Three plant cDNA libraries were expressed in yeast (Saccharomyces cerevisiae) and screened on agar plates containing toxic concentrations of aluminum. Nine cDNAs were isolated that enhanced the aluminum tolerance of yeast. These cDNAs were constitutively expressed in Arabidopsis (Arabidopsis thaliana) and one cDNA from the roots of Stylosanthes hamata, designated S851, conferred greater aluminum tolerance to the transgenic seedlings. The protein predicted to be encoded by S851 showed an equally high similarity to D6 fatty acyl lipid desaturases and D8 sphingolipid desaturases. We expressed other known D6 desaturase and D8 desaturase genes in yeast and showed that a D6 fatty acyl desaturase from Echium plantagineum did not confer aluminum tolerance, whereas a D8 sphingobase desaturase from Arabidopsis did confer aluminum tolerance. Trivalent cations are toxic to most plant cells. The increased prevalence of soluble aluminum (Al 31 ) cations in acid soils is a major limitation to plant production around the world. Aluminum disrupts a range of cellular processes, including nutrient acquisition, cell wall loosening, nuclear division, cytoskeleton stability, cytoplasmic calcium homeostasis, hormone transport, and signal transduction (Taylor, 1988;Kochian, 1995;Matsumoto, 2000). Many of these symptoms occur rapidly and some workers have concluded that aluminum toxicity is initiated by interactions occurring in the extracellular compartment (Horst, 1995) and cell membranes. Aluminum accumulates rapidly in the highly charged cell wall and near the fixed charges and polar groups on the plasma membrane surface, which can displace calcium from critical sites in the apoplasm, alter physical properties of the plasma membrane, change membrane lipid composition, block ion channels, and disrupt signal transduction processes by interfering with phospholipase C metabolism (Haug