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Bioscience in brief

Maize

Did you know?
Maize was absolutely central to several civilizations that flourished before the arrival of the Europeans in what is now Mexico and Guatemala. Indeed, according to the the creation myth of Popol Vuh, a sacred body of narratives from the K’iche’ Kingdom of Guatemala, after creating the world the gods decided to create people, but their first attempts were unsuccessful: creatures made of mud or wood lacked soul, intellect and speech. Finally, they succeeded: “only dough of corn meal went into the flesh of our first fathers, the four men, who were created. […] And as they had the appearance of men, they were men; they talked, conversed, saw and heard, walked, grasped things; they were good and handsome men“. The first four women were also created from maize dough, when the men were asleep.


MAIZE

Maize or corn (Zea mays) was first domesticated in Central America 7 500–12 000 years ago and has since spread across world, including Africa, where it is now the most important staple crop – more than 300 million people on the continent depend on it as their main food source. According to the Food and Agriculture Organization of the United Nations (FAO), global maize production in 2013 exceeded 1 billion tonnes, of which 75 million tonnes were produced in Africa. Nearly one third of the maize produced worldwide is estimated to have been genetically modified (GM).

Give me five
Despite the huge importance of maize, the identity of its undomesticated ancestor was a mystery, because no plant in the wild that looks like maize. However, it was eventually found that maize can hybridise naturally, forming fertile seeds with a short bushy grass called teosinte, which has small kernels enclosed in a hard casing – in maize the soft kernels are exposed. Later studies confirmed that both grasses have the same number of chromosomes and that the arrangement of genes along these is conserved.

Because the plants look so dissimilar, it came as a surprise that teosinte was the progenitor of maize, and that only five genes were responsible for the big differences between them. This discovery was important in showing that large evolutionary steps can occur as a result of changes in single genes that dramatically alter they way they function.

Y for yellow
White maize varieties are strongly preferred to yellow ones in African countries, although yellow maize varieties are slightly more nutritious that their white counterparts. The colour of the kernel comes from the colour of the nutritive tissue (called the endosperm) that surrounds the plant embryo in the seed – the yellow colour is due to the presence of pro-vitamin A. A single gene, called Y, is responsible for production of this, a precursor of vitamin A. Because only a single functional copy of the Y gene is needed to make pro-vitamin A, the yellow kernel characteristic is dominant. As a result, when yellow and white maize varieties are grown in the same field, the white kernel varieties are eventually lost, something African farmers know well.

THREATS AND CHALLENGES
A new foe rises in the east: maize lethal necrosis disease
In September 2011 a new maize disease was reported in Southern Rift Valley of Kenya. Symptoms include dying of the leaves from the edges, eventually affecting the entire leaf; death of young leaves (“dead heart”); stunted plants that die prematurely; failure to develop tassels and ears, and rotting cobs. Plants of all ages, from seedlings to fully mature plants, are susceptible. The disease, called maize lethal necrosis disease (MLND), is a result of a combination of two viruses, which are carried by insects over large distances – the disease has spread in a very short time to Tanzania, Uganda, South Sudan, and more recently Rwanda, representing a serious threat, in particular for smallholder farming communities as it can destroy an entire crop. Farmers are advised to immediately remove and discard infected plants, and not to eat infected grains, as secondary infections by fungal pathogens (which produce dangerous toxins) are very common.

A new base to fight back
The response of the scientific community has been to counterattack on several fronts. In September 2013, the International Maize and Wheat Improvement Center (CIMMYT), in partnership with the Kenya Agriculture and Livestock Research Organization (KALRO), established the Maize Lethal Necrosis Screening Facility at KALRO-Naivasha to focus on tackling MLND, with funding from the Bill and Melinda Gates Foundation and the Syngenta Foundation. KALRO and CIMMYT jointly carried out an evaluation of CIMMYT inbred lines and pre-commercial hybrids to uncover the genetic basis of disease resistance and identify suitable varieties for breeding progammes. This is urgent: an initial trial featuring 119 commercial maize varieties in Kenya under artificial inoculation revealed that 117 varieties were susceptible to MLND.

Efficient maize for nitrogen-poor soils
Nitrogen is the single most important input for maize production. Nutrient depletion in farming soils in Africa combined with the high price of fertilisers mean that smallholder farmers are often powerless to address this problem.

The Improved Maize for African Soils (IMAS) project explored natural genetic variation for nutrient-use efficiency in tropical maize varieties, and bred new varieties through marker-assisted selection (MAS) that are more responsive to low levels of nitrogen fertilisers. Forty-one Africa-adapted nitrogen-use efficient maize varieties, some with improved resistance to MLND, are ready for release in nine African countries through 24 seed companies. The aim is to raise maize yields by 50 per cent and benefit up to 60 million maize farmers in eastern and southern Africa. Importantly, the released varieties also perform well when there is no shortage of nitrogen.

Water efficient maize for Africa
Water is essential for life, and while no plant can live without it, some plants are better than others at coping with short periods of drought. The Water Efficient Maize for Africa (WEMA) project aims to develop improved varieties that are better adapted to African drought-prone fields, by conventional breeding techniques, marker-assisted selection (MAS) and genetic modification (GM). The resulting improved varieties will be made available, royalty free, to smallholder farmers in Sub-Saharan Africa through African seed companies. In September 2014 WEMA released two improved varieties in South Africa, developed by conventional breeding. These are now being tested by farmers.

 

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