A possible mechanism and sequence of events that lead to the Al3+-induced [Ca2+]cyt transients and inhibition of root growth.

Fluorescence resonance energy transfer (FRET)- sensitized emission imaging of Arabidopsis thaliana roots expressing the yellow cameleon 3.60 calcium (Ca(2+)) reporter showed that the concentration of calcium in the cytosol ([Ca(2+)]cyt) increased upon aluminum ion (Al(3+)) treatment in root cells from the transition zone within seconds. The Al(3+)-induced [Ca(2+)]cyt transients were biphasic and were modified by Ca(2+) channel blockers and by an antagonist of neuronal glutamate receptors, 2-amino-5-phosphonopentanoate (AP-5), and by the anion channel blocker, 5-nitro-2-(3'-phenylpropyl-amino) benzoate (NPPB). The [Ca(2+)]cyt transients were not uniquely associated with Al(3+) toxicity mechanisms since lanthanum (La(3+)) and gadolinium (Gd(3+)) also elicited [Ca(2+)]cyt transients that were similar to those induced by Al(3+). Here a testable model that describes a possible mechanism and sequence of events that lead to the Al(3+)-induced [Ca(2+)]cyt transients and inhibition of root growth is proposed. This model can be applied to study also the signal-response coupling of the trivalent ions La(3+) and Gd(3+).