Helping crops photosynthesize better can boost yields

Nature does things pretty well, but not perfectly well. Take photosynthesis, the natural process whereby plants convert Co2 and energy from sunshine into food in the form of chemical energy. Many plants produce toxins as unproductive byproducts during the conversion process called photorespiration, which leads to a loss of energy and thus efficiency.

Scientists can help out, though.

A team of researchers from the United States have come up with a solution: a metabolic pathway that helps plants photosynthesize more effectively so that less energy is lost. In field trials with tobacco plants, which were chosen because they are easy to manipulate, the researchers succeeded in making cultivated plants become 40% effective than tobacco plants in the wild.

They believe that by applying the same technique to staple crops like wheat, rice and soybean worldwide, we could boost food production enough to feed hundreds of millions of people with no extra cost or effort. “Meeting food demands for the growing global human population requires improving crop productivity, and large gains are possible through enhancing photosynthetic efficiency,” the researchers write in a landmark paper published in the journal Science.

“Photorespiration can reduce C3 crop photosynthetic efficiency by 20 to 50%,” they explain. “Although various strategies exist for lowering the costs of photorespiration, chamber- and greenhouse-grown plants with altered photorespiratory pathways within the chloroplast have shown promising results, including increased photosynthetic rates and plant size.”

The reason that plants like wheat, rice and soybean photosynthesize less than ideally involves the enzyme known as ribulose-1,5-bisphosphate carboxylase-oxygenase (or RuBisCO for short), which binds carbon dioxide molecules onto the compound ribulose-1,5-bisphosphate (RuBP). Some 20% of the time, however, RuBisCO mistakes a oxygen molecule for carbon dioxide molecule.

This mishap results in the creation of two toxic compounds: glycolate and ammonia. Plants rid themselves of these harmful toxic byproducts through photorespiration, but doing so requires them to expend extra energy, which stunts their growth.

“Photorespiration is anti-photosynthesis,” says lead author Paul South, who is a molecular biologist at the US Department of Agriculture Agricultural Research Service. ”It costs the plant precious energy and resources that it could have invested in photosynthesis to produce more growth and yield.”

Making matters worse is that in warmer temperatures this glitch tends to happen more often, leading to even less efficiency. That is a growing concern in the face of ongoing climate change. “RuBisCO has even more trouble picking out carbon dioxide from oxygen as it gets hotter, causing more photorespiration,” observes co-author Amanda Cavanagh from the University of Illinois.

The scientists set out to remedy this flaw in plants through a technique called Realizing Increased Photosynthetic Efficiency (RIPE). They isolated genes from other organisms such as the bacterium E. coli and green algae. These were then grafted onto the genes of tobacco plants in three different combinations to see which yielded the most energy-efficient pathways. One combination was especially outstanding by improving plants’ metabolic activity spectacularly, resulting in a 40% boost in efficiency.

In field studies that lasted more than two years, the researchers found that their engineered plants developed faster, grew taller and produced about 40% more biomass, most of which manifested itself in significantly larger stems. “Much like the Panama Canal was a feat of engineering that increased the efficiency of trade, these photorespiratory shortcuts are a feat of plant engineering that prove a unique means to greatly increase the efficiency of photosynthesis,” stresses Stephen Long, chair of Crop Sciences and Plant Biology at the University of Illinois.

If RIPE works out just as well on other staple crops such as wheat, rice and soybean (which has yet to be seen), the results could be spectacular. “We could feed up to 200 million additional people with the calories lost to photorespiration in the Midwestern U.S. each year,” says Donald Ort, professor of crop sciences at the university. “Reclaiming even a portion of these calories across the world would go a long way to meeting the 21st Century’s rapidly expanding food demands — driven by population growth and more affluent high-calorie diets.”