Uptake of Al across the plasma membrane of plant cells

Measurements of intracellular, cytosolic Al are plagued with technical difficulties. An accurate quantification of Al uptake into the cytosol relies on the effectiveness of the methods that desorb Al bound to the cell wall. However, published desorption methods are not completely effective in removing cell wall Al. Using giant algal cells of Chara corallina, where a physical separation of the cell wall and the cytosol can be achieved surgically, it was shown that up to 99.99% of the total cellular Al accumulates in the cell wall. Even when 95% of total Al present in intact cells was desorbed, still over 20 times more Al was left in the cell wall than in the cytosol. Therefore, without physical separation of the cell wall and the cytosol, minute amounts of cytosolic Al need to be measured in the considerably larger background of the cell wall Al. Consequently, up to several orders of magnitude lower uptake rates of Al were measured across the plasma membrane of intact Chara cells in comparison to currently available values on higher plant cells (Triticum aestivum, Glycine max, Phaseolus vulgaris), where at least some of the cell wall Al was attributed to the intracellular, cytosolic Al. Uptake of Al across the plasma membrane of Chara cells occurs without a delay at a very low rate that is directly proportional to Al concentration in the uptake medium. Moreover, residual Al left in the cell wall after desorption can be taken up into the cytosol of Chara cells during subsequent growth in the artificial pond water. For measuring Al uptake into roots of higher plants, the Secondary Ion Mass Spectrometry is the best available technique because it appears to overestimate the cytosolic Al to the lower extent than any other currently used analytical method for determination of Al.