Plants are pretty incredible, given their ability to grab sunlight and carbon dioxide from the air to make sugars for fuel.
For a time in the Earth's history, this process was relatively easy because there was more CO2 in the air, but as oxygen came to dominate, plants learned to filter out oxygen molecules and latch onto that precious CO2. This means plants waste energy while trying to make the energy they need to survive — and, of course, produce the oxygen and food we need.
Scientists at the University of Illinois and the U.S. Department of Agriculture's Agricultural Research Service have hacked plants to make them more efficient by helping them avoid grabbing those unnecessary oxygen molecules. It turns out that when plants can more efficiently fuel themselves, they can increase their biomass by 40 percent.
Helping plants recycle better
To grab CO2, plants rely on a protein called ribulose-1,5-bisphosphate carboxylase-oxygenase, more commonly called Rubisco because — well, look at that full name. Rubisco isn't very picky, and it will grab oxygen molecules from the air roughly 20 percent of the time. The result when Rubisco combines with oxygen is glycolate and ammonia, both of which are toxic to plants.
So instead of using energy to grow, the plant engages in a process called photorespiration, which essentially recycles these toxic compounds. Recycling these compounds requires the plant to move the compounds through three different compartments in the plant cell before they're recycled enough. That's a lot of wasted energy.
"Photorespiration is anti-photosynthesis," Paul South, a research molecular biologist with the Agricultural Research Service who works on the Realizing Increased Photosynthetic Efficiency (RIPE) project at Illinois, said in a statement. "It costs the plant precious energy and resources that it could have invested in photosynthesis to produce more growth and yield."
Since recycling requires a lot of energy, some plants, like corn, have developed mechanisms that stop Rubisco from grabbing oxygen, and those plants fare better than those that haven't developed this strategy. Seeing these evolutionary countermeasures in the wild inspired researchers to try and simplify the recycling process for plants.
The researchers turned to tobacco plants to develop a more efficient photorespiration process that also took less time. Tobacco plants are easy to genetically engineer, easy to grow and they grow a leafy canopy that is similar to other field crops. All of these traits make them useful test subjects for something like figuring out the best way to simplify photorespiration.
Researchers engineered and grew 1,200 tobacco plants with unique genes to find the best combination of recycling. The plants were starved of carbon dioxide to encourage Rubisco to grab oxygen and create glycolate. Researchers also planted these tobacco crops in a field over a two-year period to gather real-world agricultural data.
The plants with the best genetic combinations flowered a week earlier than others, grew taller and were about 40 percent larger than unmodified plants.
The researchers outlined their findings in a study published in Science.
Long road ahead
It would be easy to think that this was just a bit of scientific tomfoolery since, as we're all constantly told, there's more and more CO2 in the atmosphere. It would follow then that good old Rubisco wouldn't be struggling as much with more CO2 to choose from, right? Well, not quite.
"Increased atmospheric carbon dioxide from fossil fuel consumption boosts photosynthesis, allowing the plant to use more carbon," Amanda Cavanagh, a research associate at Illinois explains in a post for The Conversation. "You might assume that this will solve the oxygen-grabbing mistake. But, higher temperatures promote the formation of toxic compounds through photorespiration. Even if carbon dioxide levels more than double, we expect harvest yield losses of 18 percent because of the almost 4 degrees Celsius temperature increase that will accompany them."
And harvest yields are ultimately what making photorespiration more efficient is all about. According to Cavanaugh, we have to increase food production by 25 to 70 percent to have "an adequate supply of food" by 2050. Currently, we're losing 148 trillion calories a year in unrealized wheat and soybean crops due to the inefficient nature of photorespiration. That's enough calories, Cavanagh writes, to feed 220 million people for a year.
Which is why researchers are moving on to test their genetic combinations in other crops, including soybean, rice, cowpea, potato, eggplant and tomato. Once the food crops have been tested, agencies like the Food and Drug Administration and the U.S. Department of Agriculture will test the crops to make sure they're safe to eat and don't pose a risk to the environment. That process can take up to 10 years and cost $150 million.
That's all to say, don't expect bigger eggplants any time soon.