELLA proteins form a complex with CYCLOPS and other proteins
Initiates early colonisation and arbuscule formation
How do plants differentiate between beneficial and parasitic relationships?
Plants downregulate immunity during mycorrhizal colonisation, allowing the fungi to enter and develop within the root system
What is the hormonal crosstalk between SA and JA during mycorrhization?
SA is a key hormone in immunity against biotrophic pathogens. It is often downregulated during colonisation because it inhibits mycorrhization
JA regulated defence against herbivores and necrotrophs, it promotes mycorrhization
AM fungi manipulate JA-SA crosstalk to suppress strong plant defences while maintaining a low-level of defence in the host plant
Myc factors vs PAMPs
Myc factors are released by AM fungi, they promote symbiosis by triggering the CSSP
PAMPs (e.g. chitin) are produced by pathogenic microbes. They rigger immune responses
How does calcium spiking differ in response between AMF and pathogens?
AMF cause oscillatory Ca2+ spiking in root cells (detected by CCaMK/DMI3)
Pathogens cause a rapid, non-oscillatory Ca2+ burst (leads to activation of MAPK cascades and production of ROS to help fight infection)
What are the three main ways of immune suppression by AM fungi?
- Downregulation of defence genes: prevents excessive defence activation
- Modification of hormone signalling: modulating JA-SA crosstalk
- Secretion of effectors: some block pattern triggered immunity (PTI), allowing fungal colonisation
How do plants control AM colonisation based on their nutrient needs?
When soil Pi is high, plants reduce AM colonisation to reduce C costs
High Pi triggers production of CLE peptides in roots, binding to receptor kinases to inhibit CSSP
What is the timescale of mycorrhizal responses?
Unlike rapid immune responses against pathogens (minutes to hours to days), mycorrhizal colonisation unfolds over days to weeks, with long term benefits
Hours-days: plant percieves Myc factors, activating CSSP
Days-weeks: hyphae penetrates root, forming arbuscules, nutrient exchange
Weeks-months-years: mycelium extends into the soil, forming common mycorrhizal networks (CMNs)
How can mycorrhiza enhance crop productivity?
Drought and stress resilience
- Improves water uptake by enhancing root hydraulic conductivity by regulating aquaporins
Modulating abscisic acid responses
- Accumulates ABA during drought, triggers stomatal closure
- Mycorrhizal associated plants stomatas can respond more effectively
Enhanced soil health
- Mycorrhiza stabilise soil structure by enmeshing soil particles together with hyphal networks and secreting hyphal exudates (GRSPs), act as a glue
What is an example of how phosphorus poor soils affect AMF?
Mycorrhizal maize and wheat show increased biomass and grain yield in phosphorus poor soils
Enhanced nutrient acquisition increases vigour, root and shoot biomass and higher yields
Yield increases of ~20% in AMF inoculates crops
