L37: Legumes Flashcards
What are legumes and examples
- plant family in Rhizobia
- diverse morphology, ecology
examples: arctic annuals, tropical trees, crop plants, peas, beans, soybean, clover, alfalfa, peanut, etc
relationship between legumes and N
can grow in N poor soil if nodulate dby N2 fixing Rhizobia
they can supply plant with N which is an adv over non-legumes within that niche
– the plant supplies the N2 -fixing ‘bacteroids’ within the nodules an energy source in the form of C4 -dicarboxylic acids
– plant is in control, and does not invest energy in nodulation and symbiotic N2 -fixation if N is not needed 2
Explain what Rhizobia is
genera of soil bacteria like Rhizobium, Sinorhizobium, Bradyrhizobium
form root nodules on specific legume plant hosts
– symbiotic N2 -fixation within the nodules
– free-living rhizobia generally do not fix N
Explain rhizobia host range
limited
– interaction between plant and bacteria usually very specific
– most rhizobia can only nodulate closely-related legumes
– a few species (Rhizobium sp. strain NGR 234) have a very
broad host range
– only one non-legume genus (Parasponia) is known to have species that can be nodulated by rhizobia
Overview steps of root nodule formation
Step 1: Species-specific plant flavonoid signals
Step 2: Rhizobial response
Step 3: Plant response to specific Nod signal
Step 4: Infection
Step 5: Nodule and bacteroid development
Step 6: Nutrient exchange
Step 1: Species-specific plant flavonoid signals
– the roots of each species of legume exude a unique cocktail of
organic compounds
– root exudates include low concentrations of (phenolic) flavonoid
compounds such as flavones and isoflavones
– the exudates of different legume species each contain a unique
cocktail of flavone and isoflavone compounds that function as
species-specific chemical ID signals
– soybean: genistein, daidzein (isoflavones)
– alfalfa: luteolin (flavone)
Step 2: Rhizobial response (1)
COLONISATION
– rhizobia living in the soil can sense & respond to the flavonoid
signals of their specific host plant
– do not respond to signals from non-host plant species
– rhizobia colonize rhizosphere, especially near root hair tips
– quorum sensing determines if a sufficient number of rhizobia are present for successful nodule formation
Step 2: Rhizobial response (2)
B. Induction of nod genes & species-specific Nod signals
– bacterial nod genes are induced only in response to the specific
flavonoids of their host plant
– i.e., in S. meliloti, the gene regulatory protein NodD1 binds
to the alfalfa flavonoid luteolin
– NodD1-luteolin then turns on the rest of the nod genes
– the nod genes encode enzymes that synthesize a unique lipochitooligosaccharide (LCO) Nod signal or Nod factor
– short chitin backbone
– fatty acid side chain
– unique chemical ‘decorations’ for host specificity
– unique Nod signals function as species-specific chemical ID
response signals that stimulate the specific host plant to initiate
root nodule formation
Explain Nod factor synthesis and specificty
– Nod signals are species-specific
– each Rhizobium species makes (a) unique signal(s)
– legumes only respond to LCO’s from the correct rhizobial
species
– enzymes that synthesize the Nod signal sugar (chitin) backbone
and add the fatty acid side chain are encoded by the common
nod genes:
– nodABC
– enzymes that decorate Nod signals in a species-specific
manner are encoded by host specificity nod genes:
– sulphation, acylation (length, double-bonds), etc
– see R-groups on the Molecular Signaling figure
Step 3: Plant response to specific Nod signal
– plants only respond to Nod signal/Nod factor (NF) made by their specific symbionts: complex biochemical responses to NF result in changes in gene expression and cellular regulation:
– root hair curling: shepherds’ crook
– de-differentiation of root inner cortex cells
– cell division begins nodule formation
Step 4: Infection (1)
A. Infection thread:
– rhizobial cells penetrate into the crook of a root hair
– a tube-like infection thread (IT) forms within root hair => contains a polysaccharide matrix
– bacteria grow along with the IT until they enter the plant cells
Step 4: Infection (2)
B. Contact recognition
– plant recognizes that the bacteria in the IT are the correct species
– plant is on verge of a defense reaction and will abort nodule
if it decides the wrong species may be present
– this prevents infection by pathogens
– recognition via perception of species-specific bacterial cell
surface determinants:
– exopolysaccharides (EPS), lipopolysaccharides (LPS)
Step 5: Nodule and bacteroid development
– infecting Rhizobia are enveloped by a ‘peribacteroid’ membrane
– plant and bacterial cells develop into the specialized nodule
tissues necessary for N2 -fixation to occur
– bacteria within peribacteroid membrane stop growing but
replicate many copies of their genomes
– become terminally-differentiated ‘bacteroids’
– bacteroids synthesize nitrogenase then fix N2
– leghemoglobin produced by the plant functions to:
– facilitate rapid transport O2 to the bacteroids
– protect nitrogenase from O2 damage by binding free O2
Go through bacteriod development
slide 20 lecture 37
Step 6: Nutrient exchange
– photosynthesis by the plant yields sugars that are:
– transported to root nodule cells
– converted to C4 -dicarboxylic acids
– succinate, fumarate, malate
C4 -dicarboxylic acids:
– the bacteroids’ sole source of energy, provided by the plant
– not used for growth by the bacteroids
– oxidized directly via the TCA cycle
– this generates energy and reducing power for N2 -fixation
– bacteroids provide fixed nitrogen to the plant
– NH4+ and alanine
Nutrient exchange is at the heart of the rhizobium-legume
symbiosis
