L20 - Potassium

Question 1

What is K needed for in plants?

Answer 1

• Ribosome function
• Metabolism
• Osmotic Relations

Question 2

What are the basic mechanisms by which a plant will get K+ from the soil?

Answer 2

• Apoplastic, symplastic, trans-cellular pathways
• K+ measurements allow prediction as to whether passive or active transport needed

Question 3

Name a key enzyme used in the active transport of K+

What does it do and how does it function?

Answer 3

• H+ - ATPase
• Sets pH gradient to drive symporters by extrusion of H+ from roots
• Extrusion of H+ ions also sets membrane voltage, used to open channels
• Autoinhibitory R domain is displaced by phosphorylation, activating H+ extrusion (regulates activity)
• H+ - ATPase expression high at nutrient transfer hot spots (root epidermis, xylem parenchyma)

Question 4

Describe K+ uptake in K+ deplete soils

Give an example of one of the transporters used and describe its activity

Answer 4

• Multiple genes for K+ transporters, all membrane spanning integral proteins. Upregulated expression under starving
• E.g. Deficient Arabidopsis = upregulated HAK5 (High Affinity K Transporter 5 - also in cereals)
• HAK5 expression repressed by K+ re-supply
• Altering pH in yeast shows HAK5 is a K+-H+ symporter
• However, HAK5 is Cs permeable - risk of Cs poisoning (caution needed with GM use)

Question 5

Describe K+ uptake in K+ replete soils

Give an example plant

Outline the procedure used to test for channel function

Answer 5

• Energetics suggests uptake at root PM possible through channel
• Group of channel sub-units identified in Arabidopsis
• 4 subunits, each w/ pore region or 2 subunits w/ 2 pore regions each needed
• AKT1 protein functions as channel
• Patch electrode used to control membrane voltage
• If open channels present K+ ions flow through, measured as negative current
• ak1 mutant lacks K+ influx current, confirming AK1 function

Question 6

Describe how K+ is loaded into the xylem and why this is needed

Give an example for this

Answer 6

• SKOR (Stelar K Outward Rectifier) channel allows K+ efflux from stelar cells into xylem across plasma membrane
• Energetics show channel needed to load xylem
• AtSKOR expressed in Xenopus oocytes gives I-V relation of outwards rectifier (showing K+ flow)
• Atskor mutant doesn’t show I-V relation
• Atskor mutant accumulated less shoot K+, showing importance of AtSKOR

Question 7

Describe the function of K+ in the phloem and how it is unloaded

Answer 7

• K+ important for loading sucrose into phloem
• > 50% of K+ moving to shoots returns to roots in phloem
• Channel ATK2 likely used to unload K+ from phloem in the roots (mutant analysis)

Question 8

Describe the use of vacuolar K+ accumulation and how it is done

Give an example

Answer 8

• K+ accumulation in the vacuole crucial for osmotic relations

K+ replete conditions required:
- Two H+ pumps in tonoplast drive K+ uptake by an H+ antiporter
- Named the V-ATPase and the V-PPase
- V-PPase stimulated by cytosolic K+

• NHX1 in Arabidopsis acts as K+ - H+ antiporter to fill vacuole

Question 9

Describe the use of vacuolar K+ release and how it is done

Give an example

Answer 9

• Vacuolar K+ release “top up” cytosol
• Done through channels, apart from under starvation when active transport needed
• TPK1 identified in Arabidopsis
• Increased Ca2+ conc. facilitates channel activation
• Also activated by 14-3-3 proteins (phospho-dependent)

Question 10

Describe the 6 events that occurs when low K+ is sensed

What hormone is also triggered by this?

Answer 10

1) Ethylene production triggered
2) Inhibits elongation of primary and lateral roots (K+ needed to maintain turgor for cell expansion so this stops use of K+ in roots)
3) Causes root hair elongation (to get more K+)
4) Triggers HAK5 expression
5) Increased suberin deposition in endodermis (retains more K+ and water!)
6) Down-regulation of SKOR transcription

Hormone ABA used