Microbial Symbiosis Flashcards
Examples of symbiosis
-Rhizobium species with legumes
-Frankia species with woody plants and trees
-Vibrio species with squid
-Bacteria convert nitrogen to ammonia, plants cant - fixed nitrogen is provided to the plant as amino acids for growth
-Symbiosis involving fungi include association of mycorrhizae with many different plants - uptake of phosphorous, water and minerals from soil and transfer to plant
Establishment of symbiosis between Rhizobium and plants
-Flavenoids released by plant root signal to rhizobia in the rhizosphere
- In reponse these bacteria produce nodulation factors (Nod factors) that are recognised by the plant to activate the symbiosis signalling pathway
-Rhizobia gain entry into the plant root through root hair cells that grow around the bacteria attached to the root surface
-Bacteria are trapped inside a root hair curl. Infection threads (invasive invaginations of the plant cell) are initiated at the site of root hair curcls and allow invasion of the rhizobia into the root tissue
-Nodules initiate below the site of bacterial infection
-The infection threads grow towards the emergent nodules and ramify within the nodule tissue
-In many cases, the bacteria released into the membrane-bound compartments inside the cells of the nodule, where the bacteria can differentiate into a nitrogen fixing state.
What does the nitrogen-fixing nodule host?
Symbiotic Rhizobium bacteroids
What do the symbiotic Rhizobium bacteroids do?
-Func as specialized nitrogen fixing organelles that exchange fixed nitrogen (in form of amino acids) for plant photosynthesis
-Aerobic bacteroid metabolism is at odds with the O2 sensitivity of nitrogenase which converts N2 to ammonia
What is the main route of O2 diffusion?
Through the nodule apex which generates a longitudinal O2 gradient
-as a result, the free oxygen conc drops to less than 50nM in the central nitrogen-fixing zone containing the Rhizoid bacteroids
-oxygen diffusion is facilitated in the central zone by a high conc of lehaemoglobin
-therefore Rhizobium bacteroids fix nitrogen in a microaerobic, nitrogen rich environment
What kind of environment does Rhizobium bacteroids fix nitrogen in?
A microaerobic, nitrogen rich environment
What are nodules formed by on soybean wild type and a supernodulating mutant- nod++?
Bradyrhizobium japonicum
Why cant some soybean produce nodules?
-Inability to form a symbiotic relationship with nitrogen fixing Bradyrhizobium japonicum
-they appear stunted and pale
What is the appearance of supernodulating mutants?
Stunted in stature due to investing too much energy into forming nodule structure
What is Frankia?
A genus of Gram pos actinomycete bacteria that fixes nitrogen in association with plants
-Symbiotic interactions occur between N2-fixing actinomycetes of the genus Frankia and the roots of trees and other woody plants
Many actinorhizal plants- colonised by Frankia - are pioneers of disturbed sites and early successional sites and therefore have ecologically important impacts on late successions
What plants do the Frankia species form associations with?
Alder, birch, beech, walnut, buckthorn and roses
What are bacterial populations in root nodules involved in?
Nitrogen fixation and substantial biosynthesis and energy-generating pathways
Establishment of the symbiosis between arbuscular mycorrhizal fungus and plants
-Strigolactone released by the plant root signals to arbuscular mycorrhizal fungi (AMF) in the rhizosphere
-In response, these fungi produce mycorrhizal factors (myc factors) that are recognized by the plant to activate the symbiosis signalling pathway
-AMF invasion involves an infection peg from the hyphopodium that allows fungal hyphal growth into the root epidermal cell
-The fungus colonizes the plant root cortex through intercellular hyphal growth
-Arbuscles are formed in inner root cortical cells from intercellular hyphae
Mutalistic associations between mycorrhizal fungi and plants
-Provides the fungus w access to plant carbohydrates e.g glucose and sucrose - which are translocated from their source (leaves) to root tissue and onto plants fungal partners
-Plant gains the benefits of the myceliums higher absorptive capacxity for water and mineral nutrients due to the large surface area of mycelium to root ration, thus improving the plants mineral absorption capabilities
-Phosphate uptake and trnslocation to the plant is an important role for mycorrhizal fungi
-Sometimes - the transport of water, carbon and nutrients could be done directly from plant to plant through mycorrhizal networks that are underground hyphal networks created by mycorrhizal fungi that connectr individual plants
How much of the biomass can microbes make up?
40%
Examples of sponge specific microbes?
-Poribacteria
-Spongibacter
What is the ‘fixed’ nitrogen used for in plants?
Nitrogen that has been converted into ammonia by symbiotic bacteria can be used by the plant for amino acids for growth
What can Rhizobium nodulate?
Soybean, alfalfa, clover, peanuts
Purpose of Alvibrio fischeri in squid?
To porvide liught for squid to avoid predation
