A comparison of the kinetics of aluminum (Al) uptake and distribution in roots of wheat (Triticum aestivum) using different aluminum sources. A revision of the operational definition of symplastic Al

Abstract

The lack of information about the movement of aluminum (Al) across the plasma membrane presents a significant barrier to the elucidation of resistance mechanisms which may involve exclusion of Al from the symplast. An understanding of mechanistic aspects of exclusion requires the estimation of symplastic Al levels. Such measurements may be achievable through the use of a kinetic approach. A kinetic protocol was developed to characterize the accumulation and distribution of Al in various cellular compartments in roots of wheat (Triticum aestivum L.). The kinetics of uptake and desorption were similar when Al was supplied as AIK(SO4)2 or as AlCl3. When both salts were supplied at low concentration (50 μM), Al bound to a purified cell wall fraction could be reduced to less than 10–20% of non‐exchangeable Al, if roots were washed for 30 min in citric acid following exposure. In contrast, when AlK(SO4)2 was supplied at a high concentration (200 μM), a strong linear phase of uptake into cell wall material was observed, which accounted for approximately 48% of non‐exchangeable Al in roots. These results suggest that the use of low concentrations of Al in simple salt solutions is required to minimize accumulation of non‐exchangeable Al in the apoplasm. A series of multiple‐desorption experiments confirmed that citric acid was effective in removing Al from the cell wall compartment of roots exposed to Al for short periods (3 h). However, long exposures (48 h) appeared to create conditions conducive to the accumulation of non‐exchangeable Al in the cell wall. In experiments where uptake from solutions containing 50 μM AlCl3 was followed by desorption in citric acid, non‐exchangeable Al in microsomal membrane fractions represented less than 4% of total non‐exchangeable Al. Thus, we can exclude the plasma membrane and cell wall as major sites for accumulation of non‐exchangeable Al in short exposure studies. Although we cannot provide unequivocal evidence for the localization of Al within the symplast, use of simple salt solutions followed by desorption in citric acid provides the best kinetic technique currently available for the quantitation of Al in the symplasm.