Symbiosis Flashcards
3 types of microbial symbiosis
- Commensal - bacteria & host tolerate eachother
- Parasitic
- Mutualistic - obligate interaction, need eachother
Examples of microbes forming symbiotic relationships with hosts in which both benefit (3)
- Rhizobium species with legumes
- Frankia species with woody plants & trees
- Vibrio species with squid
What is released by the plant root to signal rhizobia
Flavonoids released by the plant root signal to the rhizobia in the rhizosphere
What do bacteria produce in response to flavonoids
Nodulation factors (Nod factors)
What does the recognition of Nodulation factors by the plant activate
The symbiosis signalling pathway
How do rhizobia gain entry into the plant
Through the root hair cells
What are bacteria (rhizobia) trapped in once they gain entry through root hair cells
Trapped inside a root hair curl
What are initiated at the site of root hair curls and what do these allow
- Infection threads
- These allow invasion of the rhizobia into the root tissue
Where are nodules initiated
Below the site of bacterial infection
What are the bacteria released into
Membrane bound compartments inside cells of the nodule
In the membrane bound compartments, what can the bacteria differentiate into
Differentiate into a nitrogen fixing state
What does the nitrogen fixing nodule host
symbiotic Rhizobium bacteroids
Function of the nitrogen fixing nodule
Function as specialised nitrogen fixing organelles that exchange fixed nitrogen (in form of amino acids) for plant photosynthates
Why is there an O2 gradient in nodules
Aerobic bacteria at odds w O2 sensitive nitrogenase (converts nitrogen to ammonis)
(Bacteria need O2 vs enzyme sensitive to it.)
What is the main route of O2 diffusion in nitrogen-fixing nodule & what does this generate
Through the nodule apex, which generates a longitudinal O2 gradient
What does the O2 conc drop to in the central N2 fixing zone containing Rhizobium bacteroids
Drops to less than 50 nM
What is O2 diffusion in the central zone of nodule facilitated by
A high conc of leghaemoglobin
What kind of environment do the Rhizobium bacteroids fix nitrogen in the nodules
- Microaerobic - due to O2 gradient - nitrogenase enzyme needs low O2
- Nitrogen rich - bacteria fixing nitrogen
How is the growth of soybean plants unable to form nodules
Stunted and pale compared to the wild type - due to inability to form a symbiotic relationship with nitrogen fixing Bradyrhizobium japonicum
How is the growth of supernodulating soybean plants
Supernodulating mutants are significantly stunted as a result of investing too much energy into forming nodule structures
Which genus of Gram-positive bacteria fixes nitrogen in association with plants - especially the roots of trees and other woody plants
Frankia
What is the ecological significance of actinorhizal plants (colonised by Frankia)
They are pioneers of disturbed sites and early successional sites
What key functions do Frankia bacteria perform inside root nodules?
They are involved in:
* Nitrogen fixation
* Biosynthesis
* Energy generating pathways
symbiosis between
arbuscular mycorrhizal fungus and plants
What is released by the plant root to signal arbuscular mycorrhizal fungi (AMF) in rhizosphere
Strigolactone
symbiosis between arbuscular mycorrhizal fungus and plants
What do the AMF produce in response to strigolactone
Mycorrhizal factors (myc factors)
symbiosis between arbuscular mycorrhizal fungus and plants
What does the recognitition of myc factors by the plant activate
the symbiosis signalling pathway
symbiosis between arbuscular mycorrhizal fungus and plants
Explain AMF invasion into plant
An infection peg from the fungal hyphopodium allows fungal hyphal growth into the root epidermal cell
symbiosis between arbuscular mycorrhizal fungus and plants
What does the fungus colonise and how
- Colonises the plant root cortex
- Through intercellular hyphal growth
symbiosis between arbuscular mycorrhizal fungus and plants
What are formed in the inner root cortical cells and from what
- Arbuscules are formed in inner root cortical cells
- They are formed from the intercellular hyphae
Mutualistic associations between mycorrhizal fungi & plants
- Plant provides fungus access to plant carbohydrates - glucose & sucrose
- Plant benefits from myceliums higher absorptive capacity for water & minerals - due to higher surface area
- Fungi increases phosphate uptake
- Mycorrhizal network transports water,carbon & nutrients from plant to plant
2 sponge specific microbes
- Poribacteria
- Spongibacter
What % of marine biomass do microbes make up
40%
What are bacteria in sponges enclosed within
Mesohyl matrix
What are the roles of microbes in microbial symbiosis and microbial ecology (9)
- food source
- pathogens
- mutualists
- photosynthesis
- nitrogen fixation - convert atmospheric nitrogen to ammonia (useful form)
- denitrification - excess nitrogen converted to nitrogen gas
- sulphur metabolism - process sulphur compounds
- elimination of toxic metabolites
- provide chemical defense mechanism
what bacteria provides the light for squid to avoid predation
Alivibrio fischeri
