THE ROLE OF THE APOPLAST IN ALUMINUM TOXICITY AND RESISTANCE OF HIGHER-PLANTS - A REVIEW

In acid mineral soils excess of aluminium ions (Al) is one of the most important factors determining plant species and ecotype distribution, and limiting growth and yield of crops. Aluminium preferentially accumulates in the root tips as sites of cell division and cell elongation. Whether inhibition of cell-division rate is due to direct interaction of Al with the chromatin in the nuclei is rather questionable because of the low radial mobility of Al in the root and the rapidity of cessation of root elongation after Al addition to the growth medium. Externally applied Al instantaneously binds to binding sites in the apoplast. Cross binding of pectates by Al may affect extensibility and water permeability of the cell wall. Interaction of Al with other cell-wall constituents is most likely but needs clarification. Aluminium also affects plasma-membrane characteristics. Ca2+ influx and K+ efflux are inhibited, and synthesis of callose is induced. Induction of callose suggests an increase rather than a decrease in cytosolic Ca2+ as initial response to Al. there is little evidence suggesting major disruption of plasma membrane and cytoplasmic functions by Al K+ uptake, H+ extrusion, Fe(III) reducing capacity and lipid peroxidation are hardly affected even in roots severely inhibited in elongation by Al. Al uptake and physiological/bio-chemical effects of Al on intact plant roots can be mimicked even more sensitivity using cell suspension cultures which, therefore, represent a powerful tool for the study of Al toxicity. Large differences in Al resistance exist between plant species and cultivars of a species. Root elongation-rate and callose formation can be used as indicators for al injury. Since short term Al injury in mainly expressed in the apoplast, Al resistance requires exclusion of Al from or/and inactivation of Al in the apoplast. Generally, Al-resistant genotypes are characterized by lower Al accumulation of the root apical meristems. This is achieved by a lower cation-exchange capacity/surface negativity or complexation of Al through root exudates (mucilage, organic acids). Long term exposure of plants to Al also inhibits shoot growth via induction of nutrient (Mg, Ca, P) deficiencies, drought stress and phyto-hormone imbalances. Such longer term effects have to be taken into consideration when selecting genotypes for high yielding capacity on acid oils high in available Al.