Targets for improving photosynthesis: case studies

Question 1

Summary and future directions:

Answer 1

The food and energy needs of more than 7 billion people (soon to be 9 billion) depend on the flow of the Sun’s energy into organic carbon compounds through the process of photosynthesis.

Can we engineer carbon-concentrating mechanisms, greater Rubisco efficiency or photorespiratory bypasses into our crops to harvest more of the Sun’s energy?

Rubisco catalyzes two competing reactions, and carboxylation and an oxygenation. Many organisms have evolved carbon-concentrating mechanisms to minimize the oxygenation reaction.

Question 2

The combination of drought and heat is particularly challenging

Answer 2

Drought decreases carbon assimilation by decreasing uptake of CO2, thereby lowering Ci (internal concentration of CO2).
Heat deactivates Rubisco.
Drought + heat stress combine to severely inhibit photosynthesis.

Finding balance is complex – read paper:

Individual and combined effects of drought and heat stress on the net CO2 assimilation rate (A) at a given intercellular CO2 concentration (Ci). The A–Ci response of cotton Monseratt Sea Island plants under well-watered conditions (data from Carmo-Silva et al., 2012) was modelled. Drought-induced diffusional limitations lower the Ci and heat-induced metabolic limitations caused by Rubisco deactivation decrease the slope of the A–Ci response. In semi-arid environments, drought and heat stress can combine to severely inhibit photosynthesis. Stars denote the values of A and corresponding Ci under each circumstance.

Question 3

Targets for enhancing photosynthesis

Answer 3

aspects that could be modified:
CONSIDER INTERCONNECTEDNESS OF THE PATHWAYS: be aware of knock on effects and how to mitigate them

Light harvesting:
- alter size of antenna
- synthesise pigments that absorb in far red
- introduce new antennae

Photochemistry
- introduce new types of reaction centre

Ratio of ATP to NADPH produced:
- control amount of cyclic and psuedocyclic electron flow
- modify ATP synthase

Alternative electron flow:
- introduce electron escape valves (Flv, PTOX)

Photoprotection and response to fluctuating light
- qE quenching
-state transitions
-PSll repair
-ROS prevention
-Regulate delta pH/ membrane potential

Question 4

Intensive farming and crop canopies

Question 5

Algal raceway ponds

Question 6

Beyond the red limit

Answer 6

Pigments of PSII and PSI (680 and 700 nm) viewed as representing the red-limit of photosynthesis.

New evidence shows a role for Chlorophyll f in photochemistry in Chroococcidiopsis thermalis

Far-red-light (750 nm) grown C. thermalis cells have ~90% Chl a, ~ 10% Chl f, <1% Chl d

See example paper:
Red limit – plants cant use light above certain wavelengths
Nürnberg et al., Science 360, 1210–1213 (2018).

Question 7

NPQ kinetics

Answer 7

(see diagram)
Fast NPQ activation (sec); slow relaxation (min-hrs); up to 30% CO2-fixing and yield loss.

Amplify NPQ pathway by increased expression of ZEP, VDE, PsbS

Transgenic tobacco with amplified NPQ have increased yields (14-20%)*

*See example paper:
Changing NPQ kinetics by upregulation of certain chemicals
Jarchum I. (2017). Photosynthesis gets a boost. Nature Biotechnology 35:29.

Aiming to modulate the kinetics of the xanthophyll cycle, which influences energy dissipation, Kromdijk et al.1 introduced genes encoding additional copies of three key cycle enzymes into tobacco plants, effectively increasing their level of expression.

Question 8

Targetting photorespiration

Answer 8

Rubisco oxygenase activity generates toxic by-products (2-phosphoglycolate and glycolate).

Photorespiration detoxifies and recoups some of the carbon.

Energy and carbon loss imposes a yield penalty of 20-50% in C3 crops.

Silencing PLGG1 and engineering alternative pathway (1-3) genes in transgenic tobacco increased biomass by 40% for pathway 3.*

• see example paper:
  South et al. (2019). Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field. Science 363, eaat9077.

They installed into tobacco chloroplasts synthetic glycolate metabolic pathways that are thought to be more efficient than the native pathway.

Question 9

Key points for photosynthesis summative

Answer 9

We are often not growing crops in their ideal conditions

We are trying to get plants to do something they might not necessarily ‘want’ to do

Consider ecology of responsible crop improvement

Bear in mind the importance of marine photosynthesis
-bacteria (cyanobacteria) and green (picochlorophytes)
-brown algae: coccolithophores have CaCO3 shells
-brown algae: diatoms have SiO2 frustule ‘glass-like’ shells