Al binding in the epidermis cell wall inhibits cell elongation of okra hypocotyl

Al inhibits root elongation at micromolar concentrations, but the mechanism s leading to this process are unknown. In these studies, Al-induced inhibit ion of cell elongation was examined using hypocotyl of okra (Abelmoschus es culentus Moench cv, Clemson Spineless) as an experimental model. One-h expo sure to Al (0.5 mM AlCl3) in the presence of 10 mu M auxin in 0.5 mM CaCl2, pH 4.0 significantly inhibited auxin-induced cell elongation of okra hypoc otyl segments. Elongation was further suppressed with increasing Al concent rations up to 1 mM, Treatment of the hypocotyl with 1 mM citrate for 10 min utes after 2-h exposure to Al resulted in significant recovery of elongatio n. The amount of Al in the cell wall relative to the total in the tissue wa s 96.0, 96.2, and 85.4%, respectively, following 1-, 2-, and 3-h exposure t o 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 i n the epidermis, and 95% of the Al in the epidermis was associated with the cell wall. Experiments using split hypocotyl segments showed that Al expos ure increased the outward bending of hypocotyl segments, suggesting that th e epidermis elongation was specifically inhibited by Al. Al inhibited the a utolysis of epidermis by about 20%, but had little effect on the autolysis of core tissue. Taken together, these results suggest that Al binding in th e epidermal cell wall inhibits critical components in cell wall loosening m echanism, resulting in inhibition of cell elongation.