Drought is the most significant environmental stress in agriculture and is likely to become even more important as the world’s climate warms and competition for water increases. Raising the yield of crops grown with little water is therefore a prime breeding objective, especially in areas where crops depend on rainfall.
Water Efficient Maize for Africa is developing drought-reoetant varieties using a combination of breeding technologies..
As water is essential for all life, it is not possible to develop crop varieties that do without it. Plants tend to respond to severe stresses in ways that improve their chances of survival – often producing fewer grains. They can, however, be bred with drought avoidance characteristics that enable them to make the most of what water is available – such as being small and maturing early. But it is critical that varieties bred for drought tolerance maintain acceptable yield levels.
The mechanisms that allow plants to tolerate short periods of drought or cope with less water are many and complex. A large number of genes are involved and are often subject to large gene/environment interactions which means that the progeny of individual drought tolerant plants may not be drought tolerant themselves in different environmental conditions.
Physiological responses to water stress include reducing transpiration through the leaf surface, lowering photosynthetic capacity, controlling water use during the plant’s life cycle and relocating resources to different parts of the plant to increase its chances of survival. Plants also have different sensitivities and physiological reactions to drought at different stages of their life cycles, with young plants and those at the stages of flowering and grain filling being particularly susceptible.
Further complications include the fact that droughts are unpredictable, vary in severity and in the timing in relation to the age of the plant, and are often accompanied by heat. And plants react differently to combinations of stresses from how they do to individual ones. No single target for breeding drought tolerance exists, so finding a “one size fits all” solution is probably not possible.
The ability of plants to keep their leaves green in situations when water becomes scarce after the plant has flowered is common and can be used to select for potentially increasing yields as it enables the plant to continue the photosynthesis needed to fill the grain. It is important to select for stay-green characteristics and yield improvements simultaneously so as to ensure that the delayed ageing of leaves is productive since leaves can also stay green longer when a plant’s fertility and productivity are reduced. The stay-green appearance of a plant is not always a good indication on how it copes with drought conditions, and analysing large breeding populations is very expensive, but stay-green traits have been associated with improved drought tolerance and yield stability in maize, sorghum and wheat.
Non-GM F1 hybrid WEMA maize varieties with improved yields during moderate drought conditions have already been made available to farmers in several African countries including Kenya, South Africa, Tanzania and Uganda.
Water Efficient Maize for Africa (WEMA)
The average maize yield in Sub-Saharan Africa is 1.8 tonnes per hectare, which is very low compared to other parts of the world because plants are typically exposed to many drought and heat (abiotic) and pest and disease (biotic) stresses, and farmers do not always have access to high quality seeds. Water Efficient Maize for Africa (WEMA) is a public-private partnership aiming to develop improved varieties to raise productivity and to make them available to smallholder farmers, royalty free.
Water Efficient Maize for Africa varieties are being developed by a combination of breeding technologies. The International Maize and Wheat Improvement Center (CIMMYT) has developed a series of maize breeding varieties suited to tropical conditions using conventional breeding techniques (marker assisted breeding and genomic selection). Non-GM F1 hybrid WEMA varieties with improved yields during moderate drought conditions have already been made available to farmers in several African countries including Kenya, South Africa, Tanzania and Uganda.
Water Efficient Maize for Africa is also pursuing a genetically modified (GM) transgenic approach incorporating the MON87460 trait licenced from Monsanto – marketed as DroughtGard in the USA, it is the first transgenic trait linked to drought tolerance ever released. It was deregulated in the USA in 2011 and commercialised from 2012, and under the WEMA partnership, MON87460 will be made available royalty-free to African farmers. This does not mean, however, that the seed will be free: WEMA varieties are sold by a number of seed distribution companies as F1 hybrids, and although these tend to be more productive than open pollinated varieties, farmers need to purchase seed each year. The GM F1 hybrids will, however, be no more expensive than their non-GM counterparts.
MON87460 contains a bacterial gene, from Bacillus subtilis, coding for a cold-shock protein that prevents RNA – ribonucleic acid which plays essential roles in cells – from undergoing some of the structural changes that occur when a plant experiences drought stress. This so-called ‘chaperoning’ minimises the negative effects of stress on photosynthesis, thereby improving yield.
In addition to tolerance to drought, WEMA GM maize varieties could also incorporate insect resistance and herbicide tolerance traits, simultaneously addressing several constraints to production by what is called ‘gene stacking’.
Prior public and political acceptance of GM technology will be required for the introduction of drought resistant varieties.
While WEMA GM maize varieties are being field trialed in Kenya, South Africa, Uganda, and, since 2016, Tanzania, a critical obstacle for their eventual release is the fact that the biosafety legislation, with the exception of South Africa, is not conducive to the commercialization of GM crops. Only three African countries currently grow GM crops commercially – Burkina Faso, South Africa and Sudan – while 12 others grow them in confined experimental field trials. Uganda has not approved its National Biotechnology and Biosafety Bill despite years of deliberation and Kenya has yet to lift its ban on the import of GM foods. And until 2016 the GM legislation in Tanzania was too restrictive to even allow field trials.
Use of these drought resistant varieties by farmers depends on the required biosafety legislation being in place and it being implemented effectively which in turn depends on prior public and political acceptance of GM technology, which remains a significant issue.
Claudia Canales Holzeis is a plant molecular biologist with a near-decade of experience in plant genetics research. She previously worked as Senior Project Officer for the International Service for the Acquisition of Agri-Biotech Applications (ISAAA), based in the Philippines. A graduate of the University of Reading in Environmental Biology, Claudia Canales gained a DPhil. in Plant Genetics at Oxford.