L21 - Nitrogen Flashcards
What is Nitrogen needed for?
Where is most leaf Nitrogen concentrated?
- Amino acids
- Nucleotides
- Secondary metabolites
- 80% Leaf N in chloroplast
- 30% Leaf N in RuBisCO
What 5 things can Nitrogen deprivation lead to?
- Stunted growth
- Decreased photosynthetic capacity
- Chlorosis
- Early senescence + leaf loss
- Poor quality grain
What is the first step in Nitrogen uptake from plants?
Which organisms perform this?
Give examples
- N2 can’t be used directly, must be FIXED - N2 converted to other N compound usuable by plants.
- Fixation by diazotrophs w/ Nitrogenase enzyme
- Symbiotic diazotrophs: E.g. Rhizobia (form root nodules in legumes + shrubs)
- Free living diazotrophs in soil: Majority, benefit from C compounds exuded by roots
How does the form of Nitrogen taken up by plants depend on pH and location?
Acidic soils: Ammonium and amino acids
Higher pH and aerobic: Nitrate
Arctic and tundra soils: Amino acids
Urea occasionally
How is does Nitrogen uptake occur?
Give examples of the transporters used
- Via proton symporters (LATS and HATS)
NRT1 family = LATs distributed throughout the plant:
- Epidermal PM LAT - converts to HAT via phosphorylation under N deprivation (NRT1.2)
- Loading of xylem (NRT1.5)
NRT2 family = HATs
- lateral root epidermal PM (NRT2.4)
- epidermal PM HAT - upregulated under N deprivation
What is problematic about Ammonium uptake?
How is this problem sorted?
Name and describe the transporters used in Ammonia uptake
Toxic to plants:
- Dissipates proton gradients (NH3 can accept H+)
- This affects uptake of other + charged ions
- NH4+ usually used, less toxic
- AMT family, inc. LATs and HATs
- HATs mostly in roots hair + epidermis
- LATs further into plant tissue e.g. endodermis
Describe the transporters used for amino acid uptake
- LATs and HATs from 3 main gene families
- E.g. charged amino acid transporters
- Diversity corresponds to transport properties of amino acids
Describe where Urea may become a large source of Nitrogen?
Describe the transporters used for Urea uptake. Give an example
- Animal waste-based fertiliser
- Proton symporters
- E.g. DUR3 in root PM in rice + maize
- Function confirmed from increased amounts of transferred rice urea symporter producing more growth in Arabidopsis seedlings with only Urea N supply
How is Nitrogen transported around the plant?
When does this happen in relation to assimilation? Give examples and explain how this may vary geographically
- Nitrate (pre-assimlation N) transported via xylem.
- Loaded by LATs or HATs in xylem
parenchyma (adjacent to xylem) e.g. NRT1.5 - Mutants retain nitrate in roots, less in shoots
- Assimilation can occur in the roots or shoots (i.e. pre or post transport)
- Roots: Pea + radish, 80% sap N organic
- Shoots: Arabidopsis, 2% sap N organic
- Tropical - mostly shoot assimlation
- Temperate - mostly root assimilation
What is Nitrogen Assimilation?
Give the two general equations for this
- Converting inorganic N into organic forms like amino acids, proteins, and nucleic acids
1) Nitrate → Nitrite → Ammonium
2) Ammonium + glutamate → glutamine
Which enzyme converts Nitrate to Nitrite?
Describe this enzyme
What factors promote transcription of this enzyme?
What factors inhibit transcription of this enzyme? How does this inhibition actually occur?
- Nitrate reductase
- Dimeric enzyme found in cytosol
- Facilitates e- transfer from NADPH or NADH to Nitrate for reduction
- e- travels along transport chain:
FAD → Cytochrome b557 → Molybdopterin - Factors showing availability of acceptors for toxic ammonium (in 2nd equation) promote transcription e.g. light, sucrose
- Factors showing assimilation pathway backed up inhibit transcription e.g. Glutamine
- Enzyme controlled by phosphorylation by kinase
- 14-3-3 protein can bind to this site to inhibit enzyme
Which enzyme converts Nitrite to Ammonium?
Describe this enzyme
What factors promote transcription of this enzyme?
What factors inhibit transcription of this enzyme? How does this inhibition actually occur?
- Nitrite Reductase
- Monomer located in plastids
- Transfer e- from ferredoxin (for photosynthetic cell) or from oxidative pentose phosphate pathway (for non-photosynthetic cell)
- Reduces Nitrite to Ammonium
Describe the next stage in Nitrogen assimilation after Ammonia has been formed
- GS-GOGAT cycle
- Used for incorporating N into organic molecules like amino acids
Describe the GS-GOGAT Cycle and the possible steps after its completion
1) Ammonia + Glutamate → Glutamine via Glutamine synthetase
2) Glutamine+2-oxoglutarate → 2Glutamate via GOGAT
3) Glutamate can be reused in the cycle, assimilating more N
4) OR Glutamate transported away, transaminases transfer NH2 to oxo acid.
5) Any amino acid pr nucleic acid then formed
Where and how is Nitrogen stored for Short Term Storage?
- Excess nitrate stored in vacuole by proton antiporters
How is Nitrogen stored in the medium term?
- Stored as amino acids
- E.g. Asparagine (transport amino acid)
How is Nitrogen stored in the long term?
- High molecular weight storage proteins
- Used for surviving dormancy or in seeds
- E.g. globulins in legumes and potatoes
- E.g. prolamins in cereals
- Both are major protein source for humans + animals
Explain the human significance of prolamins
Give the pros and cons
- Repeat proteins of 20 amino acid motif
- Gluten = composite of prolamins
- Repeats make prolamins viscoelastic + good for baking
CONS:
- Wheat gliadins (type of prolamin) can cause coeliac disease
- Hypoimmunogenic wheat could be made, but not allowed in Europe
When does the remobilisation of stored Nitrogen occur? Give 3 examples
How does remobilisation occur?
1) Higher influx of N during vegetative stage, so remobilisation during reproductive stage to keep levels high
2) In annual plant, up to 90% N remobilised in leaves and sent to seeds
3) N mobilised and sent to trunk for storage in deciduous trees when leaves lost
- Proteins dismantled to amino acids and nitrate for transport
When is the reassimilation of Nitrogen necessary?
Give the two main examples where reassimilation occurs? Provide evidence for one of these examples
- When metabolic processes release toxic ammonia
1) During remobilisation - captured by cytosolic form of glutamine synthetase
2) Photorespiration - RuBisCO adds O2, not CO2 causing ammonium release.
- captured by chloroplast glutamine synthetase
- antisensed (for enzyme) barley mutants had higher ammonia levels at ambient CO2 conc.
Give 3 points of interaction between C and N metabolism
1) Nitrate reductase regulated by signals relating to C based acceptor for ammonium
2) N availability affects photosynthetic capacity
3) Reassimilation due to photorespiration
How has Nitrogen been important in modern agriculture and how is this also problematic?
- Fertilisers containing many nitrates used in Green revolution - averted famines
CONS
1) Pollution - over 50% of nitrates leach into aquatic ecosystems = algae blooms, eutrification etc…
2) Energy use - Haber Bosch process uses 1-2% of world’s energy
