The Role of the Outer Surface of the Plasma Membrane in Aluminum Tolerance

Abstract

Aluminum (Al) toxicity is the primary stress factor in acid soils. Al tolerance is suggested to be constitutional because of its rapid response. The cell wall of the root-tip acts as the adsorption site for Al ions, but its negativity and Al adsorption capacity cannot explain the differential Al tolerance among crop plant species and cultivars. Among several strategies to tolerate Al toxicity, much evidence has been accumulating on the specific exudation of organic acids (citric, oxalic, and malic acids) from root-tip cells. However, release of organic acids was suggested to be a partial strategy for tolerance to Al ions and not to be a general and satisfactory one. The role of phenolic compounds existing outside and/or inside of root cells under soil acidity stress should be clarified more in detail. Among the apoplastic components (mucilage, cell wall, and the outer surface of the plasma membrane), the plasma membrane is considered to be the determinant of Al tolerance. The structure, function, and genetic aspects of the plasma membrane of root-tip cells that control Al tolerance are future subjects for research. For better growth in acid soils, plants should also possess additional tolerances to low nutrients (phosphorus, calcium and others) under Al stress.