Bioscience in brief
The genetic distribution of sorghum on the African continent mirrors the distribution of the main language families – Nilo-Saharan; Bantu; early Niger-Congo and Afro-Asiatic. The movement of people, shifting cultivation practices – the changing of cultivation sites as a result nomadic lifestyles – and the extensive cross-fertilisation of sorghum with compatible wild grasses have played very important roles in the evolution of this venerable crop.
Sorghum is a plant genus in the grass family, separate species of which originated independently in the Ethiopia, West Africa, Australia, Asia, Mesoamerica and the islands of the Indian and Pacific Oceans. It has been eaten by people for at least 11 000 years and today, one African native species, Sorghum bicolor, is the fifth most produced cereal crop worldwide – in Africa, it is the second most important with around 20 million tonnes produced annually. Nigeria is the main international producer. Sorghum is processed into a large variety of traditional and modern foods and is also the cereal of choice for the African beer industry. In addition, it provides animal feedstock, fibres and fuel.
A hardy crop for dry areas
Sorghum grows in areas with low water availability, which makes it a very important food-security crop in dry and semi-arid regions prone to drought. All plants need water to grow, so drought tolerance is a result of adaptations that allow the plant to conserve water and maintain normal physiological functions during periods of drought. These include adaptations in the plant’s shape and structure: sorghum has long roots to absorb water from the deeper soil layers and a thick cuticle layer, akin to skin, that reduces water loss through evaporation from the leaves and stem.Drought tolerance is also provided by adaptations at a biochemical level. Under water stress, sorghum produces soluble sugars that help protect its cells from the effects of losing water, which would otherwise result in the plant wilting. Sorghum has also evolved an improved way of converting sunlight into energy at high temperatures, called C4 photosynthesis, which originated independently in several grasses including maize.What genetic factors are responsible for this hardiness and tolerance? Drought tolerance is a very complex characteristic that is not yet fully understood and is governed by a large number of genes. However, an analysis of the sequenced sorghum genome, which was completed in 2009, suggests that recent genetic duplications of parts of its DNA may contribute to the plant’s drought tolerance.
More than a pinch of salt
It has been predicted that by 2050 more than half of the world’s arable land will be affected by high salinity, which will severely limit the yield of most crops – by this time the world will also have 2 billion more people to feed. Understanding how plants respond to high salinity and other stresses is therefore a high priority.Salinity has similar effects on a plant as drought because it effectively reduces the amount of water available to its cells. It also poses additional toxicity problems and often occurs in areas with additional abiotic stresses – non-living factors, such as the accumulation of heavy metals and/or high soil acidity.Understanding the responses of plants to environmental stresses is very complex as they are regulated by a large number of genes; plants respond differently to stresses if they occur in combination and the responses also vary depending on the growth stage of the plant – drought, for example, is particularly damaging at the seedling stage and during flowering. Nonetheless, a key requisite to start identifying these responses and breed more resilient plants is the identification of genetic variation.While rice is very sensitive to high concentrations of salt, different sorghum varieties display varying degrees of tolerance due to high levels of genetic variation for this characteristic. Sorghum is now acknowledged as one of the genetic models of choice to start unravelling the genetic basis of drought and salinity tolerance. The knowledge gained will benefit plant breeding efforts directed to maintaining yields under hot, dry and high salinity conditions in many other crops.
The purple vampire hits again
Striga or witchweed, a parasitic plant with beautiful purple flowers, is one of sorghum’s main enemies: every year it results in the loss of approximately one quarter of total production in Sub-Saharan Africa, with an estimated value of US$ 7 billion. Many lines of defence are being explored; one successful strategy has been to develop a biological control agent using a naturally occurring soil fungus originally found in Nigeria. Another approach was to determine which areas of the sorghum genome are associated with resistance to Striga and use genetic markers to introduce them into elite varieties by breeding and selection. The first sorghum varieties bred through marker assisted selection (MAS) combining resistance to Striga, drought-tolerance, high grain quality and high yielding have been released to farmers for cultivation in Sub-Saharan Africa.
Genetics lends a hand in threshing
Threshing is the process of separating the edible part of grains from the inedible chaff that surrounds them. Sorghum heads are mostly threshed by hand in smallholder communities across Africa, with common ways including trampling by cattle, stick beating and pounding – labour-intensive work often left to women. Poor thresh-ability results in post-harvest loses and in a reduction of grain quality and market value. Different sorghum varieties display varying degrees of thresh-ability, something that farmers have long known, and even sung about and incorporated into their folklore.
While environmental stresses such as late drought play an important role, a study revealed that thresh-ability is controlled by a few major genes. This information is very useful for developing improved sorghum varieties suitable for smallholder agriculture.