Photosynthesis Flashcards
Why engineer photosynthesis
Growing world population, more mouths to feed, more land lost to housing
Climate change (caused by increased CO2 in air) increases environmental stresses, limiting crop production.
Assimilation (CO2 taken in by plant) increases but so does respiration (energy/sugar burning) so net obtained through photosynthesis is 0
Ex. Area coffee is grown in Asia will be unsuitable in next 20 years
Photosynthesis
Stomata allows gas exchange (CO2 in, O2 out)
Below are spongy mesophyll, area for gas to travel through
Photosynthesis occurs in chlorophyll in chloroplasts
Light reactions in thylakoid (light energy splits water and produces ATP from ADP+Pi and NADPH from NADP+ for dark reactions of Calvin-Benson-Basham in stroma in which sugars are produced requiring CO2 bound by RuBisCO
Types of plants
C3: in mesophyll
Most plants
Intermediates have 3 carbons
CO2 carbon is added to C5 forming two 3C intermediates. The now 3C molecule becomes 6C glucose
Ex. rice
C4: C4 in mesophyll but when it evolves to CO2 the CBB cycle occurs in bundle sheath
Additional step in which C4 intermediate made from CO2 which is not a gas so can be pumped into specialised bundle sheath cells
C4 is split to evolve CO2 and fed back into traditional CBB cycle
Ex. maize, sugarcane, millet, etc
CAM: in mesophyll
C4 intermediate is made but is stored overnight and used in day (doesn’t require bundle sheath since temporal seperation)
Stomata opens at night which is cooler allowing less water loss in gas exchange
Ex. pineapple
Ways to promote photosynthesis
Evolved to be suited to resource limited natural environment and flexible, but not very efficient in one since more important to be adaptable. Photosynthesis is not efficient so room for improvement:
¬50% of energy from sun is in the photosynthetically active spectrum that plants can harness, and more lost from reflection, transmission of light and photochemical inefficiency so total a third of light from sun the plant uses for photosynthetic processes.
More energy is lost in process. For C3 energy lost from carbohydrate synthesis, photorespiration and respiration causes a total 4% of sun energy used. For C4 energy lost in carbohydrate synthesis and respiration leaves 6%.
Areas to address:
Manipulate photosynthesis to increase the efficiency of energy conversion
Enhance tolerance of high light and temperature (global warming)
Productively increase intra-chloroplast CO2 concentration (address RuBisCO inefficiency)
Maximise light harvesting
CBB cycle
Production of intermediates requires multiple CO2 molecules that come into cycle one by one
Ribulose 1,5-bisphosphate is fixed with a CO2 to create short lived 6C intermediate, broken down to 3-phophoglycerate
Series of ATP dependent reactions phosphorylates and reduces molecule to obtain glyceraldehyde 3-phosphate (G3P)
Two G3Ps are fused to create glucose (glucose production involves FBPase)
Ribulose 1,5-bisphosphate is regenerated from G3P in ATP dependent reaction (via trasketolase and SBPase)
Initial experiment to improve photosynthesis efficiency
Overexpress the rate limiting enzymes in CBB cycle: SBPase, FBPase and transketolase, in an attempt to increase rate of photosynthesis
SBPase and FBPase were successful and increased dry weight and seed yield
Transketolase produced toxic intermediate that killed plant.
Overexpressing cynobacteria with SBPase and FBPase activity was successful.
Excess light: why it’s a problem, effects on photosynthesis
Healthy plants can absorb and use more light. Stressed plants cannot tolerate more light
Drought induced stromal closure prevents CO2 uptake
High temperature increases membrane permeability and decreases proton-motive force in light reactions
Cold temperature slows enzyme catalysed dark reactions
Nutrient deficiency or toxicity affects electron transport in light reactions
At low light intensity, light is the dominant factor limiting photosynthesis, and as light intensity increases it has less of an improvement.
At high light intensities, excess light can damage photosynthetic machinery in plants in optimal conditions.
Stressed plants have a lower tolerance for light
Effect of light in plant and how they change to deal with high light
Photo-excited chlorophyll forms excited singlet chlorophyll 1Chl* which can:
Return to ground state through photochemistry (ideal), fluorescence or dissipation (ex. heat)
Convert into excited triplet state 3Chl* which can transfer energy to oxygen to produce singlet oxygen, a ROS which causes damage
Non-photochemical quenching (safety valve)
Light energy is diverted to another path creating heat and reducing energy state back to chlorophyll
Adaptations
Chlorophyll position in:
Low light- clusters on surface to harvest as much light as possible
High light- moves to sides of cell and chloroplasts stack on top of one another to shade those below
Some plants can curl into ball and go into dormant state to avoid stress
Antennae complexes (light harvesting complexes) can be chopped off to harvest less light
Change proportion of light harvesting components: Light harvesting complex decreases. RuBisCO is rate limiting in high light so levels increase
Effects of plant in elevated temperature, reason, modifications to address
Rate of photosynthesis increases (increase in kinetic energy causing more molecular collisions) then past optimal temperature, rapid decreases with increasing light due to increased photorespiration and protein denaturation (lots of enzymes are involved in photosynthesis)
Reason: RuBisCO activity
Ex. Wheat performs worse as temmperature increases. After dropping temperature the plant can recover quickly (except if temp. increases past 75C causes permanent damage)
Cotton is more resistant to heat stress but has lower RuBisCO activity (less efficient) in optimal conditions (differs in protein sequence)
RuBisCO is most abundant protein in planet due to inefficiency. Modifications are difficult due it’s essential function so directed evolution experiments in E. coli are used
RuBisCO activase (RCA) uses ATP to activate RuBisCO. It is temperature sensitive and can be engineered instead to improve photosynthetic efficiency in heat stress.
Two isoforms are expressed in wheat (RCA1beta and RCA2beta). 1beta isn’t heat tolerant and 2beta is more efficient to attempt to combine traits.
Photorespiration: definition, cause, effect, solution
On hot, dry days there’s inefficient use of light energy (stomata close to conserve water but reducing supply of CO2 for photosynthesis) so photorespiration occurs in which RuBisCO adds O2 instead of CO2 into CBB cycle so the plant loses carbon instead of producing sugars
Cause: Due to similarity of in chemistry of O2 and CO2, and little O2 in air when RuBisCO evolved, there was no selective pressure for the enzyme to differentiate the two
Effect: Photorespiration can drain as much as 50% of the carbon fixed by the CBB cycle
Benefits is it limits the damaging products of light reactions that build up in the absence of CBB
Solution: concentrate carbon around RuBisCO to reduce use of O2
CAM photosynthesis
C4 photosynthesis
Carboxysomes (algal)
Natural carbon concentrating mechanisms, way to utilise to engineer more efficient photosynthesis
-CAM photosynthesis
Prevents photorespiration as it separates light and dark reactions with temporal control
-C4 photosynthesis
Prevents photorespiration as it spatially separates light and dark reactions with bundle sheath cells (have higher CO2)
-Carboxysomes (algal)
Structures in chloroplast that form protein shell and CO2 via C4 intermediates are pumped into shells, increasing levels
Engineering:
Attempt to transfer carboxysomes into plants
Transform C3 plant into a C4 (produce bundle sheaths) or CAM (temporal control of gene expression) plant
Engineering C3 plant into C4: why, steps, outcome
Chose to engineer C3 (ex. rice) into C4 (ex. maize) not CAM (ex. pineapple) to increase carbon concentration around RuBisCO because:
1% increase in efficiency comparing C3 to C4
Maize and rice are closely related
Rice grows in warm parts of world so can see benefits improving photosynthesis (unlike C3 wheat that grows in colder climates)
C3 into C4 has happened multiple times in evolution so chemically possible
CAM is used for drought tolerance so need flashier fruits and is harder to produce
Engineered rice to have bundle sheath cells and intermediate C4 step. Steps in transitioning C3 plant into C4:
Genomic preconditioning (gene/genome duplications)
Increase in vein density (bundle sheath cells surround vein)
Enhancement and activation of bundle sheath cells (produce chloroplasts within)
Change biochemistry in plant to establish C4 cycle (able to produce and pump malate C4 intermediate that’s pumped into sheath cells)
Optimise C4 cycle
Outcome:
Overactivation of GOLDEN2 transcription factor increased chloroplast accumulation in bundle sheaths
Transferred 5 genes from C4 into rice successfully but didn’t function as required (didn’t interact with required proteins). 16 total are needed but lacking of funding halted further experiments
Needed cell to favour our process over the plants (in future turn off proteins to prevent WT process)
Synthetic carboxysomes: function, research progress
Algae have evolved to have carboxysomes protein shells to limit diffusion and have increased CO2 concentration
Pumps carbonic acid dissolved in water and pump into cell and chloroplast to increase CO2 production. Carbonic anhydrase converts carbonic acid into CO2 to be used by RubBisCO
9 transgenes are sufficient to generate carboxysomes however plant cells don’t efficiently utilise (divert RuBisCO into carboxysomes and need additional bicarbonate pumps to increase carbonic acid movement
Maximize light harvesting: light absorbing pigments, how to engineer, current use
Rate of photosynthesis vary dependent on light. Different coloured light has different functions in different crops (predominantly red and blue light in photosynthesis)
Photosynthetic pigments in green plants:
Chlorophyll a - blue and red
Chlorophyll b (plants, green algae and cyanobacteria) - blue and orange
Carotenoids (antennae proteins) - blue
Phycocyanin (in cyanobacteria) - orange
Phycoerythrin (in cyanobacteria and non-green algae) - green
In algae and plants chlorophyll absorbs most of light, and the carotenoids funnel light absorbed to photosystem
In cyanobacteria, has lots of antennae funnels light through differently
Adding cyanobacterial systems to plants allows more absorption of light (ideally produce a black plant that absorbs all wavelengths)
Alternatively replace photosystem I with bacterial reaction center that uses longer wavelengths. Difficult to do since involves ripping out existing light harvesting complex and adding new from cyanobacteria
Vertical farming (ex. grow plants in dessert)
Self contained growing modules with LEDs to precisely control the growth environment and air conditioning systems
Utilisation of LEDs is efficient but brings its own complications
Dynamic lighting: cause, effect, methods to alter
Dynamic light in day from changes in cloud cover, taller plants shading, etc
Wheat yield is significantly decreased due to delayed reaction time altering rate of photosynthesis in response to light. Up to 10mins increasing rate moving from dim to light. Yield can be significantly improved
Stomata are the primary drivers of this delay due to time taken to open (¬10mins) and close (drought tolerance). The increased amplitude and speed of stomata opening means more gas exchange, so more photosynthesis
Smart canopy:
Currently plants grow to the same height.
Dense canopy means lower leaves have access to less light energy
Varied heights with upper leaves with smaller antenna complexes and more vertical orientation permits more light to reach lower leaves for greater overall efficiency
Algae biofuels production:
Those in center of vat have less access to light
More mixing
Smaller antennae (phcocyanin truncation mutants) to make those outside less efficient absorbing light so more penetrates in
