Al Binding in the Epidermis Cell Wall Inhibits Cell Elongation of Okra Hypocotyl

Abstract

Al inhibits root elongation at micromolar concentrations, but the mechanisms leading to this process are unknown. In these studies, Al-induced inhibition of cell elongation was examined using hypocotyl of okra (Abelmoschus esculentus Moench cv. Clemson Spineless) as an experimental model. One-h exposure to Al (0.5 mM A1CI3) in the presence of 10 /uM auxin in 0.5 mM CaCI2, pH 4.0 significantly inhibited auxin-induced cell elongation of okra hypocotyl segments. Elongation was further suppressed with increasing Al concentration s up to 1 mM. Treatment of the hypocotyl with 1 mM citrate for 10 minutes after 2-h exposure to Al resulted in significant recovery of elongation. The amount of Al in the cell wall relative to the total in the tissue was 96.0,96.2, and 85.4%, respectively, following 1-, 2-, and 3-h exposure to the Al solution. The total and cell wall Al content was decreased by half after the citrate desorption treatment. Furthermore, 95% of Al was found in the epidermis, and 95% of the Al in the epidermis was associated with the cell wall. Experiments using split hypocotyl segments showed that Al exposure increased the outward bending of hypocotyl segments, suggesting that the epidermis elongation was specifically inhibited by Al. Al inhibited the autolysis of epidermis by about 20%, but had little effect on the autolysis of core tissue. Taken together, these results suggest that Al binding in the epidermal cell wall inhibits critical components in cell wall loosening mechanism, resulting in inhibition of cell elongation.