L23 - Micronutrients Flashcards
Give a brief overview of Fe in plants
- Most abundant transition metal in plants
- Useful Fe source in human diet
- Not readily bioavailable from cereal seeds due to chelation by phytate
In what forms is Fe found in plants and what is its function?
Where is the bulk of Fe held in the plants?
- Chelated by haems and sulphur clusters
- Acts as enzyme co-factor
- Fe in co-factor acts in respiratory + photosynthetic e- transport chains
- Bulk found in chloroplasts
Why is control of Fe bioavailability important?
What protection exists against excess Fe?
- Fenton catalyst in free form = produces hydroxyl radicals (potent ROS)
- Excess Fe captured by ferritin (globular storage molecule) to avoid toxicity
Where can Fe be stored? Give two places
Name the transporters used for one of the places
- Vacuole
- Apoplast (bound to pectin)
- VIT1 and FPN2 transporters used to load vacuole
How abundant is Fe in soil and how easy is uptake of Fe?
What are the effects of Fe deprivation?
- Fe can be abundant
- Not bioavailable at normal soil pHs (hard to take up)
Fe Deprivation:
- Chlorophyll synthesis inhibited = chlorosis
- Impaired growth and reproduction
- N assimilation impaired
There are two strategies for Fe uptake.
Divide broad plant groups into these two strategies
Strategy 1: Dicots and non-grass monocots
Strategy 2: Grasses
Describe Strategy 1 uptake of Fe in 4 steps
1) Root plasma membrane H+-ATPase acidifies rhizosphere, solubilising Fe3+
2) Soluble Fe3+ captured by secreted chelators
3) PM ferric-chelate reductase reduces Fe3+ to Fe2+
4) Fe2+ transported into cell
Name the genes that encode the proteins used in Strategy 1 in Arabidopsis
Give an example of a transcription factor that is upregulated in At upon deprivation.
How is transcription activated?
- AHA2 encodes H+ pump
- FRO2 encode ferric chelate reductase
- IRT1/2 encodes uptake transporters
(All induced upon deprivation) - bHLH TFs upregulated and bind to FRO2 and IRT1 promoters
- Mediator multi-protein complex links these to RNA polymerase Pol II to activate transcription
How large are the changes seen in RSA under Fe deprivation?
What are the changes seen?
- Subtle changes
- Increased root hair elongation
- Some elongation of primary and lateral roots
- (Additionally ABA production induced, promoting root uptake and vacuole efflux)
Describe Strategy 2 uptake of Fe
1) Phytosiderophores secreted in rhizosphere
2) Phytosiderophores chelate to Fe
3) Taken up by specific transporters e.g. YS1 H+ symporter (without reduction)
Describe an experiment showing that YS1 is taking up Fe3+, not Fe2+
1) Express IRT1 and YS1 in separate Fe transport deficient yeasts
2) Grow yeasts on medium containing 1) DMA as the phytosiderophore and 2) BPDS to chelate Fe2+
3) Supply only Fe3+ ions to both mediums
4) Result: IRT1 can’t support yeast growth (no Fe2+ ions to uptake) but YS1 can
How is Fe transported around the plant?
Describe the possible outcomes of the Iron once it reaches the desired cell
- Transported in xylem, chelated w/ citrate
1) Uptake to chloroplast/plastids by CIP1 permease (At)
- Fe incorporated into haem and Fe-S clusters
2) Unknown substrate released from mitochondria by ATM3 transporter
- Fe-S cluster assembled in cytosol
When and how is Fe remobilised?
- Remobilised from stores under deprivation
- Sensing unknown but potentially Fe available for Fe-S synthesis detected by organelles
- Fe retrieved from apoplast
- Fe efflux from vacuole stores via NRAMP transporters
- Fe chelated to nicotianamine (NA) and redistributed via phloem
- OPT transporters retrieve Fe-Na from phloem
Give evidence that the OPT transporter is needed to redistribute Fe from the shoots to the roots
- Rice OsOPT3 (in leaf phloem) mutated
- Roots show deprivation response
- Shows insufficient Fe travelling from shoots to roots
Explain why the balance of Fe and S is important and how it is maintained
- Fe and S balance crucial for Fe-S cluster assembly + to avoid Fe toxicity
- ++Fe = S uptake increased and vacuolar S efflux increased via upregulation of SULTR transporters
- –S = phytosiderophore secretion and induction of Fe uptake transporters suppressed
Explain why the balance of Fe and Pi is important and how it is maintained
- Pi deprivation can cause Fe toxicity
- –Pi = IRT1 downregulated and ferretin production upregulated
Why is it so important for plants to balance Boron levels precisely?
Geographically where is toxicity and deficiency common?
- Narrowest range of all minerals, easily limiting or toxic
- Toxic in semi-arid regions e.g. Australia
- Deficient in high rainfall areas e.g. SE Asia
What is B needed for and what can toxicity and deficiency cause?
- Needed for wall integrity, binds to pectin
Toxicity:
- Leaf necrosis
- Root growth inhibition
Deficiency:
- Stunted growth
- Male sterility
In what form and how does B uptake occur?
How is B then distributed and redistributed?
What happens under deprivation and replete conditions?
- Present as H(BO)3 and uptake in this form
- Uptake by NIP channels
- Casparian strip forces symplastic flow
- BOR transporters efflux B to xylem for distribution
- Redistribution via the phloem
- NIPs and BORs upregulated under B deprivation and suppressed when B replete
Give an example study that compares B toxicity tolerance between two plants
- QTL analysis on B-tolerant non-agronomic barley landrace (Sahara) and B-sensitive Australian commercial variety (Clipper)
- Sahara had high B efflux rate from roots + low shoot B
- Efflux due to B efflux transporter in root tips BOT1 being more efficient in Sahara
- And more BOT1 copies in Sahara
- Only 2 amino acid difference in BOT1s
