Aluminium toxicity in plants: internalization of aluminium into cells of the transition zone in Arabidopsis root apices related to changes in plasma membrane potential, endosomal behaviour, and nitric oxide production.

The extent of aluminium internalization during the recovery from aluminium stress in living roots of Arabidopsis thaliana was studied by non-invasive in vivo microscopy in real time. Aluminium exposure caused rapid depolarization of the plasma membrane. The extent of depolarization depends on the developmental state of the root cells; it was much more extensive in cells of the distal than in the proximal portion of the transition zone. Also full recovery of the membrane potential after removal of external aluminium was slower in cells of the distal transition zone than of its proximal part. Using morin, a vital marker dye for aluminium, and FM4-64, a marker for endosomal/vacuolar membranes, an extensive aluminium internalization was recorded during the recovery phase into endosomal/vacuolar compartments in the most aluminium-sensitive cells. Interestingly, aluminium interfered with FM4-64 internalization and inhibited the formation of brefeldin A-induced compartments in these cells. By contrast, there was no detectable uptake of aluminium into cells of the proximal part of the transition zone and the whole elongation region. Moreover, cells of the distal portion of the transition zone emitted large amounts of nitric oxide (NO) and this was blocked by aluminium treatment. These data suggest that aluminium internalization is related to the most sensitive status of the distal portion of the transition zone towards aluminium. Aluminium in these root cells has impact on endosomes and NO production.