Building the Road to a Revolution

The Realizing Increased Photosynthetic Efficiency (RIPE) project is a multi-institutional effort to increase the yield of staple food crop in developing countries by improving C3 photosynthesis through the following steps. 

Step 1: Engineer improved photosynthetic efficiency in silico

Photosynthesis is the best known and also most conserved process in plants. It is much the same in cassava and rice as it is in tobacco, wheat and soy. The genes, proteins, biophysics and biochemistry is well known, as is organization at the plastid, cell, leaf and canopy level.  Photosynthesis is analogous to a factory production line with carbon dioxide and light entering at one end and sugar leaving at the other to power all processes and growth of the plant. Each of the one hundred plus steps can be seen as a stage along the production line. To make the process more efficient we need to identify the weak links of the system – and either increase their capacity or short-circuit them with a different way of processing. We can increase capacity by increasing the expression of the gene that codes for the protein controlling that step, or engineer a process from another organism that is more efficient. It would take many, many years to test the millions of permutations of changes in the different components that could be changed in practice. However, the high performance computers used by the team can work through these millions of combinations to identify those changes that would likely give the greatest benefit.

Step 2: Determine how molecular changes affect photosynthesis

Based on Step 1, the most promising changes are brought about in plants. First, pieces of DNA that contain the genes of interest and their promoters are assembled for production of the functional protein. In some cases, several genes are assembled into one piece of DNA. The DNA is added to a bacterium, which is then used to infect plant tissue, which transfers the new DNA into the plant. The transformed plant is then tested for photosynthetic performance. 

Initially, RIPE will be using tobacco as a test crop because tobacco can be engineered far more easily than our target crops due to well-developed transformation methods, easy propagation and well-studied genomes. Once we identify the most valuable constructs using tobacco, we can then engineer cassava, rice, and potentially other food crops. 

RIPE has six diverse pipelines for improvement of the efficiency of conversion of sunlight into food, via photosynthesis. These pipelines have a variety of obstacles to implementation.

Algal mechanisms 
Insert carbon concentrating mechanics from algae into plants to elevate carbon dioxide concentrations around the carbon-fixing enzyme, Rubisco.

Photorespiratory bypass 
Replace the carbon cycling pathway for photorespiration in crop plants with a more efficient bacterial pathway.

Transplanting Rubiscos
Replace the inefficient Rubisco enzyme present in modern crops with better Rubiscos from algae and wild plants that have higher specificity and faster binding rates.

Carbon metabolism
Optimize the investment of resources in the Calvin cycle by altering gene expression of several enzymes within the leaf.  

Relaxing photoprotection
Increase the speed of recovery from photoprotection, which minimizes damage caused by high light levels while decreasing photosynthesis.

Optimizing canopies
Optimize plant architecture to maximize light energy absorbed by the entire plant canopy. 

Mesophyll conductance
Optimizing CO2 diffusion inside of the leaf to increase its concentration at the site of carboxylation.

Step 3: Confirm increased yield in field environments

The microenvironment in which plants are bioengineered can leave an epigenetic effect that can alter performance in another laboratory or the field. To avoid this issue, RIPE is using a single test-bed in its pipeline. All constructs, which are pieces of designed DNA, from the different institutions are engineered into one tobacco cultivar at the Illinois facility. The engineered plants are multiplied and placed into replicated block trials on the Illinois farm. The constructs that are shown to statistically result in higher yields in the field than the control plants will then be inserted into cassava and rice for further testing.