Nitrogen Fixing Cereals Flashcards
(13 cards)
What does the Haber Bosch process do?
CH4 + N2 -> NH3
Reacts atmospheric N2 with a hydrogen source to produce liquid ammonia
N2 is very stable so this must be done at high temps and pressure
Ammonia is then reacted with concentrated nitric acid to produce ammonium nitrate, or with CO2 to produce urea
How is nitrogen fixed in rhizobia?
- Denitrogenase (coded for big nifHD) forms a hetero-tetramer containing a MoFe factor
- This is transiently associated with denitrogenous-reductase (coded for by nifH), which contains a FeS cofactor
- This transfers electrons to MoFe in order to reduce N2
How can cereals be engineered to become nitrogen fixers?
Engineer nitrogenase genes into crops
Not successful so far
What is leghaemoglobin?
Found in root nodules, binds to O2, reducing concentration within the nodule sufficiently for nitrogen fixation, but still allows bacterial respiration
What are the 3 steps of plant-rhizobial symbiosis?
- Recognition
- Entry
- Nodule development
What happens in the first step of plant-rhizobial symbiosis?
RECOGNITION
- Roots exude flavonoids, which are sensed by the rhizobia, which produce NOD factors in response
- Plant expressed specific compatible receptors for NOD factors (LysM-RLKs, such as NFR1, NFR5 in Lotus japonicus)
- It is a specific interaction between a plant and its rhizobial partner
What happens in the second step of plant-rhizobial symbiosis
ENTRY
- Root hair deforms, traps bacteria into an infection pocket
- Bacteria proliferates, creating an infection foci
- Infection thread extends into the lower cortical layers of the root, where root organogenesis has begun
What is bacterial infection and nodule formation dependent on?
Bacterial infection and nodule formation is dependent on NOD factors
What was found in Medicago about nodule formation?
- NFR1 and NFR5 (from lotus japonicus) were introduced into Medicago
- This allowed it to form symbiosis with L. japonicus’ rhizobial partner
- In Medicago, NFP is a homologue of NFR5, and is essential for all stages of NOD factor signalling
- LYK3/HCL is a homologue of NFR1, is critical for infection thread formation
- This gave rise to a theory that there is a low affinity receptor for perception and a high affinity receptor that only regulates bacterial entry
What happens in the third stage of plant-rhizobial symbiosis and what was found about this step?
- 2 mutants were identified, which undergo spontaneous nodulation, suggesting these factors act downstream of NOD factor perception
- The genes involved indicate a role for calcium and cytokinins in nodule development
- It was shown that after NOD factor application, there is a CA2+ flux, followed by oscillations
Out of a number of NOD mutants, what 3 groups of CA2+ signatures are there?
- No Ca2+ flux or oscillations (LysM-RLKs: NFR1 and NFR5)
- Ca2+ flux but no oscillations (e.g. SYMRK)
- Ca2+ flux and oscillations (e.g. CCaMK, NIN)
What leads to spontaneous nodulation?
Mutations in CCaMK
What happens to Ca2+ during the nodulation pathway?
- Perception of NOD factors by SYMRK results in an influx of Ca2+
- Oscillations are decoded by CCaMK, activating transcription factors including NIN
- Cytokinin signalling is believed to be involved in transmitting an intercellular signal to the inner cortical cells to initiate cell division
