Background and aims: Enhanced aluminum (Al) resistance has been observed in dicots over-expressing enzymes involved in organic acid synthesis; however, this approach for improving Al resistance has not been investigated in monocots. Among the cereals, oat ( Avena sativa L.) is considered to be Al resistant, but the basis of resistance is not known. Methods: A hydroponic assay and hematoxylin staining for Al accumulation in roots were used to evaluate Al resistance in 15 oat cultivars. Malate and citrate release from roots was measured over a 24 h period. A malate dehydrogenase gene, neMDH, from alfalfa ( Medicago sativa L.) was used to transform oat. Results: Oat seedlings were highly resistant to Al, as a concentration of 325 M AlK(SO 4) 2 was needed to cause a 50% decrease in root growth. Most oat cultivars tested are naturally resistant to high concentrations of Al and effectively excluded Al from roots. Al-dependent release of malate and Al-independent release of citrate was observed. Al resistance was enhanced in a transgenic oat line with the highest accumulation of neMDH protein. However, overall root growth of this line was reduced and expression of neMDH in transgenic oat did not enhance malate secretion. Conclusions: Release of malate from oat roots was associated with Al resistance, which suggests that malate plays a role in Al resistance of oat. Over-expression of alfalfa neMDH enhanced Al resistance in some lines but was not effective alone for crop improvement.
