Lecture 15 - Microbial symbioses with microbes, plants, and animals Flashcards
what are lichens?
mutualistic relationship
between a fungus and an alga (or
cyanobacterium)
what is Chlorochromatium aggregatum?
bacterial mutualistic relationship; consists of green sulfur bacteria and a flagellated rod-shaped bacterium
true/false
The green sulfur bacteria are
obligate anaerobic phototrophs
true
what is Methanotrophic consortia?
couple the
activities of two anaerobes to effectively
oxidizing methane to carbon dioxide in
anoxic marine sediments; Use “nanowires” for direct interspecies
electron transfer
what does the mutualistic relationship between legumes and nitrogen-fixing bacteria lead to?
Infection of legume roots by nitrogen-fixing
bacteria leads to formation of root nodules that
fix nitrogen; leads to significant increases in
combined nitrogen in soil
what is a cross-inoculation group?
a group of related legumes that can be infected by a particular species of rhizobia.
what is nitrogenase and what is it inhibited by?
enzyme that fixes nitrogen, inhibited by high oxygen levels
what is leghemoglobin?
an oxygen-binding protein in the nodule that binds the free oxygen; protects nitrogenase from free oxygen
how does the leghemoglobin function?
the heme group of leghemoglobin cycles between the oxidized form (Fe3+) and reduced form (Fe2+) to supply enough oxygen for bacterial respiration while also keeping free oxygen within the nodule low
what happens to the rhizobia after infection?
they divide rapidly within the nodule, change shape (bacteroids) and form a symbiosome within the nodule
what is involved in attachement?
cell surface proteins of rhizobia and plants (rhicadhesin)
what do the roots of leguminous plants secrete?
organic compounds that stimulate the growth of rhizosphere microbial communities
what happens if there are suitable rhizobia in the soil?
they will form large populations and then attach to the root hairs
what is an infection thread?
a cellulosic tube formed by the plant, induced by the bacterium, that spreads down the root hair
true/false
plant cells divide to form a tumor like nodule that consists of plant cells filled with bacteroids
true
what are the major organic compounds transported to the bacteroids and what are they used for? (2 paths)
malate, succinate, fumerate
used in citric acid cycle leading to the electron transport chain to generate ATP
pyruvate as electron donor
what does nitrogenase convert?
nitrogen to ammonia, which is assimilated to glutamine and asparagine and transported through the plant
what is mycorrhizae?
mutualisms between plant roots and fungi
plant transfers carbohydrates to fungi
fungi transfer inorganic nutrients
what are the two classes of mycorrhizae?
endomycorrhizae and ectomycorrhizae
what are the characteristic of ectomycorrhizae?
- fungus stays outside the plant roots
- found primarily in forest trees (boreal and temperate)
- some trees can form multiple myccorhizae associations with multiple fungi (allow exchange of nutrients and carbon between trees of the same and different species)
what are characteristics of endomycorrhizae?
- fungal mycelium becomes deeply embedded with root tissues
- 5 classes, most common is arbuscular mycorrhizae
- more common than ectomycorrhizae
- cannot be cultured in pure culture
explain arbuscular mycorrhizae root colonization
- plant roots release hormone strigolactone (stimulates growth of plant root system and fungal germination and mycelial branching)
- fungi produce oligosaccharide singaling molecules to initiate mycorrhizal state
- fungal mycelium forms attachement structure HP (hyphopodium), penetrates through epidermal cells and cells of outer cortex
- mycelium can spread intercellularly or intracellularly in the outer cortex
how can microbial symbionts be acquired?
- environmental reservoirs (horizontal transmission)
- parents (herritable transmission) (obligate, lacking a free-living replicative stage)
two categories based on host dependancy
- primary symbionts (required for host reproduction)
- secondary symbionts (not required for host reproduction)
