Engineering Photosynthesis III: Turbo-Charging CO2 Assimilation

Question 1

Structure

Answer 1

1. C3 plants
2. Biogeography
3. C4 plants
   4.

Question 2

C3 photosynthesis

Answer 2

• most plants
• all reactions happen in one cell

Question 3

50Mya biogeography

Answer 3

• India is migrating North across the ocean
• it crashes into mainland Asia
• Himalayas begin to erode
• granite-silica rocks pushed up
• rapid weathering

Question 4

Himalayan erosion reaction

Answer 4

2CO2 + H2O + CaSiO3 -> Ca2+ + 2HCO3- + SiO2
- Ca2+ + 2HCO3- -> CO2 + H2O + CaCO3

Question 5

35Mya

Answer 5

huge drop in atmospheric CO2

Question 6

describe photorespiration throughout biohistory

Answer 6

• > 35Mya: 1500ppm (not much)
• now: 420ppm (LOADS)

Question 7

How did C4 evolve?

Answer 7

• low CO2: plant starvation
• evolution can’t change RUBISCO’s gaseous affinity, but it can change its environment

Question 8

Describe solubility

Answer 8

• O2 50x less soluble
• affects diffusion acores cell wall to outside
• chloroplast positioning around outside

Question 9

C4 plants - the basics

Answer 9

• photosynthesis distributed between 2x cells; reaction partitioning
• photosynthetic cells concentricall6 arranged around leaf veins
• less protein content (less RUBISCO)
• alter O2:CO2

Question 10

C4 plants - the specifics

Answer 10

• stabilised as malate
• huge conc. gradient; diffusion
• decarboxylate: high CO2 conc in neighbouring cells (trapped)

Question 11

malate

Answer 11

4C

Question 12

C4 cellular arrangement

Answer 12

• CO2- concentrated cells
• surrounded by a layer of carbon-pumping cells
• connected by plasmodesmata (to facilitate O2 demand)

Question 13

Suberin layer

Answer 13

• surrounds BS cells
• keeps CO2 in
• O2 shield

Question 14

Describe the advantages of C4

Answer 14

• more CO2 fixed per unit N2, H2O, light energy, leaf area
• due to minimal photorespiration

Question 15

Veins

Answer 15

• increased density in C4 (maize vs rice)
• in C3: mesophyll cells compete for CO2

Question 16

Given the same resources, C4 plants

Answer 16

• higher productivity
• higher yield
• produce more biomass
• maize, Echinochioa, rice (same age)

Question 17

Echinochioa

Answer 17

weed

Question 18

C4 evolution

Answer 18

• 30Mya: rapid
• > 70 independent occurrences (2 major clusters; grasses, Charyophyllales)
• continuous throughout embryophyte emergence
• 17/18 Families
• phylogenetics: molecular clock
• unequally powerful e.g. Chloridoideae, Andropogoneae, Flaveria
• huge diversity in anatomy and arrangement

Question 19

Andropogoneae

Answer 19

• sorghum, sugarcane, maize
• crops!

Question 20

Zea mays

Answer 20

• model
• Kranz anatomy

Question 21

Give the classical examples of 3 major biochemical subtypes; carboxylase

Answer 21

1) Zea mays (NADP-ME)
2) Bassia scopoura (NAD-ME)
3) Bienertia sinuspersici (phosphoro-pyruvate carboxylase)

Question 22

Single cell C4

Answer 22

• spatially segregate chloroplasts
• 4CC stabilisation (outer)
• decarboxylation (inner)

Question 23

NADP-ME

Answer 23

NADP malic enzyme

Question 24

Engineering prospects?

Answer 24

• wheat, rice
• producibility gains
• decreased inputs

Question 25

Problems with C4 engineering

Answer 25

1. no blueprint 2. anatomy

Question 26

no blueprint

Answer 26

- incomplete biochemical understanding - complex enzymatic repertoire - metabolite transporters of BS cells

Question 27

Our current understanding of maize C4 biochemistry

Answer 27

- CO2 outside leaf; diffuses in through stomata -> mesophyll - carbonic anhydrase capture - PEPC adds phosphoenol pyruvate: oxaloacetate - chloroplastic import - malate stabilisation - plasmodesmata diffusion -> BS cell - NADP-ME: decarboxylation; CO2 liberation - pyruvate regenerates cycle by diffusion - RUBISCO fixes CO2

Question 28

Attempting to engineer C4 into rice

Answer 28

- CA, PEPC, NADP-ME oe in right cells - validate expression with TR - measure 13CO2 incorporation and release - nonfunctional w/o BS chloroplast transporters; no decarboxylation

Question 29

the anatomical problem

Answer 29

- we need to insert more veins - reorganise M + BS cells for balance

Question 30

LOF vein mutagenesis and screens

Answer 30

- sorghum - EMS: point induction - vein no./mm : LM - poor photosynthetic rate, seed set - cells larger; vein expansion

Question 31

C4 veins

Answer 31

- parallel - stripes tip -> base

Question 32

GOF vein mutagenesis and screens

Answer 32

- 100,000s EMS - smaller cells, increased vein density, smaller leaf, smaller plant - affects brassinosteroid biosynthesis

Question 33

Temporal separation?

Answer 33

- CAM photosynthesis - same enzymes - antithetical stomatal dynamics - (de)carboxylation between day and night

Question 34

night

Answer 34

- cool, less wind - less transpiration - water loss is a non-issue - open stomata - carbon fixation - malate: vacuolar transport; accumulates

Question 35

day

Answer 35

- light: photosynthesis; ATP, NADPH - close stomata to prevent O2 entry - vacuolar malate export: decarboxulation

Question 36

CAM evolution

Answer 36

- 30Mya - continuous - >60 times - scattered origins (lycophytes, monilophytes) - ecological advantage: succulence

Question 37

succulents

Answer 37

- agave - cacti - orchids - bromeliads

Question 38

anatomy and biochemistry of CAM

Answer 38

- substantial diversity

Question 39

engineering succulence

Answer 39

- inspired by grapevines (swell w/ water) - VvCEB-bHLH TF controls swelling - At oe - all cells bigger; ;larger vacuoles - more water /g of leaf tissue, leaf area - reduced internal space - much worse @ photosynthesis - fewer photosynthetic cells - much more water-use efficient