AlTSB, a major gene of the Multidrug and Toxic Compound Extrusion (Mate) family confers tolerance to aluminum toxicity in sorghum. This gene is a transporter gene that is responsible for the exudation of citric acid in the presence of toxic level of aluminum in the soil. The citrate complexes with the toxic aluminum forming a nontoxic compound. During the past several years, isogenic sorghum breeding lines, both cytoplasmic male-sterile lines (A and B-lines) and fertility restoring pollinator lines (R-lines) for this AlTSB gene have been developed at Embrapa Maize and Sorghum. These isogenic lines for AlTSB were used to develop sixteen isogenic sorghum hybrids with zero, one and two alleles for tolerance to aluminum toxicity. These sixteen hybrids are essentially genetically equal but with variation in the dose of the AlTSB allele. Seedlings of these isogenic hybrids were evaluated for root growth in nutrient solution with 0, 11, 20, 27 and 39μM aluminum for seven days at four intervals (0, 3, 5, and 7days). One dose of the gene had a very significant effect on maintaining root growth up to a concentration of 27 μM aluminum. A second dose of the gene continued to have a positive effect for some of the isogenic hybrids confirming the effect of partial dominance for this gene. The presence of the AlTSB gene in sorghum cultivars used in regions with acid soils or subsoils will contribute to the development of a better and deeper root system and promote greater and more sustainable productivity.

Aluminum (Al) toxicity is a major constraint for crop production on acid, Al toxic soils, that occupy over 50% of the potentially arable lands in the world. Aluminum is ubiquitous in soils and when solubilized at pH values below 5.0, becomes highly phytotoxic as Al3+, causing severe yield reduction to sensitive crops. Al tolerance in species such as wheat, barley and sorghum appears to rely largely on a small number of genes within each species. However, quantitative inheritance for Al tolerance appears to be the general pattern observed in rice and maize. In maize, citrate exudation has been suggested as an important tolerance mechanism. However, lack of correlation between citrate exudation and differential Al tolerance observed in some maize lines suggests that other Al tolerance mechanisms may take place in maize. In this study, we undertook a physiological study of Al tolerance mechanisms focusing on citrate exudation in selected maize recombinant inbred lines for which a QTL map had been previously developed. Our results are being assessed in light of the genetic constitution of these RILs for the major Al tolerance QTLs segregating in the population. Our results support the role of citrate exudation as a major Al tolerance mechanism in maize but suggest that other mechanisms probably take place in this population.