Symbiosis

Question 1

3 types of microbial symbiosis

Answer 1

1. Commensal - bacteria & host tolerate eachother
2. Parasitic
3. Mutualistic - obligate interaction, need eachother

Question 2

Examples of microbes forming symbiotic relationships with hosts in which both benefit (3)

Answer 2

1. Rhizobium species with legumes
2. Frankia species with woody plants & trees
3. Vibrio species with squid

Question 3

What is released by the plant root to signal rhizobia

Answer 3

Flavonoids released by the plant root signal to the rhizobia in the rhizosphere

Question 4

What do bacteria produce in response to flavonoids

Answer 4

Nodulation factors (Nod factors)

Question 5

What does the recognition of Nodulation factors by the plant activate

Answer 5

The symbiosis signalling pathway

Question 6

How do rhizobia gain entry into the plant

Answer 6

Through the root hair cells

Question 7

What are bacteria (rhizobia) trapped in once they gain entry through root hair cells

Answer 7

Trapped inside a root hair curl

Question 8

What are initiated at the site of root hair curls and what do these allow

Answer 8

• Infection threads
• These allow invasion of the rhizobia into the root tissue

Question 9

Where are nodules initiated

Answer 9

Below the site of bacterial infection

Question 10

What are the bacteria released into

Answer 10

Membrane bound compartments inside cells of the nodule

Question 11

In the membrane bound compartments, what can the bacteria differentiate into

Answer 11

Differentiate into a nitrogen fixing state

Question 12

What does the nitrogen fixing nodule host

Answer 12

symbiotic Rhizobium bacteroids

Question 13

Function of the nitrogen fixing nodule

Answer 13

Function as specialised nitrogen fixing organelles that exchange fixed nitrogen (in form of amino acids) for plant photosynthates

Question 14

Why is there an O2 gradient in nodules

Answer 14

Aerobic bacteria at odds w O2 sensitive nitrogenase (converts nitrogen to ammonis)

(Bacteria need O2 vs enzyme sensitive to it.)

Question 15

What is the main route of O2 diffusion in nitrogen-fixing nodule & what does this generate

Answer 15

Through the nodule apex, which generates a longitudinal O2 gradient

Question 16

What does the O2 conc drop to in the central N2 fixing zone containing Rhizobium bacteroids

Answer 16

Drops to less than 50 nM

Question 17

What is O2 diffusion in the central zone of nodule facilitated by

Answer 17

A high conc of leghaemoglobin

Question 18

What kind of environment do the Rhizobium bacteroids fix nitrogen in the nodules

Answer 18

1. Microaerobic - due to O2 gradient - nitrogenase enzyme needs low O2
2. Nitrogen rich - bacteria fixing nitrogen

Question 19

How is the growth of soybean plants unable to form nodules

Answer 19

Stunted and pale compared to the wild type - due to inability to form a symbiotic relationship with nitrogen fixing Bradyrhizobium japonicum

Question 20

How is the growth of supernodulating soybean plants

Answer 20

Supernodulating mutants are significantly stunted as a result of investing too much energy into forming nodule structures

Question 21

Which genus of Gram-positive bacteria fixes nitrogen in association with plants - especially the roots of trees and other woody plants

Answer 21

Frankia

Question 22

What is the ecological significance of actinorhizal plants (colonised by Frankia)

Answer 22

They are pioneers of disturbed sites and early successional sites

Question 23

What key functions do Frankia bacteria perform inside root nodules?

Answer 23

They are involved in:
* Nitrogen fixation
* Biosynthesis
* Energy generating pathways

Question 24

# symbiosis between arbuscular mycorrhizal fungus and plants What is released by the plant root to signal arbuscular mycorrhizal fungi (AMF) in rhizosphere

Answer 24

Strigolactone

Question 25

# symbiosis between arbuscular mycorrhizal fungus and plants What do the AMF produce in response to strigolactone

Answer 25

Mycorrhizal factors (myc factors)

Question 26

# symbiosis between arbuscular mycorrhizal fungus and plants What does the recognitition of myc factors by the plant activate

Answer 26

the symbiosis signalling pathway

Question 27

# symbiosis between arbuscular mycorrhizal fungus and plants Explain AMF invasion into plant

Answer 27

An infection peg from the fungal hyphopodium allows fungal hyphal growth into the root epidermal cell

Question 28

# symbiosis between arbuscular mycorrhizal fungus and plants What does the fungus colonise and how

Answer 28

* Colonises the plant root cortex * Through intercellular hyphal growth

Question 29

# symbiosis between arbuscular mycorrhizal fungus and plants What are formed in the inner root cortical cells and from what

Answer 29

* Arbuscules are formed in inner root cortical cells * They are formed from the intercellular hyphae

Question 30

Mutualistic associations between mycorrhizal fungi & plants

Answer 30

1. Plant provides fungus access to plant carbohydrates - glucose & sucrose 2. Plant benefits from myceliums higher absorptive capacity for water & minerals - due to higher surface area 3. Fungi increases phosphate uptake 4. Mycorrhizal network transports water,carbon & nutrients from plant to plant

Question 31

2 sponge specific microbes

Answer 31

1. Poribacteria 2. Spongibacter

Question 32

What % of marine biomass do microbes make up

Answer 32

40%

Question 33

What are bacteria in sponges enclosed within

Answer 33

Mesohyl matrix

Question 34

What are the roles of microbes in microbial symbiosis and microbial ecology (9)

Answer 34

1. food source 2. pathogens 3. mutualists 4. photosynthesis 5. nitrogen fixation - convert atmospheric nitrogen to ammonia (useful form) 6. denitrification - excess nitrogen converted to nitrogen gas 7. sulphur metabolism - process sulphur compounds 8. elimination of toxic metabolites 9. provide chemical defense mechanism

Question 35

what bacteria provides the light for squid to avoid predation

Answer 35

*Alivibrio fischeri*