Unraveling the complexities of plant science

By Joseph Opoku Gakpo

February 21, 2018

In Sub-saharan Africa, cassava serves as a major staple for most people; in fact it is the third largest carbohydrate source after rice and maize. Some of the more than 500 million cassava consumers in the region eat is as often as three times a day. It’s a dependable crop because cassava grows well in poor soils with little rainfall.

But two main diseases continue to damage the crop on the continent, causing farmers huge losses. First, there’s the cassava mosaic disease (CMD) virus. It causes stunting in the plants and eventually kills them. Annually, it is estimated to cause cassava yield loss of up to 30 percent, mainly in West and East Africa.

Then there is the cassava brown streak disease (CBSD) virus, which can ruin an entire crop. This disease is currently the biggest cassava constraint in East Africa, as it destroys the edible roots of the cassava even when rest of the plant looks healthy.

Since 2005, a global collaborative effort to deal with these challenges has been ongoing by researchers at the Donald Danforth Plant Science Center in the US, the National Crop Resources Research Institute in Uganda, the National Crop Research Institute in Nigeria, the Kenya Agricultural and Livestock Research Organization, and a number of other partners. The VIRCA Plus Project (Virus Resistant Cassava for Africa Project) is the result of a merger of two separate projects: the VIRCA project, which was aimed at making the cassava resistant to the two diseases; and the Bio Cassava Plus Project, which was aimed at improving upon the nutritional value of the cassava.

But the work has revealed the complicated nature of science. There are a number of cassava varieties in East Africa that have natural resistance to the CMD virus. The VIRCA Plus project used modern biotechnology processes to introduce the genes responsible for resistance to CBSD into the ones with natural resistance to CMD. The result: the natural CMD resistance was lost.

“VIRCA took varieties with natural CMD resistance…. Then went on to add transgenic CBSD resistance…. The process of tissue culture caused them to lose resistance to CMD… but transgenic CBSD resistance remained,” explained Dr. Andrew Kiggundu, project manager of the Institute for International Crop Improvement at the Donald Danforth Plant Science Center at St. Louis, Missouri, in an interview with the Alliance for Science.

Kiggundu attributes the situation to inexplicable circumstances that demonstrate how complicated plant science can be. But he says work is ongoing to correct that and re-introduce the lost resistance into the variety so it can have both. “The mission is to provide resistance to both diseases… So they are doing field work to back cross the CMD resistance… Now there are about 300 lines that are transgenic and have both CMD and CSBD resistance… and they are crossing it back to non-transgenic, which is CMD resistant… Now they have the first generation,” he said.

“We are sure that once we get CBSD resistance in there, it will help millions, because cassava is a food security crop for many people. Cassava production will be sustained,” he added.

The Donald Danforth Plant Center is also working with its partners under the same project to enhance the nutritional value of cassava. Although cassava provides a lot of calories, it does not contain sufficient levels of key nutrients.

Iron and zinc deficiency is a key nutritional problem in sub Saharan Africa. It causes stunting, disrupts the cognitive development of children and increases their risk of contracting diarrhea. Some 75 percent of preschool children and 67 percent of pregnant women in Nigeria are affected by iron deficiency anemia, while 24 percent of the people in sub-Saharan Africa are at risk of zinc deficiency due to inadequate dietary intake. Deficiency in vitamin A, which is known to cause night blindness as well as increased risk of infection in children and maternal mortality in women, is a big issue in Africa as well.

The VIRCA project was working on introducing these three nutrients into local cassava varieties in Nigeria. But it halted work to introduce carotene, which is a vitamin A precursor, into the cassava as a result of inexplicable complications.

“We were able to make golden cassava. We had orange roots,” said Dr. Nigel Taylor, director of the Institute for International Crop Improvement at the Donald Danforth Plant Science Center, in an interview with the Alliance. “But as you put the beta carotene in and that level goes up, the starch goes down in the roots. Unknown science, unknown biological phenomenon, and right now, we can’t fix it. It’s not like it was broken. This is an unknown biological phenomenon that has been there forever,” he explained, noting that there are conventionally bred cassava varieties that have a higher content of beta carotene.

“The farmers grow cassava for the starch and if the starch content goes down, they won’t plant it,” Taylor said. “That is why we dropped it, because we know we will not be able to deliver it to the farmer.  We are focusing on getting it to the farmer… What we are doing, it has never been done before. And anytime you do this, you discover these things.”

These examples of anomalies encountered by the VIRCA project speak to the complexities of science. But clearly, the same science is being used to overcome them.