Physiology of nutrient acquisition: Ion Uptake LECTURE 1

Essential minerals

inorganic chemical elements

• Required for the completion of life cycle of plant and animals

Where found?

• Cycle continuously through all organisms

How enter the plants

enter the biosphere via plant roots

• uptake from the soil

As plants are autotrophs

Once plant has all the inorganic elements and water and sunlight

→ Can synthezize all organic compounds it needs

How many minerals nutrients needed

16

• including C O and H

Due to this

• plants can be cultivated hydroponically

• without any soil

• used commercially

If not enough nutrient

• Cause diagnostic deficiencies

• negatively impact plant perfermance

Diagnostic symptoms

1. Chlorosis

2. Anthocyanin accumulation

3. Necrosis

4. Stunted growth

Relevance for food security

• most agricultrual soils are deficient in mineral nutrients

  • e.g N, P or K

• agricultural yields depend on fertiliser addition to reach critical concentration

• Relationship between growth/yield and nutrient concentrations depends on zones

  • deficiency

  • Adequacy

  • toxicity

• → varies between different plant species

Critical concentration

• Minimum tissue concentration that

• correlates with maximum growth/yield

Plant mineral nutrition is extremely active area of research:

• Nutrient uptake

• nutrient use efficiency

• development of alternative fertilization strategegies

Needed for

• sustainable feeding growing population

• with changes ecosystems

Root architecture

• Develop an extensive surface area

  • for soil exploration

e.g 16 week-old rye

• surface area is over 500m2

Soil

Heterogenous

Due to this

• Root system architecture (RSA) will determine the abiltiy of a plant to secure mineral resource

Basic anatomy of all root types

1. conserved

2. Varies between monocots and dicots

3. Varies between species

Dicots RSA

1. Prominent central axis

2. Tap roots

3. have lateral roots radiate (allorhizic)

4. net formation of veins in leaves

Monocots RSA

1. many main roots

2. give rise to lateral roots

3. parallel veins

→ creates fibrous root system

• secondary homorhizic

How varies between species

• shallow or deep

• branched or unbranched

What soil is made of:

Mixture:

1. organic and inorganic particles

2. water with dissolved minerals

3. Air (CO2, O2, N2)

How to mineral ions move towards root surface

1. Bulk flow→ with water

2. Diffusion

What affects mineral uptake

1. transpiration rate

2. mineral ion concentration in the soil

3. root ahir SA

4. Due to soil chemistry

   • charges

   • pH

1. transpoirtaion rate and mineral ion concentration

• determine rat of water flow

• thereby amount of mineral provided to the root

3. Root hair

• make intimate contact with all soil phases

• increase root surface area for ion uptake

Trichoblast

• have 30x the SA of an atrichoblast

4. Soil chemistry

• Organic and inorganic soil particles are polyanionic

  THEREFORE→ adsorb cations (K,Mg, Mn, Ca, NH4)

  → forms nutrient reservoir

4. Then how to plants get these cations if attracted to soil?

Cation exchange:

• + changed nutrient replaced

• plant can then extract cations from soil

Cation Exchange Capacity

• Inherent soil characteristic

• depends on soil compositions

  • high CEC→ high clay or organic matter

    → able to provide more nutrients to plants

  • low CEC→ sandy soil

4. Root growth pH favoured at

pH 5.5-6.5 slightly acidic

How this pH happens?

• Root respiration (production of energy for root growth

• Decay of organic matter

→ generate CO2:

• equilibriates with soil water

  → produces H+ ions

• contributes to the acidity of the soil

What about anions?

1. Nitrate

2. phosphate ions

Repelled by the polyanionic particles surfaces

1. Nitrate NO3-

• remain in the soil fluid

• prone to leaching via water movement

• very mobile in soil

• concentrations can really vary

2. Phosphate H2PO2-

• preciptates at alkaline and acidic pH

• extremely immobile

• extremely low abundunce

Plant demand for N vs P

• 10x higher demand for N than P

But short fall of P

• P concentrations are 1000x lower than N

  → 100x shortfall

Due to this

• not enough P in soil for plants

• N often in run off (concentrations really vary)

  • eutrophication

→ limits plant growth

Roots can

1. modulate the pH of soil

2. sense low availability of nutrients

1. Modulate soil pH

Via extrection of protons

2. Sense low avilabiltiy of e.g phosphate

• Induce the production and release of organic acids (anions)

  → Anion exchange

Specialised proteoid roots (cluster roots) for phosphate scarcity

Used for anion exchange

1. e.g in Lupinus and Macademia

2. Produce vast amounts of carboxylate ions

3. aid phosphate mobilization

Radial root anatomy

1. Root epidermis (rhizodermis)→ ion uptake

2. Cortex→ storage and ion uptake

3. Endodermal cells

   • casparian strip

4. Stele

1. Root epidermis

• trichoblasts

• atrichoblasts

→ for ion uptake

• root hair increasing SA

2. Cortex

• Ground tissue

  → high storage capacity

  (at higher ion availability → ion uptake)

3. Endodermal cells

• Radially surrounded by hydrophobic casparian strip

  → provides diffusion barrier between the root centre and periphery

4. Stele

• Contains vasculature for long distance transport

  • of photosynthates (phloem)

  • water and minerals (xylem)

As the xylem exchanges freely with the liquid phase…

• ions enetering into the stele apoplasts

• also enter into the transpiration stream

• driven by the negative pressure potential

Route taken by minerals into the stele

1. Symplastic

   • through plasmodesmata

2. Apoplastic

Apoplastic way

1. Apoplastically in cell wall space

2. only until the hydrophobic casparian strip

3. enter the symplasm

4. Once in the stele

   • minerals releaesed into apoplast

5. Enter the pressure-driven bulk flow of water

   • into xylem vessels

Functionally similar to animal intestinal epithelia

1. Resemble and inverted gut

2. Selective uptake mediated by epithelial brush border or root epidermis (selective uptake)

3. tight junctions and Casparian strip (barrier to diffusion)

   • seal neighbouring cells and block diffusion

4. Polarity→ peripheral central polarity in the root

   • basolateral polarity in the animal epithelia

How identidied plant ion transporters

• plant gene rescue→ yeast complementation

  1. Yeast strains mutated in transporter function of interest

  2. transformed with a population of cDNA from plant roots

  3. restoration of growth indicated functional complementation by plant gene

K+ inward rectifying channels in Arabidopsis

Shaker type K+ channels

• AKT1 and KAT1

Features:

• 4 subunits

  each with

  • 6 TM domains

  • voltage sensing region of segment 4

AKT1

• one of the two major uptake systems for K+ from the soil

Recap of uptake methods

Complex kinetics→ high and low affinity transport systems

1. e.g Potassium uptake

2. e.g Nitrate uptake

1. Potassium uptake

• biphasic kinetics→ two distinct transport proteins involved

1. At 1mM → high afffinity

   • active symport

   • potassium starved

2. At higher concen→ low affinity system

   • through K+ channels

   • high potassium fertility→ so need a lower affinity

2. Nitrate uptake→ biphasic due to dual affinity transporter

Transporter CHL1

• Low nitrate availability

  • CHL1 phosphorylated

  • convert low affinity into high affinity transporter

• High nitrate availability

  • CHL1 not phosphoylated

  • operates at low affinity

THEREFORE CHL1

Operates as

• nitrate sensor

• regulates nitrate induced gene expression

Once the ion has reached the stele

1. exists symplast at pericycle and xylem parenchyma

2. loaded into xylem

3. further distribution through plant

→ion efflux is highly regulated

How ion efflux regulated

• involves dedicated channels/carriers

  • e.g SKOR (outwardly rectifyng K+ channel)

  • e.g NRT1.5 (low affinity transporter)

How is SKOR regulated

• Expression is inhibited by stress hormone ABA

What this suggests

• control of K+ translocation to shoot

• part of plant drought response

Plant ion influx/efflux activies

• involves diverse channels/carriers

• with discrete roles in

  • root uptake

  • sub-cellular partioning

  • xylem loading/unloading

  • re-distribution

• e.g Arabidospsis has 73 genes coding for different NO3 transporters)