How to fast-track cowpea breeding

Growing cowpea plants in a screenhouse to select for desirable traits. Photo by IITA.
Growing cowpea plants in a screenhouse to select for desirable traits. Photo by IITA.

IITA and partners in the Netherlands and the United States are developing genomic tools that will facilitate the development of improved cowpea varieties with traits such as drought tolerance.

An IITA-Lukas Brader Postgraduate Fellow, Eugene Agbicodo, who carried out the genetic analysis of drought tolerance in cowpea and subsequently constructed a linkage map of the crop, identified portions on the cowpea genome where genes that have effects on drought tolerance and resistance to bacterial blight could be located.

His findings, a landmark in marker-assisted selection in cowpea improvement, will help shorten the plant breeding process.

Similar work has been reported by researchers at the University of California, Riverside, and researchers at the two institutions are comparing notes on the outcomes of their research to see areas of agreement and possible collaboration, according to Christian Fatokun, Cowpea Breeder, who supervised Agbicodo’s genomic mapping work at IITA.

“If both parties are able to find areas of agreement or concurrence, such areas of the genome would be of immense benefit when marker-assisted selection is applied in cowpea breeding. So what will normally take about 10 years to accomplish could be done in three years or even less,” he said.

Cowpea seedlings. Photo by IITA.
Cowpea seedlings. Photo by IITA.

With about 70% of world cowpea grown in the savanna region of Africa, the protein-rich legume provides not only incomes but also improves the health of its consumers. However, cowpea faces several production constraints, among which are diseases, insect pests, parasitic weeds such as Striga, and drought which is becoming increasingly important in the cowpea-producing zones of sub-Saharan Africa.

Agbicodo phenotyped and genotyped a set of cowpea recombinant inbred lines generated at IITA in Ibadan. Phenotyping was carried out in Ibadan and Kano, Nigeria, while the genotyping was carried out at the University of Wageningen, The Netherlands.

Consequently, he constructed a cowpea genetic linkage map using the data obtained from genotyping and phenotyping. The linkage map showed molecular markers that defined quantitative trait loci (QTLs) with effects on drought tolerance and resistance to bacterial blight among others.

Designer (cowpea) plants

Christian Fatokun, c.fatokun@cgiar.org

Plants can be designed to order. Science has long found a way to combine good and useful characteristics in a plant by studying the genes for such traits, and putting them together in a process called “genetic engineering.”

Caterpillar boring into a cowpea pod. Photo by S. Muranaka
Caterpillar boring into a cowpea pod. Photo by S. Muranaka, IITA

Cowpea is grown mainly for its protein-rich grains and quality fodder for livestock. At present, biological control and conventional breeding methods are proving inadequate in developing cowpea varieties resistant to destructive pests, such as the legume pod borer Maruca vitrata.

M. vitrata is the most widespread cowpea pest. The adult moth lays eggs on the plant. The larvae that emerge from the eggs damage plants in the field, particularly during the reproductive stage, through feeding on young succulent shoots, flowers, pods, and seeds. This pest can cause significant grain yield reduction, between 20% and 80% if not controlled with insecticides.

Farmers usually spray insecticides to protect the cowpea crop from Maruca and other pests. Purchasing chemicals, however, adds to the production cost, thus reducing the farmers’ profit. Also, farmers are not well equipped to protect themselves when using such toxic chemicals. In some farming communities, adulterated chemicals that do not control the pests are sold to farmers. The development of cowpea varieties with resistance to Maruca and other insect pests would benefit the most resource-poor African farmers who grow the crop.

Cowpea is grown extensively in the savanna region of sub-Saharan Africa (SSA). At least one major insect pest attacks cowpea at every stage in the life cycle, including seeds in storage. These pests are significantly responsible for the low grain yield in farmers’ fields.

Through conventional breeding, some varieties have been developed that show resistance to some of the pests, such as aphids and flower thrips, and low levels of resistance to the storage weevil. However, not much progress has been made in host plant resistance, especially M. vitrata.

Efforts continue to identify parasites and predators that could be used as biocontrol agents. When deployed, such agents would greatly reduce the population of the Maruca larvae in the field, giving the cowpea plant some respite for the production of flowers and pods containing whole and well-formed seeds.

Using conventional breeding, several hundreds of accessions of cultivated cowpea and its wild relatives have also been screened for resistance to this pest. Accessions belonging to Vigna vexillata were found to be resistant to M. vitrata. These accessions were found to be closest to cowpea in a phylogenetic study of diversity in the Vigna species. The study was based on data obtained after DNA genotyping. Efforts were made to cross cowpea with V. vexillata but without success.

This strong cross-incompatibility makes gene exchange between the two species impossible. This is where biotechnology comes to the rescue. Two major steps are needed to develop genetically modified cowpea with resistance to M. vitrata. First is developing a transformation system and the second is identifying the transgene that would be effective against the pest when introduced into cowpea. Since Maruca is a Lepidopteran, some of the genes from Bacillus thuringiensis (Bt) should be effective against the insect’s larvae. IITA screened several Bt protoxins on Maruca by incorporating different concentrations in the diet fed to the larvae. The protoxin of Bt gene ”Cry1ab” was found to be most effective even at very low concentrations in the artificial diet. This Bt gene (Cry1ab) was therefore selected as the candidate gene for designing Maruca-resistant cowpea.

Scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia developed the transformation system using an IITA-developed breeding line ”IT86D-1010” derived from a cross between ”TVx4659-03E” and ”IT82E-60”. CSIRO scientists have now obtained cowpea lines containing the Bt gene. Monsanto donated the Bt gene used to transform cowpea by licensing it to the African Agricultural Technology Foundation (AATF) for use in Africa. The Rockefeller Foundation and USAID funded this cowpea transformation project.

Farmers transporting cowpea harvest. Photo by S. Muranaka
Farmers transporting cowpea harvest. Photo by S. Muranaka, IITA

The Bt cowpea had been tested in the CSIRO laboratory in Australia and found to be effective against the larvae of another Lepidoptera, Helicoverpa armigera. The Bt gene in cowpea is expected to be effective against M. vitrata, the cowpea pest, but needs to be tested in an environment where Maruca thrives. Apart from Burkina Faso, none of the African countries where cowpea is an important crop has a biosafety law in place. A few lines of the Bt cowpea were, therefore, taken to Puerto Rico for field testing. The field trial was carried out in late 2008. If the Bt gene in the cowpea lines is found to be effective against Maruca, the next step would be to transfer the Bt gene into popularly grown cowpea varieties selected from interested countries. The line presently containing the Bt gene is not high yielding, and farmers are not likely to accept it readily.

Under the international biosafety protocol (Cartagena protocol on biosafety) it is necessary to carry out risk assessment on the Bt cowpea before it is introduced to another country. The data obtained from risk assessment form part of the dossier that accompanies applications requesting for importation to any country. Risk assessment would entail studies on gene flow, the effect of the transgene on nontarget organisms, food safety, and resistance management strategies.

A meeting of experts in these various fields is planned in March 2009 at the Donald Danforth Plant Science Center, St. Louis, Missouri, USA. The experts would design studies to address the different questions that may arise from biosafety regulators in the countries where the Bt cowpea is meant to be grown. Many of the proposed studies are necessary, because cowpea is an indigenous food crop in SSA where cross-compatible wild relatives are found growing in agroecologies similar to farmers’ fields. Biosafety reviews in the African countries would, therefore, be rigorous.