9. Improving Photosynthesis - C4 to CAM

Question 1

Structure of the lecture

Answer 1

1. The issue with Rubisco
2. C4 plants
3. CAM plants
4. Comparing C4 and CAM

Question 2

What is the fundamental issue with Rubisco?

1

Answer 2

Rubisco has not evolved to contend with the current carbon environment of Earth

1. Has not evolved since Himalayan weathering events
2. Accumulation of ~1 amino acid change every 7.2 million years
3. Rubisco is in a high O2 environment, slowing its efficiency
4. We cannot wait for evolution to occur! GGC are NOW.

Question 3

When was the last time that Rubisco evolved?

1.1

Answer 3

Rubisco has not evolved since ~35mya when India crashed into the rest of Asia, and the Himalayas underwent weathering. This caused CO2 levels to plummet as CO2 was converte into seashells and limestone

Question 4

How slowly does Rubisco evolve?

1.2

Answer 4

One amino acid change every ~7.2 mya

Has only undergone 5 amino acid changes since Himalayan weathering event

Rubisco is failing to adapt to the current carbon environment of Earth

Question 5

How is Rubisco limited by current Oxygen levels?

1.3

Answer 5

When the stomata open to let CO2 in, they also let O2 in. However, O2 is 50 times less soluble in water than CO2, meaning that O2 in surrounded in a 10:1 (O2:CO2) ratio, decreasing its efficiency

Question 6

When did C4 plants evolve?

2.1

Answer 6

First evolved after the substantial drop in CO2 after the Himalayan weathering event ~30 mya

Appears to have evolved separately >70 times since then

This multitude of different C4 origins have led to great diversity in C4 anatomy and biochemistry

Question 7

How do C4 plants work?

2.2

Answer 7

C4 plants distribute photosynthesis across Mesophyll and Bundle Sheath cells to alter CO2:O2 ratio for efficiency

Mesophyll cells uptake CO2 and stabilise it to Oxaloacetate (C4 compound). This is then converted to malate, which is sent to the Bundle Sheath cell, where the Calvin cycle occurs

Question 8

How are C4 plants structured differently to normal plants?

2.3

Answer 8

Photosynthetic cells are concentrically arranged around leaf veins in C4 plants, allowing for metabolic partitioning and cell organisation (Example: Suberin/Cork surrounds the cells to create a shield that precent the CO2 from leaving, creating high CO2 levels)

C4 plants also have higher vein density for this

Question 9

Why are C4 plants advantageous?

2.4

Answer 9

Produce higher biomass than normal plants, since do not waste their energy on photorespiration

Question 10

What are the two primary issues with developing C4 photosynthesis?

2.5

Answer 10

1. We do not yet have a complete blueprint for C4, as we are missing understanding of transporters in the Bundle Sheath Cells. There have been multiple attempts to engineer C4 metabolism in rice, and most have failed
2. C4 photosynthesis requires a more balanced number of Bundle Sheath and Mesophyll cells, would would require insertion of additional veins between existing ones via the synthesis of genetic mutants

Question 11

When did CAM plants evolve?

3.1

Answer 11

Evolved after the atmospheric drop in CO2 that was associated with Himalayan weathering

Like C4, CAM plants exhibit substantial diversity in anatomy and have evolved separately >60 times

Question 12

What mechanism do CAM plants operate by?

3.2

Answer 12

CAM allowsxs for the temporal partitioning of photosynthesis

Stomata are open during the night, allowing CO2 in. This leaads to the enormous production of malate, that is then stored as malic acid in the vacuole.

Stomata are then closed during the day. Malic acid moves from the vacuole into the cytoplasm, where it is converted to oxaloacetate then back to CO2 for fixation

Question 13

How does CAM influence the structure of plants?

3.3

Answer 13

CAM is often associated with succulence due to the closing of the stomata during the day

Question 14

Give an example of how CAM has been tested for agriculture

3.4

Answer 14

Lim et al., 2020

Testing in A. thaliana

VvCEB-bHLH transcription factor was used, and caused leaves to swell with succulence

However, they did not get any further than the creation of succulence, and photosynthesis actually became less efficient.

It’s a start, but much more progress is required

Question 15

What are the 5 ways that C4 and CAM plants differ?

Answer 15

1. Carboxylation module
2. Decarboxylation module
3. Storage of CO2
4. Changes to diurnal biology
5. Changes in leaf anatomy

Question 16

How do C4 and CAM compare in their partitioning of photosynthesis?

4,1

Answer 16

C4 separates photosynthesis over space and CAM separates photosynthesis over time

Question 17

How do C4 and CAM differ in their carboxylation modules?

Answer 17

C4 - Carboxylation module is mesophyll cells during the day

CAM - Decarboxylation module is mesophyll cells durin the night

Question 18

How do C4 and CAM plants differ in their decarboxylation modules?

4.2

Answer 18

C4 - Decarboxylation module is in the bundle sheath cells

CAM - Decarboxylation module for bundle sheath cells only during the night

Question 19

How does storage of CO2 differ in C4/CAM cells?

4.3

Answer 19

C4 - Transient (seconds) storage of CO2 in C4 acids

CAM - Long term (24 hours) storage of CO2 in C4 acids

Question 20

How have C4 and CAM plants undergone changes in their diurnal biology?

4.4

Answer 20

C4 - No need to change their diurnal biology, the stomata are open during the day, and closed at night

CAM - Changes to their diurnal biology - stomata are open during the night, and closed during the day

Question 21

How do C4 and CAM plants differ in their structures?

4.5

Answer 21

C4 - Changes in leaf anatomy to increase the number of veins

CAM - Changes in leaf anatomy to engineer succulence into the leaf.