From an engineering point of view, photosynthesis is inefficient. One out of every five tries, the enzyme that plants use to catch carbon dioxide mistakenly grabs on to oxygen, producing toxic glycolate instead of carbohydrate building blocks. This misstep is called photorespiration. Plants use a tremendous amount of precious energy to correct it. In fact, photorespiration reduces crop yields by an estimated 20 to 50%.
Now researchers have demonstrated a better way to correct this photosynthetic error. Tobacco plants genetically engineered to more efficiently break down glycolate showed 40% greater crop yields in field trials (Science 2018, DOI: 10.1126/science.aat9077). If the findings can be replicated in food crops, farmers could produce more calories using less land and fertilizer.
“This is a demonstration that you can alter photorespiration in a fundamental way and get significant gains in yields—and that’s exciting,” says Berkley Walker, a plant biologist at Michigan State University who was not involved with the research. Walker’s modeling work suggests that if photorespiration were eliminated, farmers in the midwestern US would produce 320 trillion more calories from their crops annually on the same amount of land (Annu. Rev. 2016, DOI: 10.1146/annurev-arplant-043015-111709).
Many efforts to fix photorespiration have focused on the error-prone carbon dioxide-capturing enzyme called rubisco. But making rubisco more selective for carbon dioxide slows it down, so engineered plants don’t grow any bigger or faster. Other efforts focused on supplementing plants’ native pathways for breaking down glycolate, inserting genes for glycolate-scrubbing enzymes from bacteria or algae into their genomes. This work, mostly done in the model plant Arabidopsis, helped plants grow bigger in greenhouses, but hadn’t yet been translated to field studies. And the gains in crop yield weren’t as large as agronomists believed could be possible. One reason is that these engineered plants didn’t turn off their inefficient native photorespiration machinery, says Paul South, a molecular biologist with the US Department of Agriculture’s Agricultural Research Service, based at the University of Illinois at Urbana-Champaign.
South and his collaborators tested a novel pathway that added a glycolate-processing enzyme from the algae Chlamydomonas and a malate synthase from a pumpkin, and blocked some aspects of native photorespiration. Part of the reason photorespiration is so costly for plants is that it requires transporting reactants between different compartments inside the cell tens of times. The Illinois researchers used RNA interference to block the first transport step, trapping glycolate in the chloroplast. There the added enzymes work in concert with native ones to free the carbons wasted in glycolate, forming two molecules of carbon dioxide.
The team engineered tobacco, which is a model crop plant, to express this new pathway. In 2017, the team conducted a large field trial of the tobacco. On average, the engineered plants yielded 40% more biomass at harvest than unmodified tobacco.
Veronica Maurino, a botanist at Heinrich-Heine University Düsseldorf who developed an earlier version of the genetic pathway built upon by the Illinois group, says demonstrating yield increases in field studies is an important step forward. But researchers still need to determine whether these changes “will also boost productivity in other plant-crop species and in the parts of the plants that are used as food such as seeds, roots, and fruits,” Maurino says. The Illinois group is now trying to engineer this pathway into soy and plants that are major sources of calories in the developing world, including cassava and cowpeas.
This story was updated on Jan. 3, 2019, to correct the spelling of Amanda Cavanagh's name in the photo caption.