L13 - Establishment and functioning of mycorrhizal associations

Question 1

Which group do land plants form a monophyletic group with?

When did they arrive on land?

Which modern day plants do the early land plants resemble and why is this significant for symbiosis theories?

Answer 1

• The green alga Charales
• 470 Mya
• Resemble extant liverworts
• Rootless, suggests a symbiosis w/ fungi needed to provide nutrients

Question 2

What evidence is there for plant-fungi symbiosis in the fossil record?

Answer 2

• Oldest fossils of intact land plant organs dated to 400Mya
• Fungal structures seen in primitive vascular plant Aglaophyton major
• Closely resemble structures formed by Glomeromycotina in root cells today

Question 3

Describe the Glomeromycotina phylogenetically and functionally.

Answer 3

Phylogenetically:
- Belong to phylum of Mucoromycota which forms monophyletic group w/ Asco- and Basidiomycota

Functionally:
- Asexual organisms (raises Q of how they’ve survived so long w/o sexual reproduction)
- Obligate biotrophic life-style (rely on plant association)
- No host specificity! (But gradient of compatibility)

Question 4

Outline the 4 kinds of mycorrhizal fungi

Answer 4

Arbuscular mycorrhizal fungi
- Occurs in all plant lineages (74% of species)
- Restricted to 300 fungal species in Glomeromycotina

Orchid mycorrhizal fungi
- In 9% land plant species
- Many species in Asco- and Basidiomycota

Ericoid mycorrhizal fungi
- In 1% land plant species
- Many species in Asco- and Basidiomycota

Ectomycorrhizal fungi
- In 2% land plant species
- Many species in Asco- and Basidiomycota
- Remain extracellular during colonisation

Question 5

What is the main benefit that plants derive from mycorrhizal associations?

Answer 5

• Hyphal exploration created enlarged soil volume for mineral acquisition
• Fungal mycelium can even replace lateral roots

Question 6

List the 4 key stages of arbuscular mycorrhizal symbioses

Answer 6

1) Pre-symbiotic cross talk
2) Root penetration
3) Arbuscule formation
4) Vesicle and spore formation

Question 7

What is pre-symbiotic cross talk, why is it needed and how does it work?

Answer 7

• Reciprocal exchange of signals in rhizosphere for mutual recognition
• Required for reprogramming in fungi + host in anticipation of symbiosis
• Plants release strigolactones to stimulate fungus to branch start interaction
• Stricolactones are unstable in soil fluid = high conc. gradient shows host present
• Fungus releases chitin based signals : lipo-chitooligosaccharides (LCO) and chitin tetra + pentamers (CO4/5)
• Ca spiking response in root epidermal cells induced
• Fungal hyphopodia forms on root surface

Question 8

Describe the next stage of AM symbiosis formation: root penetration

Answer 8

• Cell nucleus reposition to below hyphopodium
• Nucleus moves to opposite side of cell, pulling dense cytoplasmic + ER material to form subcellular tunnel
• Fungus penetrates through cell lumen

Question 9

Describe the next stage of AM symbiosis formation: Arbuscule formation

Answer 9

• Highly branched arbuscules formed in inner cortex cells
• Host cell architecture highly reorganised to accomodate this, e.g. new PM synthesis to envelope it
• Becomes enveloped by plant derived membrane, creating ideal exchange surface for exchange
• Arbuscules live for several days then collapse, cell returns to normal

Question 10

Describe the final stage of AM symbiosis formation: Vesicle and spore formation

Give a relevant example

Answer 10

• Asexual daughter spores produced, contain many nuclei (unknown mechanism)
• Spores and vesicles contain lots of lipids
• Evidence suggests arbuscules are prerequisite for asexual life cycle to obtain lipids from plant

E.g. Members of Glomeromycotina lack genes for FA synthesis
- Arbusculated plant cells induce FA biosynthesis + deliver large amounts to fungus for asexual reproduction
- Hence fungi is an obligate biotroph

Question 11

Describe the AM symbiotic phosphate uptake route in 5 steps (a well characterised example of nutrient exchange)

Give an example that shows just how important AM symbiosis is for nutrient uptake

Answer 11

1) Inorganic phosphate taken up by extraradical fungal hyphae from soil fluid
2) Pi polymerised to polyP inside fungus
3) polyP transferred to intraradical fungal structures within vacuolar compartment
4) polyP hydrolysed into Pi + released from exchange surface
5) Plant encoded Pi transporters uptake Pi to cell

• OsPT11 in rice shown to be localised to peri-arbuscular membrane, especially around fine branches
• Radioactive tracer studies showed mycorrhizal rice plants take up >70% total Pi via symbiotic route

Question 12

Describe what is meant my “common mycorrhizal networks”

Answer 12

• Mycorrhizal mycelia develop commonly shared hyphal networks that connect associated plants!
• Leads to redistribution of minerals and carboydrates

Question 13

Describe the experimental evidence for different “terms of trade” seen in common mycorrhizal networks

Answer 13

Reciprocal reward strategy:
- Amount of minerals exchanged per plant corresponds to amount of C received
- Evidence from sterile tracer experiment

Asymmetric terms of trade:
- Evidence from tracers in common mycorrhizal network between Sorghum and Flax
- Sorghum delivered more than twice the amount of C to network than Flax
- Flax obtained up to 94% of all N and Pi
- Asymmetric trade!

Question 14

Give an example of myco-heterotrophy

Answer 14

• All orchids achlorophyllous upon germination = rely on AM for nourishment from common hyphal networks (C from other plants)
• Monotropacean species are myco-heterotrophic their whole lives

Question 15

Give an example of where common AM hyphal networks are used to spread information between plants

Answer 15

• Mycorrhizal bean plants infested w/ aphids send warnings to neighbours via hyphal network
• Neighbours produce aphid repellants!