Optimising carbon fixation part 2 LECTURE 4

CAM

Crassulacean Acid Metabolism

CAM vs C4

• C4→ Spatially separated CO2 assimilation and fixation

• CAM→ temporal assmilation and fixation

How many plants use CAM?

• 5% of all plant species

• more than 35 plant families

  → much more widespread than C4

Where are CAM plants found?

• Semi -arid

BUT ALSO

• Can be aquatic

Examples of CAM plants

1. Succulents

2. Pineapple

3. agave

4. vanilla

5. Cacti

6. Orchids

7. some ferns

8. strangling figs

CAM plants native to the UK

1. Sedum anglicum

2. Sedum telephium

3. Sedum acre

SEDUM family

How does CAM work

1. NIGHT: Fixation: sugars→ C3 acids→ PEPC with HCO3- to make C4 acids (malic acid)

   • Done overnight (instead of in separate cell)

   • so need a lot of sugars at night to fuel this!

2. Malic acid accumulates overnight→ cell vacuole

3. DAY: decarboxylation

   • C4→C3 acids and CO2 out (for RUBisco) → back into sugars

4. SO then can be used by Rubisco in the light

Temporal separation→ Different stages Pt1

Phase 1: NIGHT→ Stomata open

• Vapour pressure high

• transpiration low

  → PEPC activity:

  • Malic acid increases→ into vacuoles

  • Glucan increases as it is converted into Malic

  • CO2 fixation increased due to PEPC activity

  The stomata are open for this

Phase 2: EARLY MORNING→ Stomata open

• Malic acid→ stop increasing (starts to decrease)

• CO2 → Spike and then decrease (as stomata then shut!)

• Glucan→ slight increase?

WHY:

• Transition of PEPC → Rubsico acitivity

  → burst of CO2 uptake until stomata close

• (one of two transient daytime phases where additional CO2 fixation may occur)

Temporal separation→ Different stages Pt2

Phase 3: Morning and afternoon

• Malic acid→ decreases

• CO2 assim→ low then increase

• Glucan→ increases

WHY:

• stomata close

  → Malic acid is Decarboxylated (decreases)

• High concentrations of CO2 made (under closed doors) (low CO2)

  → used to suppress photorespiration

• This replenishes large storage of carbs used at night (glucan increases)

Temporal separation→ Different stages Pt3

PHASE 4: Evening

• malic→ slight increase

• CO2 assim→ slight increase then plataeu

• Glucan→ plateau

WHY:

• Stomata re-open→ if well supplied with water

• Direct C3 photosynthesis:

  • takes in CO2

  • glucan gonna be made

  • Malic not really made

How to measure the CAM activity?

1. Take leaf samples at dusk and dawn

2. back titrate the sap extract to neutrality

3. Allows to calculate how much malic acid present

C3 vs CAM anatomy 1

C3:

1. smaller cells

2. high intercellular airspace

3. thin leaves

C3 vs CAM anatomy 2

CAM plants

1. Large cells

2. Large vacuoles

   • to store malate

3. Small intercellular airspace

   • tightly packed to reduce CO2 leakage

4. Thick leaves with thick waxy cuticle

5. Smaller and fewer stomata

   • prevent water loss

Adaptation to dry environments and to store a lot of malate (because it is not getting moved)

Stomata: Commelina communis vs CAM plants

• smaller

• behave differently

  • open at different times

Commelina communis:

• Where: high humidity, sufficient water

• Adaptations: large stomata, high stomatal density

CAM

• Where: semi arid, water limitation

• Adaptations: small stomata. low stomatal density

Stomata are regulated differently

C3 and C4 stomata open when:

• daylight→ blue light

• sufficient water

• low internal CO2

Close when

• high CO2→ low stomatal conductance

• lack of light

• low water→ ABA

CAM→ opposite

How open at night?

1. At night PEPC activity sequesters CO2 into C4 acids

2. Internal CO2 concentrations are LOW (coz its being converted constantly)

3. Low CO2→ no closing signal for stomata at night

   → Stay open

How closed in the day?

1. In the day: blue light could trigger opening BUT

2. PEPC→ Rubisco: So CO2 concentrations increase dramatically

   • due to carboxylation

3. The CO2 increase outweighs the light increase

   → Stomata close

Therefore: the principles are the same but Stomata close due to the internal CO2 concentrations

But to achieve the temporal separation

• need to regulate when PEPC enzyme is available

• Cant just put it in a different cell like in C4 plants

How is this done?

1. At night: de novo transcription of PEPC kinase

2. PEPC is activated by phosphorylation

3. The phosphoylated form is active because Malate does not inhibit it

4. In day: Phosphatase dephosphoyrlates it

5. Malic acid builds up

6. Dephosphylated enzyme is inhibited by malic acid

7. So PEPC becomes deactivated during the day

How is PEPC kinase controlled?

Under circadian control

Facultative CAM plants

• Can switch CAM on and off

• In response to drought or high light

Facultative CAM e.gs

1. Ice Plant Mesembryanthemum crystallinum

• Responds to salinity and drought

2. Clusia pratensis

3. Talinum triangulare

2. Clusia pratensis → use CAM when stressed

1. Water withheld

2. Switched to gas exchange at night→ CAM

3. Switch back to the day when water provided

2. Talinum triangulare→ CAM when stressed

1. Drought

2. Increase in night-time acid storage→ CAM

Ecological Advantages of CAM

1. Desert

   • Good at conserving water

2. Tropical forests

   • greater CAM biomass than in deserts

     • orchids, bromeliads

3. Aquatic

   • macrophytes

     • e.g fern ally, Isoetes, shoreweed

   • plants

     • hydrilla

       → Although lacks Kranz anatomy

Has convergently evolved in many families!

How advantageous in aquatic habitats?

• CO2 diffuses 10000x more slowly

THEREFORE:

• need a CO2 concentrating mechanism

• also use sediment CO2 in carbon concentrating mechanisms

Other carbon concentrating mechanism found in

1. Diatoms

2. Dinoflagellates

3. Macroalgae

In the ocean→ IMPORTANT for primary producers to cope with infficiencies of Rubisco

Energetic requirements for CAM

1. Large amount of carbs needed at night

2. A lot of organelle storage for

   • carbs and organic acid

3. Transport between cell organelles

OVERALL: very high ATP and NADPH demand

Compared to C3

• Even higher than C4

• Much higher than C3

Due to this…

• CAM plants can grow very slowly

Exception? Agave tequilana

Can be as productive as C3 and C4→HOW?

• carbon-concentrating effect of stomatal closure in the day

• sufficient to offset the energetic cost of running CAM

IF: nutrient availability are comparable

Why cant C4 and CAM operate at the same time?

1. Differences in metabolism

   • same enzymes needed by at

     • different times

     • different cells→ Rubisco and PEPC would compete for CO2 in the same cell mesophyll!

2. Differences in leaf anatomy

   • C4→ need Kranz anatomy

   • CAM→ need large mesophyll cells (not Kranz anatomy)

Exception→ Portulaca oleracea

C4 species but capable of expressing CAM photosynthesis

• as a stress response

  • to drought

When water available

Uses C4

When water scarse

• Uses CAM

How is this possible?

• temporal up regulation of different isoforms of key enzymes

At night:

• malate synthesised

• in mesophyll cell

  → CAM

In day:

• Malate released from mesophyll

• transported to bundle sheath

• decarboxylated: CO2→ Rubisco

Why care about this?

• Growing population

• Need to feed→ need crop yields to increase

  BUT ALSO

• Climate change

  • less water

  • less land

  • higher temperatures

• Must limit fertiliser use

What need to do to do this?

1. Increase photosynthesis

2. Increase water efficiency

Steps to improve photosynthesis

1. Pigments which use more of light spectrum

2. Engineered Rubisco with higher carboxylation efficiency

3. C4 pathways into C3 crops

4. Synthetic photo respiratory bypasses

5. Engineer improved water use efficiency

1. Pigments which use more of light spectrum

• Could use pigments from other algae or bacteria

• which uses light from non visible parts

  • intr-red and UV A

2. Engineered Rubisco with higher carboxylation efficiency

• Many diverse forms of Rubisco across photosynthetic organism

  • O2 sensitivity, catalytic efficiency

• Could harness this diversity

• Could enhance carbon fixation rates

• Use synthetic biology techniques

3. C4 pathways into C3 crops

• C4 evolved over 60 times

• So might be kinda easy to engineer C4 out of C3 crops

• e.g trying to do this with rice

• BUT: need to change

  • leaf anatomy

  • cell-type-specific expression of Calvin cycle enzymes

4. Synthetic photo respiratory bypasses

• develop a shortcut from the long recycling procress of phosphoglycolate

• enable to save energy

• boost crop yields

1. Engineer improved water use efficiency

• changes in stomata density or

• stomata sensitivity to the environment

Issues with breeding stategies?

• limited genetic diversity present in photosynthetic enzymes and pathways

Solution to this

Use

• genetic engineering

• Synthetic biology

But this can be difficult…

• requires in-depth understanding of biological systems involved

• adanved technologyies

• synthetic biology tools

• comprehensive genomic data of crop plants