Plant lecture 10 - C4 photosynthesis

Question 1

C4 photosynthesis discovery

Answer 1

• 3 research groups simultaneously found in 1960, one = Hatch + Slack in Australia
• In certain tropical grasses, 1st formed products after 14CO2 feeding are labelled 4C compounds x 3C
• 14C label quickly moves into usual 3C compounds

Question 2

Anatomy of C4 plants

Answer 2

• Kranz anatomy
• Large bundle sheath cells (BS) surrounding vascular bundles contain prominent chloroplasts arranged around their outer walls
• Vascular bundles are separated by 2 layers of intervening mesophyll cells
• Explains difficulty in finding Rubisco (Rubisco present but entered contained in bundle sheath cells)

Question 3

C4 pathway

Answer 3

1st step = carboxylation phase in mesophyll cells. Co2→HCO3- which combines w/ phosphorylation-pyruvate → oxaloacetate

2. = decarboxylation phase inside thick-walled bundle-sheath cells, malate→pyruvate w. release of CO2
   - Shuttling of CO2 from mesophyll to bundle sheet. PEPC = v efficient E + draws down CO2 in mesophyll to 150ppm (2000 in bundle sheath)
   - Bundle sheath cells have chloroplasts w/ thylakoid lamellae deficient in PSII so x O2 evolution in BS cell. Good for rubisco (suppresses photorespiration) but lacks NADPH so Calvin cycle x have enough reducing power
   - Means activity of Calvin cycle is split as only 1/2 NADPH is supplies to drive cycle, 1/2 = supplied inside the mesophyll cell

Question 4

C4 advantages

Answer 4

• Provides Rubisco w/ ↑ CO2 for carboxylase reaction

- Suppresses oxygenase activity, overall v efficient C fixation

Question 5

Key E in the C4 pathway

Answer 5

1. Carbonic anhydrase
   CO2 + H2O → H2Co3 → HCO3- + H+
   - Rate of spontaneous hydration of CO2 = 10,000 x too slow to provide bicarbonate at rate needed
   - So present at ↑ quantity + ensures CO2 hydration = good rate
2. Phosphoenol pyruvate
3. Decarboxylase e.g. malic E
   malate + NADP+ → pyruvate + CO2 + NADPH
   - CO2 released builds up locally to ↑ conc. inside bundle sheath cell, helps Rubisco bet at max efficiency
4. PEP regeneration e.g. PPDK
   - Extra E requirement for pathway (2 ATP per CO2 fixed)

Question 6

3 subtypes of C4 plant

Answer 6

• Variation lies mainly in localisation of decarboxylating E in bundle sheet
  1. NADP-malic E (chloroplast)
• ↑ common form, malate enters bundle sheath + is ox. decarboxyl. to pyruvate + frees CO2
  2. NAD malic E (mitochondria)
• Aspartate into bundle sheath. Transaminated to oxaloacetate
• Malic E converts malate → pyruvate + CO2
  3. PEP carboxykinase (cytosol)
• more complex
• Asp coming in → oxaloacetate → PEP, uses ATP + releases CO2
• To support ATP requirement, ↑ activity of malic E in mit.

Question 7

Tissue specific gene expression

Answer 7

• C4 photosynthesis involves partitioning of photosynthetic E btw 2 cell types (mesophyll + bs)
• Hypothesis for control at molecular level = PPDK or PEP carboxykinase = expressed in mesophyll x bs cells
  Model A = C4-specific sequence elements are present in upstream promoter region of gene although both C3 + C4 have needed TF. Thought correct
  Model B = PPDK has upstream promoter sequence in C3+4 plants but gene is only transcribed in C4 as TF present in C4 cells

Experiment

• Upstream promoter sequence from PEP carboxykinase of C4 is ligated to Gus reporter gene
• When gene is expressed in a tissue if sugar substrate is produced, blue dye is expressed. Blue = mesophyll cells
• If use C3 promoter, blue = mesophyll + BS
• Conclude = specific sequence elements present in promoter of C4 gene that can confer specificities of expression. Supports A

Question 8

Key properties of PEPC

Answer 8

• ↑ specific activity in C4
• Effective Km for CO2, lower than Rubisco in C3 plants
• Regulated by reversible phosphorylation
• Allosterically activated by G6P, Gly, Ala
• Allosterically inhibited by malate, OAA, Asp

C3 vs C4

• C4 = distinct Ser774 in region 5. C4 determinant
• C3 = Alanine774
• Phosphorylation site of PEPC = at N terminus. One phosphorylated, C4 has ↑ affinity for substrate + ↓ affinity for PEP
• Also phosphorylated C4 has ↓ sensitivity for inhibition by malate

Question 9

CO2 diffusion gradients in C3 vs C4

Answer 9

• C4 plants confer ↑ efficient H20 use so can grow in hot + semi-arid environment
• Greater draw down of CO2 in C4 plants = direct function of kinetic properties of PEPC vs Rubisco + directly leads to ↓ transpiration rates

Question 10

Ecological characteristics of C4 plants

Answer 10

• Total 7500 species (around 3% of all flowering plants)
• Have high max rate of photosynthesis and growth
• More H20 efficient than C3 plants
• 2 main ecological groups = tropical + subtropical grasses vs highly stress-tolerant plants

Question 11

C4 advantages / disadvantages

Answer 11

Advantages
- C4 photosynthesis effectively eliminates photorespiration, more water/nitrogen use efficient + permits growth under ↑ temperature conditions

Disadvantages
- ↑ energetic cost + intrinsic cold sensitivity