Describe the step 1 of root nodule formation
- Species specific plant flavanoid signals
- The root exudates of different legumes species each contain a UNIQUE COKCTAIL OF FLAVONE AND ISOFLAVONE (organic) compounds that function as species-specific chemical ID signals
- Soybean: Genistein, daidzein (isoflavones)
- Alfalfa : Luteoln (flavone)
Describe the step of nodule formation
- Rhizobial response
A. COLONIZATION
- Rhizobia living in soil can sense and respond to flavonoid signals of their specific root plant (don’t respond to non-host plant species)
- Rhizobia will colonize the rhizosphere, especially near root hair tips
- Quorum sensing determines if sufficient number of rhizobia are present for successful nodule formation )
B. induction of NOD GENES and species-specific NOD SIGNALS
- Bacterial nod genes are induced (only) in response to the specific flavonoid of their host plant
- The nod genes encoded enzymes that synthesize a unique lipochitoologosaccharide (LCO) NOD SIGNAL or NOD FACTOR
- Short chitin backbone
- FA side chain
- Unique R-group decorations for host specificity - UNIQUE NOD SIGNAL function as species-specific chemical ID response signals that stimulate the specific host plant to initiate root nodule formation
What are structural characterstics of the Nod signal or nod factor?
- Short chitin backbone
- FA side chain
- Unique R group
The enzyme that synthesize the NOd signal sugar backbone and adds the FA is encoded by which nod genes?
NodABC (common nod genes)
The enzymes that decorate the NOd signals in a species-specific manner are encoded by what nod genes?
HOST SPECIFIC nod genes
Describe step 3 of the nodule formation
- Plant reponse to specific Nod signal
- Plants only respond to Nod signal/factor made by their specific symbionts : complex biochemical responses to NF result in changes in gene expression and cellular regulation
- root hair curling
- De-differentiation of root inner cortex cells
- Cell division begins nodule formation
Describe step 4 of nodule formation
- Infection
A. INFECTION THREAD
- Rhizobial cells penetrate into the crook of a root hair
- 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
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 bacterial cell surface determinants:
- EXOPOLYSACCHARIDES (EPS)
- LIPOPOLYSACCHARIDES (LPS)
Describe step 5 of nodule formation
- Nodule and Bacteroid development
- Infecting Rhizobia are enveloped by a peribacteroid membrane
- Plant and bacterial cells develop into the speialized nodule tissues necessary for N2-fixation to occur
- Bacteria within peribacteroid membrane stop growing
- 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
What is the function of leghemoglobin?
Facilitate rapid transport of O2 to the bacteroids
Protect nitrogenase from O2 damage by binding free O2
Describe step 6 of nodule formation
- Nutrient exchange => The heart of rhizobium-legume symbiosis
Photosynthesis by plant tields sugards that are:
- Transported to root nodule cells
- Converted to C4-dicarboxylic acids
- Succinate, fumarate, malate, malate
C4-dicarboxylic acids:
- The bacteroids sole source of energy - Not used for growth by the bacteroids - Oxidized directly via TCA cycle
This generates energy and reducing power for N2-fixation
Bacteroids provide the plant with fixed nitrogen => NH4+ and alanine
Why do bacteroids stop growing?
Free-living rhizobia can grow using malate as carbon and energy source but bacteroids can only use it as an energy source
=> Malate CANNOT be used for growth by bacteroids
=> bacteroids cannot express PEP carboxykinase=> enzyme needed for growth on malate
- Bacteroids cannot be cultured from crushed nodules
- Control of bacteroid growth prevents parasitism/pathogenicity
=> Energy invested by plant is used to fuel N2-fixation
What benefit do Rhizobia derive from the symbiosis?
Release of undifferentiated bacteria from the nodule
=> When the nodule or plant dies
- The very small number of infecting cells give rise to many times their number within each nodule
- Cells grow during infection and some are maintained in an undifferentiated viable state in the infection threads
- Undifferentiated cells can be cultured from crushed nodules
Some strains of rhizobia produce rhizopines within the nodule
- Rhizopines are tailor-made NUTRIENTS that can feed rhizobia of the same strain (but not other bacteria) in the rhizosphere
