Aluminum effects on the kinetics of calcium uptake into cells of the wheat root apex

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Abstract
The effects of aluminum on the concentration-dependent kinetics of Ca2+ uptake were studied in two winter wheat (Triticum aestivum L.) cultivars, Al-tolerant Atlas 66 and Al-sensitive Scout 66. Seedlings were grown in 100 μM CaCl2 solution (pH 4.5) for 3 d. Subsequently, net Ca2+ fluxes in intact roots were measured using a highly sensitive technique, employing a vibrating Ca2+-selective microelectrode. The kinetics of Ca2+ uptake into cells of the root apex, for external Ca2+ concentrations from 20 to 300 μM, were found to be quite similar for both cultivars in the absence of external Al; Ca2+ transport could be described by Michaelis-Menten kinetics. When roots were exposed to solutions containing levels of Al that were toxic to Al-sensitive Scout 66 but not to Atlas 66 (5 to 20 μM total Al), a strong correlation was observed between Al toxicity and Al-induced inhibition of Ca2+ absorption by root apices. For Scout 66, exposure to Al immediately and dramatically inhibited Ca2+ uptake over the entire Ca2+ concentration range used for these experiments. Kinetic analyses of the Al-Ca interactions in Scout 66 roots were consistent with competitive inhibition of Ca2+ uptake by Al. For example, exposure of Scout 66 roots to increasing Al levels (from 0 to 10 μM) caused the K m for Ca2+ uptake to increase with each rise in Al concentration, from approx. 100 μM in the absence of Al to approx. 300 μM in the presence of 10 μM Al, while having no effect on the V max. The same Al exposures had little effect on the kinetics of Ca2+ uptake into roots of Atlas 66. The results of this study indicate that Al disruption of Ca2+ transport at the root apex may play an important role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars, and that differential Al tolerance may be associated with the ability of Ca2+-transport systems in cells of the root apex to resist disruption by potentially toxic levels of Al in the soil solution.