photosynthesis Flashcards
give an example of a C3, C4 and CAM plant with their yield and WUE values
C3; rice: 25,000 tonnes of yield/h/y. 28,000 tonnes water/h/y
C4 Maize: 49 t/h/y: 20/t/h/y
CAM: agave: 43 t/h/y. 4.5 t/h/y
describe the C4 rice project
- 3.5 billion people depend on rice for more than 20% of their daily calorie intake
- Rice is C3 so has a light conversion efficiency of 4.5%, if the C4 pathway could be input into rice (8.5% efficiency), yield could be increased
- funded by bill and melinda gates
describe 4 future challenges regarding plants
1)climate change; warmer weather and unpredictable rainfall; 50% of USA has experienced extreme drought since 2012
2)loss of arable land due to
a)population growth (1%; 80 million new people annually)
b)urbanisation
c)desertification
desertification
32% of dryland in hyper arid-semi arid - UN
rate of desertification is 1.3%
3) increased food demand; population increase and change to meat diets
4) fertiliser synthesis (non renewable) requires energy
discuss 4 future challenges regarding agriculture
1) desertification; 1.3% rate of desertification. amount of arable land has decreased by 15% in last 10 years.
2) increased food demand; 1.1% population increase(80 million new people annually), move to meat diets
3) climate change; unpredictable weather
4) fertilisers are non renewable
introduce photosynthesis
- green plants
- conversion of water (roots) and co2 (stomata into carbohydrates and oxygen byproduct
- chlorophyll
- 6co2 + 12h20»_space;> C6H12O5 + 6o2
- 50% aquatic, 50% terrestrial
- light reactions capture light and convert it to energy (ATP and NADPH
- dark reactions uses chemical energy to fix CO2 into a carbohydrate
discuss photorespiration
conversion of oxygen to carbon dioxide
-rubisco evolved millions of years ago when O2 concentration was much lower; enzyme wasnt any less efficient
-produces PG, which requires ATP and NADPH to detoxify
-
what organic acids are used by C3, C4 and CAM plants
C3: no organic acid
C4: malate
CAM: malic acid
what are the solutions to current plant issues
1) increase plant yield by GM
- the amount of biomass partitioned to edible grain has been maximised
- cant change the amount of light energy available
- future efforts must increase
- light capture efficiency
- photosynthetic inefficiency
2) increase land use
- use CAM plants on arid land
3) decrease demand
- go to a plant based diet
4) decrease pollution to stop climate change
how can plant light capture efficiency be increased
- increase at which the canopy develops
- increases the length of time leaves stay green
- size of leaves
- top leaves more vertical and smaller
- lower chlorophyll content at top leaves
what are three key eras of agriculture
- 10,000 BC (neolithic aera) transition from hunter gathering to settled agriculture
- 17th-19th century; (agricultural revolution); applying science to farming; using new crop varieties, doing crop rotation and using mechanisation
- 20th century (green revolution); Norman bourlag improved food security in Mexico with varieties of wheat which are semi dward. disease resistant and high yield
- Mexico went from importing 50% of its wheat to exporting wheat
- Bourlag did a similar thing in India with Rice (doubled yield without using any more land)
what is the toxic product rubisco makes during Photorespiration
phosphoglycerate
describe atmospheric changes of over time
- life emerged 3.5bya; CO2 very high
- photosynthetic bacteria emerged 3bya; carboxylase activity caused a gradual increase in oxygen in the atmosphere; life began diversifying
- 300mya when CO2 was very high, there was selection methods to overcome rubisco inefficeicny; CCMs emerged
what determines the net CO2 uptake
rubiscos specificity factor for CO2 relative to O2
- plants have a specificity factor of up to 85
- red algae have a specificity factor of 200
discuss the improtance of photosynthetic organisms
perform 50% of global photosynthesis
- half is done by algae, half by bacteria (cyanobacteria)
- despite algae doing 25% of global photosynthesis, they only make up 1% the biomass of land plants which do 50%
why can be photosynthesis in aquatic environments be challenging
CO2 is limited
CO2 diffuses slower than in gas
CO2 is converted to bicarbonate which is unavailable to photosynthetic organisms without modification
discuss cyanobacteria CCM
how is the CCM regulated
differences between alpha and beta cyanobacteria
how concentrated in CO2 in carboxysomes
three challenges to engineerign this into higher plants
- biophysical
- aquatic CO2 is converted to unavailable bicarbonate (HCO3) due to pH
- bicarbonate is pumped into cyanobacteria (pH regulated so that it isint converted to CO2; it would diffuse out of cell)
- bicarbonate is transfered to carboxysome compartment within chloroplasts; pH regualted so that carbonic anhydrase converts bicarbonate to CO2 (leak barrier prevents diffusion out of carboxysome)
- transcription inducible when CO2 concentration goes low
- energetically expensive so only occurs during sunlight`
-alphas live in seawater where nutrients and light are more limiting than CO2 concentration. betas live in lakes and estuaries where CO2 is most limiting factor; beta have a more sophistociated CCM
1000 more concentrated CO2 than external environment
challenges:
- engineering expression of active pumps
- engineering carboxysomes into chloroplasts
- CA removal from stroma; diffusion would occur
what are the differences between the CCM in blue-green algae and cyanobacteria
would the blue-green algae CCM have better potential for engineering other plants than bcyanobacterial CCM?
for blue-green algae;
1) chloroplast compartment is called a pyrenoid
2) CCM not as efficient; Carbon dioxide concentrated to 100X that of external environment, not 1000
3) separate pumps comapred tobacteria
possibly;
- eukaryotic mechanisms
- pyrenoid has a very simple structure (12 different proteins needed)
- easy to engineer?
- what are hornworts
- describe their CCM
- early land plants
- 50% have CCM
- pyrenoids
- evolved and lost many times; not correlated to low CO2
- plants exist in wet environments; inducible when covered in water
introduce rubisco
- Ribulose 1-5-carboxylase-oxygenase
- most abundant protein on earth
- slow kinetics
- promiscious
name the four types of rubisco and their key properties
form 1;
- most common and most sophisticated.
- found in higher plants, algae and cyanobacteria
- 8 large subunits (cylinder) and 8 small (4 below cylinder, 4 below)
form 2; plankton and bacteira
-2 large subunits
form 3
- archaea
- subunits arranged into 2s or 5s
form 4 (rubisco like protein);
- bacteria
- 2 large subunits
- cant fix CO2 from -RUBP
in form 1 of rubisco, when genes are expressed as the small and alrge subunits
small: rbc S (nucleus)
large: rbc L (chloroplast)
details of rubisco structure
what experiments have been done to alter structure
- large subunit is important because it contains the active site
- in the Large subunit, 30% of AAs are involved in holding dimers together and 20% are highly conserved across higher plants so cant be altered
- 50% of AAs are not conserved and can be altered in the hopes of creating a better rubisco
- DNA shuffling (DNA from high plants and from red algae (high specificity factor) are fragmented and denatured (ss) before being annealed into a “shuffled protein”; no success so far
- small subunit perhaps plays role in specificity factor (shuffling with red algae)
describe rubisco assembly
name an important chaperone and its role
- chaperones
- light activated transcription
- if youa re to engineer rubisco into a plant you must also express rubisco
1) mRNA attatchs to ribosome and it translated
2) transit peptide is used to dock the protein in the chloroplast membrane
3) combiantion of small and large subunits at membrane using a binding protein (chaperone)
rbcX2 (molecular stapler); ensures large subunits bind properly by preventing incorrect carboxy terminus interactions. without this the small subunits couldn’t bind
describe rubisco activation
-the enzyme (E) must bind to CO2 and then magnesium to become activated (known as ECM) and bind RUBP
If RUBP binds to inactivate rubisco it cannot be activated, so Rubisco activase exists to remove RUBP
rubisco inhibitors such as CA1P also exist which bind to the activase site to prevent the plants own proteases acting on the active site (also allows plant to alter rubisco activity), which can be removed by Rubisco activase
RA mode of action involves binding to rubisco and changing the conformation of it so that inhibitor/rubp are released
describe rubisco catalysis (intermediates between RUBP and 3-phosphoglycerate
1)RuBP
(enolisation)
2)2,3 enediol
(CO2 addition)
3)3-keto-2-carboxy-arabinitol
(hydration)
4)3-keto-2-carboxy-arabinitol (hydrate
(C2-C3 cleavage)
5) 3-phosphoglycerate and another molecule which is converted to 3-phosphoglycerate
6) 3PG is used to make G3P which is used to make carbohydrates
much of the 3PG is used for regeneration
which factors effect photosynthesis
factors whcih effect rubisco and therefore assimilation rate
supply of CO2
- stomata number
- stomata regulation
- barriers to entry
how CO2 is processed
- Rubisco biochemistry
- calvin cycle activity and the light reactions needed to fuel this
- carbohydrate synthesis reactions
describe the Aci curve
compensation point; point at which the line crosses the X axis; co2 concentration at which CO2 assimilation begins
carboxylation efficiency; the slope of the line as CO2 increases; represents variation in demand and processing of CO2
Pa; point on x axis which is ambient CO2 cocnentration
Pi: point on X axis which is optimum internal CO2 concentration; work it out by drawing a line between Pa and the point where the line begins to plateau
how can the rate of photosynthesis be altered by plants
1)alter supply of CO2 by
a) changing stomata shape
b) alter stomata distribution a
c) changing leaf anatomy (how densely packed leaves are)
2)alter co2 demand
a) alter rubisco biochemistry; Vcmax (max velocity) depends on number of active sites and catalytic turnover; turnover is fixed but active site nu,ber isint
b) alter amount of CO2; desert plants are limited by water so nitrogen is invested in roots rather than rubisco
c) alter how rubisco is activated; expression of rubisco activase meant CO2 assimilation began quicker
what is the toxic product of photorespiration (PCO) and how is it detoxified
Does PCO cycle exist in plants with biophysical CCMs
- phospho-glycolate
- chloroplasts, mitochondria and peroxisomes are used
- PG converted to glycerate which can take part in the calvin cycle
- energy requiring
- loss of CO2
details of pathway needed??
- pathway is incomplete in most CCM plants; does not occur
- in aquatic plants with CCMs, PCO cycle does exists so that plant can not use the CCM if needed (energetically expensive)
what are the four strategies used by macrophytes to overcome low CO2 concentration
1) unlock co2 from soil at the bottom of lakes
2) using bicarbonate and exist in a low photorespiritory state (biophysical)
3) use C4 acids to concentrate co2 in chloroplasts (biochemical)
4) have ariel leaves
does does temperature effect photorespiration and why
- enhances photorespiration more than it enhances photosynthesis
- reduced specificity of rubisco for CO2 compared to CO2 (greater free energy between 2,3-enedoil and oxygen comapred to between 2,3-enedoil and co2)`
- more reduction in (CO2 solubility compared to O2
what are the 5 areas for improving rubisco activity and CO2 fixation
describe rationale (R) successes (S), methods (M)and failures (F) in each area
1)alter (decrease) amount of rubisco
R: plants thought to overinvest
M; antisense; plants had 20% that of wt plants
S: under experiemntal conditions (low light) a dramatic decrease in rubisco needed to decrease photosynthesis; some rubisco could be invested elsewhere
F: under realistic light conditions, small rubisco reductions led to increased content
2)alter regeneration of RUBP
R: SBPase (enzyme which replaces RUBP thought to have a high flux control value on the pathway
M: overexpression
S: RUBP was replaced quicker and plants grew twice as big
F: n/a
3)alter Rubisco activase
R: RA is sensitive to high temperatures so global warming could limit photosynthesis
M: overexpression/antisense
F: reducing RA led to reduced rate of activation.
S:
-overexpression increased thermal stability and increased rate of rubisco activation
-plants in different cliamtes have rubiscos with different thermal profiles; future work involving transgenic plants with rubiscos with thermal profiles suited to high temperatures
4)modify photorespiration pathway
R: PCO is energetically expensive
M: removal of different enzymes
F: GOGAT and GOX removal caused decreased assimilation and tiny plant size
S:
-GDC/SHMT overexpression made plants grow very large (relieves a bottleneck)
-bacterial GDH insertion (glycolate processed in chloroplast rather than peroxisome) made plants larger
conclusion: photorespiration is key but bypassing parts can be successful
5)alter rubisco structure
R: eliminate/reduce oxygenase activity may increase productivity
M; site directed mutagenesis
F: activity against CO2 decreased and activity against O2 increased upon mutagenesis
S: Loop 6 identified as a region which holds the substrate in place andis key to specificity factor; replacing AAs and cleaving residues in Loop 6 removed carboxylation function but adding residues to the C terminus increase carboxylase activity
discuss the difficulty involved in getting plants to express modified rubisco
- many potential areas involve modifying rubisco (adding the the Loop 6 C terminus, reshuffling so that red algae specificity factor is induced etc)
- these proteins often arent expressed in high enough levels and are not folded properly due to the fact that the different subunits come from different cellular compartments
- chaperones like rbcX have helped move this along
- cyanobacterial rubisco has been expressed in tobacco but the plant was very small
- next step involved making sure the plant expressed enough og the modified protein
describe C4 taxonomy
name economically important C4s
2% of higher plants 25% of plant biomass; they are so productive 50% are grasses worst weeds maize, sugar cane, sugarcane
describe kranz anatomy and the reactions which occur in each cell type and the CO2 concentration in each
Mesophyll cell (MC);
co2 diffuses in and is converted to bicarbonate by carbonic anhydrase
bicarbonate is combined with PEP to make OAA by PEPC which is converted to malate
CO2 concentration is low (100ppm) becase PEPC is very efficient
Bundle sheath cell (BSC)
malate is transported here and decarboxylated releasesing CO2 and pyruvate, which is moved back to the MCs for conversion back to PEP by PPDK
CO2 concnetration is high (2000ppm) because rubisco is inefficient)
describe biochemical diversity within C4 plants
characteristics of each type
what is the significance of the sub types
diversity invovled which is main enzyme responsible for malate decarboxylated (many C4 plants have more than one enzyme)
-NADP-ME (chlorophyll);
-malate moves into BSC
-pyruvate moves into MC
most abundant and simple enzyme
-only type to only have PS1 in BSC
- NAD-ME (mitochondira)
- aspartate moves into BSC
- alanine moves into MC
-PEPC kinase (mitochondria and cytosol) -aspartate moves into BSC -PEP moves into MC -most complicated
significance of subtypes not understood
how is C4 photosynthesis regulated
1) regulate the flux of metabolites between cell types
2) regulation of carboxylase enzyme
describe single celled C4 photosynthesis
what evidence exists to support the existance of single celled C4
name a plant species which does single celled C4
- entire C4 pathway occurs in a single cell
- rubisco and PEPC are kept in chloroplasts at opposite ends of the cell (alternatively rubisco chloroplasts in middle of cell and PEPC chloroplasts on outside)
-staining showed the plant has C3 anatomy but C4 organic acids and PEPC activity
asian desert plants (B. aralocaspica)
how are PEPC and rubisco regulated
1) sunlight casues rubisco activation via RA
2) rubisco produces G3P which diffuses into MC and is dephosphorylated which changes pH, causes calcium release and activation of calcium dependant protein kinases (CDPK)
3) CDPKs activate PEPC kinase
4) PEPC kinase activates PEPC by reversable phosphorylation, which changes its kinetic properties
a) malate can no longer interact with the enzyme and inhibit it because the enzyme isint sensitive to (low Ki)
b) affininhibition for PEP is increased
name 6 ways to improve C4 photosynthesis
1) overexpress aquaporins so CO2 delivery to PEPC is increased (leaky membranes)
2) overexpress CA so more bicarbonate substrate exists
3) improve rubisco function (catalytic turnover/number of active sites)
4) increase amount of rubisco
5) alter calvin cycle to improve regeneration of RUBP
6) improve the ETC
differences between C3 and C4 plants on an Aci curve
- c4 plants have a higher max rate of photosynthesis
- c4 plants have a lower compensation point
- c4 plants have a steeper slope (high carboxylation efficiency)
- OXYGEN:C4 plants are uneffected by increasing oxygen concentration (c3 plants are
- CARBON DIOXIDE: C4 assimilation rate remains high even at low concentrations
- LIGHT: C4 plants have a steeper slope compared to C3 plants under same light conditions
when do C3 plants perform better than C4 plants
- C4 light use efficiency (CO2 taken up per light molecule)
- C4 light use efficiency is high and steady throughout
- C3 light use efficiency is low at high temperatures (photorespiration occurs)
- C3 LUE is very high at low leaf temperatures (no photorespiration and no energetically expensive C4 pathway)
- C3 plants dominate areas with low temperature such as UK
do C4 plants perform photorespiration
- enzymes are present; could occur if oxygen was very high (unrealistic)
- does occur if CO2 leaks from BSC to MC
what is the max quantum yield and the yield if 11% leakage occurs?
max: 0.07 mols co2/photon
leakage: 0.06 moles co2/photon
are C4 plants more productive than C3 plants
what is a good measure of overall productivity that considers all factors
- generally yes
- productivity isint directly related to rate of photosynthesis though
- respiration rate
- carbon allocation to leaves compared to roots etc
- leaf longevity
efficiency to intercepted light to biomass conversion
discuss nitrogen use efficiency in C3 and C4 plants and the ecological significance
- C3 plants have to allocate lots of N to rubisco because its inefficienct (50% of total protein compared to 20% for C4 plants)
- C3 plants have to allocate more N to PCO enzymes
- C4 plants do need an additional enzyme (PEPC) but it is very efficient so not much enzyme is needed (5% for C4, 0.5% for C3)
- C4 plants can invest more nitrogen in roots to improve nutrient acquisition (those in nutrient poor soils) or in creating more leaf area meaning better competition for light (those in fertile soils)
discuss c3 and c4 WUE
- c4 plants have a higher WUE
- closing stomata causes C3 plants to decrease in assimilation rate whereas no real change in C4 plants occurs
- during drought, C3 plants close their stomata and decrease assimialtion rate sooner (8 days) than C4 plants (18 days)
what are the stats for climate change
what have experiments shown about how Maize will fare under high co2 conditions
- co2 concentration will rise from the current value (380ppm) to 550ppm (2050) then 700ppm (2100)
- increased c3 productivity due to Co2 concentration due to less PCO
- FACE experiments (free air concentration enrichment) allow for controlled fumigation
- during early season (elevated rainfall) maize will have increased CO2 assimilation rate (CO2 had been limiting factor)
- during late season (lower rainfall) the assimilation rate will be unaffected
- increased tassel formation (growth of edible structures) due to early advantage
evidence that C4 plants will benefit from increased CO2 as well as C3 plants
discuss C4 plants in relation to temperature
why don’t C4 plants exist in cold climates
- their optimum temp for photosynthesis is 10 degrees higher than C3 plant
- C4 plants will outperform C3 plants because of lack of photorespiration
- C4 enzyme PPDK is sensitive to low temperatures; modification could mean C4 plants could be grown in colder climates
- C4 plants exist at regions of higher temperatures (low altitude) where as C3 plants do better in colder climates (high altitude)
when and why did C4 photosynthesis evolve
-evolution occurred 30mya (very recent)
during a period of decreasing Co2 concentration
-great diversification occurred 5mya, during another period of reduced CO2 concentration
-C4 has evolved independently at least 66 time suggesting it is easy to evolve
-genome duplication necassary; second set of genes develop new functions and are regulated differently
how were C3-C4 intermediates discovered
name the first group discovered
how many species exist
- low compensation point is a characteristic of C4 plants
- Aci curves were created for a group of plants and some plants have a compensation point in between that of c3 and c4 plants
Mulga grass (Neurachne species) shown to have intermediates as well as C3 and C4 species
not many species exist; perhaps this is an evolutionary dead end
what are the different types of C3-c4 intermediates, and what are their characteristics
-C3-C4; intermediate compensation point
-C4-like; C4 leaf anatomy
C3/C4;complete C3 and C4 pathway which are seperated temporally or spatially (c3 in water but c4 terrestrially)
-hybrids of a C3 x C4 cross
what are characteristics of C3-C4 intermediates
1) small portion of BS cells
2) compensation point changes with light intensity (unique)
3) compensation point decreases when oxygen concentration does; indicates photorespiration hasn’t been eliminated
4) intermediate leaf anatomy: enlarger BS cells with many organelles
5) crossing an intermediate with a C3 plant gives the intermediate anatomy but a C3 compensation point;; indicates a biochemical mechanism is involved too
what differentiates the two groups of c3-c4 intermediates (Moricandia and Flaveria)
- Flaveria has a lower compensation point than Moricandia despite having lower number of organelles in the BSC
- perhaps Flaveria has more C4 enzyme
discuss how the C4 pathway could be engineered into C3 plants
what differences would need to be induced in C3 plants
- C3 plants already have all the necessary genes for C4 pathway (celary stems have C4 acids despite the plant being C3)
- plants closely related to arabidopsis have C4 members (cleome) are being looked at
differences
1) division between MC and BSC in terms of enzymes present; no rubisco in MC
2) more BSC which are bigger and arranged in a kranz way (surrounding veins which deliver water)
3) increased veination
4) more organelles in in BSC
5) genome dupication (occured during C4 evolution) so genes can be regulated differently
6) enzymatic differences;
a) light inducible PEPC transcription in MC and higher expression of PPDK
b) higher rubisco expression in BSCs and removal of PEPC
needs to be established if C3-C4 intermediates are a stepping stone or a dead end
rice cultivars which naturally close veins have been found. PEPC transcription has been boosted. current effort to separate enzymes between cells
how is CAM similar to and different to C4
- separation of photosynthesis (temporally rather than spatially)
- evolved many times (convergent evolution)
- malic acid rather than malate
- rather than the organic acid being pumped into another cell it is pumped into the vacuole for storage
what are the four phases of CAM
PEPC (1); nighttime; CO2 uptake occurs through open stomata and malic acid increases in concentration
PEPC-rubisco (2): morning; stomata close and rubisco is activated
Rubisco (3); afternoon; stomata fully closed and maic acid breakdown occurs; as CO2 concentration increases CO2 diffuses into chloroplasts
Rubisco-PEPC (4); evening; stomata being to open and rubisco is deactivated which PEPC is activated
what is still unknown about CAM
- how stomata open at night
- thought to involve bicarbonate concentration changes
discuss regulation of PEPC in CAM plants
the amount of PEPC in CAM plants doesnt change throughout the day,
- however the amount of inhibitor (malate) needed to inhibit it does increase at night; malic acid can accumulate
- insensitivity to malate is due to reverabsle phosphorylation from PEPC kinase
- at day time, not much malate is needed for inhibition; malate does not accumulate (due to dephosphorylation by PP2A)
- indeed clusia (which can fix CO2 well into the daylight) is dephosphorylated later than most CAM plants; despite this malate was accumualted to the same level; perhaps malate concentration signals for PEPC kinase and PP2A activation (malate accumulation stops transcripton of PEPC kinase; PEPC becomes sensitive to malate)
discuss CAM plants in relation to water
- they open stomata at night rather than during the day; it is more humid at night so less water is lost
- there is less reliance on rainfall due to succulent leaves
- CAM plants have lower xylem tensionindicating more water is present (less defecit than C3)
what are the different types of CAM plants
- constitutive CAM; CAM performed regardless of conditions
- Facultative CAM: C3 plants which perform CAM when under water stress
- CAM cycling: look and act like C3 plants (daytime CO2 uptake) but also perform overnight malic acid accumualtion using respiritory CO2
- CAM idling; stomata closed for 24 hours due to drought conditions. respiritory CO2 captured and used for make carbohydrates. plants can go up to 6 years like this
what are examples of facultative CAM plants
clusia
kalanchoe
discuss CAM ecology
- found in deserts due to good WUE
- large diversity in tropical rainforests as epiphytic plants (grow on the surface of other plants and get water from the moisture in the air); water supply can be very unpredictable during the dry season
- some temperate plants exist for example semi aquatic plants in CO2 limited environments (remember CAM exists to be CO2 concentrating; water saving is just a byproduct)
name some CAM plants
pineapple, cacti, fern, orchid
why and when did CAM plants evolve
100 mya
limited water/co2 environment
what changes occur when facultative CAM plants induce the pathway
- rubisco is only active during the day
- PEPC is only active at night
- an ability to accumulate malic acid and break it down at night
- PEPC upregulation
how many genes would need to be re-regulated if CAM is to be induced in to C3 p
- no new genes; simply temporally regulate pre-existing genes
- upregulation of CA, PPDK etc
- comparative transcriptomics on facultative CAM plants such as the ice plant which induces CAM in saline conditions
- other genes are also induced to deal with salt stress; another CAM model organism is involved
why is clusia a good potential model for studying CAM genes
- its facultative CAM; comaptitive transcriptomics between wet and dry conditions
- only group of plants to have CAM tree species
- drought tolerant C3 clusias exist which could be compared to CAM clusia species to investigate which genes are involved in CAM
are people wrong to say CAM plants are slow and unproductive
- yes; its the desert environment that some CAM plants live in which limites productivity
- some CAM plants are very productive; prickly pear cactus was introduced to Australia in 20th century and became out of control because it was so productive
could CAM plants be used for bioenergy feedstocks?
if so why?
what approaches exist?
-yes; could be grown in areas which arent needed for food production because they are so dry (no ethical dilemmas); known as Advanced Biofuels
- have WUE,
- high drought tolerance (only need 25mm rain per year)
- heat tolerant (65 degree optimum temp compared to C3 plants which is 45)
- UV-B radiation resistant
- most resources are put into above ground structures so plants grow quickly
approaches
1) use pre-existing mega-CAM species which are very productive such as Agave, Aloe vera, prickly pear etc
- future efforts to figureout temperature limits and how the plant will perform in a high CO2 world are needed as well as modification of hydrocarbons produced
2)transfer CAM pathway to C3 crops so these can be grown for biofuels
what was annes proposal and its rationale
-transfer CAM pathway to C3 crop poplar which is already used for biofuels but has a low WUE so can only grow where there’s lots of water
rationale:
1) CAM has evolved many times so evolution is fairly easy; genes already exist in poplar but need to more highly expressed
2) C4 rice was funded, which is likely to be more complex than CAM poplar because no spatial seperation is needed
3) facultative CAM exists, but facultative C4 doesn’t; no metabolic incompatibilities exist between CAM and C3 plants
-$14million received from US energy department
how does CAM poplar biodesign occur (what steps are involved)
1) define parts list; which genes need to upregulated; CAM plant sequencing and use of bioinformatics to see which genes are common to all CAM plants
- model CAM plant has genome duplication compared to C3 plants (more PEPC, PEPC kinase enzymes)
- of the 4 genes which code for PEPC, one was shown to be more highly abundant at night compared to day (and in older leaves of kalanchoe)
- gene flipping investigated; those genes in CAM which show an abundance profile opposite to C3 plants are thought to be related to stomatal regulation (CAM stomata opens in reverse)
2) investigate identified genes in a CAM model plant (Kalanchoe) by knockout/overexpression
3) end hope: design different CAM modules (identify genes involved in carboxylation, decarboxylation, stomatal regulation)
why is Kalanchoe a model organism for CAM
- small; many can be packed into a greenhouse
- short life cycle; dont have to wait years for next gen
- C3 leaves to begin with but CAM leaves later in life
- small genome; easy to sequence
questions which remain unanswered regarding CAM biodesign
challenges
updates
how many genes
is there a magic bullet transcription factor which will regulate everythign correctly
how are stomata regulated
challenges;
- large number of genes may need to be introduced
- if succulence is needed this will need to be engineered
updates
- introducing a grape transcription factor into arabidopsis has induced succulence (thicker leaves, more air space between cells, faster growth)
- differences in starch between C3 and CAM guard cells has been identified; starch isint broken down overnight and is thought to play a role in stomatal regulation
