Cassava as a 'strategic starch reserve': why the US is sequencing the plant's genome

Cassava or manioc is a root crop grown all over the tropics and the sub-tropics by subsistence farmers. After rice, it is the second most-widely used crop in the developing world, where billions consider it to be their staple food. In a country such as Congo, cassava makes up more than half the entire daily calorie intake of the Congolese (see the FAO's Food Balance Sheets).

Despite the fact that such a large segment of the world's population relies on the crop to meet its dietary needs, the plant is typically understudied and underutilized. Few international research organisations, multinationals or biotech companies are interested in funding cassava research. This also means virtually no efforts are made aimed at increasing the productivity of the plant or at utilizing it in innovative ways to add value, with which to boost the incomes and the food security of the subsistence farmers who make a living from it. The main reason for this lack of interest: cassava is not consumed by people in the West, and those who do eat it are poor people.

But recently, this situation has changed. Earlier we reported on how a U.S. biotech research center received a US$15 million grant from Monsanto to create a transgenic variety of the crop (earlier post), while cassava has also been chosen by the U.S. Department of Energy's Joint Genome Institute to have its genome sequenced (earlier post).

Strategic starch reserves
There can be only one reason for this sudden interest into a plant that was previously seen as being of 'marginal' importance: to create 'strategic starch reserves' around the world to supply the carbohydrate economy of the future.

Cassava roots are very rich in starch (with a starch content of up to 35%), but poor in protein and other nutrients. This starch can be easily transformed into sugars, which are the building blocks of a bioeconomy in which petrochemical products will be replaced by sugar-based products (from plastics to fuels).

The high-profile research interest into cassava gives a clear indication that even the US will be looking abroad for future reserves of cheap starch. After all, cassava yields far more of it than corn, the main source currently used for the production of ethanol in the US. Contrary to the North America or Europe, vast parts of the Global South have plenty of land available for the sustainable production of energy crops. Here, they can be grown competitively and cheaply.

Even though the R&D efforts into cassava may have a finality that ultimately goes beyond improved food production, in the shorter term they may benefit the billions of subsistence farmers who depend on the root crop:
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The University of Arizona, which played a major role in sequencing the genome of rice, offers us a better view of the rationale behind the Joint Genome Institute's interest in cassava. It is providing one of the multi-institutional team's main researchers, Steven Rounsley, associate research professor of the BIO5 Institute and of the Department of Plant Sciences.

"The cassava is a very important crop for subsistence farmers in Africa, and to obtain more information that farmers can use will help them build better crops for the poorest people," says Vicki Chandler, director of the BIO5 Institute.

The cassava, which is second to rice in the developing world as a main source of food, has a unique potential for conversion into ethanol because of its high percentage of starch, Rounsley said.

Interest in the cassava from the science community is still small, Rounsley said, but since the U.S. Department of Energy's Bioenergy Initiative, the government has taken interest in bioenergy and turning things like corn or other starchy material into ethanol fuel. "The cassava is not generally an interesting crop for the scientific community, but the recent energy proposal kicked it all together and there is a reality for a potential energy use," Rounsley said.

From a humanitarian perspective, the cassava genome sequencing project will benefit those people who eat it on a daily basis. "The problem with the cassava is that it's so full of starch. There is very little in nutrition," Rounsley said. "Protein is missing and there is a cyanide compound in raw cassava that makes it poisonous to eat."

In sequencing the cassava genome, there are hopes to make it more nutritious and safer to consume, and to protect the root from disease. "Plant diseases can wipe out entire crops, and if this happens to a farmer in Africa he not only loses his source of income, but also his food source and the food source of the community," Rounsley said.

Africa and South America are one of the largest producers of cassava because the root can survive draught conditions and can grow in poor soil, Rounsley said. "The cassava is a major crop in Latin America, and the cassava is not a trivial crop, and the genome sequence of the cassava will be helpful in identifying ways to improve the crop," said Robert Leonard, head of the plant sciences department.

The sequencing project is still in the pilot phase, said Rounsley, meaning the DOE is still evaluating the data to pick the next step. After the DOE has completed evaluating the data, the information will be passed on to researchers such as Rounsley to interpret and evaluate the data and find a meaning.

"Sequencing a genome is like reorganizing a textbook where the chapters, words and sentences are all mixed up," Rounsley said. "I try to define a beginning and end to come to a meaning." It takes about a year to generate a DNA sequence, and the resulting genome can have a DNA sequence 800 million letters long, which is the same amount as a grain of rice, Rounsley said.

"In terms of getting useful information to breeders, farmers and scientists, we will try to get it to them as quickly as possible, but it may take one to three years," Rounsley said.