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Cassava is RIPE for yield increases

By Joan Conrow

Photo by GFAR

Though many Westerners have never heard of cassava, it's the third-most important food crop in the tropics, right after rice and corn (maize).

Cassava — also known as manioc, yuca, tapioca and mandioca — feeds some 800 million people in Africa, Asia and Latin America. It's especially important in sub-Saharan Africa, where it comprises 30 to 50 percent of all calories consumed.

Yet despite its immense value to growing populations and small-holder farmers, yields have not improved in the last 25 years. Now scientists working with the Realizing Increased Photosynthetic Efficiency (RIPE) program, funded by the Bill & Melinda Gates Foundation, have identified 14 different paths to boost cassava production, including tweaking the process of photosynthesis.

"Having an impact on farmers particularly in the poorest countries of the world is very important to myself and my group,” Stephen P. Long, a professor of crop sciences and plant biology at the University of Illinois, Urbana-Champaign, told the Alliance for Science in a recent interview. “We’ve devoted quite a chunk of our lives to this in the hope that this is going to provide germplasm (seed) that the farmers in these countries can really benefit from. Because if these predictions of food shortage really do develop, these will be the communities that are affected first. We see this seed as being their insurance against this future possibility of shortage.”

As Long and his co-authors, Amanda P. De Souza and Lynnicia Massenburg, reported in an article on TheConversation.com:

Hacking photosynthesis has long been considered to be a holy grail of plant biology. Photosynthesis is the process in which green plants use the energy of sunlight to synthesize food from carbon dioxide and water, fueling their growth. It is directly or indirectly the source of all of our food, as well as many of our fibers and most of our fuel. By simulating the process on supercomputers, we identified points where we might intervene to speed up the process.

Our research demonstrates that this theory can now be translated into real productivity increases in crops, and that the potential payoff is significant. By genetically modifying tobacco plants, we increased the amount of plant tissue that they produced by 14 to 20 percent in real-world, replicated field trials where light, rainfall and other factors are unpredictable. We used tobacco because it is easily modified, but also produces many layers of leaves, making it a good proxy for other crops. The process we modified is common to all plants, which strongly suggests that this approach should work just as effectively in cassava and other food crops.

This research is especially timely in view of an assessment by a United Nations Food and Agricultural Organization project that determined the world will need about 70 percent more primary food sources, such as corn, rice, wheat and soy, by the year 2050. The challenge of meeting that goal is intensified by climate change.

“We really need to able to do that on the land we’re already using in agriculture,” Long told the Alliance. “We don’t want that to spill over into more land and see more tropical deforestation. Because that will even accelerate climate change further. So now when we impose climate change on top of this we’re gonna see [higher] temperatures which are suddenly affecting cereal production in the tropics, and we’re also seeing a rise in what we call the vapor pressure deficit. This is basically the drying power of the atmosphere. Crops will use more water because of this.”

As a result, plants will need to become super-efficient. As Long and his co-authors explained in their article on TheConversation.com:

Here’s how our approach works: In full sun, plants receive more energy than they can use. If they can’t get rid of this excess energy, it will bleach their leaves. To protect themselves, plants induce a process called photoprotection, which converts this excess energy harmlessly to heat.

But when a cloud passes overhead, it can take minutes to hours for the plant to fully recover and begin photosynthesizing at maximum capacity again. In the shade, lack of light limits photosynthesis and photoprotection causes the plant to waste precious light energy as heat.

Using a supercomputer, they determined that the recovery process could reduce yields by 7.5 percent to a huge 40 percent, depending on the type of plant and prevailing temperature.

Long's team then collaborated with researchers at the University of California, Berkeley, to develop a “cassette” of genes to speed up the recovery process by boosting the amount of three proteins involved in photosynthesis. Two of the modified plant lines they developed consistently achieved 20 percent higher productivity than unaltered tobacco plants, while the third was 14 percent higher.

Buoyed by those results, they've now moved on to cassava, using the same gene “cassette” to improve its photoprotection recovery process.

But that's not the only research in the works, according to The Conversation article:

Other “synergistic improvements” on our radar include steps such as engineering plants to produce fewer leaves; improving the way leaves are arranged to better capture light; and altering leaf color to reduce shading of lower leaves. We are also working to reduce losses from photorespiration, a parasitic process that occurs during photosynthesis when oxygen is accidentally used instead of carbon dioxide. Photorespiration causes plants to burn as much as 40 percent of the energy they have produced through photosynthesis. This problem will increase along with rising temperatures from climate change.

It typically takes 15 to 20 years to move advances like these from the lab to farmers’ fields at scale. Because of that lag, in a world with a fast-growing population, we are just one crop breeding cycle away from starvation. It is therefore essential to start improving yields of staple crops like cassava now, so that we will have these solutions when we need them.

Their work may have other profound implications for farmers in Sub-Saharan Africa, who often work small farms of an acre or less.

“Being able to improve the yield on that land can make a very big difference to that livelihood,” Long said. “They may just be producing enough food for their family. If we can provide them with germplasm which gives them 30 percent more, they have enough for their family and now some to sell to pay for other services, like education for their children.”

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