Transforming photosynthesis:
frontier biology for a changing world
Colin Osborne (University of Sheffield)
Global Food Security in 21st Century
Last 50 yrs:
• global popn + grain prodn doubled
• 1 billion people are hungry
Next 40 yrs:
• global population to stabilize at 9 bn
• 70-100% increase in crop production
http://gli.environment.umn.edu
We already farm 40% of Earth’s land area
http://gli.environment.umn.edu
Yield stagnation in major crops
Rice
http://gli.environment.umn.edu
Yield stagnation in major crops
Wheat
http://gli.environment.umn.edu
Photosynthesis
sunlight
water + carbon dioxide
sugars + oxygen
“makes life and oxygen out of water and thin air”
Cyanobacteria
“a small step for a bacterium,
but a giant leap for biology”
O2
4 H+ + 4 e2 H2O
ATP
NADPH
Cyanobacteria enslaved within plants as chloroplasts
www.microscopy-uk.org.uk/mag/artnov00/dwelodea.html
Photosynthesis powers life on our planet
NASA SeaWIFS
http://archive.org/details/SVS-3451
Rubisco: the most abundant protein on Earth
Rubisco
CO2
sugar
Calvin
cycle
NADPH
ATP
• Ribulose-1,5-bisphosphate carboxylase/oxygenase
• ~5-10 kg for every person on the planet
• “C3 photosynthesis”
http://cdiac.ornl.gov/trends/co2/sio-mlo.html
CO2 in geological time
Cretaceous chalk
100 myr ago
Carboniferous limestone
300 myr ago
CO2 and climate in geological time
http://droyer.web.wesleyan.edu/GSA_Today.pdf
Rubisco works well at high CO2 …
Rubisco
CO2
Calvin
cycle
… but it’s less efficient at low CO2
Rubisco
CO2
photosynthesis
O2
photorespiration
• uses ATP, NADH
• releases CO2
• CO2 : O2 in the modern atmosphere = 1 : 500
• photosynthesis : photorespiration = 3 : 1
Evolution of carbon-concentrating mechanisms
ATP
CO2
CO2
Evolution of carbon-concentrating mechanisms
cyanobacteria
green algae
Evolution of carbon-concentrating mechanisms
C4 plants
CAM plants
The C4 carbon-concentrating mechanism
CO2
malate
C4 pathway requires anatomical adaptations
How many times did C4 photosynthesis evolve?
• DNA sequences used to reconstruct tree of life
C3
C4
C4 evolved
• DNA sequences used to reconstruct tree of life
C3
C4
C4 evolved
C4 photosynthesis evolved more than 70 times!
How did C4 pathway evolve so many times?
C4 cycle delivers malate to
bundle sheath
Enzymes in bundle sheath
remove CO2 from malate
All plants have metabolic components of C4 cycle
Xylem transports malate from
roots to photosynthetic cells in
stem
Enzymes in these cells
remove CO2 from malate
CO2 is fixed by photosynthesis
http://eprints.whiterose.ac.uk/113/1/quickwp1.pdf
All plants have metabolic components of C4 cycle
Xylem transports malate from
roots to photosynthetic cells in
stem
Enzymes in these cells
remove CO2 from malate
CO2 is fixed by photosynthesis
Evolution builds new machinery from existing parts!
Why don’t all plants use C4 photosynthesis?
Why are some lineages more likely to evolve C4?
Why are some lineages more likely to evolve C4?
C4 photosynthesis evolved after CO2 dropped
C4 evolved
http://www.sciencedirect.com/science/article/pii/S0960982207023445
C4 photosynthesis evolved in hot, sunny places
Why don’t all plants use C4 photosynthesis?
1. Only some lineages had the necessary
anatomical arrangement for C4 photosynthesis.
2. CO2 drop 30 myr ago drove evolution of C4
photosynthesis in hot, sunny environments.
3. C4 evolution made use of metabolic “parts”
already present in every plant.
C4 grasses are the most productive plants on Earth
Echinochloa polystachya
100 t / ha
8 m water rise
Miscanthus giganteus
60 t / ha
[wheat ~20 t / ha]
Can we harness C4 photosynthesis in agriculture?
C4 crops already important for food and fuel
maize
sugarcane
miscanthus
Can we harness C4 photosynthesis in agriculture?
• Rice uses C3 photosynthesis in hot, sunny climates
• It has reached a yield plateau
• C4 pathway could boost photosynthesis by 50%
C4 Rice Project
• Introduce C4 into rice
• Discover genes involved
• C4 rice crop in 20 years?
www.3to4.org
www.c4rice.irri.org
Combining algal and plant photosynthesis
Algal carbon-concentrating mechanisms into plants?
http://cambridgecapp.wordpress.com
osbornelab.group.shef.ac.uk
@sheffieldplants