The identification of aluminium-resistance genes provides opportunities for enhancing crop production on acid soils

Acid soils restrict plant production around the world. One of the major limitations to plant growth on acid soils is the prevalence of soluble aluminium (Al3+) ions which can inhibit root growth at micromolar concentrations. Species that show a natural resistance to Al3+ toxicity perform better on acid soils. Our understanding of the physiology of Al3+ resistance in important crop plants has increased greatly over the past 20 years, largely due to the application of genetics and molecular biology. Fourteen genes from seven different species are known to contribute to Al3+ tolerance and resistance and several additional candidates have been identified. Some of these genes account for genotypic variation within species and others do not. One mechanism of resistance which has now been identified in a range of species relies on the efflux of organic anions such as malate and citrate from roots. The genes controlling this trait are members of the ALMT and MATE families which encode membrane proteins that facilitate organic anion efflux across the plasma membrane. Identification of these and other resistance genes provides opportunities for enhancing the Al3+ resistance of plants by marker-assisted breeding and through biotechnology. Most attempts to enhance Al3+ resistance in plants with genetic engineering have targeted genes that are induced by Al3+ stress or that are likely to increase organic anion efflux. In the latter case, studies have either enhanced organic anion synthesis or increased organic anion transport across the plasma membrane. Recent developments in this area are summarized and the structure-function of the TaALMT1 protein from wheat is discussed.