Cassava is a starchy, tuberous root first domesticated about 10,000 years ago in South America. Also dubbed manioc and yucca, cassava may be more familiar to North Americans as tapioca—tiny pearls of starch used to thicken pies and jams. For millions of people in the tropics, however, it is a staple, not a baking aid. Now, concerted efforts at crossbreeding and genomic selection have created novel versions of cassava that could dramatically boost yields, ward off malnutrition and grow in a wide range of conditions.
The typical cassava shrub produces unassuming brown roots with snowy white or creamy colored interiors. A cassava crop is perennial—after maturing for at least eight months roots can be harvested for a few years. New plants grow easily from cuttings. The root is carbohydrate-rich, protein-poor and must be boiled, roasted, fermented or otherwise processed to tame compounds that can produce toxic hydrogen cyanide during digestion. Nevertheless, an estimated 800 million people worldwide eat cassava. In Africa 500 million depend on the root as their main staple.
Because many cassava consumers live in developing countries, the plant has not received the intense breeding that has benefited crops more familiar to the Western world such as corn, wheat and rice. In the past decade, however, cassava has started to garner attention. China and Thailand use it to make high-quality starch, and some countries see the crop as a potential biofuel. What’s more, cassava will likely do well in the world’s changing climate; it survives drought when other crops have failed and flourishes in warmer temperatures.
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Big boost in yield
Among the latest and most impressive breeding successes comes from Nagib Nassar, a cassava breeder and professor emeritus of genetics at the University of Brasilia. He has developed a new variety that could dramatically boost yields. Each of his plants produces about 14 kilograms of edible roots after one year whereas traditional varieties yield just two to three kilograms.
Apart from their massive roots, the new cassava plants don’t look too strange until examined under a microscope. Nassar has created chimeras—a fusion of two individuals—by carefully grafting a common cultivar with a wild species and treating the results with a synthetic plant growth hormone. The outer, or epidermal, layer of the new plants’ tissues is from cultivated variety and the inner tissues are from the wild parent.
Encouraged by the vigorous growth of the chimeras, Nassar hopes to investigate the possibility of more crosses. With his graduate student, Nayra Bomfim, he published a paper on the second chimera in the online journal Gene Conserve in January (pdf). “He is a tireless scientist,” says Hernan Ceballos, a cassava breeder at the International Center for Tropical Agriculture in Colombia. Ceballos, who was not involved in the work, adds that the chimeras are interesting but that further work to demonstrate the plants’ productivity is needed.
Nassar’s chimeras aren’t the only new cassava variety out there. Ceballos himself has been working on a variety that would tower high above future farmers’ heads. The "Asparagus Cassava" has no branches—its leaves grow directly from a massive stalk. This variety can be planted more densely than traditional cassava and produce higher yields. Other varieties include a cassava high in vitamin A, which turns the root orange, and one with extra protein.
Getting to the next generation
These promising varieties could be just the beginning in a cassava revolution. “There is a growing recognition for the importance of food security in the most food-insecure areas," says Jim Lorenzen, a senior program officer at the Bill and Melinda Gates Foundation, a major supporter of cassava research. The foundation aided researchers in the sequencing of the cassava genome and, along with the U.K.'s Department of International Development, awarded $25 million in late 2012 to a massive international effort called the Next Generation Cassava Breeding (NEXTGEN) project, which aims to jump-start genetic improvement of cassava.* "It’s a very good time for cassava research," Lorenzen says. He notes that this attitude is reflected in more researchers focusing on cassava and intense interest from African leaders, including Nigeria’s Minister of Agriculture and Rural Development, Akinwumi Adesina.
The project unites researchers at breeding programs in Nigeria, Uganda and the U.S. The scientists involved are currently homing in on key traits needed to breed better cassava plants. Robert Kawuki, a cassava breeder at the National Crops Resources Research Institute in Uganda, is working to identify molecular markers associated with resistance to brown streak disease, one of cassava’s greatest threats in Africa.
Kawuki’s first step in finding genes responsible for disease-resistance is looking at many plants as they grow in the field for the target traits. For brown streak disease, that requires a lot of effort: "You have to uproot the plant from the ground and score each root individually," he says. That may require digging up over 1,600 plants for a single trait. The most promising plants are genotyped, and their data is entered into a computer model. That model will identify molecular markers and decide which plants should be crossed to produce the best genetics for the next generation. The whole approach, called genomic selection, is already used to great effect in the dairy breeding industry, Lorenzen explains.
The ability to accurately predict crosses promises to save a lot of time. Typically, breeders had to wait until their cross-breeds grew to see how they would turn out. New varieties were tested in multiple locations over several growing seasons before release. Aided by the computer model, NEXTGEN’s approach can compress that time line into three to five years by how offspring will fare, says Chiedozie Egesi, an assistant director and head of cassava breeding at the National Root Crops Research Institute in Nigeria. The project is also targeting resistance to cassava’s other plagues: green mites that devastate leaves, cassava bacterial blight that browns stems and defoliates the plants and cassava mosaic virus that yellows leaves and stunts plant growth.
This past weekend Egesi joined a group to meet and discuss the NEXTGEN project’s progress. "The first year was very successful," Lorenzen says. "But it will take time to see how it works out in practice."
Millions of small famers and their families stand to benefit from cassava researchers’ efforts. By the end of this year, more than 9,000 farmers are scheduled to grow Nassar’s chimeras in Brazil. Expect to hear more about the starchy root in the future, as new varieties help feed the tropical world.
*Editor's note (3/26/14): This sentence was edited after posting. The original omitted crediting the U.K. Department of International Development for its role in funding the NEXTGEN project.
More:
Breeding Cassava to Feed the Poor, by Nagib Nassar and Rodomiro Ortiz, Scientific American, May 2010