Low-pH-mediated elevations in cytosolic calcium are inhibited by aluminium: a potential mechanism for aluminium toxicity

Aluminium, the most abundant metal in the earth's crust, is highly toxic to most plant species. One of the prevailing dogmas is that aluminium exerts this effect by disrupting cellular calcium homeostasis. However, recent research gives strongly conflicting results: aluminium was shown to provoke either an increase or a decrease in cytosolic free calcium concentration (Ca2+]c). To solve this question, we have adopted a novel approach: Ca2+]c measurements in intact plant roots as opposed to isolated cells, and the correlative measurements of intracellular and external pH. The results obtained show that plant roots respond to low external pH by a sustained elevation in Ca2+]c. In the presence of aluminium, this pH-mediated elevation in Ca2+]c does not occur, therefore any potential calcium-mediated protection against low pH is likely to be irreversibly inhibited. The severity of the inhibitory effect of aluminium on Ca2+]c depends on the concentration of external calcium, thus perhaps explaining why the effects of aluminium toxicity are ameliorated in calcium-rich soils. It seems possible that a primary toxic effect of aluminium might be to impair calcium-mediated plant defence responses against low pH.