Cereal roots

Lead Organization:

Embrapa Maize and Sorghum

Community of Practice:

East & Southern Africa


Kenya and Ethiopia




The savanna regions of Africa and Latin America are very old land surfaces that are at an advanced stage in the weathering process. This means that the soils are acidic and have low levels of plant available nutrients, especially phosphorus (P) and nutrient cations, and high levels of potentially exchangeable aluminum (Al). When these soils are strongly acidic, aluminum becomes soluble and interacts with phosphorus and calcium to render them unavailable. Since soluble aluminum is toxic to the roots of most plants, these soils are considered to be highly marginal for the production of most food crops, unless special management strategies are employed. Strongly acid soils can be rehabilitated by liming. Increasing soil pH to 5.5 eliminates the aluminum toxicity problem and increases the range of crop types that can be grown, although nutrient additions are still required to achieve good crop productivity. Liming, however, does not correct the acidity of the subsoil below the plow layer (0-20 cm) where root growth is restricted in susceptible cultivars causing susceptibility to drought and restricted nutrient uptake. Also, in many poorer areas of the world where soil acidity is a significant constraint, agricultural lime and fertilizer inputs are not readily available or affordable.Where lime is not a viable solution to the soil acidity problem and where acid subsoil persist, farmers must rely on Al-tolerant plants. A number of important tropical food crops, such as cassava and sweetpotato, are able to grow in soils with a pH as low as 4. Special Al-tolerant genotypes have also been identified within otherwise Al-intolerant crop species, such as maize and sorghum. These Al-tolerant genotypes modify the rhizosphere environment by secreting organic acids that detoxify Al as well as allow the plant to access P more efficiently. Over time, Al-tolerant crop genotypes have been selected by farmers and breeders working in areas with acid soil problems. Today’s plant breeders hope to accelerate the creation of new Al-tolerant and P-efficient food crop varieties by using marker-assisted breeding and genetic transformation.The second phase of the project “New Approach for Improving Phosphorous Acquisition and Aluminum Tolerance for Plants in Marginal Soils” is a joint activity between Embrapa, Brazil, Moi University, Kenya, Purdue University, USA and USDA-Cornell University, USA undertake both basic and applied research modules aimed at improving the productivity of maize and sorghum in the Brazilian and East African acid soils.The team’s research activities seek to advance the understanding of key plant and soil factors that influence Al-tolerance and P-acquisition and use efficiency in maize and sorghum. The plant studies focus primarily on the molecular, genetics, physiological and breeding aspects of Al-tolerance and P efficiency, while the soil studies are concerned with understanding how various soil components influence and/or interact with the plant responses. This information will provide a better understanding of the soils components and on the plant mechanisms of Al tolerance and P efficiency acquisition in order to generate maize and sorghum cultivars with better adaptability and improved yields under stressful acid soils.

Grant Aims:

The goal of this project is to develop knowledge of factors affecting crop Al tolerance and P acquisition and use efficiency in plants; to develop Al tolerant and P efficient maize and sorghum cultivars that will help increase crop productivity, sustainability and food security in regions where acid soils currently limit maize and sorghum productivity; to create an effective model for South-South collaboration.

Outputs and Outcomes:

Soils sampled in maize growing areas from Kenya and Brazil showed high Al saturation and low P availability, while the soils from Uganda and Tanzania had lower P fixation capacities. The limitations of Kenyan acid soils for crop development were confirmed by field experiments that clearly demonstrated the significantly increase of maize yield under application of P fertilizers combined with lime. Then, maize and sorghum cultivars with superior Al tolerance and P efficiency are recommended to increase crop sustainability in both countries, and improved germplasm from Brazil are likely to promote a great impact in Kenyan breeding programs.Organic acids added to both Kenyan and Brazilian soils in quantities compatible with potential plant exudation into the rhizosphere increases phosphate solubility several fold.A major aluminum tolerance gene (AltSB) was cloned in sorghum, controlling citrate exudation in root tips. The two major Al tolerance QTLs in maize harbor homologues of the AltSB gene, indicating that both crops may share, at least, one common Al tolerance mechanism.A wide collection of sorghum and maize genetic materials were characterized and developed using Kenyan and Brazilian germplasm. As genetic stocks contrasting for Al tolerance and P efficiency can be highlighted near-isogenic lines, near-isogenic hybrids and segregating populations, which will be used in advanced molecular and physiological studies. Additionally, elite lines and hybrids of maize and sorghum were improved by conventional and/or marker-assisted breeding achieving superior levels of Al tolerance and P efficiency. A considerable number of maize lines and crosses are now available in Kenya combining Al tolerance, P efficiency and disease resistance, mainly for GLS.The screening conditions for Al tolerance were established and can be performed at Moi University as well as at Embrapa or USDA/Cornell. We currently defined an early screening condition to differentiate maize seedlings contrasting for P efficiency, which is an important achievement towards a large scale germplasm characterization, combined with the automated imaging system developed to analyze root architecture. Improvements on growth systems and software for 3D imaging and quantification will continue provide great opportunities for studying the dynamic growth responses of roots systems under a range of nutrient and stress environments.Microorganisms from Cerrado soil showed ability to solubilize P from rock phosphate, which can be applied to improve the amount of soluble P from different sources of phosphates.Specific markers were developed to tag the superior Al tolerant allele of the AltSB gene. The markers are easily scored in agarose gels and have been validated in a sorghum diversity panel in order to be applied in sorghum breeding programs worldwide.Transgenic maize seedlings over-expressing the sorghum AltSB gene under control of ubiquitin promoter presented a superior Al tolerance after growing for 5 days under {60} uM activity of Al, indicating that this gene may be effective to improve Al tolerance in other species.