Mycorrhizal Symbiosis

Question 1

What is mycorrhizal symbiosis?

Answer 1

a mutualistic association between soil fungi and plant roots (both the mycobiont and phytobiont benefit)

Question 2

What are the 4 types of mycorrhizal associations?

Answer 2

Ectomycorrhizae (ECM)

endomycorrhizae (arbuscular - AM)

Ericaceous (Ericoid, Arbutoid, Monotropoid)

Orchid

Question 3

How does the mycobiont strategy for ECM differ from AM?

Answer 3

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)

Question 4

How does the number of plant and fungal species vary for ECM vs AM? What implications does this have?

Answer 4

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

Question 5

What plant taxa are involved in ECM?

Answer 5

mostly trees (and mostly conifers)

All Pinaceae, Cupressaceae, Salicaeae and others

many Myrtaceae and leguminous trees (ex. Acacia)

Question 6

What fungal taxa are involved in ECM?

Answer 6

mostly Agaricomycetes (from Agaricales and Boletales orders)

some Ascomycetes (Pezizomycotina - including truffles)

Question 7

What plant taxa are involved in AM?

Answer 7

huge range of herbaceous and woody plants

Question 8

What fungal taxa are involved in AM?

Answer 8

ONLY species from Glomeromycota

Question 9

What are the 2 distinguishing features of ectomycorrhizae?

Answer 9

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

Question 10

What are the distinguishing features of endomycorrhizae?

Answer 10

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

Question 11

What are chlamydospores?

Answer 11

asexual spores produced within hyphae

likely function to allow fungus to persist through adverse conditions

Question 12

Where are AM distributed?

Answer 12

most habitats around the world

Question 13

Where are ECM distributed?

Answer 13

they are dominate in boreal and alpine forests

common in temperate broad-leaved forests

some tropical, subtropical savanna, rain forests

Question 14

What are non-mycorrhizal or facultatively non-mycorrhizal plants?

Answer 14

bryophytes, hydrophytes, some ferns
and plants growing under extreme conditions (water, salinity, disturbances)

Question 15

What are the 4 benefits of mycorrhizae to plant nutrition and physiology ?

Answer 15

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

Question 16

How do mycorrhizae increase water and nutrient uptake in associated plants?

Answer 16

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

Question 17

What are 3 methods of access of mycorrhizal fungi to insoluble nutrient substrates?

Answer 17

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

Question 18

How do mycorrhizae provide protection to plant hosts?

Answer 18

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

Question 19

How might plants be resource sharing if they have a shared mycorrhizal network? what are the benefits of this?

Answer 19

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

Question 20

What are the costs of mycorrhizae?

Answer 20

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

Question 21

Which form of mycorrhizae likely evolved first? what evidence supports this?

Answer 21

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

Question 22

What applications do mycorrhizae have?

Answer 22

reforestation - ECM
agriculture - AM

Question 23

How can ECM be involved in reforestation efforts? what are the challenges?

Answer 23

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

Question 24

How can AM be involved in agriculture? what are the challenges?

Answer 24

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