Metabolism and root exudation of organic acid anions under aluminium stress

Numerous plant species can release organic acid anions (OA) from their roots in response to toxic aluminium (Al) ions present in the rooting medium. Hypothetically OA complex Al in the root apoplast and/or rhizosphere and thus avoid its interaction with root cellular components and its entry in the root symplast. Two temporal patterns of root OA exudation are observed. In pattern I, OA release is rapidly activated after the contact of the root with Al ions while in pattern II there is a lag phase between the addition of Al and the beginning of OA release. Compounds other than OA have been detected in root exudates and are also correlated with Al resistance in plants. Plant species like buckwheat and tea show mechanisms of Al tolerance, which confer them the capacity to inactivate and store Al internally in the leaves. Disturbances in metabolic pathways induced by Al are still obscure and their relation to the altered OA concentration observed in roots under Al stress is not yet established. High concentrations of OA in roots do not always lead to high rates of OA release even when the spatial distribution of these two characteristics along the root axis is taken into account. Al induces high permeability to OA in young root cells and anion channels located in the cell membrane have been proposed to mediate the transport of OA to outside the cell. Genetically modified plants that overexpress genes involved in the biosynthesis and transport of OA as well as in Al toxicity events at the cell level have been generated. In most cases the transformations resulted in an improved ability of the plant to cope with Al stress. These promising findings reinforce the possibility of engineering plants with superior resistance to Al-toxic acid soils. The environmental impact of the large amounts of root exudates possibly conferred by these genetically modified plants is discussed, with special emphasis on soil microbiota.