Metabolism and root exudation of organic acid anions under aluminium stress

Abstract

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.

Transcriptome profiling identified novel genes associated with aluminum toxicity, resistance and tolerance in Medicago truncatula

“…In A17, it is possible that although Al can induce expression of the MATE gene, subsequent activation of organic acid eZux might be delayed or inhibited. We also did not observe up-regulation of genes encoding organic acid biosynthetic enzymes in our study, which is not surprising since several studies have shown that there is no apparent correlation between Alinduced expression of organic acid biosynthetic enzymes and increased activity and exudation of organic acids (Mariano et al 2005). Future work will involve determining whether the M. truncatula MATE gene is a citrate eZux transporter and whether its over-expression will result in enhanced Al resistance.…”

“…Although, release of organic acids, which are primary metabolites, is one of the major mechanisms of Al resistance in many plant species, several studies have shown that there is no apparent correlation between Al-induced expression of any of the organic acid biosynthetic genes and increased activity or exudation of organic acids (Mariano et al 2005). Also, under stressful conditions, downregulation of certain metabolic processes may be necessary to allow cells to redirect resources towards adaptation mechanisms.…”

Molecular Basis of Aluminium Toxicity in Plants: A Review

“…Eleven families of pteridophytes presented different nutritional imbalances, mostly in Ca, Mg, P, K depending on Al accumulation [86]. In maize, Al resulted in negative effects on the uptake of micro (Mn and Zn) and macronutrients (Ca and Mg) [87] and K [88] than the Al-sensitive genotypes. Both sensitive and tolerant genotypes of wheat had presented a decrease in K and Mg contents in roots, whereas Ca, Al, Si contents increased [89].…”

Nutritional adaptations of native plants of the cerrado biome in acid soils

“…Aluminum toxicity is an important research topic since many cultivated plants are susceptible in acid soils and their growth and yield are adversely affected by high levels of available Al in the soil (Mariano et al, 2005;Silva et al, 2002;Vitorello et al, 2005). However, very little attention has been paid to native plant communities which tolerate acid soil conditions over large areas in different biomes.…”