Rhizobium-Legume Symbiosis Flashcards
Symbiosis
Nodules on root - bacteria take in atmospheric N and turn into ammonia (reduction)
Expensive so get photosynthate from plant
Gets N into soil
Most rhizobia on root, some on stem
Nodule controls atmospheric O2 within nodule
Try and expand beyond legumes
Features
Enzyme nitrogenase encoded only by prokaryotes - sensitive to O2 so low O2 env needed inside specialist structure
High energy reaction so energy from plant so high rates of respiration needed - bacteria have e- transport chain w O2 at end
Oxygen is buffered:
Leghaem binds O2 when it is high
Releases O2 when it is low
Respiration consumes this oxygen
Plant controls nodulation - no need if soil N is high
Nodules
Indeterminate:
‘temperate’
All stages in 1 nodule - cells at different stages
Meristem continues to grow
Determinate:
‘tropical’
All cells at same stage
Meristem doesn’t persist
Steps - plant POV
Needs to recognise friend
Release secondary metabolites into soil to shape microbiome into rhizosphere
Release of flavinoids which are recognised by bacteria
Bacteria guided by flavinoids to tip of root
Root curls over - protected env for bacteria to make hole in plant cell wall
Infection thread down root hair
Formation of meristem gives rise to nodule
Plant cells have lots of bacteria - some cells uninfected to provide carbohydrates and metabolise ammonia
Steps - bacteria POV
NodD protein binds flavinoids and activates bacterial nodulation gene expression
Bacteria swim up conc gradient then attach
Exchange of signals and recognise symbiotic partners
Chemical signal from bacteria causes root hair curling
Makes hole in cell wall, moves and grows down thread
Grow towards promordia
Detection of microbe
Microbial associated or damage associated molecular patterns
Triggers plant defence response
Release of effectors
Inkect proteins to dampen defences and deactivate response
Nod genes
Detect plant flavinoids and turn on other genes associated with symbiosis
Common - no effect on host specificity; recognisable to all
Host specific - differences in sequences; different signals exchanges
Nod genes form nod factors as nodule forms
Hydrophylic backbone, hydrophobic tail
Diff rhizobia produce diff nod factors - difference in place of attachment of tail
Induce Ca spiking around nucleus in root hair only in plants that nodulate - downstream events
Info encoded in oscillations - certain freq. associated w certain gene expression
MC and rhizobia
Many plants have assocations w MC
Myc factors from MC similar to rhizobia, similar machinary and present in many plants
Different receptors associated
Causes root branching
Nod factors
Receptors in plant cell membrane similar to those involved in plant defence responses - detecting signal outside of cell
Identify receptors associated w nod factors:
Nod factors bind to receptors and trigger downstream events
Interact w another receptor and send signal to another protein
Generate Ca spike around nucleus
Act on these spikes to give rise to infection
Gives rise to root branching
Removing Ca spikuing doesn’t always prevent symbiosis as many pathways available
Host cell proliferation
Inner cell layers activated as root hair curling induced
Pericycle cells opposite protoxylem undergo some cell divisions
Inner cortical cells lose identity and enter cell cycle - form nodule primordium in inner cortex
Outer cortical cells enter cell cycle - arrested and form preinfection thread
Root nodule differentiation
Cells at apex persist as meristem
Peripheral tissue limit gas exchange
Central tissues host rhizobia
Bacteria become more Y shape as peptide signals perceived from plant
N fixation structures
Structure to fix N needs to be present
Nif genes: N fixation genes
Nif A - detects oxygen, when low genes for N2 fixation turned on
Fix genes: found in nodulating strains, form effective symbiosis
Plant genes
Early nodulation genes: development of symbiosis
Late genes: operation of symbiosis
Transport/metabolism genes - glutamine synthase converts ammonia into amino acid; uricase for N metabolism; sucrose synthase degrades sucrose and provides energy
Leghaem - binds O2 tight to keep low; rapid rate of respiration to fuel reaction; allows nitrogenase to be expressed
