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»Funding dates
Phase I: 3/1/2002-2/28/2006
Phase II: 7/1/2006-6/30/2010
»The problem
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.
»The approach
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.
»The goal
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.
Major achievements
- Soils from the Cerrado Region of Brazil are similar chemically and mineralogically to soils from the maize growing regions of Kenya west of the Rift Valley. On the other hand, soils around Mt. Kenya have significantly greater P sorption capacities and higher exchangeable Al.
- Soils sampled in maize growing areas of southeastern Uganda and near Morogoro and Dar es Salaam in Tanzania are similar chemically to soils previously sampled in Kenya and central Brazal, with the exception that the Ugandan and Tanzanian soils have lower phosphorus fixation capacities. 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 and detailed molecular and physiological analyses have been conducted in order to better understand its role in the tolerance mechanism.
- A candidate for the first maize aluminum tolerance gene, which may be responsible for the largest aluminum tolerance QTL, was identified. The candidate is a member of the MATE gene family and a homolog of the sorghum aluminum tolerance gene, AltSB.
- Maize cultivars contrasting for mycorrhiza colonization, Al tolerance and P efficiency were identified and further investigation is underway to determine the molecular, genetic and physiological components underlying these differences.
- Isogenic sorghum lines and hybrids for Al tolerance were developed and are being studied and compared in both field conditions and in nutrient solution.
- The genetics of P efficiency in maize was studied and new breeding strategies to enhance P use efficiency are being developed.
- Brazilian P-efficient, aluminum-tolerant and acid soil-adapted elite maize germplasm were transferred to Kenya and locally adapted acid soil and P efficient maize genotypes are being generated at KARI (Kenyan Agriculture Research Institute).
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